"Oil Furnace Airconditioning Airflow Problems"

The design of the some OIL furnaces' with their ultra large heat exchanger coming to near the top of the furnace and the blower set to the side at the bottom of the furnace, can be an engineered airflow problem. The blower set to the side blows against the heat exchanger and the back of the furnace which blocks directional airflow velocity thus generating high initial velocity & static back pressures against the blower.

Most installers set the A-Coil directly on top of the furnace with no transitions resulting in another restriction, between the HT-EX, and more blocking of directional velocity airflow and a huge leap in (ESP) static pressure.]Thermo pride states that the E-Coil must be at least 3" above the furnace. Three inches above furnace might work for a small 1.5 or 2-ton A/C, but what about a 3.5 or 4-Ton A/C's required airflow?

MOLO Plumbing & Heating sets the A-Coil at least 6" above a Thermo Pride OL 11 oil furnace. They know the importance of unrestricted airflow! http://www.molocompanies.com/plumbingandheating/index.html

In my opinion, these Thermo Pride OL 11 Low Boy Oil Furnaces' should be designed with a transition-space above the heat exchanger depending on the airflow requirements of the air conditioning application size to be used. There should also be a transition beginning at the top of the furnace and transitioning to the intake area of the evaporator coil.

This would reduce the back-pressure and improve airflow. The worst place to lose velocity and generate ESP back-pressure is below the evaporator coil. Where it needs the velocity and static pressure is at the diffusers.

The low airflow probable cause is "an unbalanced airflow heatload through the evaporator coil, along with "back pressure and extreme turbulence," due to the evaporator coil being too close to the very large oil furnace heat exchanger.

With the DX coil set perhaps illegally close to the heat exchanger thus causing an airflow restriction and high turbulence, a few of the A-coil's circuits may be unevenly heat-loaded. Since the liquid refrigerant is not completely evaporated it will cause the outlet line that the TXV sensor bulb is on to be too cold and the TEV will shut-down the refrigerant flow, which can greatly reduce the BTUH capacity of the DX coil and the entire system.

Additionally, the return air intake should be at the ceiling level, in order to properly heat load the evaporator coil. Old gravity flow supply registers should be converted to diffusers, in order to achieve the proper air throw across the room. To achieve maximum airflow efficiency, --the supply air and return air ductwork must be properly sized, along with oversizing the filter grille areas.

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 back-pressure/turbulence and an imbalanced DX coil circuitry heatload!

Static Regain & Deadly Turbulence: Every time the fps velocity is reduced by poor equipment & duct design, there is a conversion from fps velocity to static pressure. In this case, with the blower off to the side, it not only loses fps velocity airflow energy due to hitting the heat exchanger & furnace side walls, it also hits the evaporator drain pan & the area it sits on, this skyrockets turbulence & static pressure, greatly reducing the blower's ability to deliver the required CFM!

Additionally, they install the evaporator directly on top of the furnace which restricts the airflow into the entry of the e-coil. Therefore the blower loses a great portion of its designed airflow potential before it gets through the e-coil. Increasing blower motor HP is NOT the solution, - the poor airflow design must be corrected!

The required main trunk Supply Air velocity is lost between the heat exchanger and the evaporator drain pan, and therefore there is insufficient velocity and static pressure at the SA diffusers to deliver the throw and requisite CFM!

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The blower curve graph on my ThermoPride OL 11 illustrates the devastation of doubling the static pressure. Coupled with the return air and supply air being at the floor level, and supply air registers that do not throw the cold air upward, the cold air recycling will cause an under heat loaded evaporator coil!

My model OL 11 Thermo Pride, has a quarter HP blower motor, as there is no central air conditioning. (It is a Belt Drive Blower.)

With return air registers and supply diffusers at floor level you have a recycling of cold air which makes it extra difficult to heat load the evaporator coil! An adequate "Temperature Difference" between return air and supply air is absolutely necessary for the evaporator coil to absorb the rated btuh heat load and transfer it to the outside condenser coils.

In northern colder climates, we need some innovative engineering to make it easy to switch from floor to near the ceiling Supply Air/Return Air operation in the summer. The system should also be engineered to provide adequate throw across rooms from the supply air diffusers!

Required fan motor HP varies as to the cube of the rpm blower speed.

My ThermoPride OL 11 Oil furnace has a quarter ¼ HP "belt drive blower motor" producing (without an AC Coil) around 982-cfm. At 700-rpm the graph shows around 0.33" SP at 982-cfm. I have pleaded filters over three (14" X 25") over floor return grilles and another (20" X 25") in the furnace, for a total of 4 filters, three non-inline one 20X25 inline. (With NO EVAPORATOR COIL to cause the above problems with my brother's AC system!) I use room A/Cs. 1.5-Ton needs a 1/3-HP motor on these drive belt units.

Keeping the static pressure as low as possible for air conditioning is the first requirement in an efficient system design.

BTW, what is the average pressure drop across the new +90 high efficiency furnace condensers? That pressure drop should be published by all of the companies!
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One Performance Assessment research revealed:
Several recurring factors were found to account for the inadequate flows:

    * Return ducts and return grills were often undersized
    * Fans were set to medium rather than high speed for cooling operation
    * Filters and cooling coils were dirty with high flow resistance
    * Duct system static pressures were elevated due to circuitous runs, pinched ducts, turbulence, etc.
    * Larger outdoor units were installed without changing the indoor unit. (Wow!)
    * Devices had been added which increased system static pressures.

(Air Conditioning Systems) Typical Static Pressure difference before the fan to after the coil in existing installations averaged 0.54 inches of water column (134 Pa). Darrell U.

My Thermopride oil furnace is rated at 140,000-btuh input with a one/gal per/hr nozzle, however it is 119,000-Btuh input with the (0.85) gal/hr nozzle that is installed.

At the nominal 100-psi oil pressure to the nozzle my furnace has a .85-gal/hr nozzle yielding 119,000-btu/hr input, at a tested 78% efficiency, it should yield around 92,820-btu/hr output.

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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.

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 SA +0.40" I.W.C. plus a RA minus -0.20" I.W.C. equals a total static pressure reading of 0.6" I.W.C.

Record the pressure readings on a service 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.

Darrell Udelhoven
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