Did I do a temperature rise right?


Old 12-20-16, 10:21 AM
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Did I do a temperature rise right?

I have a 10 year old gas furnace. Last year the entire heat exchange unit was replaced because of a crack. The house is 1700sf 1st floor, 800sf 2nd floor, and about 1700sf basement.
The returns are high on the wall.
There is a bypass humidifier that that takes hot air and sends it through the humidifier into the return.

There are 21 ducts. 4 go to the basement; 4 go to the 2nd floor, 4 go to the 1st floor master suite, and 9 go to the rest of the 1st floor.
3 of the ducts going to the basement were closed, as was one going to the 1st floor. One basement duct is sealed off completely; the rest leak pretty badly even though the dampers are closed.

The master suite is cold in the winter and hot in the summer. After 4 years of ignoring the problem, I decided to balance the system. I did that by closing 6 of the 8 open vents going to the 1st floor. The result was an even heat through out the house; but people warned me I might damage the furnace by restricting air too much.

I measured the air temperature at 129 with a new digital thermometer, and 126 with an old dial thermometer. (the new digital measures 72 when next to other thermometers measuring 72; the old dial one doesn't go that low.)
I measure in the main duct about 10" above the furnace before anything branched off. Is that adequate?

A foot above the humidifier the return air measured 79. Does that make sense? Maybe because it is drawing air from the top of the rooms? I measured 85 below the humidifier; I suppose because of the introduced hot air?

If you take the 129 and 85, then I am only 46 degrees.
The furnace says it can handle between 35 and 65 degrees; so I seem to be good.

Now the interesting part. When I turn the humidifier to Summer, which block much of the bypass air, the temperature in the return below drops to 82 degrees; which I suppose is what you expect.
When I then measure the hot air, it has dropped to 126, retaining the 46 degree temperature rise. Does that make sense?

I would appreciate criticism on everything I have done; as I don't want to damage anything.
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Old 12-20-16, 06:35 PM
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Since you are within the temperature rise specification, you should be fine. The difference above & below the humidifier is due to the cooling effect of evaporating water.
Old 12-21-16, 05:18 AM
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Your method to measure heat rise was OK. I would have blocked the hot air to the humidifier to prevent mixing hot air with the return air to measure the air entering the furnace. One method to increase the heat rise is to reduce the blower speed if it is adjustable.
My opinion is the return air of 79 makes the return ducts a heat source for the wall cavities and space surrounding the ducts unless you have the t'stat set at 79. What is the air temp. at the height of the return registers? If its 79 you are OK. To minimize heat loss, the living space temperature should be comfortable and as close to outside temp as possible. Hope this helps.
Old 12-21-16, 08:58 AM
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A big section of the return crosses, and it in contact with a section of heat going out. I suspect some heat is transferred between the two there. When I take the temperature before that point it is 73.

So, the return air 6 feet from the furnace is 73.
The return air at the furnace is 85 between the humidifier and the filter (between the heat transferred between the ducts and the heat that comes through the humidifier)
The temperature of the air leaving the furnace is 129.

The temperature rise of the system is 129-73-56.
The temperature rise across the heat exchanger is 129-85=44.

Both are within the 35-65 requirement, but which is the meaningful number?
Old 12-23-16, 04:50 PM
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I'm an HVAC tech of twenty one years and from what I've read, you are just fine. But to take the supply temperature, you aren't supposed to measure it in the main plenum on the furnace. You are supposed to take the reading on the trunkline close to where the takeoff is off the plenum. That's so you don't measure the radiant heat off of the heat exchangers of the furnace.

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