What Should Furnace Supply Air Temperature Be

jaysgardenDecember 19, 2011

Two floor house with an unfinished basement.

Have a Goodman-single stage - natural gas - forced air - 100K BTU furnace. Last year I foil taped all joints in basement trunk line and 6" branches. Also used some mastic.

Main trunk line in basement runs perpendiclar to joists then take offs run in between basement joist cavities then booted up thru subfloor to supply 1st floor vents.

2nd floor vents are supplied by duct runs from basement up thru 1st floor wall cavities. Pretty standard installation I believe?

I only mention this to indicate there are no duct runs in a crawlspace or cold attic.

Now for question:

At the supply register approximately what should the temperature of the supply air be?

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No way to answer that question as each specific furnace has a specification for outlet temperature and temperature rise.

The TR will be dependent upon how the furnace was set up (blower speed) and other factors.

All registers (runs) are not created equally. Depending on the design of the duct system, and the length of the trunks and runs, there will be differences in airflow through each. The longer the run, the slower the airflow, the greater the loss of heat will be along the run.

At a minimum, if you post the full model number of your furnace, someone can give you the specs mentioned above. Any information on blower speed will be helpful.


    Bookmark   December 19, 2011 at 1:56PM
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Goodman model GMPN100-4. Think I need to do my readings after furnace has been burning for at least 10 minutes.

    Bookmark   December 19, 2011 at 3:43PM
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The temperature rise for your furnace is listed as 40 - 70*F. This is the difference in temp of the return air immediately before entering the furnace and the supply air, a few feet downstream of the furnace (out of sight of the HX), after a/c coil or in duct.

If you feel that the air coming out of the registers is cooler than with a previous furnace, it is likely due to the fact that it needs to blow more air to reach the higher AFUE value.

The installation guide (http://igate.northernplumbing.com/specsheets/goodman/gmpn.pdf) does not list a maximum outlet temperature. Try the furnace data plate.

Again, the temperature of air leaving the registers is subject to variation, as I indicated previously.



    Bookmark   December 19, 2011 at 4:51PM
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Thanks....I was going to ask where to take input air temp readings and output temp air readings to check the rise but you answered that above.

I take it if I have to drill a small hole in duct just downstream of A/C coil that is not a problem just cover it with foil tape.

    Bookmark   December 19, 2011 at 5:29PM
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That's fine. What I use are small (flexible) strip magnets.

Makes routine testing easy.


    Bookmark   December 20, 2011 at 1:17PM
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