Below is an example of this problem with a (Thermo Pride OL
11 oil furnace). OIL HEAT
FUEL COST COMPARISONS New!
The low airflow probable cause is "an unbalanced airflow heatload
through the evaporator coil, along with an airflow restriction
due to the evaporator coil being too close to the large oil furnace
heat exchanger.
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
When installing evaporator coils on an oil
furnace, due to the extra-large heat exchanger that is near the top of
the furnace, the coil should be set well above the heat exchanger using
sheet metal to channel the air into the air intake of the evaporator
coil. This will optimize the directional velocity which will increasing
the flow and throw through the supply diffusers.
When the evaporator is set directly on top of the furnace it
results in a restriction of airflow along with extreme backpressure
turbulence, which results in a loss of both velocity and static
pressure in the main supply trunk. The mfg'ers need to re-engineer the
distance from the heat exchangers and design effective transitions to
the intake area of the evaporator coils.
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.
===============
This is merely an illustration of some
useful oil heating airflow testing procedures. This airflow test
is easy to do using a thermometer, --so bear with me. The results only provide an
estimate of cfm airflow, the real clincher for summer airconditioning,
is whether, the system is merely "recycling the cold air" due
to the return air and supply air being at the floor level.
The chemical energy of furnace fuel oil is nominally
rated at 140,000-Btu-per-gal (140,000 Btu/gal)
Fuel oil pressure to the nozzle, on residential oil heat is nominally
rated at a setting of 100-psi.
My oil furnace is rated at 140,000-btuh input with a one/gal per/hr
nozzle and 119,000 input
with a (0.85) gal/hr nozzle.
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 78%
efficiency, it should yield around 92,820-btu/hr output.
Use the room return air temperature subtracted from the supply air
plenum temperature or the closest Supply Air register
temperature. You can read the temperature from a lot of the fan center switches located on the
furnace or up on the plenum.
Formula is: CFM= Btuh output
of 92,820-btuh / divided by (i.e., Temp rise
90ºF times 1.08 or 97.2ºF) = 954-CFM
========
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 through 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 Hp motor "belt drive blower" producing 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") floor returns and
another (20" X 25") in the furnace, for a total of 4 filters.
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!
====================================
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 at least 6 inches above this
model oil furnace to achieve adequate airflow! Use 1/3-HP bklower motor
on 1.5-Ton system.
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.
(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.
On an upflow
furnace, External Static Pressure is measured before the E-Coil to
determine your the mfg'ers ESP Ratings.
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 true graph chart, worthy of printing and
discerning the variables of 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!
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-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 states that the A-Coil must be at least
3" above the furnace. That might work for a small 1.5 or 2-ton A/C, but
what about a 3 to 5-ton A/C's required 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.
===============
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.
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 Supply Air diffusers
to deliver the throw and requisite CFM!
Darrell Udelhoven
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