Fireplace insert near HVAC return - good or bad?

murinsky · Jan 11, 2011

Hi, I have lurked here for awhile, this is by far the most well informed wood burning group of folks I've found. I have a question that seems like it has been covered (yes I do know how to use the search function!) but most of the answers start out with...."well, it depends....." so I thought I would throw this out and see if anyone has an opinion on my specific situation. If not, that's cool too.

In the attached photo (hope it works), there is a 16x16 HVAC return that services the entire 1st floor, under the ottoman. The fireplace is where my insert will go. I am trying to figure out if the return being so close to the insert will cause draft problems, (bad) and/or be helpful to circulate the warm air from that room throughout the house (good). I would consider the fireplace to have so-so draft, although I am not sure if that matters at all for the new insert.

Let's assume it will be a "medium sized" insert installed "correctly" although when I get to that point I may post a few questions. From reading here extensively I am pretty clear on how to install correctly. The chimney is in the center of the house, but it will only be about 12' from the top of the insert to the top of the chimney.

For reference, the first floor where the fireplace is, is 1250 sq ft, finished basement is about 850 sq ft. The forced air is all 1 zone but the basement does have it's own return. I installed an Evergreen ECM blower motor in the air handler this summer, so running the fan all the time would not be a problem, and I have lots of different speed options as well.

What do you guys think? Is this a mistake that is going to make my living room hot and smoky and the rest of my house freeze? Or is there a chance I can get a nice warm house throughout and cut my oil bill in half?

Thanks!

Mike

1869_Caboose · Jan 11, 2011

I don't have any hard numbers to support my hunch, but my gut tells me you might have a problem, especially before the fire's been lit and you've established a positive draft up the chimney. I have a 14 x 24 grille in my upstairs return that pulls air for about 1800 sq. When the system goes on, the suction coming from that grille actually shuts bedroom doors (up to 5 feet away) that were left open just a hair (enough so that the latchbolt was touching the strikeplate).

begreen · Jan 11, 2011

Code requires the return to be at least 10ft away. A 12ft flue can give anemic performance, even with an insulated liner. Adding a few feet to it could make a nice difference.

murinsky · Jan 11, 2011

Interesting, is it 10' to an open fireplace as well?

pgmr · Jan 11, 2011

Does it cause a problem now with the FP? HVAC systems are supposed to be balanced (same amount of air leaving air handler as returning) so the house doesn't become pressurized or vacuumized (hey, new word!) relative to outside air pressure. Are they in reality? Probably not very many. Does yours have enough of an imbalance to cause a problem? We can't answer that.

I can't think of a reason that an insert would cause any more of a concern than the fireplace, other than the draft might not be as strong when not burning the insert (which means less HVAC heated air going up the chimney).

You can do some experimenting with airflow using an incense or punk stick.

grrzoot · Jan 11, 2011

Just noticed this post couple things.

you have a prefabricated chimney to which you will have to install an insulated liner. You will not be able to install a medium sized stove without breaching the manufactureds installation, i think you will be stuck with a small one. But

you wont have any problems with the return if you install a fresh air intake to the insert, or block open the existing fresh air so that it can feed the stove, that should provide enough fresh air, although it may make the fireplace cold, but will eliminate any kind of pressure problem by allowing the insert to pull air from the existing prefab. If the stove has its own supply of air the cold air return will not matter.

Renovation · Jan 11, 2011

Welcome murinsky, and kudos for you diligence!

The answer is Bad. Code requires returns be at least 10' away from a stove, for two reasons I can think of, and perhaps more:

1. Negative pressure from the return could cause your stove to backdraft when your HVAC fan is running.

2. If your stove does backdraft, or malfunction for some other reason, concentrated gases will be sucked right into your HVAC, and pumped into your bedroom while you sleep.

However, outlets near a stove are a good thing, for they help push the heated air away from the stove.

I suggest moving your return away from your stove, putting an outlet near your stove, and returns in cold, peripheral areas to suck heated air towards them.

amellefson · Jan 12, 2011

I have a 1400 sq ft ranch, forced air hvac and a return about 6 ft away from my insert on the adjacent wall. I am experimenting with fans, off or on, and so far have had no problems. The insert is medium sized and when I first light the fire or reload I leave the door cracked 5 min, then close and continue to adjust the controls. I will note that there is a return and supply vent in every room of the house, so maybe my set up is equalized better. The couple heating guys I have talked to say adding more return airs should not hurt anything.

Would it be a good idea to move it up high on the wall or ceiling cause heat rises, then circulate with the fan? Just a thought.

Lynch · Jan 12, 2011

well that kinda sucks to hear.

my return grate is only 3-4 feet away from where i was planing on putting my stove.
im supose to get a price on chimney install from a guy ill ask him what he thinks.

murinsky · Jan 12, 2011

Thanks for the pointers, had not considered the clearance issue but it is a good point and enough to make me not want to do the project without moving the return or figuring a new place for the insert.

Also to address a few comments:

Pgmr, it is pretty balanced, and doesn't cause problems now with the fireplace. The draft is equally so so when the blower is on or off. The house is from 1954, a lot of original windows, pretty loose anyway. I guess this is why I didn't consider the clearance problem in the first place.

FoxChim, I don't think it is a fabricated fireplace, it may look like it because of the new doors I installed. But it is all cinder block down to the basement and up to the roof, or maybe my definition of a prefab chimney is misguided?

Curious George, the concept of using the return to pull cold air and supply to push out warm air from the room makes a lot of sense and had not occurred to me. I am still thinking about this....

Lynch, let me know what you find out....I was wondering the same thing....If I had the local stove company come out to my house and I was standing there cash in hand, would they have turned a blind eye to the return, if I didn't know any better??

Thanks again for all your help guys, looks like it's back to the drawing board for awhile.

Mike

Renovation · Jan 12, 2011

murinsky said:
Thanks for the pointers, had not considered the clearance issue but it is a good point and enough to make me not want to do the project without moving the return or figuring a new place for the insert.

Also to address a few comments:

Curious George, the concept of using the return to pull cold air and supply to push out warm air from the room makes a lot of sense and had not occurred to me. I am still thinking about this....

Mike

You're welcome Mike.

It's one of those insights that are particularly useful because they're counter-intuitive. Search the threads if you want--folks have had good luck with it.

And kudos for heeding advice!

Burn long, and prosper!

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Fireplace insert near HVAC return - good or bad?

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