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Old 12-19-2011, 01:09 PM   #1
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What Should Furnace Supply Air Temperature Be


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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Last edited by hammerlane; 03-05-2013 at 10:30 AM.
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Old 12-19-2011, 03:19 PM   #2
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What Should Furnace Supply Air Temperature Be


If you need to see if the furnace is heating at the right temp. Then you need to check heat rise.

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Old 12-19-2011, 03:27 PM   #3
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What Should Furnace Supply Air Temperature Be


Just noticed on the rating plate there should be a temp rise of between 40 to 70 fahrenheit. SHould I check the return air temp(input air temp) right behind the air filter for the beginning temp? And still check the supply air(output air temp) at a register or as close to the plenum as possible?
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Old 12-19-2011, 03:40 PM   #4
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What Should Furnace Supply Air Temperature Be


Any were close to supply of the unit and return of the unit. Ps make sure filter is clean
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Old 12-19-2011, 03:41 PM   #5
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What Should Furnace Supply Air Temperature Be


roger that... so not a big deal if I drill a small hole into plenum to stick thermometer probe to obtain output air temp.

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Old 12-19-2011, 03:47 PM   #6
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What Should Furnace Supply Air Temperature Be


You sure can
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Old 12-19-2011, 03:47 PM   #7
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What Should Furnace Supply Air Temperature Be


but watch out for a/c coil
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Old 12-19-2011, 03:53 PM   #8
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What Should Furnace Supply Air Temperature Be


Quote:
Originally Posted by hammerlane View Post
roger that
You also need to make sure that the gas input is correct, manifold pressure should be 3.5 inches of water collum. there is no set temperature that is supposed to be coming out of the registers, I can tell you that in the old days when gravity heat was the norm, mom could dry her laundry on the vents. Now a days we call high efficiency heat "cold heat".
you can get higher register temps by bumping up the gas pressure (slightly never above 3.9 wc) or by slowing down the blower if it is not already on low speed.also try running a pleated , or more restrictive filter.Just keep in mind that the hotter you run your furnace, the faster you put nails in its proverbial coffin.
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Old 12-19-2011, 03:59 PM   #9
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What Should Furnace Supply Air Temperature Be


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Originally Posted by harleyrider View Post
You also need to make sure that the gas input is correct, manifold pressure should be 3.5 inches of water collum. .
I can check the air temp but as you suggested above A little above my pay grade.

Also good call on slower blower spead and pleated filter.

@Tator....thanks for reminder on evaporator coil
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Old 12-19-2011, 04:15 PM   #10
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What Should Furnace Supply Air Temperature Be


are those round takeoffs just raw sheet metal if they are your heating the basement with them as your trying to get the MAX temps up into the occupied...
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Old 12-19-2011, 04:24 PM   #11
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What Should Furnace Supply Air Temperature Be


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Originally Posted by biggles View Post
are those round takeoffs just raw sheet metal if they are your heating the basement with them as your trying to get the MAX temps up into the occupied...
Yes they are 6" round ducts which are uninsulated. I know I'm losing some temperature to the surrounding cooler basement air.

Havent decided if I want to buy and install the round duct insulation.

Wish I knew what the temperature increase in supply air upstairs would be if I did insulate the ducts.

If it only increased 2 or 3 degrees then in my opinion not cost effective to insualte ducts
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Old 12-19-2011, 04:28 PM   #12
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What Should Furnace Supply Air Temperature Be


just a matter of time if it's not too late already before those metal ducts rust. probably already rusted on the inside. that is what insulation is for, partly.
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Old 12-19-2011, 04:44 PM   #13
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What Should Furnace Supply Air Temperature Be


temperature rise has nothing to do with the ducts outside of having proper air flow so size. what is happening is you are losing or gaining heat after the heat extraction (in cooling) or losing heat (in heating) mode which takes place in the evaporator (colling) or heat exchanger/furnace (heating).

In cooling mode is where you really stand the chance of rusting out those ducts. The cold, dry air on the inside mixes with the wamer and usually more humid air on the outside and so the moisture in the air condenses on or in the duct.

Seen it many times, even with (improper or old and worn) insulation. All ducts need replaced eventually and then you really are going to be
spending some bucks, thousands.

Not to mention you are throwing money out the window as your system will run a lot longer, higher electrical bill, etc.
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Old 12-20-2011, 08:36 AM   #14
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What Should Furnace Supply Air Temperature Be


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All ducts need replaced eventually
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Old 12-20-2011, 08:51 AM   #15
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What Should Furnace Supply Air Temperature Be


Quote:
Originally Posted by Doc Holliday View Post
temperature rise has nothing to do with the ducts outside of having proper air flow so size. what is happening is you are losing or gaining heat after the heat extraction (in cooling) or losing heat (in heating) mode which takes place in the evaporator (colling) or heat exchanger/furnace (heating).

In cooling mode is where you really stand the chance of rusting out those ducts. The cold, dry air on the inside mixes with the wamer and usually more humid air on the outside and so the moisture in the air condenses on or in the duct.

Seen it many times, even with (improper or old and worn) insulation. All ducts need replaced eventually and then you really are going to be
spending some bucks, thousands.

Not to mention you are throwing money out the window as your system will run a lot longer, higher electrical bill, etc.
Really, that means that I should open up a duct replacement shop, cause there isn't an installation here that insulates ducts. If you are down south I could see where condensation would form on the outside of the duct when using AC, but not on the inside.

As well as heat loss of those 6" round ducts is lost to the basement, heat rises which warms the floors above, which warms the room above the floors.

Yes if your basement walls are not insulated there will be some heat losses, but nothing really to be gained by insulating the ducting in your basement.

Sure look like there are alot of heat runs for a furnace of 96.5K output, what is the size of the home you are heating, and did the installing contractor do a Manual J to size the furnace properly.

Mark

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