Fireplace insert combustion air intake?
Good morning all,
Anybody have experience with ways of providing combustion air for a fireplace woodstove insert, like repurposing the old pathway which was the ash cleanout (now underneath and behind the insert) fitted with piping or other scheme? Or a dedicated intake pipe? When we had the insert installed the distributor as well as their installers were totally closed to the idea of providing intake air other than just through the house, which means it finds every possible leak and draft.
They said it wasn’t worth it and that it would have to be fairly big to work. I never agreed but they weren’t worth fighting with.
The insert is a Jotal with glass doors which stay fairly clean unless you try to operate it closed down to minimize the loss of warm household air up the flue. I figure the CFM’s must be substantial and if I could lessen the waste of heated air the efficiency would be much better. I think some manufacturers actually provide for it, but I’m not sure how.
We’re in the process of weatherizing the house and fixing old insulation and weatherstripping, but the woodstove seems to be the culprit compounding the drafts.
Any ideas? They thought I was crazy and said nobody does that. I already knew I was and knew some folks who did it! Guess they were crazy, too.
Thanks in advance for any comments, serious or otherwise
Bob
Replies
I was required to put one in for an outdoor fireplace. Simply because it was part of an enclosed deck. They didn't care that it was outside the houses building envelope.
I'd check with your town's building department. Freqently, they require a permit for new fire appliances.
They are crazy for thinking it's not important. We always install them, or I should say our fireplace installers install them. I don't know the details but do know it's a good idea.
Thanks Mike, Just as I suspected, now I just have to find out the most reasonable way to do it.
Hope this isn't an affront to your knowledge / sensibilitiesthe rule in gas fireplace arrangements is a two part pipeconcentric circles - say a 4" diameter exhaust pipe inside a six " pipe
those outer two inches of diameter supply intake - would think it would work once you got draft going
obvious to many / new to me 6 yrs ago
I'm sorry the subject line should have mentioned woodburning not gas insert. The masonary flue was relined with an insulated s.s. liner properly sized to the Jotul insert. I think possibly Vermont castings uses a flue sourced intake but I'm really not sure about it. Thanks for the reply.
Bob
Is the unit designed for an outside air intake ?If it is then it will have a place to make the proper connections
I'm pretty sure there is a connection on the back of the unit will need to check. Mostly I'm curious as to how intakes would typically be routed and sized. My initial post had to do with whether intakes are considered to be a more or less standard component of a woodburning insert system and key to getting efficiency for heating vs. pulling in a lot of cold drafts and wasting heated household air up the flue.
Thanks
AFAIK outside air intakes vary from stove to stove and aren't standardized. For mobile homes, some kind of outside air source is required (since these fine homes are so tightly sealed :)), but many stoves don't have a ductable air intake (the outside air kit is simply a vent into the room). I found a handy way to figure the air volume: the air:fuel weight ratio is about 35:1, and air takes up 466 cu. ft. per 35 pounds. If you burn 5 lbs. of wood per hour, that's 2330 cf of air per hour or 38.8 cfm.(edit: 466 cu ft. is the volume of 35 lbs. of air, not one. The rest of the calculation is the same.)
Edited 12/7/2008 12:46 pm ET by KHWillets
Great way to figure it out!
Thanks a bunch.
Digging up the manual suggests
"If appropriate, use 3" noncombustible duct to connect an outside air inlet to the air inlet collar at the back of the firebox"
I guess I just have to find a pathway for it.
Do you have the owner's handbook and the installation instructions?
Sounds like you're in the land of "That's the way we've always done it" and they aren't about to do what you want cuz they're doing it their way.
Look at the Jotul web site or your original docs and see what they say, sounds like there is already most of an air inlet through your ash clean out.
Post your model number and someone here will know the answers.
Joe H
It's a Jotul C450 and I think the ash door scenario is one that my uncle used on a similar install in Minn. My installers claimed they'd have to drill through the back of the existing firebox of the old heatilator unit and through the brickwork. and Besides "it wouldn't really make any difference" Actually thinking back on it I was shocked, they'd been installing for 25 yrs and the store had been selling stoves for 30 yrs with an excellent reputation.
Thanks for the encouragement, now at least you have all motivated me to at least call some more fireplace installers if I can't figure it out myself.
There's not much difference calorifically, because bringing in cooler input air just subtracts heat from firebox and chimney gases, compared to using preheated interior air. Either way, cold air comes in, gets heated, and goes up the chimney. However the duct may be better than cold air seeping in through outlying rooms.
This is the main issue and perhaps where I would hope I could learn from the experience of folks who use outside air intakes. Does it make a big difference, as I would hope, or not really because as you suggest the cold intake air lowers the firebox temp and the flue, also.
Admittedly, we are trying to get the most heat out of the unit that we possibly can, because the boiler system is oil.
This whole issue resurfaced in my thinking as I am upgrading insulation and attempting to better the fuel to comfort ratio.
Your CFM formula should help make sense out of it quantitatively in the context of the overall energy/warmth/heat loss equation.
Perhaps it comes back to how and when to use the wood heat, maybe when it's -10 deg. outside more could be gained using the boiler instead in conjunction with better insulation. We only have 2x4 walls-house built in the 70's. Planning to redo the siding and thinking about adding insulation in the process.
Thanks for your thoughts and if you think of anything else please don't hesitate.
Bob
I wouldn't worry too much about it. With the 5 lbs wood/hour situation above, you're using 175 lbs. of air, with a specific heat of .24 btu/lb-F. So a 50 degree temperature difference works out to using 2100 btu/hr heating indoor supply air, out of ~35000 btu in wood heat (6%). Also, the efficiency rating on the insert (usually 63%) may include that loss already (I haven't checked), so be careful not to double-book it.Oil probably uses a similar amount of air too.
Very interesting information. I just this minute got back to the computer and will have to digest all of this.
Once again Thanks a lot for all your excellent information very helpful and much appreciated.
Bob
I thought that other than preventing drafts from low air pressure pulling unheated air through cracks at door and windows and so on, that providing outside air through a duct for combustion prevented the stove and other furnaces from back drafting. I think backdrafting of your boiler would not be desirable. (I thought I read somewhere that you did have a boiler or furnace as you main source of heat?)
Excellent point!
I also realized in the process of this discussion that continually bringing in fresh very cold air contributes to extremely dry household air which we find we need to mitigate with humidifiers. At least an intake would help lessen that use of energy.
Maybe the intake would help in several small ways and not just the obvious cold air drafts or waste of warm interior air up the chimney.
All the posts have added something to the picture.
Again thanks to everyone.
Bob
We have the smallest Vermont Castings stove (Aspen?).It came with a fitting to connect to a 3" tube. I installed 3" glav tube from the outside under our front porch. I wish that I had upsized the tube to 4" and reduced right at the stove. The run from the tube opening to the stove is only about 5', but there are three 90s (couldn't avoid it). It seems that it might draft a little slower than it should, but since I've never had the same stove without the intake air set-up, I'm not sure.I do think it helps keep the house a little more moist. I've noticed that if it's below zero a little condensation forms on the tube behind the stove.Our house is so tight we have to watch what we're doing with exhaust fans or we can backdraft the flue. Not a problem now (house is five years old) that we learned to be mindful of it.I wouldn't go without outside intake air again. It way not save much heat (as someone said), but you can't feel the cold when it pours straight into the stove instead of leaking in everywhere else.
I hope all of your posts have been helpful to others as they have been to me. Thanks very much for taking the time to add your thoughts to the conversation. Useful points of view.
best wishes, Bob
Excellent info! I'm putting in a freestanding wood stove this week and plan on using an outside air source but it's good to get some actual feedback from someone using this setup.It is a little odd that it is a requirement in modular houses but not regular construction but I bet the req. is based on a CO level or other archane FHA requirement rather than worries of "tight" construction.My biggest issue was purchasing a stove that would allow a hidden intake through the stove base and look okay. W/O a wall to back up against I didn't want an ugly vent coming straight out the back, the fan and wiring are ugly enough! Went with a Pacific Energy brand 'Fusion' - we'll see how it goes.Thanks for the input!
We have the smallest Vermont Castings stove (Aspen?).
Got the same one, mine's close to the wall so the inlet comes straight into the back. It works well, but don't open the door to add wood with the bathroom fan on.
Joe H
Combustion air for woodstove wood insert
I see this is an older post, but find it strange that more energy efficient builders haven't chimed in here. Bob is absolutely right that combustion air is a good idea when installing a woodstove inside any conditiioned space where it is practical. Remember, for every CFM out a CFM must come in. You can either control where those CFM's come in or let them find the path of least resistance. There are many resons to define that path and bring the air directly to the unit I'll list a few, I'm sure others can add more. The airflow for woodstove draft I'm familiar with (from BPI certification classes) range from 200-400 CFM and are much higher (10x) than the numbers cited above. If the house is not well sealed the stove will make it cold and drafty just like a small blower door by putting the house under negative pressure. Drawing in freezing air across the living space will significantly diminish comfort and will not prove more "efficent than bringing that cold air right into the firebox. Think about it, you'd be preheating the combustion air by introducing ice cold air into the air your trying to heat! Either way 1 cfm out = 1 cfm in, and either way that cfm is coming in from outside at the same temp. With up to 400CFM coming in, you want that in your fire box not in your conditioned space.
1. Gaps around windows, doors, electrical boxes and other small holes in the building envelope will draft in making the perimeter of the house much less comfortable. Proper combustion air creates a sealed combustion appliance and does not negatively pressurize the building.
2. Air infiltration follows the path of least resistance. Therefore, if you are trying to start a fire in your woodstove and the path of least resistance is back down the flue, you will have unpleasant smokey backdrafts until you establish a draft through stack effect (warm air rising). Of course you can start opening windows and doors and cool your room down, but proper combustion air will virtually eliminate that exercise in heat loss.
3. If the house is built tight you run the risk of back drafting other combustion appliances such as water heaters that do not have sealed combustion / direct vent systems. This is a health and human safety issue that must be considered when installing wood burning appliances in superinsulated / tight buildings.
One way to accomplish the combustion air install on Bob's insert w/ insulated stainless chimney liner would be to seal the chimney at the top letting the liner pass through a sheet metal cap. Seal the chimney at the bottom, above the insert, with a sheet metal pan that provides a hole to let the flue liner through. Put a hole near the the bottom of the chimney in the masonry chimney itself (if structurally feasible). 4" is a good size because you can use a 4" metal wall vent cap and remove the backdraft damper so air can flow in, but leave the bug screen intact. If the intake hole is above the metal pan over the insert, add a 3" hole in the pan to let the metal combustion air flex line to the port on the fire box. Then seal the front of the unit's surround from the condtioned space. If there is no port in the unit you can run the flex line to the ash box or just leave it out entirely. What you are trying to do is create a new path of least resistance for the combustion air that doesn't mix with the conditioned air. If the fireplace is sealed at the top, a sealed stainless liner is used for exhaust, and the chimney is not being used to exhaust anything else, the chimey between the upper and lower pans becomes a plenum for the combustion air. If the intake port is well below the sealed top of the chimney, this will be the path of least resistance. If there is a 3" hole in the lower pan the insert will draw its air through that and not under your front door or down your water heater vent.
There are many other ways to accomplish this. I have the system described above and it works wonderfully on my Vermont Castings Montpelier insert. I also have a Hearthstone freestanding stove with combustion air plumbed up through the flloor directly into the unit. No drafts in the house, great draft in the stoves. I wouldn't install a wood burning appliance without combustion air and I wouldn't install a gas heater without "direct vent". If your installer tells you they don't work or aren't worth it look elsewhere. Do the research and grasp the science and I think you'll agree.
Direct Venting Your Vermont Castings Montpelier Insert
Thank you for your post. We're at the tail end of a deep energy retrofit and have gotten the house so tight that in order to safely use our Montpelier Insert, we have to supply make-up air. Failing to do so will put us at risk of backdrafting our oil furnace. Vermont castings does not sell a make-up air venting kit, but the insert does have a port where an air supply can be directly connected. Did you use this port to directly supply air to your stove? If so, can you supply any more detail on this specific application, please? Thank you again. Eric Zimmerman