Lately I had been noticing that the temperature of the "heated" air
coming out of my gas furnace was barely above room temperature, 75
degrees by my infrared thermometer. Filter is nearly new, flame is a
Turns out that the problem was I'd opened up the registers to a couple
of rooms, and the "redumbdant" register in the living room. After I
closed the registers again, the temperature of the heated air shot up
to 110+ degrees at the furnace.
Now, my gut is telling me that this furnace is undersized for the house
if I can't have airflow to more than one register in the living room,
one in the breezeway and one in the main hallway without the register
temperatures dropping to barely-above-ambient. Any expert opinions on
this? House is 1300sq ft, furnace is 64,000 BTU, location is Rochester,
NY. Former owners were snowbirds and the furnace never had to do more
than keep the house at 50 over the winter.
Is it able to maintain a decent temperature in the house with outside at
zero or 10 below that would be common in your area? is the infrared
giving the air temperaure or hte d uct temperature? The purpose of an
ifrared thermometer is to get a reading of "things" that it targets so your
reading may not be accurate. Try a probe or bulb and see what you get.
There is also a specificatio for the heater saying what hte temperature rise
will be. You measure both entering and leaving air and see the difference.
That will tell you if the heater is functioning at capacity.
They are likely the same when the furnace is running,
with the thermometer fairly close to the register.
Raytek Minitemps have an 8:1 beamwidth, so you'd
want to be within 8x4" of a 4" wide register.
Not enough information to judge the size of the furnace. 75F is
pretty pathetic though. But can you trust that measurement?
As for load/sizing, only an HVAC guy on site looking at your house's
layout, the ductwork, the efficiency of the furnace (which you don't
mention), and a blower door for good measure to judge infiltration
would be able to say with any certainty. Based on square footage
though, it doesn't sound so bad.
Have you ruled out duct leakage on that run?
This is an Amana furnace 80% efficient with a heat rise rating of 30 to
60 degrees. Can't remember the exact model number. Installed in 1999
I'm using the infrared thermometer to measure the temperature of the
inside of the register boxes about 7 minutes into the 10-minute cycle.
The air temperature should be no more than a couple degrees hotter at
75 degrees is fairly accurate. Given the cheap IR thermometer, it may
be a couple of degrees off, but that's about it. My main complaint was
the cold drafty feeling instead of the warm cozy feeling every time the
furnace kicked on. Now that I've got the heat rise back, I've got the
warm cozy feeling back. But I've got all the registers but three
completely closed off with the dampers down in the basement.
The house is warm enough to suit me, but there is a 7 degree difference
in temperature from the living room to other rooms in the house (72 vs
65). One of the registers in the living room is completely dampered
off, and the other one is wide open now, but damping that one doesn't
seem to make much difference. Living room is right over top of the
furnace and main plenum, so I expect some heat transfer happening there.
OK. Doesn't sound like the furnace is necessarily undersized.
It should be capable of producing 120-140F air outlet temps.
The other factor that hasn't been considered here is fan speed.
If it is set to run at one of the speeds above LOW speed,
then outlet temps can drop a lot.
There are a lot of tricky unknowns with duct layout and sizing, etc.,
but fan speed should at last be looked into.
If this is in typical Rochester tract, talk to some neighbors
with similar homes to get their experiences.
One of the trick to a good hot air system is balancing the flow by opening
and closing registers. You may have to play around a bit to get exactly
what you want. We intentionally keep bedrooms cooler, no added heat in the
kitchen, etc. What works for us may not for you so experiment.
Blower speed is a factor also. Some keep the blower on constantly at a low
speed claiming more even temperatures.
64K BTU may not be unreasonable for only 1300 sqft.
I'm in Cleveland w/similar weather and 69K (95%) on a
somewhat larger house.
Instead of the IR probe, take a common outdoor thermometer
or similar and place it right in the air flow to check temps.
If this furnace is a high efficiency condensing model,
they are designed to have quite low plenum temps.
The air from the registers may indeed "feel" cool
if you are used to withering blasts of 160F air from
an old furnace.
If you want to find out what size furnace is needed without
all the complicated measurements (and assumptions),
record what percentage of an hour the thing burns when the
outside temp is (X) deg F. Do this after the house has
stabilized at some comfort level. From here you can
extrapolate the BTU loss for the house per degree (you will need
to know the furnace efficiency, or at least assume some number).
From there it's easy to get the BTU required for the coldest
I've used this scheme and you can get numbers (BTU loss) which
are very repeatable and represent actual losses within a few
On 24 Jan 2007 06:28:56 -0800, email@example.com wrote:
IMHO, don't use a IR therm, use a dial thermometer. You might be
actually pointing at an insulated part. Rather than poking a whole in
your vent, like I did, you can go to registere and take a reading off
that, since that is the actual temp supplied into a room.
Just a guess....
tom @ www.YourMoneySavingTips.com
I'm wondering if there is a major air leak in the return air.
Perhaps the furnace is sucking in only cold cellar air.
It's one of those situations where someone has to come out and
look, and look over the entire situation.
I'm in the Rochester area. What part of the city are you in?
Christopher A. Young
You can\'t shout down a troll.
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