Below
is an example of this problem with a (Thermo Pride OL 11 Low Boy Oil
Furnace).
OIL HEAT
FUEL COST COMPARISONS New! | Oil Furnace Airfow Problems HVAC-Talk FORUM @#10
The design of the oil furnace with its ultra large
heat exchanger coming to near the top of the furnace and the blower set
to the side of the furnace is 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/E-Coil directly on top of
the furnace with no transitions resulting in another restriction and
more blocking of directional velocity airflow and a huge leap in (ESP)
static pressures'. Thermo pride engineer states that the E-Coil must be
at least
3" above the furnace to protect the condensate pan. At least 6" above
oil furnace for airflow!
In my opinion, these low boy oil furnaces should be
designed with 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 heat exchanger
and transitioning to the intake area of the evaporator coil. This would
greatly reduce the backpressure and improve airflow. The worst place to
lose velocity and generate static back-pressure is below the evaporator
coil. Where it needs the velocity and static pressures' 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 restrictions
and wicked turbulence, a few of the coil's circuits may be
unevenly heatloaded. 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. HVAC
Efficiency
Overview E-Mail me if Audio Works > udarrell @ pcii. net Works off my hard-drive here
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 explained:
every time the velocity is reduced there is a conversion to static
pressure. In this case, it not only loses all of the velocity airflow
energy due to hitting the heat exchanger , furnace side walls, and
evaporator drain pan, which also skyrockets static pressure, greatly
reducing the blower's ability to deliver the required CFM!
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 North Country Oil furnace A/C or heat pump
scenario:
Any
good Tech should already know what I am going to illustrate here; this
is for Homeowners & any who can use the least costly way to deal
with this too common a problem in the cold North Country.
We
are moving into the cooling season; however, any changes in equipment
should consider how every component matches with optimal airflow
efficiencies.
My
lowest cost solution to the cold climate oversized Oil furnace with a
small tonnage A/C evaporator coil which requires nearly half the
airflow as heating:
I
ran a check on my late brother’s home & the summer cooling heatload
is about 14,000-BTUH.
The
original scenario, had much less airflow than required for 1.5T
cooling; its 2-T A-Coil wouldn’t flow 1150-cfm for heating
I
did a lot of other figuring; 45,000-BTUH output should heat that small
single story home.
However,
with the 140,000-BTUH input, 112,000 output Oil furnace installed; the
nozzle size can be dropped from one gal an hour (we’ll use 139,000-BTUH
input) to .75 @100-psi, the BTUH drops to 103,500 input, this furnace
tested at .74% efficiency; *103,000 = 76,590-BTUH.
Using
a 90F heating temperature rise, which Thermo Pride can stand; (90*1.06
here) 76,590 / 95.4F is 803-cfm.
Therefore,
the 2-Ton A-Coil will handle the CFM; mounted +6 inches above the
furnace, it has the flow capacity to work okay in both heating &
cooling modes.
With
a third HP belt-drive motor, you could simply adjust the RPM down by
turning the adjustable motor pulley out enough to get to 700 or 600-CFM
for the 1.5-Ton A/C.
The
other solution is to install a multiple speed direct drive motor with
the fan relay energizing the cooling speed tap.
This
rather common situation in cold climates seems not to be properly
addressed my Techs, & the HO does not know why things don't work
well.
We
should all work to improve efficient use of high cost energy ... so
America & everyone wins.
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Required fan motor
HP varies as to the cube of the rpm blower speed.
Also,
at 700-rpm & .2" SP
for heating my Thermo Pride OL 11 with its
quarter Hp motor will deliver 1200-CFM; add a cooling coil, &
at
.5 SP it will deliver
only 400-CFM.
Keeping
the total static
pressure as low as possible and within mfg'ers ESP requirements 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! See blower
curve graph linked below! More
proofs in below Article to get
airflow right.
Why
HVAC Systems are Less Efficient than You May Think
MAY
4, 2011 2:45 PM, BY ROB 'DOC' FALKE
We
pulled test data from over 20 systems from our CommonCents™ database.
This is an online testing and diagnostic software that NCI members use
to collect and analyze information and present their findings to their
customers. These were systems that had complied with his program’s
energy standards and all had received substantial incentive payments.
Here’s what we found:
•
The average operating efficiency was 63% of equipment
rated capacity at
the time of the test.
•
The average total external static pressure was operating at .86-in. of
pressure with an average maximum fan rated pressure of .57-in.
•
The system delivered airflow at the supply registers averaged 261 CFM
per ton at registers. (400-cfm per/ton is normal airflow).
http://link.contractingbusiness.com/u.d?U4Gp0d4hVVSpWIc7C7xs=41
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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.
My scan of my Thermopride
OL 11
http://www.udarrell.com/Blower_Curve_Graph.tif
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 & using
an anemometer to check atual airflow is a must! The Thermo Pride OL
11 requires 1164-CFM of airflow to keep temp rise within 90-F.
That would require at
least a 2.5-ton A-coil at 1125-CFM to get close to enough aiflow, a
2-Ton coil is only 900-CFM.
Darrell Udelhoven
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