Heating water with a forced air wood furnace

[Jules083]

I am currently building a new house, and my furnaces are backwards from what they should be. What I mean by this is I have a forced air wood furnace, and an oil fired boiler. I am running ductwork through the basement for the forced air, and the boiler is going to warm the basement floor for use as a backup heat. The House is setup as a very open design, and the heat from the basement will rise up and heat the whole house.

What I would like to do is heat the basement floor with the forced air wood burner, if possible. I don't wish, or expect, the forced air furnace to completely heat the house just through the floor. What I am thinking is that if I were to run some water lines through the furnace in such a way that the water also went through the oil boiler, it will help to warm the floor a bit.

My idea is that at the top of the firebox to run some 2" heavy wall pipe going back and forth, with maybe a 1" gap between the pipes. Coming out of the back of the furnace would be an inlet and outlet, and that would be plumbed inline with the oil boiler. What this 'should' do is heat the water to a point that the floor is kept somewhere between 'less cold' and 'warm'. The forced air aspect should still work as before, but possibly the blower would kick on a bit less because of the cool water running through the top.

To make the bends tight enough to keep the pipes close I would have to build basically a tight 180 degree turn, which wouldn't be a problem. The water would zig zag across from one end to the other, then exit on the hot side. The circulator pump would be always on in order to keep the water from boiling in the wood furnace. If the house cooled down below the temp setting on the oil boiler it would kick on and pick up where the wood boiler left off.

Worthwhile project or waste of time? I'm an X-ray certified welder, and am very used to working in very tight places. Welding everything in is not an issue at all. I would probably end up TIG welding every root then 7018 from there, it takes longer but I'm used to it and I'm confident in the welds when done.

Is there a better way? I'm a boilermaker by trade, but I work on power plant boilers that are typically 200' tall and 60' across. This boiler is a bit smaller than that and the budget has way less 0's. :msp_biggrin:

 

[mtfallsmikey]

Nothing personal, my friend, but I'm thinking fail on this...
But, I'm a plumber...

 

[Deereman76]

Have you considered a Radiator in the hot air outlet, Kind of in reverse of what the OWB folks use? Might be simpler, and Cheaper....

 

[laynes69]

I think it's a waste of time. You cannot pull enough btus from a coil in a forced air woodfurna e to heat a slab. I wouldn't bother with ductwork either, if you want to heat with wood install a wood boiler. There are very efficient boilers on the market that would work well tied into the main.

 

[dave_dj1]

I just went through this same scenario, not gonna happen with pipe, the only way is to have a loop inside the firebox (that's what I'm doing).
There just isn't enough heat (btu's) in the hot air, you need the fire.
Search on here and a site called **********, very knowledgeable people over there too. I got lot's of good ideas and opinions (they talked me out of wasting time trying the same thing you want to do).
good luck!
dave

 

[Jules083]

I think it's a waste of time. You cannot pull enough btus from a coil in a forced air woodfurna e to heat a slab. I wouldn't bother with ductwork either, if you want to heat with wood install a wood boiler. There are very efficient boilers on the market that would work well tied into the main.

A wood boiler will probably be purchased eventually, but the forced air one is only 3 years old. I'm trying to use what I have right now.

I just went through this same scenario, not gonna happen with pipe, the only way is to have a loop inside the firebox (that's what I'm doing).
There just isn't enough heat (btu's) in the hot air, you need the fire.
Search on here and a site called **********, very knowledgeable people over there too. I got lot's of good ideas and opinions (they talked me out of wasting time trying the same thing you want to do).
good luck!
dave

It would be inside the firebox, at the top.

 

[timbrjackrussel]

I tried something similar with 1 1/4" pipe in a Carmor down draft woodstove. I used about 3 feet of pipe around the top of the firebricks. The pipe always gummed up on the outside likely because I couldn't get the fire large enough and hot enough. The firebox was not as big as the Surefire wood hot air furnace that I have now. Your idea may work better than my attempt if you have a large fire, dry wood and good combustion. I would try smaller diameter piping to get more surface area perhaps 1". Lots of safety concerns but I am sure you can handle them. Check out this link
Heating Water With a Wood Stove
I have a good system heating my domestic hot water with a hot air wood furnace if you are interested.

 

[dave_dj1]

well as long as your are going in the firebox you should be good. Just don't forget a pressure relief.
good luck and keep us posted!
dave

 

[Jules083]

I tried something similar with 1 1/4" pipe in a Carmor down draft woodstove. I used about 3 feet of pipe around the top of the firebricks. The pipe always gummed up on the outside likely because I couldn't get the fire large enough and hot enough. The firebox was not as big as the Surefire wood hot air furnace that I have now. Your idea may work better than my attempt if you have a large fire, dry wood and good combustion. I would try smaller diameter piping to get more surface area perhaps 1". Lots of safety concerns but I am sure you can handle them. Check out this link
Heating Water With a Wood Stove
I have a good system heating my domestic hot water with a hot air wood furnace if you are interested.

Good site, thank you for the link. I was thinking bigger pipe for the flow rate, if and when the oil boiler kicks on it will need more flow than a 1" pipe will provide I think.

I could always weld fins onto the pipe to increase surface area. We finned a friend's homemade waste oil furnace a few years ago, it made a very big difference. His was made out of a water tank and used to glow a dull orange when running, when we finned it the garage got warmer and the tank quit glowing as much.

My concern now is something that was touched on in that site, which is what to do it the power goes out or the pump goes bad. A pressure relief would keep the whole thing from exploding, so I guess I'd be alright there. I have a log home, so if there's a fire there's about 30 tons of well seasoned pine that's going to burn long and hot. :angry2:

Edit* I use the word 'pipe' loosely, the material I would be using would be 2" boiler tube with a 1/4" wall thickness. I have access to short sections that have been removed from the power plant, when the wall thickness gets down to .20 or smaller we generally replace the tube.

 

[atlarge54]

"I am currently building a new house." Odd that a boilermaker would be asking questions about boilers.

Two mistakes I can see are:

Why try to reinvent the wheel?

Why fight the laws of physics?

 

[timbrjackrussel]

Good site, thank you for the link. I was thinking bigger pipe for the flow rate, if and when the oil boiler kicks on it will need more flow than a 1" pipe will provide I think.

I could always weld fins onto the pipe to increase surface area. We finned a friend's homemade waste oil furnace a few years ago, it made a very big difference. His was made out of a water tank and used to glow a dull orange when running, when we finned it the garage got warmer and the tank quit glowing as much.

My concern now is something that was touched on in that site, which is what to do it the power goes out or the pump goes bad. A pressure relief would keep the whole thing from exploding, so I guess I'd be alright there. I have a log home, so if there's a fire there's about 30 tons of well seasoned pine that's going to burn long and hot. :angry2:

Edit* I use the word 'pipe' loosely, the material I would be using would be 2" boiler tube with a 1/4" wall thickness. I have access to short sections that have been removed from the power plant, when the wall thickness gets down to .20 or smaller we generally replace the tube.

Fins sound like a good idea, how may feet of tube do you think you could get into the furnace? What make of furnace is it?

 

[Jules083]

"I am currently building a new house." Odd that a boilermaker would be asking questions about boilers.

Two mistakes I can see are:

Why try to reinvent the wheel?

Why fight the laws of physics?

The smallest boiler I have ever worked on was roughly 90' tall and 40' across, coal fired, and they typically run with a max pressure of 2500psi and over 1100 degrees fahrenheit.

I'm not trying to reinvent the wheel or fight physics I don't think, I just want to warm up a bit of water.

Fins sound like a good idea, how may feet of tube do you think you could get into the furnace? What make of furnace is it?

It's an Energy King. I could probably get 8' of tube inside. Any more and I would start losing capacity for wood.