Below
is an example of this problem with a (Thermo Pride OL 11 Low Boy Oil
Furnace)
Scan of My Oil Furnace Blower Curve
While the image loads switch
back & read this page
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?
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!
========
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.
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!
====================================
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.
====================
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 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.
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.
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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