gas furnace register temperature question


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 perfect blue.
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.
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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.
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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.
Nick
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snipped-for-privacy@rochester.rr.com writes:

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?
-- Todd H. http://toddh.net /
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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 IIRC.
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 that point.
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.
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snipped-for-privacy@rochester.rr.com wrote:

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.
Jim
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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.
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snipped-for-privacy@rochester.rr.com wrote:

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 day expected.
I've used this scheme and you can get numbers (BTU loss) which are very repeatable and represent actual losses within a few percentage points.
Jim
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On 24 Jan 2007 06:28:56 -0800, snipped-for-privacy@rochester.rr.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
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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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