Heat Ducting for Wood Stove

I have always wondered if the fans are worth the electric$$ especially if you are located in a high priced region. Would just using the excess heat to increase the basement temp be of equal or greater benefit? I guess much depends of how the basement is insulated and how much heat exchange occurs between the basement and the lived in space.

Your answer brings back bittersweet memories of one of my first projects as a young newly first time homeowner of a large Victorian. It was Boston 1976 and the US Northeast was still reeling from the shock of the oil embargo of 1973-4; I as a card carrying "Earth Day activist" and as a person of small financial means was faced with the horror of enormous heating oil bills.
By the end of our second heating season my husband and I decided we needed to replace the oil furnace (converted from coal (in fact there was still a half bin of coal stored in the basement), single pipe gravity fed steam radiator system to a gas fired forced hot air system. We elected the forced hot air by virtue of two facts one: the original heat system for our 1890 Queen Anne had been gravity hot air so the duct orifices were still there (yes some of the duct work as well), and two: the expense (and unappealing aesthetics) of baseboard copper tubed hot water heat system was way beyond our means.
I am neither an engineer or a physicist; at the time I was just a humble computer programmer at a small community bank, but I was then and am now willing to do the research and thus make competent decisions when it comes to parting with large portions of my hard earned cash.
All your calculations reminded me of the hours I spent in the library researching heat loads, the qualities of various heat systems, and then the hours logged home measuring windows, calculating wall dimensions, and assessing the thermal qualities of the materials used. Just wish the Internet had been around back them but back then I was programming for an NCR Century One mainframe the daytime capacity was 32K night time 64K.

Finally it was time to contact heating contractors and pick one to design and install our new system. What a total bust, after calling 10 (some said "have your husband call me"),meeting with 5 contractors, and having none of them willing to do the work; I threw in the towel and had my husband takeover the project. Still took some time and me playing dumb and a lot less picky; but we got a new system installed before year's end. Maybe it was just a case of there being a lot of "low fruit" around and those guys were already pretty busy. There were lots of folks finally motivated to replace inefficient outdated systems and it seems most folks just trust "the expert" to do right by them. At the time my thoughts were murderous and otherwise, my feminist sensibilities were sure rankled!

Thanks for the chance to have and share these memories!

westbye, I think the reason there has been no activity after Tornado's comment is that Tornado has it right.

Well done, and a green thumb for Tornado.

Ok gang I appreciate it. The basement is about 45 degrees so the loss will be less - so much the better. This form is invaluable.

Thanks again,

I think Tornado is spot on. You will also have the advantage of any heat loss in the basement isn't 100% lost. It will warm the upper levels of your basement and therefore your floors slightly.

I am considering a similar operation and wondering how you are going to capture the heat from the wood stove. Will you use some sort of hood?

Thanks

I'm building a 6" dropped plenum across the wall above the wood stove with a few grilled openings. The plenum will be hollow. I'll cut through into the wall cavity within the plenum and run duct down thru the wall and into the basement; this will feed into a 10 or 12" insulated round which will contain a fan or two which will be controlled by a reversed thermostat on the wall above the wood stove. When the heat gets above say 85 - 90 degrees they will switch on. I'll put a directional air register in the floor or just up the wall two rooms away to which all this will connect. Then I'll be warm and happy heating my whole house with wood which I harvest myself from my wooded lot.

Hi, If it were me I would change the chimney for one with a heat exchanger in it, this can be used to heat water to either heat a hot water cylinder and piped to other areas to heat radiators, if the wood stove itself is generating excess heat I'd make a surround from pipes wrapped around the sides and back,depending on the type of stove, and link the pipes to the pipes of the heat exchanger, again depending on the type of stove you could clad the pipework in a heat proof box so all the excess heat is channelled into the pipe work.

hope this is of some use.

Bazzer

That is a great idea. It is much easier and probably cheaper to just install a wood

boiler. They work great!

Tornado seems to have wrapped it up well.

Just go for it.

The only thing I would add is do remember that you need to circulate air, what you take out of the room above, you need to replace it. Generally its better to take the excess from below and feed it up to the room above.....at as near the same rate as the warm air coming down....take the excess air for down low in the basement too where its colder.....

If you don't do this you will either create drafts of cold air from outside, or drafts of warm air from inside......

Don't underrate the fans, forcing warm air down several meters requires quite a lot of power......the same as forcing cold air upwards....variable speed would be good.

Also remember larger fans running slow are generally much quieter and longer living than small fans running fast.....

Something like this needs to be done:-

Absolutely right. GA

This will improve the efficiency by not cooling the air from new inputs into the stove area. He might need some fresh air control if the building is too hermetically closed.

To improve mixing of different temperatures and prevent cold drafts: maybe the duct to the basement should be extended to somewhere near the basement's floor, and likewise the upgoing duct to the livingroom's ceiling?

Not a good idea.

You always want to take the hot air above the stove near to the ceiling downwards and release it in the upper part of the basement, as you fill a room with warm air from the top downwards to stop causing drafts of the warm air rising suddenly.....

You suck the cold heavy air from near to the floor in the basement and place it at floor level in the living room, where the coldest air is, ready to be warmed up by the stove, rising as it warms up until it gets to the ceiling level.

If you pushed the cold air up to the ceiling, it would form an invisible "water fall" of cold air as it fell to the floor level and it would make a uncomfortable draft for anyone in there.....ever sat next to a single pane window, in a warm room on a cold day? Same effect!!! Not pleasant, which is why central heating radiators are placed under windows to correct this effect - but the effect is, we simply then forget that the effect happens.....

Good point.

To take it a step further, why not use the exhausted air from the 45 deg. basement as the combustion air for the stove? It wouldn't require a fan since it would be pushed out by the positive pressure from the down-duct fan.

Sounds like a great improvement to me! Thanks.

Dont insulate the duct. There is no need, the heat losses would be inside the house and redistribute the heat in the home. You have to see the home as a package and your moving within that package.

10 round is ok, round is better than rectangular, 12 would move more air. I suggest that you use the fan to push the air rather than pull it. ?? The fan can not suck the air in the duct, locate the fan at the inlet and blow - pressurize -- the duct so that you move more air.Some people forget that air expands and suction is weak at the end of the duct.

Second item is that the duct will only work if there is a path of return air. Many supply air and forget that it needs a return, with LOW restriction like a door that is open.

I would consider moving the colder air to the stove if that is possible, reverse of what you were thinking. This would bring colder air into the stove and allow a greater heat transfer, plus you could duct it to the bottom of the stove and possibly save banging the duct into your head. Just another approach.

When you pull the cold air into the stove the warm replaces it in the basement. This allows the fan to be in the basement and save the noise issue also. Just need a remote for the fan control like a simple extension cord for temporary operation or a relay for a more permanent installation.

I say extension cord as it temporary. THe install may not be what you want and the best laid plans .... Live the fan and see if you want to make a change or quit it all together. My approach, Woody

http://www.ct-texas.com/images/furnace_pipe_004.jpg

I have never used our insulating coating on piping from a wood stove, I have coated steam pipes and boilers. I have dropped the surface temperature of those metals down from 267F to 155F. This was a 35% reduction in heat loss per the hospital engineer.

Our contractor in Texas coated the exhaust pipe of a ceramic kiln and dropped the surface temp from 400F down to 100F.

East Bay Regional Park Distridt painters coated the heater duct under a 33 ft mobile home in one of their parks. They did a before and after test of the unit, measurinf the air temp, what that temp topped at and how quickly it all happebed. After coating it they realized an 84% drop in heater runtime for the trailer to reach 72F inside.

PM me your contact information and I can mail you a cured sample on polyester roofing cloth. You can place the sample against the hot metal surfaces and take your own readings to see what it will do for your situation.

Many companies have told us our coating does better than the foams for their particular applications and they accomplished their needs at a fraction of the cost vs the foam insulation.

Hal Skinner

CT-TEXAS.COM

CR4 Admin - email removed
From the Site FAQ: Do not post phone numbers or email addresses.

Westbye, I have found in my travels and experiences - the K.I.S.S. principle works best. Especially when dealing with wood heat and passive solar. If you have a ceiling fan, use it. If you can cut a hole in the floor close to the stove, do it. If you can cut a hole in the floor at the other end of the house, wonderful. Now, get a room to room, or, a ceiling exhaust fan with variable speed (router speed control works good). A cooling thermostat and grates for the holes in the floor.

Blow the air Down (with the fan), and allow the air to flow back Up to the 'hotter' woodstove location. This would be by convection method and there will be uneven heat spots. But, it would surprise you the difference in the woodstove area.

Keep in mind the more cfms you use to blow 95 degree air around, the more actual heat you will lose, by mixing with the cooler air @ the intake.

But, your way sounds good too! Good Luck!

Thanks everyone; my house is a 1820s cape farmhouse which I've made as tight as I can. That said, the old part of the basement has a open piled stone foundation. (You could put a stick through in places and come out with dirt on it) I've sprayed 3-4" of foam from the ceiling including the joist spaces down to about a foot below the frost line (3') and have thick plastic going down from there and silicone sealed to the cement floor. (When doing this one needs to be careful not to go too far below the frost line with foam as the stone foundation was designed to leak water in, in order to minimize hydrostatic pressure and avoid being pushed inward when the ground freezes.) All this to minimize heat loss and cut down on moisture coming in. It works pretty well and minimizes the spiders as well. However going with the duct allows me to direct the heat where I want it and the return air will simply drift thru the house back to the wood stove via open doors.

Thank you all for the support and thoughtful help

Westbye

You may find that instead of cold air returning to be warmed, the warm air returns as it is far lighter......Substantially reducing the efficiency.

Time will tell as to whether or not the effect achieved is good enough or not.

Let us know please.

Hi Westbye,

Were you able to get this setup?? I'm thinking about a similar approach, and am curious if you had any luck with it. Any details you have on fan/duct sizing would be appreciated.

Thanks!

Vin

My thought would be to cut a duct from one room to the other if it is a straight run horizontal and box it if you wish. You would not have to insulate the duct because it is in the condition space, and would give you the option to use the heat in other spaces. You can use a cooling T-stat that is easy to come by for your fan(s) due to it closes on temp. rise.

As a thought you might want to add a high side wall/ ceiling return to your existing system to move the excess heat through out the house using what you now have in place. This would only require duct work and a damper or two and using the low speed fan on the system that is in place now.

If you want to make it automatic you could use a by-pass damper system in the return air duct to switch from the current return air to the high wall/ceiling one.

Adapting Hotwater's approach, here's a simple thought:

Put a louver high in the wall from the stove room to the next room(s), a quiet 12" fan from the next-to-last into the last room, and a return louver from the last room into the hall. (That's just a guess about the layout.) No duct, no insulation.