# What Should Furnace Supply Air Temperature Be



## hammerlane (Oct 6, 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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## Tator1076 (Dec 22, 2009)

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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## hammerlane (Oct 6, 2011)

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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## Tator1076 (Dec 22, 2009)

Any were close to supply of the unit and return of the unit. Ps make sure filter is clean


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## hammerlane (Oct 6, 2011)

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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## Tator1076 (Dec 22, 2009)

You sure can


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## Tator1076 (Dec 22, 2009)

but watch out for a/c coil


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## harleyrider (Feb 20, 2007)

hammerlane said:


> 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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## hammerlane (Oct 6, 2011)

harleyrider said:


> 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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## biggles (Jan 1, 2008)

are those round takeoffs just raw sheet metal :wink: 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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## hammerlane (Oct 6, 2011)

biggles said:


> are those round takeoffs just raw sheet metal :wink: 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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## Doc Holliday (Mar 12, 2011)

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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## Doc Holliday (Mar 12, 2011)

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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## hammerlane (Oct 6, 2011)

Doc Holliday said:


> All ducts need replaced eventually


:laughing::laughing::laughing::laughing:


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## Jackofall1 (Dec 5, 2010)

Doc Holliday said:


> 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.
> 
> ...


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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## hammerlane (Oct 6, 2011)

Jackofall1 said:


> Really, that means that I should open up a duct replacement shop


Im no heating contractor but glad someone else found a little humor in that. That why I threw in a couple of :laughing::laughing::laughing:



Jackofall1 said:


> 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, did the installing contractor do a Manual J to size the furnace properly.


House is about 2800 ft2. There are a total of (25) 6" takeoffs from the main trunk line. (10) feed 1st floor, (10) feed 2nd floor and (5) feed basement.



Jackofall1 said:


> did the installing contractor do a Manual J to size the furnace properly.


Do not know who the contractor was. Been in house for 2 years but was built in 1997. Can you elaborate on what is needed for this.


Also this morning I knew the furnace would run for at least 20 minutes to bring house up to temp from the night's setback. So I set my thermometer in the closest register to plenum and output air read 126 F. Input air was probably 65 so about a 60 degree rise that falls within the parameters.


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## Jackofall1 (Dec 5, 2010)

Sounds like your furnace is developing the necessary temp rise and you mentioned 20 minutes for the house temp to increase to the Tstat setpoint this morning, which sounds good in my opinion, you didn't mention the increase in setpoint.....65 to 70??

Are you in southern Ohio?

A "Manual J" is a comprehensive spread sheet which totals all heat losses in a structure, which is then used to size a furnace accordingly. 

Mark


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## Doc Holliday (Mar 12, 2011)

Jackofall1 said:


> 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.
> 
> ...


 
I am down south and yes, everything and I mean everything (no basements,attic) is always insulated and again, "usually much more humid air" and yes, inside and out.


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## Doc Holliday (Mar 12, 2011)

All metal duct is insulated in both residential and commercial, return plenums and supply plenums as well. Internally _*most*_ times on rectangle duct and externally on round.


Always down here.


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## Doc Holliday (Mar 12, 2011)

The one home I have ever been to that has a basement here had insulated metal rectangle and round cut running thourgh it, none of this "heat rises and rots your floorboards" stuff happening down here. 

It simply can not or will not work uninsulated. 

You should move down here and then start a duct insulation company but you'd go out of business due to the fierce competition from people who actually know what they're doing. :whistling2:


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## hammerlane (Oct 6, 2011)

Jackofall1 said:


> Are you in southern Ohio?


northern ohio...15 miles south of Cleveland


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## hammerlane (Oct 6, 2011)

Doc Holliday said:


> You should move down here and then start a duct insulation company but you'd go out of business due to the fierce competition from people who actually know what they're doing. :whistling2:


Doc.... my ducts are about 14 years old. Some can be seen in the 2nd photo on my post #1. As you can see they are uninsulated. There are no signs of rust either.

In your opinion when should I plan on replacing them due to rust


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## HVACDave (Oct 16, 2007)

Just goes to show you the different concerns for different areas of the country. Doc's obviously working on A/C 90% of the time in a much different climate zone than Ohio. 

Some of the rest of us would have a much different split of heating to cooling demand, at a greater variation of humidity and construction methods. Insulating ductwork in my area is rare, but 99% of houses have a basement, and most of those are finished as living areas.


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## Jackofall1 (Dec 5, 2010)

Doc Holliday said:


> The one home I have ever been to that has a basement here had insulated metal rectangle and round cut running thourgh it, *none of this "heat rises* and rots your floorboards" stuff happening down here.
> 
> It simply can not or will not work uninsulated.
> 
> You should move down here and then start a duct insulation company but you'd go out of business due to the fierce competition from people who actually know what they're doing. :whistling2:


Really the heat doesn't rise south of the Mississippi? Now that is some crazy stuff there and heat is usually used to drive off moisture don't understand the rot thing, not in the least bit.

But yes in the south, because of cooling and high humidity ducting will reach dew point and condensate if not insulated...:thumbsup:

As for experience thats a rather bold statement as you have no idea who I am or what my capabilities are...


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## Jackofall1 (Dec 5, 2010)

If you are interested hammerlane (I take it you like to go fast by that name..:no there are Manual J calc sheets available on the web, just search will cost you about $40 for a single use copy, but I guess as long as your furnace keeps up on the coldest days you would be fine.

Believe it or not it is more efficient to have a furnace run for longer cycles or even continuously for that matter as you don't lose on the heat-up of the heat exchanger on each new cycle.

Mark


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## hammerlane (Oct 6, 2011)

Jackofall1 said:


> Believe it or not it is more efficient to have a furnace run for longer cycles or even continuously for that matter as you don't lose on the heat-up of the heat exchanger on each new cycle.
> Mark


Thats easily understandable. 

Also I watched a manual J calculation video from here:

https://www.acca.org/acca-files/MJ8ae.swf


I don't think I want to go any further than what I have already discovered. 

I'm thinkin/hoping the current furnace will last at least a couple more years. I shop vac the inners at the beginning of each heating season and change the filters regularly. I installed an hour meter I had last year and tied it into the gas valve. I change the filter every 125 hours. When I need a new system I'll make sure the contractor provides a Manual-J spreadsheet.

Thanks for all the comments to everyone even you Doc. 

All have a good Christmas.


****ITS HARD TO SOAR LIKE AN EAGLE WHEN YOU'RE FLYING WITH TURKEYS*****


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## Doc Holliday (Mar 12, 2011)

Okay, so I realize things are different in geograhical regions, why I originally said (again), "in most cases more humid" but I guess most should've just been down south by the gulf. 

Sill, to myself it makes no sense to have heat escape thourgh exposed metal duct to then try and make it up through floor for heating a roOm. Sounds highly inefficient compared to delivering the full potential of heated air through a well insulated duct into a well insulated roOm via the vent. 

Sounds bassackwards but that's just me from way down here in Insulationville, USA.


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## jkv (Oct 23, 2011)

Hea Doc your on the right track. With energy audit coming in more and more eveyone will have to insulate more and start sealing more no matter what region you may live in, if we can get the envelope to 3.5 air changes per hour per BPI standards then people would not have so many ?s about the house being to cold. Keep sealing and insulating those ducts. Your good at what you do keep it up.


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## hammerlane (Oct 6, 2011)

I see everyones point. But look at this angle:

What is the "main"point of insulating? Probably to save money on fuel costs. But you cannot win. You will not save in the long run. 

If we keep insulating and using less fuel, the gas or electric company just raises their rates. 

Say "Natural Gas Company OHIO " sells 1,000,000 CCF of gas annualy at $1.00 per ccf. So their revenue is $1,000,000 a year.

When customers start insulating making their home/businesses more efficient "Natural Gas Company OHIO" may now only sell 900,000 CCF of gas. So to get at their $1,000,000 revenue mark they now charge $1.11 per CCF.

So in the long run THE MAN wins again.

The above scenario occurred with the Cleveland water Department....people using less water(low-flow shower heads, low usage toilets, etc)

http://blog.cleveland.com/metro/2011/03/cleveland_water_department_has.html


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## dseuser (Dec 22, 2011)

There is a different approach for utilities if the state legislatures approve, called decoupling. It allows the electric utility company to make profits from reduced consumption. Currently, as consumption increases, the utility has to build a new plant and pass the costs to consumers, with no incentive to stop this cycle.

Decoupling looks at the cost of the plant and in effect says, make money by helping your customers save money, and save the money of building the new generating plant.


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