"Within your particular conditions, select an air conditioning system and Service Tech to optimize the unit's BTUH output and SEER level to achieve optimal comfort, efficiency, and savings. "A lot of older furnace air handlers and duct systems, are not delivering anywhere near the AC Unit's BTUH and SEER Ratings. This is primarily due to inadequate cubic feet per minute (cfm) of a balanced air flow through the evaporator coil circuits, and/or dirty fins/coils and lint filled blower wheel blades. Also, improper location of supply diffusers and return air grills can result in inefficient floor level recirculation of the cold conditioned air, resulting in a lack of a proper heat load through the evaporator coil.
An unbalanced airflow through the evaporator coil (DX coil) circuits can cause a large reduction in heat absorption capacity. The non heat loaded vapor or ultra cold liquid will cause a TXV to shut down the flow of refrigerant to the coil. Superheat charging will be inaccurate when the coil is fed by a piston orifice. Total BTUH capacity could drop 15 to 30% or more. An 18,000-BTUH unit losing 30% of design capacity would be delivering only 12,600-BTUH.
Especially if your system is oversized or there are a lot of low AC load days use an adjustable differential room TSTAT.
TH Differential: Differential is defined as the difference between the cut-in and cut-out points as measured at the room thermostat under specified operating conditions. For example, if the thermostat turns the COOLING EQUIPMENT ON AT 78-F & OFF at 76-F that is a 2 degree differential setting; heating equipment on at 70 degrees F and turns the heating equipment off at 74 degrees F, then the differential is 4 degrees F. Some have half degree increment settings over several degrees of differential spread.
California Research Report on EER SEER pdf - download 07/23/08, SEER Payback Savings cannot be accurately represented! AHIR - SAVE ENERGY - CALCULATORS | Find Your Best Payback Investment Return
Measuring Low Airflow
I normally would measure the airflow with a flow hood, also called a capture hood. You should normally have around 400 CFM (Cubic Feet per Minute) per ton of cooling. Half of the systems I measure have [a mere] 200 CFM per ton, OR LESS. This will be aggravated by a dirty air filter, Some Restrictive high efficiency air filter's or grilles closed in rooms that you are not using. Normally, do not turn the thermostat down below 70º [74º 76º -better] degrees. A/C Tech guru, Kevin O'Neill, CM
First, before doing anything else check the sizing, and thoroughly seal and properly insulate all the ductwork!
Design Engineering and Installation Objectives should be focused towards achieving the most efficient and effective means toward a conditioned space that is within the "Human Comfort Zone, and within an affordable investment 'payback' period."
Summer Comfort Zone
Maximum Comfortable Temperature
Minimum Comfortable Temperature
The above comfort zone was found to be acceptable to 90% of test subjects drawn from a range of age groups and genders, with work and life-styles involving varying levels of activity and clothing. An air conditioning system that establishes and maintains indoor conditions within this zone will provide thermal comfort. It will produce a neutral sensation, occupants will feel neither too hot nor too cold. Above chart and findings From: Home Energy Magazine Online September/October 1996) Sizing Air Conditioners: If Bigger Is Not Better, What Is? by John Proctor and Peggy Albright Toward Optimal Occupant Comfort
If you over pay for over capacity equipment, --you will be paying more every month and will not be as comfortable as you would sizing it right to also achieve the appropriate humidity levels!
The Supply Air & the Entering Return Air delta-T, - tends towards less & less as the EER goes higher,
7 EER or less
'Max' condenser air temp 'delta-T'
15 to 25
14 to 24
Max temp drop 'across' E-Coil
'Max' SA/Return Entering Air 'Delta-T'
therefore, dehumidification could become more difficult at the highest EER levels. The EER & SEER levels widen, as SEER sky rockets.
When a typical HVAC contractor quotes the efficiency of the Air Conditioning equipment's SEER & Btu/hr, and leads you to believe the new equipment will automatically deliver that SEER efficiency & Btu/hr rating, think again. Typically, --installed equipment only operates at 55% to 70% of rated capacity. Oversized equipment is the worst combination there is because the duct system airflow and heatload on the cooling coil are often way off what is required!
Equipment Ratings are only the 'potential efficiency' of that component of the system under perfect conditions." Over half of the system's efficiency depends on correct equipment sizing toward adequate run-time, on the duct system sizing, i.e., on the quality of the complete field-installation!
What you want & need is right sized equipment operating at its optimal ratings within varying conditions, for your optimal comfort and savings.
When considering initial cost and pay-back period, -- is there too much emphasis on ultra-high seer ratings when considering the most effective engineering and marketing path towards achieving the most affordable "Human Comfort Zone" Goals?
"A lower cubic feet per minute (cfm) airflow when air is at 50% Relative Humidity or below, will condense and absorb a larger portion of the air's latent heat." With a properly sized system coupled with a properly adjusted TXV the system will better adjust to varying conditions.
I believe that optimal efficiencies under variable latent heat loads could be effectively achieved through the use of computerized engineering.
The total latent and sensible evaporator heat load needs to be optimized at your normal operating conditions. This will also optimize the condenser's heat-load.A/C OWNERS: Measuring the air temperature rise across the outdoor condenser coils is the easiest check point to determine the total amount of latent and sensible BTUH of heat your air-conditioner is actually transferring to the outside. You will enjoy doing it, doing it could lead to making changes that could considerably improve your Air Conditioning System's performance, thus improving your total comfort while in most cases greatly reducing your cooling bills. SEE CHARTS BELOW.
It is best to have the supply air and return air near the ceiling where the warmest air is located.
By way of background: ARI introduced the Energy Efficiency Ratio "EER" in 1975. This was an "HVAC" industry instituted and policed way "to determine the relative efficiencies" of one unit to another in the cooling mode. "EER" was determined by dividing the published steady state capacity by the published steady sate power input at 80°F dB/ 67°F Wb indoor and 95°F dB outdoor. This was quite objective yet unrealistic with respect to system "real world" operating conditions.
The SEER of a system is determined by multiplying the steady state Energy Efficiency Ratio (EER) measured at conditions of 82°F outdoor temperature, 80°F dB/ 67°F Wb indoor entering air temperature by the (run time) Part Load Factor (PLF) of the system. A major factor NOT considered, is the actual part loading factor of the indoor evaporator cooling coil, that greatly reduces the unit's listed btuh capacity and SEER efficiency level.
Determining which metering device the system has without physically looking
If you do not absolutely know whether the metering device is a TXV, or a fixed orifice device or cap tube.
Hook up your manifold gauges, block off considerable condenser air intake for a short time.
If the suction pressure starts rising, you have a piston, or a cap tube.
If only the high side goes up, you have a TXV.
Have things with you in your van or truck to block-off the condenser air for a short time.
Check every time you are not certain what metering device it has.
There will be a lot of guessing in the future.
Do this procedure on known metering devices to observe the difference.
Report back to me how well it works for you.
In some situations, that could save you from cutting a hole in the plenum.
Squirrel cage wheels with forward curved blades on residential systems
unload when discharge air is blocked off too much & will overload
when there is no static pressure.
There is a preferable ESP range for each Air Handler blower design, that ought to be listed on the blower; they vary at the point of serious unloading.
If you amp-probe check enough of those blower motors, if the amp draw is too low according to its rating, you can begin to tell that the External Static Pressures (ESP) is too high.
Additionally, mfg'ers could list the amp draw at various design ESP numbers, then we could amp-probe & know if it was too far above the amp rating, a duct maybe off,
if amp reading is too low, it is time to check all static pressures & delivered CFM to each room.
I lot of us used to set a nearly empty R-22 cylinder on top of a condenser to warm it a little. Back then fan motors had more HP
& higher amp draws, therefore it didn't seem to cause any harm, just more noise.
Back in the 1960's & 1970's there were a far number of TXV metering devices & some table top condensers' that had the fan underneath blowing up through the coils.
Well, where there were cottonwood trees, nearby clothes dryer lint vents, or a lot of leaves or other debris under the unit, the fan motors would be blocked overload & burnout.
I don't understand the engineering genius of that moronic design.
However, on hot days & a heat-loaded E-Coil,
You could move your wrist over the condenser from outlet up to inlet, & tell if the liquid was taking up too much area of the coils; an overcharged system. - udarrell
Always get the CFM airflow correct, first, if it is a piston or cap tube, use the superheat method to charge it.
If it is a TXV, subcooling is the way to charge it, but check the Superheat to verify the TXV is holding within its known specs
Optimizing the "Evaporator Heat Load" will Optimize the Condenser BTUH Heat Load
Most evaporator coils are too under heatloaded when operating at the normal room temp setting!The airflow should be adjusted to fully load the evaporator coil at the normal room temperature setting! This airflow adjustment will optimize your air conditioner's BTUH and SEER performance. Most air conditioner's have an underloaded evaporator coil at the room temp thermostat setting, where the vast majority of its run time will take place!
Too low an air flow can greatly reduce the capacity of your AC unit. In the case of a thermostatic expansion valve (TEV) (liquid refrigerant metering device); it will simply shut down the flow of liquid refrigerant into the evaporator coil to keep it at the TEV's usual 10ºF Super-Heat setting.
A flow rater type metering device will continue to feed to much refrigerant into the coil which can cause it to drop below freezing temperatures which will block air flow and also can flood liquid back to the compressor causing severe damages to it. Additionally, the refrigerant charge will not be accurate unless it is weighed into the system. There can be NO accurate measure of Superheat without an optimally balanced heat load through all the evaporator coil circuits.
Take the condenser entering air temp and leaving air temp, subtract for the temp-split. As a double verification: You can use the high-side (SCT) Saturated Condensing Temperature minus the outdoors-ambient temperature; the difference gives you the condenser temperature-rise or temperature/split. There is NO excuse for not utilizing this important diagnostic check. Always use an accurate volt meter and amprobe to make sure you are not overloading the compressor's Wattage Service Factor and check the compressor discharge line to see that it is under 225-F.
On wrap-around condenser coil top air discharge condensers' --first, check the condenser entering air dry-bulb temp., and the condenser dry-bulb discharge air temp., while moving the TH around in the air stream. (This will usually be around a 18 to 24 degree condenser temperature split rise. Older units run a higher temperature split.)
Techs should get the condenser air flow data in CFM from the manufacturer's data. Because all the heat discharged by the condenser air flow also includes the converted latent heat of the evaporator's absorbed condensation heat, you can determine the total BTUH of heat exhausted by the AC condenser and thus determine if it is getting anywhere near its BTUH rating. You also need to add the additive heat of the condenser's compressor and fan motor. The indoor blower motor is also a heat contributing factor, not figured in this formula.
You can make up the charts for 10, 12, and 14 SEER units for specific makes. One chart might include many different makes. The 14 SEER is a whole different bucket of bolts, as it uses a larger condenser and a very high CFM for a lower temp-split.
For the uninitiated, Delta-T is the difference between the air temperature entering and leaving the outdoor AC condensing unit. This is a good diagnostic check because it measures the latent heat of condensation as well as the sensible heat absorbed by the vaporizing refrigerant in the indoor evaporator coil. I'm betting when you find out approximately how many BTUH that the AC system is actually transferring outside, you may be shocked. Many new Packaged Units have a very high condenser CFM airflow and a LOW TEMPERATURE SPLIT! Very high SEER units have oversized condenser coils and very low temp-splits!To get the gross BTUH Heatload the Evaporator (DX) Coil is absorbing (which includes both latent, sensible heat) (These are ARI Formulas) There are many ways to figure the amount of heat the evaporator is transferring to the condenser.
First, determine the Gross Rated BTUH the condenser is ejecting.
Condenser’s Gross Btuh = Condenser’s rated CFM X’s Temp Split X’s 0.88
Brother’s Example: Heil, 1.5-ton, with 2-ton DX (evaporator) coil with a TEV refrigerant control, -Condenser Rated at 18,400-BTUH, with a 13-SEER rating.
1400-cfm X’s (13-temp rise X’s 1.08) = 19,656-Gross BTUH heat ejected, subtract the 6,562.5-btuh motor heat additive = only 13,093-NET BTUH transferred from the evaporator (DX) Coil to the condenser, compared to a net heat transfer rating of 18,400-btuh! A loss of 6,307-btuh or over half a ton loss, or over a one-third loss of heat transfer! A one ton condenser would have done almost as much! As the rooms cool it is only a 12-F temp-split or 11,582-btuh output! The actual lack of an adequate DX coil heatload would only require a small one-ton condenser!
Brother Don’s 18,400-Btu/hr Heil central A/C unit.
1400-cfm (outdoor) condenser *Xs 1.08 *Xs 12-F split = 18,144 minus 8,591-Btu/hr motor heat = 9,553-Btu/hr net Xs .80 sensible = 7,642 sensible 1,911 latent.Formula: 7642/1.08/16-F indoor split = "a mere 442-cfm" | 7642/1.08/14-F indoor split = 505-cfm [I want 750-cfm; supply and returns at floor level!]
Also, could be an unbalanced load on the evaporator circuits causing the TXV to shut down the refrigerant flow; among other things.
CONDENSER TEMP-SPLITS - My Brother's Heil 12-SEER Condensing Unit
1.5-Ton - Rated at 18,400-BTUH, Condenser fan CFM 1400 (Total Cond. Watts 2221 X's power Factors0.88 X's= 1887 X's = 1954.48 * 3.413 = 6,670-BTUH Motor Heat additive +18400= MotorPower "Rated Gross Heat Ejection" is 25,070-BTUH / 1400 = 17.9-F Temp Rise Cond/Split. The condenser only gets a 10 to 13-F temp-rise-split, depending on the heat load in the house. Supply air and return air are both at the floor level recirculating the coldest air in the room to the DX coil, the evaporator is NOT being supplied with an adequate temp split heat load or, an unbalanced heatload on the DX coil's circuitry.
The probable cause is "an unbalanced airflow heatload through the evaporator coil. "It's a (Thermo Pride OL 11 oil furnace). Those oil furnaces have a very large round heat exchanger that goes to near the top of the furnace, --due to a low basement ceiling the DX coil sets perhaps illegally close to the heat exchanger causing a few of the coil's circuits to be under heatloaded. Since the liquid refrigerant is not completely evaporated it will cause the outlet line that the TEV sensor bulb is on to be too cold and the TEV will shut-down the flow, which greatly reduces the BTUH capacity of the DX coil and the system. On piston refrigerant control systems, they may flood back liquid which could damage the compressor, unless the system is way under-charge. Thermo Pride could install airflow turning vanes just above the heat exchanger to funnel the air directly into the DX coil, instead of most of the airflow hitting the bottom of the DX's drain pan causing extreme turbulence back-pressure and an imbalanced DX coil circuitry heatload!The chart split listed below is at Condenser Design conditions: Indoor Return Air 80-F dry bulb 67-F Wet Bulb or 50% Relative Humidity as you go up to 99% RH the condenser split could increase by up to 6-F; down as much as 4-F at a low humidity of 55-F Wet Bulb.
(The below condenser temp-splits span 1.5 to 5-ton 12-SEER units. For the 12-SEER units, my figures get 17-F for a 1.5-ton and 23-F for a 2-ton condenser, 1400-cfm listed for both condenser fans). This is a good performance measure that should be part of any maintenance check since there are no duct variables to contend with, the coil is easy to inspect, thermometer calibration isn't much of an issue (if you use the same thermometer for both in and out air), wet bulb temp doesn't matter as you are measuring the latent heat removal too, and the compressor is closely matched to the condenser (the last can vary some with built-up systems of course). About the only thing to mess you up is a slow running condenser fan (a suspect if the motor runs real hot) or an incorrect fan blade or blade position to the venturi of the shroud (easy to inspect).
You can use the high-side (SCT) Saturation Condensing Temperature on your manifold gauge's dial, minus the outdoors-ambient Temperature; the difference gives you the condenser temp-rise or temp/split. There is NO excuse for not utilizing this important diagnostic check. Always use an accurate volt meter and amprobe to make sure you are not overloading the compressor's Wattage Service Factor and check the compressor discharge line to see that it is under 225-F.First, figure the 'rated' gross capacity of the condensing unit. To determine the "Gross BTUH Heat Ejection" of the outdoor condenser: New Data = Let's take the total 'Watts' from the data sheets on an 17,500-Net-BTUH Heil condenser with a 2-ton DX evaporator coil with a TEV/TXV refrigerant control.
Add the condenser motor heat: 1591-watts X's my low Power Factor of 0.90= 1432 X's 3.413= motor heat additive of 4887-BTUH + 17,500-BTUH = 22,387-BTUH Gross condenser heat ejection.
22,387 / 1400-CFM of condenser fan = 16-F X's 1.08 = 17.3-F Rated Temp rise split off the condenser. A properly operating matched system should be within 10% of the condenser's gross temperature rise.
Take the "listed watts" of the compressor and Condenser fan and multiply that wattage by the Power Factor, they used to use 0.90, then times 3.413 to get the BTUH heat additive of the motor, then add the listed BTUH of the condenser to that figure, and then divide by the condenser's CFM. Multiply that figure by 1.08 to get the temperature rise.
Brother Don’s 17,500-Btu/hr Heil central A/C unit.
1.08 *Xs 10-F split = 10.8 X 1400-cfm = 15,120-Btu/hr (outdoor) condenser, minus 4887-Btu/hr motor heat = 10,233-net-Btu/hr Xs .76 sensible = 7,777 sensible .24 X 10,233 or 2,456 latent heat transfer.7777/16-F indoor split = a mere 486-cfm | [I want 750-cfm; supply and returns at floor level!] Also, could be an unbalanced load on the evaporator circuits causing the TXV to shut down the refrigerant flow; among other things.
For the uninitiated, Delta-T is the difference between the air temperature entering and leaving the outdoor AC condensing unit. This is a good diagnostic check because it measures the latent heat of condensation as well as the sensible heat absorbed by the vaporizing refrigerant in the indoor evaporator coil. I'm betting when you find out approximately how many BTUH that the AC system is actually transferring outside, you may be shocked by how far it is below its BTUH rating.
His condenser usually has a 10 temp rise split, the evaporator appears to be under CFM heat-loaded or, it has an unbalanced heatload on the DX coil's circuitry allowing liquid refrigerant in the return line causing the TEV sensing bulb to reduce the refrigerant flow thus reducing the DX coil's heat absorption capacity.
The probable cause is "an unbalanced airflow/heatload through the evaporator coil. "I have a Thermo Pride OL 11 oil furnace. Those oil furnaces have a very large round heat exchanger that goes to near the top of the furnace, --due to a low basement ceiling the DX coil sets perhaps illegally close to the heat exchanger causing a few of the coil's circuits to be under heatloaded. Since the liquid refrigerant is not completely evaporated it will cause the outlet line that the TEV sensor bulb is on to be too cold and the TEV will shut-down the flow, which greatly reduces the BTUH capacity of the DX coil and the system. On piston refrigerant control systems, they may flood back liquid which could damage the compressor, unless the system is way under-charged. Thermo Pride could install airflow turning vanes just above the heat exchanger to funnel the air directly into the DX coil, instead of most of the airflow hitting the bottom of the DX's drain pan causing extreme turbulence back-pressure and an imbalanced DX coil circuitry heatload!.
Do your own figuring based on this formula. Get the Motor Power Factors (PF) of the compressor and fan motor from the manufacturer.
The chart split listed below is at Condenser Design conditions: Indoor Return Air 80-F dry bulb 67-F Wet Bulb or 50% Relative Humidity as you go up to 99% RH the condenser split could increase by up to 6-F; down as much as 4-F at a low humidity of 55-F Wet Bulb.
Do your own figuring based on this formula. Motor BTU/hr additive = Watts X's PF x's 3.413 for Btu/Watts additive added to rated BTUH, divided by condenser fan CFM X's 1.08 = condenser Temp-Split. Get the Motor Power Factors (PF) of the compressor and fan motor from the manufacturers. Some of the temp-split figures need correcting, will do ASAP. Some Splits rounded.
CONDENSER TEMP-SPLITS 12-SEER units - Comfortmaker® | Heil® | Temp Star® - used 0.88 Motor Power Factors
ARI Conditions are: 95ºF-OAT; 80ºF-IDB; 67ºF-IWB or 50%RH | TVA conditions; 95-OAT; 75ºFIDB; 63-IWB or around 50%RH | Try 85ºF-OAT | Outdoor Ambient Temperature (OAT)
1.5-Ton 18,000 21-F Split Cond. CFM 1400 WATTS 1536 1.5-Ton is from actual published DATA - Only ARI Rating Conditions
1.5-Ton 18,000 @ 95ºF OAT; Indoors 75-IDB; 63ºF-IWB or near 50%RH; @ 600-CFM; Outdoor Ambient Temperature (OAT); 18ºF condenser split | @ 85ºF OAT; 67-IWB or 66.5%RH; +20ºF cond. split.
To figure this; units pressure chart, the Temps, instead of IWB the %RH, & CFM, For users, No gauges required, to check if your A/C is near specs! However, the temperatures & indoor humidity make a big differenence in the condenser split. (Airflow & proper load on evaporator!)
Take the both the indoor Supply Air & Return Air DB, WB or %RH , too! If you have an accurate airflow CFM, I can Ballpark the BTUH your A/C or Heat Pump is delivering in the cooling mode.
2.5-T 30,000 21°F Temp-S Cond. CFM 2000 WATTS 2778x.90= 2500=8533+30000=38533/19.2x1.08=20.8° (All ARI Conditions)
3-Ton 35,600 14.8°F Split Cond. CFM 2800 WATTS 3096x.90= 2786+35600=38386/2800=13.7x1.08=14.8°
3.5 T 42,500 17.6°F T-Split Cond. CFM 2800 WATTS 3578x.90=3220+42500=45720/2800=16.3x1.08=17.6°
4-Ton 48,500 19.5°F Split Cond. CFM 3400 WATTS 4174x.90=3756.6x3.413=12821+48500=61321/3400=18x1.08=19.5°
5-Ton 59,000 22°F Temp-Split Cond. CFM 3400 WATTS 5043x.90=4539x3.413=15,490+59000=74490/3400=21.9°
The new Goodman 13-SEER 1.5-Ton Condenser, 2-Ton Evaporator:==========================================================
At 675-cfm 450-per/ton cooling | 85°F OAT | 63°F-IWB | 52% RH | 20°-F ID Delta T | 18,600-Btuh
201-psig 100°F = 15°F cond. temp split - larger coil areas | 80-psig suction
http://www.udarrell.com/air_return_latent_condenser_split.jpg IE Browser's Click for Graph
Page 618, Refrigeration & Air-Conditioning (ARI) Second Edition, © 1987
Those lower SEER units had higher condenser splits than 12-SEER and higher units.
Sorry, I defiled the graph, 90-db outdoor, 80-db indoors with 67 wet bulb/50% RH represents the condenser splits shown above.
Typical matched units from major manufacturers have Sensible Heat Ratios (SHR) in the 68% to 80% range (or 32% to 20% Latent) when it is 95-F outside and 75-F with 50% relative humidity inside. Proper mixing of the air and proper distribution to individual rooms is critical for comfort.
The condenser fan speeds are slower on several of the 10-SEER Tonnage Models. We are only trying to get a figure to go by for a comparison. When new condensers and Evaporator coils "are installed on older air handlers" the new, or old, evaporator coils are usually under heat-loaded. (Always, check voltage and amp draw!)
The Base Spec sheets 12-SEER part no. 421 41 33301 03, Feb 2001. These are the Comfortmaker® units, which are nearly identical to Heil® units. I used the first rating on each tonnage class. While the "Performance Cooling Data" is listed at a 95-F outside ambient temperature, you can adjust the indoor airflow to get the Nominal BTUH Rating at the customer's normal indoor stat' temp' setting and the most outside temperature/degree operating hours.
Take the "listed watts" of the compressor and Condenser fan and multiply that wattage by 0.85 X's 3.413 to get the BTUH heat additive of the motor then add the listed BTUH of the condenser to it, and then divide by the condenser fan's CFM.
By using the various units' "base specification sheet data" from the dealer, you can determine if it is operating near its BTUH capacity rating. Some packaged units run a very high condenser discharge CFM airflow!
Some "Condenser Makes" will have different temp-splits. The 2-ton 10-SEER, Janitrol; GMC; Goodman; with the U-29 E-Coil delivers less btuh, or 23000-btuh, I subtracted a reasonable amount from the total of the wattage and come up with 19 to 20-F temp-split. That is "if" its CFM is 1400, --get the figures on the "different Makes." The figures are used to provide an idea of what the condenser temp-split should be for use by the unit's owner and the service tech.
With a properly sized system and proper evaporator airflow you will have consistent optimal nominal capacity heat absorption and removal coupled with the requisite longer run-time cycles. I believe that optimal efficiencies, with variable latent/sensible heat loads, could be effectively achieved through the use of computerized control system components.
For efficiencies sake, do this immediately. Measure the Return Air duct/chase area. If it's a round duct measure the inside diameter, I'll give you the sq.ins.
Take your air conditioner's btuh and divide it by 120, (or dividing 24,000-btuh by 150 will give you 160 sq.ins., close to a 0.10" return air duct Static Pressure drop) to get the amount of free air square inches for the Return Air duct system.
A 4 ton condensing unit, 48,000 btuh would need 400 sq.ins. or two 16" rd. ducts. A 5 ton 60,000 btuh calls for 500 sq. ins., or two 18" rd ducts for 510 sq.ins. A two ton 24,000 btuh takes 200 sq. ins., or one 16" rd. duct. For a 18000-btuh or 180 sq. ins., go with the 200 sq.in. 16" rd. duct. This will permit building more pressure at the supply air diffuser grilles providing more throw across the room.Before you make all the recheck tests, it is very important that your condenser coil and evaporator coil and indoor blower wheel be truly clean.
This AC system would have to be sized to the combined latent and sensible heat load targets (i.e., 78F/50RH) and the cubic foot volume of air changes that we would like per hour. This would need to be performed accurately to achieve the requisite run time, and CFM airflow through the cooling coil, to achieve our combined comfort zone and unit efficiency goals.
Proper duct sizing and location is important. If you have a high ceiling supply air and return air ducts should be at the floor level so you can take advantage of air stratification. There is no need to cool the air above the occupant height level. To achieve a proper evaporator heat load level with a floor level SA/RA system, an increase to 475-CFM per ton of cooling capacity may be necessary. If the conditioned space is extreme hot it might be wise to shut down some Supply Air ducts and partially cover the Return Air grilles so the condenser doesn't become overloaded.
Most older homes need reduced ambient air infiltration and more effective use of vapor barriers coupled with adequate insulation. Windows are special areas to work on. My upstairs windows around the pulley wheels allowed air to blow in unrestricted from the attic area winter and summer.
All air conditioning condenser manufacturers' should publish the CFM and normal temperature rise range across the condenser coil, so that the service tech's can measure the heat transferred from the evaporator coil. Most high efficiency units will have temperature degree rises between 18 and 25ºF. Older lower SEER condensers can have temperature rises up to 30 degrees.
Such temperature rise data provides a guide to the actual heat transfer by the evaporator coil to the outdoor condenser coil, and therefore also, whether the proper design amount of (cubic feet per minute) CFM of indoor air/per ton of cooling BTU/HR, is passing through the heat absorbing cooling coil.
Let's say you had 0.20-IWC without evaporator coil and due to the large oil furnace heat exchanger near the coil add 0.20 IWC, the coil adds 0.30 IWC for a total of 0.70-IWC . Check the static pressure with a wet coil then check the Units Blower Curve Chart to see if you are getting 375 to 450 CFM per ton of cooling, depending on the humidity removal needs. Is your motor horsepower and blower RPM up to the task?
Subtract 50 cfm from the cfm derived from the formula for the wet cooling coil cfm. It may not be getting the requisite 375 to 450 cfm per ton of cooling, especially if there is a low demand for heating in your area and a high demand for cooling tonnage, or the cooling coil is too close to the oil furnace's heat exchanger causing a restriction and a lot of turbulence back pressure, running static pressures as high as 0.75 or higher water column static pressures (view blower curve chart).
Owner's of AC systems should check the return air filters frequently and the blower wheel squirrel cage curved blades for dust and lint accumulation. If the blower wheel and motor are dirty the evaporator fins and coils will need to be checked on the air entry side, all components must be cleaned to regain btu/hr heat transfer efficiency to original specifications. It will cost you big-time on your cooling costs if you don't keep the blower wheel blades and the cooling coil clean. Also, the blower motor is air cooled and will overheat and burn out prematurely.
The condenser coils and fins must also be clean. Never use household detergents, most detergents have an oil base that will insulate coils and fins which will reduce the evaporator & condenser coil's heat absorbing efficiencies. Use only the proper AC coil cleaning fluids for for cleaning the indoor and outdoor air conditioning heat transfer coils. High pressure water can be used but you must never bend the coil's heat transfer fins, so keep the stream perfectly straight with the fins.
Typically, a system is designed where the appropriate fixed sized metering device bridges (or matches) condenser capacity to evaporator capacity as dictated by the compressor and a specified CFM at a specific temperature/humidity heatload point. With a TEV refrigerant control to the evaporator coils, the CFM range can be any workable CFM from 350 to 450 CFM per ton of condenser cooling capacity. Variable speed blower fan motors "that will provide those 350 to 450 CFM/ton, would be ideal," or adjustable speed belt drive blowers allow the technician to provide the evaporator coil with an optimal heat load at normal indoor temperature and humidity levels. If these factors are ignored and are out of the required specifications your new unit won't deliver the btu/hr or the SEER you paid for. Your new 12 or 14 SEER may be delivering only 8 or 9 SEER.
Subject: What design for lower duct static and lower blower motor HP?
Remember that many oil furnaces have a large round heat exchanger in the center up to near the top, and if the evaporator coil is set too close this can cause extreme air turbulence and back pressure which could be a huge factor in running the static pressure way up!
Need for Low Flow Resistance Residential Duct Systems
Beyond improving evaporator airflow, reducing fan power and duct leakage are two further reasons to promote proper duct design with a lower external pressure drop than those encountered in many research studies.
For instance a duct system moving 800 cfm with a pressure drop similar to that measured in a study (0.63 IWC without coil and 0.83 with the evaporator coil) would result in a power draw of 347-Watts. However, a duct system with a total pressure drop of only 0.20 IWC, or 0.40 IWC with the evaporator coil would produce a power demand of only 167 Watts -- a fan power reduction of 52%. If the compressor electrical demand was 1800 W to produce 24,000 Btu/hr (7032 Watts) of cooling at the coil (not including fan energy), the improvement would alter EER from 10.63 to 11.91 Btu/W -- a 10% net increase in cooling efficiency and capacity.
One HP = 746 watts: with S.P. @0.83" | 347 watts / 746 = 0.465 HP or a Half HP Motor | with S.P. @0.4 | 167 watts / 746 = 0.2238 or 0.25, or a quarter HP motor. Properly sized and laid out ducting is critically important to performance.
The larger AC units are usually short changed on return air filtering area! Figure the sq. in. of your furnace's return air filter. Furnace free area return air filter area should be sized for the largest AC unit it will handle!
The heat-load is determined by the amount of heat the evaporator coil is absorbing from the conditioned areas' --air flowing through it at an optimal CFM heat-load level will properly load the compressor and condenser. Let's take a closer look at the effects of a low heat load on an evaporator coil with a evaporator temperature controlling TEV refrigerant control.
The lower heat-load will cause the temperature sensor bulb to reduce the flow of liquid refrigerant into the evaporator coils resulting in a lower than normal suction pressure which reduces the volumetric capacity of the compressor, and liquid refrigerant will begin to back up in the condenser coils which also reduces its capacity.
This means that your entire cooling system, (which includes the ductwork design), would NOT be delivering the unit's rated BTUH. Lack of an adequate airflow heatload through the evaporator coil will reduce the BTUH transfer of heat by the evaporator, therefore to the compressor and condenser and on to the outside air.
Under very light heat-load conditions the subcooling might appear close to Normal. However, the BTU capacity of the system would be lowered as would the SEER rating because the total amp draw of the system does not drop enough from a fully heat loaded BTU design capacity to come close to the resultant inefficiency.
A fixed orifice would begin to flood the evaporator with liquid refrigerant reducing its capacity, because there wouldn't be a sufficient heat-load to vaporize it. Liquid refrigerant could flood back to the compressor causing irreparable damage.
The relationship between head pressure variation with liquid subcooling and suction superheat is not the same with TEV/TXV when compared to fixed orifice. With a fixed orifice, the relationship is immediately obvious to experienced tech's.
When condenser dT (temperature difference) is very low, a static pressure fan-curve-graphic chart check-up, is required procedure. "On older retro-systems" ("break out your Magnehelic / manometers") this procedure is essential before attempting to charge a TEV system or fixed orifice on older systems that may have serious airflow heat-load mismatches.
Additionally, a liquid line sight glass near the evaporator coil is a help, in that you can recover refrigerant until it begins to bubble. Then add charge according to the manufacturer's Return Air ºF match to the wet bulb / dry bulb listed Temp. difference figures, —Superheat charging table, while also monitoring liquid line Subcooling.
Service techs' put your Magnehelic gauge and Digital Micromanometer to good use to measure the static pressure and then get and apply the blower curve charts on each system you are working on, then you know you're getting the proper evaporator airflow temperature and heat-load to meet the customer's desired humidity and temperature comfort zone. It is always very good practice to measure the external static pressure on all systems; you can do this with a simple magnehelic gauge or with a digital micromanometer. In any case, static pressures above 0.45 IWC should be investigated and reduced if at all possible.
Service techs' use your sling-psychrometers' and do the job right.With an Insufficient Heat Load on the Evaporator COIL:
Suction will be LOW.
Super-Heat LOW - with fixed orifice or flow-rater control (TEV 10ºF)
Head-pressure - LOW
Subcooling - LOW
Compressor Amps- LOW
With the TEV, liquid would probably still back up some in the condenser coils reducing BTU capacity, which it doesn't need as much of with the light evaporator load.
http://www.udarrell.com/air_temperature_drop_evaporator.jpg IE Browser's
Air Temperature Drop Through Evaporator Coil (1987 Period)
Indoor temperature and humidity load variations graph.
Refrigeration & Air-Conditioning (ARI) Second Edition,
Page 624, © 1987
Knowing the operating static pressure is a first essential to revealing the operating CFM. If ductwork retrofitting doesn't solve the problem; Blower wheel RPM and blower motor Horse Power may need to be increased to achieve the optimal CFM to achieve your Unit's rated nominal BTUH and Energy Efficiency Rating. (80% don't).
There ought to be a code requiring every manufacturer of an airhandler or furnace to provide capped taps ahead of the evaporator coil and ahead of the blower for easy static pressure testing access.
Read the pressure on the gauge, and record the reading on the supply side, then on the return side. Use a (+) sign before the positive or supply side reading to show where it was taken, and a (-) sign before the negative or return side reading.
Add the two pressures. Disregard the positive and negative signs before the pressures, because both negative and positive pressures affect the fan as a force, so they must be added together to determine the total resistance the fan has to overcome. For example a +.35" I.W.C. plus a -.25" I.W.C. equals a total static pressure reading of .60" I.W.C.
Record the pressure readings on a diagnostic report or on your service ticket. Write the pressures on the cooling coil for future reference and use. Any future changes in static pressure reveals a change in the system that should be addressed.
If pressures are high check the temperature & Web Bulb at the Return Air entry to the blower wheel against data at room Return Register. Hot moist air could be entering the Return Air Duct!
Required fan motor horsepower (hp) varies as to the cube of the rpm speed: hp2 = (rpm2 /rpm1)3 x hp1 = hp2
Belt driven blower: (new 800-rpm/old 700-rpm)3 | result X's old HP .25
(1.1428571)3 =(1.306122449)= 1.492711314 X .25 = .373177828 HP or 1/3 hp within SF of 1.3
.3333 X's 1.30 Service Factor = .43329 HP maximum load. (Many have a 1.35 SF.)
Brother's 1/4th HP furnace blower curve: at .45" SP @ 700-rpm only gets 500-cfm for 18,400 Btu/hr.
Even at only 350-cfm per ton (way too low for his situation) it requires 525-cfm.
Since his supply and return air is at the floor level and the diffusers are old style for a gravity flow furnace, his needs at least 475-cfm per ton or over 700-cfm.
According to the blower curve chart the 1/3HP would deliver 700-cfm at .55-sp.
This 1/3 HP belt drive blower motor at .65"SP would only deliver 500-cfm.
Finding the "New Static Pressure" SP:SP2 = (rpm2 800/rpm1 700)2 = (1.142857143)2 = 1.306122449 X's SP1 .45-SP = .58-SP2 = or only 600-CFM, or an increase of 100-cfm.
A few calculations and presto, a matched airflow with your systems' heat absorbing coil capacities, delivering near its BTUH, EER, and SEER ratings! (80% don't meet their specs)
You will need a good service tech to make the proper tests, and perform the proper adjustments. Utilizing numerous other energy savings techniques, you'll save tons!
My Scan of My ThermoPride OL 11 Graphed Blower-Curve-Chart
Thermopride OL 11 Graph ipg image - Thank you Dave Staso, CA. for the better expandable image!
"After it loads Right click "Show Original Images" - Move cursor arrow over graph - Click + when 'over graph' for expanded image," then print on the highest quality setting.
Every manufacturer should furnish blower curve charts with their units and also put them on the Internet for service tech's to download and print. Also, air conditioning codes should be updated in respect to proper sizing of the duct work which must include all the pressure inducing factors when sizing the supply and return ducts. Also, illustrate best furnace to evaporator coil transitions, especially on oil furnaces! You should always keep the ESP to 0.5" or mfg'ers listing.The evaporator must be mounted 4 to 6 inches above this model oil furnace to achieve adequate airflow!
Also, air conditioning codes should be updated in respect to proper sizing of the ductwork, which must include all the pressure inducing factors when sizing the supply and return duct systems.
We also need the pressure drop figures on the condensers in the high efficiency furnaces, --that should be a data tag requirement!
Knowing the operating static pressure is a first order essential toward accurately identifying the operating CFM. If ductwork retrofitting doesn't solve the problem; Blower wheel RPM and blower motor Horse Power may need to be increased to achieve the optimal CFM to achieve your Unit's rated nominal BTUH and Energy Efficiency Rating. (80% don't).
There ought to be a code requiring every manufacturer of an airhandler or furnace to provide capped taps ahead of the evaporator coil and ahead of the blower for easy static pressure testing access.Read the pressure on the gauge, and record the reading on the supply side, then on the return side. Use a (+) sign before the positive or supply side reading to show where it was taken, and a (-) sign before the negative or return side reading. Add the two pressures. Disregard the positive and negative signs before the pressures, because both negative and positive pressures affect the fan as a force, so they must be added together to determine the total resistance the fan has to overcome. For example a +0.35" I.W.C. plus a -0.25" I.W.C. equals a total static pressure reading of 0.6" I.W.C.Call your local Utility Company and query them about their energy saving initiatives, if they don't have any, --request that they develop such programs ASAP.
Record the pressure readings on a record sticker on the furnace plenum as a diagnostic report for future reference and use, and on the service invoice ticket. Any future changes in static pressure will reveal a change in the system that should be addressed. Our federal government along with every state and all the Electrical Utility Companies ought to be supporting the testing and upgrading of all air conditioning systems, new and old, in order to reduce electrical demand and brown outs. Check the temperature rise across the outside condensing unit, get in touch with a good AC tech if you even have a hint your system is not operating up to its optimal efficiency level.
Therefore, every manufacturer should furnish blower line curve charts with their units and put them on the Internet for service tech's to download and print. A blower curve graph chart, for discerning the variables of ol furnace belt drive blowers.
Observe how easy it is to fall below the required CFM with a quarter horse blower belt drive motor that was standard with 112,000 btuh output oil furnaces. Measuring the static pressure of the duct system is a must!Also, air conditioning codes should be updated in respect to proper sizing of the ductwork using the "Equal Friction Method," which must include all the static pressure reducing factors in the longest duct run.
Any of the HVAC companies I list on any of my web pages have nothing to do with the information I post on any of my Web pages nor do I assume any responsibility for how anyone uses that information.
All HVAC/R work should always be done by a licensed Contractor! This information is only placed on these pages for your understanding & communication with contractors & techs.
This information is for the edification of contractors and techs. You are liable for what you do! - Darrell Udelhoven
Darrell's Refrigeration Heating and Air Conditioning - Federal Refrigerant Licensed - Retired - Licensed Contractor
Please write me if you have questions! - Darrell
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The Air Side of Air Conditioning - Static Pressure Excessive Airflow coupled with an Excessive Charge will greatly reduce AC capacity Air Conditioning SEER Levels & Evaporator Air Flow - Are you losing 15 to 40% under SEER Rating?
Optimizing the Evaporator BTUH Heat-Load Input, EER, SEER, Ratings
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Air Conditioning Engineered for Latent Heat Removal For high humidity climates
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Darrell Udelhoven - udarrell
Covering The Real Political Issues
Posted: 07/19/02; Updated: 01/08/13