Below
is an example of this problem with a (Thermo Pride OL 11 oil
furnace).
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.
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 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.
(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
-------------