Building a cold air return box

[jlmac27]

Hello guys,
I have a question,my wood furnace doesn't have a cold air return on it.Its just two 500 cfm blowers on back.Can I safely install a return box?

 

[KindredSpiritzz]

i dont see why not, looks like the back of my vogelzang furnace.I just have the filter box and filter, not connected to the furnace cold air return

 

[begreen]

If you are tying this into an existing hot air system you must be mindful of how it is connected. Otherwise you may just loop the air through the primary furnace. To avoid this add backdraft dampers (check flaps) . Here is an example connection diagram from Kuuma that show what I mean. Your setup may vary.

 

[jlmac27]

Thanks for the tips,I'm going to add a box and pull air from house to furnace in garage.This was suppose to be easy lol

 

[begreen]

If the furnace is independent then it is pretty easy. Just don't undersize the return air ducting. It should at least equal the supply size.

 

[jlmac27]

The furnace won't be independent.Im going into exsisting duck work.I also was wondering I have a 25' run from supply of woodfurnace to my plenum return on gas furnace.I know of clearances of pipe I'm running,but what about where I connect supply of woodfurnace into gas furnace.Is the supply air really hot at that point?It won't damage anything in my home furnace will it?

 

[KindredSpiritzz]

if you're running it from the garage i wouldnt think it'd be hot enough by the time it gets to the furnace to worry about. When i hooked mine up i had some concern about being to close to where the central air stuff comes into the furnace as i read that could be damaged, but i havent had any issues. You can install a back draft damper coming out of the gas furnace to prevent air from the wood furnace from flowing back into the gas one if you want.

 

[begreen]

The furnace won't be independent.Im going into exsisting duck work.I also was wondering I have a 25' run from supply of woodfurnace to my plenum return on gas furnace.I know of clearances of pipe I'm running,but what about where I connect supply of woodfurnace into gas furnace.Is the supply air really hot at that point?It won't damage anything in my home furnace will it?

That is a long run, it should be properly insulated with good duct insulation. Also, the supply doesn't get connected to the return duct. It should tie into the supply trunk duct with a check flap. Post a simple sketch of what you are proposing.

Is a furnace your only alternative? Have you cleared this with the local inspecting authority? Wood fires generally are not permitted in garages. Also check with your insurance company, they may say no way.

 

[jlmac27]

Thanks kindred,I wasn't counting the 6 foot it has also from wood furnace to ceiling.I tried my best to draw my plan out.If anyone can chime in on if there's any flaws in this.
I have a cold air plenum in attic with 3 cold air ducts that go into it.I was going to put draft dampers in those three to direct air flow correct way.Then take my supply off wood furnace from garage into the top of plenum on house furnace.

 

[begreen]

Dampers in those locations will do nothing. And that is a cold air supply plenum, not return. The backdraft damper belongs in the feed duct to that supply plenum.

Have to be honest here, I think you are a bit over your head on this one. What I am seeing says risky business all over it. If you get code and insurance approval, then work with a pro to see that this is done right and safely.

 

[KindredSpiritzz]

i'd have to agree with Begreen, i can't make heads or tails of your drawing and it looks like you have backdraft dampers all over the place. And like he said any wood burning device in a garage around here is a no no. Im not saying you can't do it, but it is a serious consideration as to whether its wise or not. Originally i thought you just wanted to add a filter box around the motors, not connect them to the cold air return system in the house.

 

[jlmac27]

Yeah you may be right,I'm pretty lost in this project.Might call my heating guy to check it out.insurance was a go.

 

[begreen]

Good plan. If you can get code and insurance approval then bring in the heating guy. Be sure he understands the need for the check flaps (back draft dampers) correctly.

 

[jlmac27]

Does anyone know a way to figure out what two 500 cfm blowers can push?I have a 12" supply off wood furnace how far of a run could I get off 12" round pipe?or split with a "Y" to 2 8 inch pipes?
Was thinking on just making it a independent system leaving out my house duct work.Thanks

 

[begreen]

There are still code issues to pay attention to. To be efficient and safe you will need to run a return duct to the house too. But first you need to get the ok to have a furnace in the garage.

 

[jlmac27]

There are still code issues to pay attention to. To be efficient and safe you will need to run a return duct to the house too. But first you need to get the ok to have a furnace in the garage.

I got the ok from insurance,

 

[jlmac27]

Here it is

 

[begreen]

I got the ok from insurance,

Are garage installations allowed in your jurisdictions?

 

[begreen]

You have a young family to watch out for and this is a serious undertaking. For a safe installation bring in someone who knows code and proper hvac installation. Don't be surprised if he finds this against code. Also, there are requirements for plenum clearance, code , return circulation, venting, etc. that one needs to be aware of. If this unit is used one has to be sure that safety controls are in place, wired correctly and working.

 

[jlmac27]

I understand begreen,I am just trying to figure out my options.I am still going to get hearing guy out to give me some guidance.I will do the labor myself.Also curious how strong my supply will blow with those two blowers.

 

[KindredSpiritzz]

I have two 500 cfm blowers on the back of mine and im not impressed with the push i get out of them and i have a lot shorter run than you do sounds like. Kind of toying with the idea of trying a regular gas furnace blower on the back of it someday. My gas furnace kicks out the air nicely, the two 500 cfm blowers not so much.

 

[jlmac27]

Another question for you fine folks.When I build my return box over blowers can I include the small combustion blower as well inside the box?This won't strain motors?Also it is ok to have blowers inside box,this won't over heat motors on them will it?

thanks again!

 

[jlmac27]

Here's a picture of the back

 

[begreen]

Another question for you fine folks.When I build my return box over blowers can I include the small combustion blower as well inside the box?

Absolutely not. That would be instantly dangerous. Just imagine what would happen when the furnace got up to temperature and the return blowers kicked on. What could possibly go wrong?

It is pretty clear that you have the wrong furnace for the intended purpose. The blowers were not designed to be enclosed.

 

[jlmac27]

Hey good morning begreen,
I see other furnaces that sell the return box as an extra item and the back of the furnaces are built like this one.Why is it a problem?