# Furnace Plenum



## ben's plumbing (Oct 6, 2011)

the opening on your new furnace is the plenum size.. Iwould not under cut this size...ben


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## Ishande (Nov 11, 2011)

Ok thanks. That is what I was thinking. Ok but what about when constructing a new one. Is there a standard rule of thumb. I will have to be creating an access spot to put in the "a" frame for the air conditioner. Will I need to get a new AC coil sense it is smaller by an inch per side. How will the coil sit on their?


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## ben's plumbing (Oct 6, 2011)

when we make a new plenum yes we adapt to accept existing coil to sit on then transistion to existing ductwork....your going in the right direction make sure all your air is going through coil.....Ben


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## hvac122 (Dec 10, 2007)

Why the larger furnace? For new addition or something? Larger furnace will require larger suffer all around.


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## Ishande (Nov 11, 2011)

Ben, thanks for the help so far. I appreciate the help.


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## Ishande (Nov 11, 2011)

hvac122 said:


> Why the larger furnace? For new addition or something? Larger furnace will require larger suffer all around.


Yes there was a remodel. It is a cape cod style home and the upstairs was recently finished, with more ductwork. As well as the downstairs. So even with one of those in line duct fan there isn't enough force to heat and cool the house anymore. Or at-least that is what I was told.


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## beenthere (Oct 11, 2008)

Ishande said:


> Yes there was a remodel. It is a cape cod style home and the upstairs was recently finished, with more ductwork. As well as the downstairs. So even with one of those in line duct fan there isn't enough force to heat and cool the house anymore. Or at-least that is what I was told.


You were probably told wrong. Good chance your old furnace was over sized. If you haven't already bought the new furnace, do your own load calc and see if your old furnace can handle the load.

http://hvaccomputer.com/talkref.asp

Your current duct system is probably under sized for the current furnace. And even with adding more duct work for the bigger furnace, unless you change out all of your main duct work, will still be under sized.

Its not the power of the blower that counts as much as it is the size of the duct work.


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## Ishande (Nov 11, 2011)

Oh, well I have already purchased the new one. On a plus side it is more energy efficient then the other one. Still yet I would like to swap out the furnaces. How do you change the duct work for the whole house? I would think that would be a huge hassle. At any rate, the plenum is my issue at hand. When making the new plenum if I am not to restrict any airflow directly above the opening am I to make a "shelf" on top of a "shelf"? Being that the AC coil is smaller than the opening. Could I put the coil in the bigger duct work above the plenum?


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## beenthere (Oct 11, 2008)

The coil can go in the plenum above the furnace. Depending how small the coil is, you could also have trouble.

If your old furnace was a 60,000 BTU input 80% efficient furnace, then to maintain a temp of 60°F it needed to move 740 CFM. if your new furnace is a 80,000 BTU input 90% efficient furnace then in order to maintain a 60°F temp rise it needs to move 1111 CFM. A 150% increase in air flow. That can make for loud duct work.


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## joepierson (Oct 26, 2011)

Ishande said:


> Oh, well I have already purchased the new one. On a plus side it is more energy efficient then the other one. Still yet I would like to swap out the furnaces. How do you change the duct work for the whole house?


You can think of the furnace as a heart, the bigger the furnace the bigger the arteries (ducts) need to be to support the flow, otherwise the furnace system is going to have high blood pressure and everything is going to fail prematurely (namely the heat exchanger because it will be constantly overheating since not enough cool air will be passing by it). In addition, high efficiency furnaces need bigger ducts then low efficiency furnaces of the same size. 

The bigger furnaces are designed to move a bigger volume of airflow through bigger ducts at the same speed, not move faster airflow through smaller ducts.

Anyhow, you don't need to replace the ducts just add more, like a heart bybass.


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## Ishande (Nov 11, 2011)

How do you add more air ducts then? So basically what you are saying is I should have bought the same BTU furnace.


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## Technow (Nov 12, 2010)

Ishande said:


> How do you add more air ducts then? So basically what you are saying is I should have bought the same BTU furnace.


No, they were saying to do the calculation determine what size furnace you really need and then fix the duct to deliver the air the right sized furnace needs.


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## joepierson (Oct 26, 2011)

Ishande said:


> How do you add more air ducts then? So basically what you are saying is I should have bought the same BTU furnace.


generally yes, the golden rule for DIY projects is to replace not upgrade (assume the original was sized correctly for the ductwork), upgrading usually requires professional help, in my opinion, because there are side effects which need to be accounted for. I wouldn't upgrade to a more efficent furnace either. 

Your new furnace might start shutting off when it overheats to save itself, or it might work ok and you would have just noisy registers.

you add more ducts by adding takeoffs from the supply plenum and running parallel paths with your existing ductwork and connecting them together at a later point downstream (i.e., helper ducts), this will reduce resistance

you said you added duct work for the remodel, but most likely you tapped off existing trunk lines and not the plenum, correct? This generally won't work well.


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## Ishande (Nov 11, 2011)

Yes, I did tap into the existing lines to run the ductwork upstairs. So apparently from what I am gathering is that I need to make a new plenum that is the correct size. Is their a way to calculate how big I need to make the new one?


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## beenthere (Oct 11, 2008)

A bigger plenum isn't all that you need. Your trunk lines are too small for the larger furnace. Without increasing the trunk lines it will cycle on its high limit. It will have a short life span.

Sell it on ebay and get the correct size.


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## joepierson (Oct 26, 2011)

Ishande said:


> Yes, I did tap into the existing lines to run the ductwork upstairs. So apparently from what I am gathering is that I need to make a new plenum that is the correct size. Is their a way to calculate how big I need to make the new one?


No, the plenum is fine , you need to increase the trunkline coming off the plenum, either by replacing it with bigger one or adding a second one to help out the original one. Use this as a guide

http://webcache.googleusercontent.c...entid=488&d=...+cfm+duct+size+typical&ct=clnk


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## beenthere (Oct 11, 2008)

Also gonna need the additional supplies to get rid of that air.


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## Ishande (Nov 11, 2011)

I appreciate all the help so far. I can clearly tell that this is a much larger job then what I had anticipated. Is it possible for me to do some calculations on things and see if the ductwork can handle the furnace. Why would you have to beef up the trunk lines and not the plenum?


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## Ishande (Nov 11, 2011)

Ok I keys read that. Bigger volume. But hypothetically can't I make the plenum bigger so it takes more air to fill up? Just because it would be the easiest thing to fix. Sense everything starts there. Then if needed add bigger trunk lines. Because that would be hard to do with the basement. Also I will run an extra duct from the plenum so I will take more volume there.


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## joepierson (Oct 26, 2011)

Ishande said:


> Ok I keys read that. Bigger volume. But hypothetically can't I make the plenum bigger so it takes more air to fill up? Just because it would be the easiest thing to fix. Sense everything starts there. Then if needed add bigger trunk lines. Because that would be hard to do with the basement. Also I will run an extra duct from the plenum so I will take more volume there.


You can make the plenum bigger but it's not going to make any difference if you then force the air through small trunk lines.

Running extra duct directly from the plenum is what you want, to the remodeled part of the house.


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## Ishande (Nov 11, 2011)

Ok. I see. But how much is the question. I don't really understand that chart that you previously sent. Is that the cubic feet of air per plenum only? How does trunk line convert to cubic feet? Do you have some other charts that I can reference?


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## joepierson (Oct 26, 2011)

Ishande said:


> Ok. I see. But how much is the question. I don't really understand that chart that you previously sent. Is that the cubic feet of air per plenum only? How does trunk line convert to cubic feet? Do you have some other charts that I can reference?


You need to figure out the heat loss from each room and that gives you the required cfm/room. Scientific results are obtained from procedures called Manual J, S, T, and D.

Rules of thumb are about .75 to 1.25 cfm/square foot, or use neighbor's data. So if you have a 400 square foot room, you might need a 12x8 duct running to that room (400cfm).


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## Ishande (Nov 11, 2011)

Sorry I haven't replied to anything recently. I have been gone on business. So I measured the trunk lines coming off of the furnace. It is a 8"x12" duct that runs 17' and 15'. From there, there is several runs to the rest of the house. Would those runs be able to hold the 100,000 btu furnace that I bought? I can easily add more lines if necessary but it would be a little hard to make the trunk lines larger.


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## beenthere (Oct 11, 2008)

The trunk line is TOO small for a 100,000 BTU furnace.


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## Ishande (Nov 11, 2011)

How are you calculating this?


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## joepierson (Oct 26, 2011)

An 12x8 duct is usually designed to carry 400cfm (this will give you the correct static pressure and velocities), you have two that is 800cfm. That is typical airflow required for a 40K furnace.

A 100K furnace wants to drive 1600 to 2000 cfm of air to be happy (Check the manual for nominal airflows for your furnace). You need a couple 22x8's or four 12x8's to handle that flow and keep the velocities and static pressure down to reasonable levels.


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## Ishande (Nov 11, 2011)

Ok, so I am guessing that when a house is built the duct system in the house is what it is and you really can't stray far from what is already there? So how is anyone to improve what they have? So I understand that if I want to use this furnace I have to add more trunk lines but how long will the other trunk lines have to be? And thanks I Appreciate the help and advice.


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## beenthere (Oct 11, 2008)

There are ways to improve the duct system. But i highly doubt that your house needs that big of a furnace. And using it will probably make you pay a lot more for heat then you have been.


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## Ishande (Nov 11, 2011)

How so? If you recall earlier discussions my original thoughts where bigger furnace more air. But clearly my ducts can not handle that furnace. And I really don't think I can add that kind of ductwork because of my finished basement. And I have added more lines off of my furnace but they all come from the trunk lines that are existing. So what size furnace should I get then. Because the one I have just doesn't cut it. I have a 55k btu now? Could I benefit from getting a 65k btu instead of the 100k btu I already bought?


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## beenthere (Oct 11, 2008)

First, is it that it doesn't cut it because the duct work may already be too small for it?

Can you check its temp rise across the heat exchanger.

Its possible that your return duct is too small, and if it was larger, or the return system had some minor alterations, it would be able to move more air.


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## Ishande (Nov 11, 2011)

How do you measure the temperature across the heat exchanger?


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## beenthere (Oct 11, 2008)

Put a probe in the supply duct 2 to 3 foot away from the plenum, and then measure the return air temp at the furnace.


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## joepierson (Oct 26, 2011)

Ishande said:


> How so? If you recall earlier discussions my original thoughts where bigger furnace more air. But clearly my ducts can not handle that furnace. And I really don't think I can add that kind of ductwork because of my finished basement.


This seems to be the root of your problem, you are prioritizing aesthetics over functionality. (Which is why most duct work is undersized to begin with.) You can add ductwork in a finished basement and enclose them, or use spiral ductwork and keep it exposed or paint it. 

http://www.agentpanelgold.com/agents/matthorton/imagelibs/private_library/4020 azlofts interior.jpg

I agree with beenthere though, you existing furnace is probably big enough, you just need more ductwork to move the air. 100K can heat a 4000 square foot house.


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