We are currently building an ICF home, and would like some kind of fireplace, primarily for 'atmosphere'. I'm currently looking into direct vent gas fireplaces, and wonder how they work in a tight, well-insulated home. I looked into woodstoves, but it's hard to find one with a low enough heat output for our small home. Gas seems like less work as we get older anyhow. Is there much of an energy hit with a direct vent unit? What about air leakage? I believe they get all of their combustion air from outside. I'm also wondering how you run the metal vent pipe through an insulated attic. There has to be a minimum 2" clearance from combustibles. How is this done with cellulose insulation? I'm envisioning a chase built around the vent pipe from the ceiling to the roof deck. The chase would have to be at least 10" wide or more inside. Is there any way to insulate the inside of the chase where it comes through the ceiling with non-combustible insulation?

Most if not all high temperature flue manufacturers have an attic insulation shield that slips over the pipe in the attic and holds the insulation back the required distance, typically 2". You can further improve this by using non-combustible rock wool insulation batts, aka Roxul, wrapped around the attic insulation shield.

Don't forget to put a fresh air intake pipe sleeve through your ICF wall near the fireplace prior to pouring the concrete.

"Is there much of an energy hit with a direct vent unit?"

Around 2500 BTUs

"What about air leakage? I believe they get all of their combustion air from outside. There has to be a minimum 2" clearance from combustibles. How is this done with cellulose insulation? I'm envisioning a chase built around the vent pipe from the ceiling to the roof deck."

Generally we would put a larger pipe around it to maintain distance and with celulose it would require a lid as well.

" Is there any way to insulate the inside of the chase where it comes through the ceiling with non-combustible insulation?"

Nothing approved that I'm aware of. This is where much of the 2500 BTUs come from

The thing with direct vent is that they should be "direct vented" out the wall. They come with coaxial in/out venting and should be "almost" air tight to the building envelope. The run them up through the attic defeats most of their pluses.

Have you looked at electric fireplaces? They have come a long way from the last century. Many people cannot tell the difference.

Posted By FBBP on 11 Nov 2013 12:34 PM

Have you looked at electric fireplaces? They have come a long way from the last century. Many people cannot tell the difference.

Most of the fireplace heat escapes through the chimney. The requirements for air tightness in passive houses cannot be reconciled with open chimneys. Once you install a fireplace in a home, it automatically disqualifies it for PH certification. I know you didn't mention PH as your goal but the point is that fireplaces and their flues are a constant air source leak in summer and winter.

Green Building Advisor

Touching on what FBBP stated, I am going with an electric fireplace. No punching holes in the walls, no flues, no gas, no carbon monoxide, no back drafting, no air leaks and no energy loss. Today the modern LED fireplaces use high-tech imagery can create a fireplace that looks 100% real. Plus it can put out heat if you want via electric radiant heat. It's technically 100% efficient since 100% of the generated heat stay indoors. If you don't want the heat, you can turn it off and just leave the fire image going with no heat.

I recommend DIMPLEX with OPTY-FLAME. It utilizes water mist to resemble the smoke. It can put out about 5,000 btu's. It comes in numerous designs DIMPLEX

It's too late to run anything through the wall other than the gas supply. Walls are already poured; in fact, I'm ready for drywall inside, which is why I'm looking into one now. Why is running the vent through the attic a bad idea? Not only are my walls poured, but the only place that would work for a fireplace shares a wall with the garage, so I couldn't go out there even if I wanted to. I could go up into the attic, and then out through the gable, as it's higher than the garage. The units I've looked at have coaxial venting.

Electric sounds good, but I'd like a back-up heat source in the event of a power outage. Between being out in the boonies, and in ice storm country, a lengthy outage is a very real possibility. The house is not intended to be PassiveHouse, or anything else, other than code compliant. With ICF and R-49 attic, plus above-average sealing, this house will out-perform most houses by a good margin. 2500 BTU doesn't sound like a deal-breaker.

There is always more leakage from a ceiling penetration then a wall penetration.

Direct vents cannot be used for backup. The gas valve will not open until positive air flow is determined. i.e. the fan comes on and flows air through exhaust. Plus there is no way to light the gas. No standing flame pilot. The point of a direct vent is the same as a sealed combustion furnace of water heater. They rely on a fan to move the flue gases so become inoperative when the power goes out.

If you still want to go with a d.v. some allow downward venting, such that you can drop down into the joist space and out that way. I probably cost less to get someone to core the ICF then to have the tall chimney.

The DV units I looked at all work without power(?) They seem to promote usage during power outages. I guess I need to learn more.

I couldn't go through the floor, either. There is a big LVL in the way one way, and in the other direction there is either a porch or a garage.

I started by looking at cheaper gas fireplaces, which were simply pre-fab wood burners piped for gas. In fact, they can be used for wood if desired, although you would have to take out the gas logs. There are ventless units, as well, and I know from experience that they don't need external power, but I'm leery of them from a safety standpoint, and they put out too much heat for a small, well-insulated house. That seems to be the snag- trying to find a source of atmosphere and supplemental heat that won't overwhelm an efficient house. The house will be heated and cooled with a mini-split system, and my generator will easily power it in a failure, but that gets old gassing and tending a genny for a week, not to mention having to listen to it.

Are you in a mild climate? I went with an outdoor fireplace. It's on the porch right outside the living/dining space. For those guests who think its too cold out they can see it through a large picture window. The rest of us actually get a lot of use out of it in the fall and winter. Wouldn't be so great though in someplace like Minnesota I reckon.

We're in a mild climate, but the outdoor fireplace is of no help during a power outage. We are going to put a wood stove in our barn once we finish a great room in there. With a 21' ceiling and minimal insulation, overheating won't be a problem!

I went back and looked at the DV fireplaces, and a number of them will run without electricity. They use a battery-powered controller and a piezo igniter. The heating blower won't work, of course, but you would get some heat out of it. None of them had any kind of fan for the exhaust.

If I did install one, there's probably some way I could insulate it against heat/cooling loss when not in season. Maybe some kind of insulated insert to block it off during the off season. Probably not code, though. The old house I grew up in had no dampers in the 4 fireplaces, but there were fitted pieces we slid into place to stop the heated air from zooming up the chimney. I'm sure they've passed laws against such devices, assuming people are too stupid to remove them prior to using the device.

There are inflatable bags or balloons that you can stick in the flue during the off season and seal it up tight. Google [chimney balloon] for more info.

"They rely on a fan to move the flue gases so become inoperative when the power goes out."

that is one definition of "direct vent". Another however simply refers to an appliance that draws it's combustion air from outside vs habitable space.

2500 btu doesn't seem like much however that s per hour all winter. If your home is tight and relatively modestly sized that would be somewhere between 5 and 10% of the total heat loss.

I have used lock top dampers to help bring down the load where my clients have old masonry fireplaces (which may cause 15,000 btu loss).

"The requirements for air tightness in passive houses cannot be reconciled with open chimneys. Once you install a fireplace in a home, it automatically disqualifies it for PH certification."
this may be a new PH requirement; it was not a few years ago, but closed airtight fireplaces were required. However, the final blower door test on our 2012 deep energy retrofit project, a house with a huge open fieldstone fireplace, was less than 0.02 (this is not a misprint) so getting the house airtight with a fireplace is not impossible with a chimney top damper.

sounds like a little regional discrepancy on the definition of "direct vent"

Bob and Joe - how and when does the chimney top close? Is it manual or automatic?

the damper is spring loaded; the default position is open; a chain which hangs down into the firplace is used to pull it closed and lock it. When the chain is released, it opens. I do not know how they hold up over time.

Posted By joe.ami on 12 Nov 2013 09:25 AM
"They rely on a fan to move the flue gases so become inoperative when the power goes out."

that is one definition of "direct vent". Another however simply refers to an appliance that draws it's combustion air from outside vs habitable space.

2500 btu doesn't seem like much however that s per hour all winter. If your home is tight and relatively modestly sized that would be somewhere between 5 and 10% of the total heat loss.

I have used lock top dampers to help bring down the load where my clients have old masonry fireplaces (which may cause 15,000 btu loss).

For it to be 2500BTU/hr every hour all winter it must mean the air velocity is downright SCREAMIN' on days when the temps are pretty mild, eh?

At 0.018 BTU per cubic foot per degree F, when it's say, 35F out on a late winter afternoon, and it's 70F indoors you have a 35F delta.  For that to be a 2500 BTU/hr or ~42 BTU/minute, that's about 66cfm.  ( I've seen bathroom fans that can't pull that hard through a 3" vent pipe!

When it's 0F outside it still takes 33 cfm, which is more air than most 60K-80K air-tight wood stoves would pull.

When it hit's 50F out it has be really roaring!

Or maybe I'm misunderstanding what you really mean by that(?).