Woodstove to preheat water in hydronic system

hot_rod

you can calculate the stoves output based on the cubic feet of wood it will hold. Google around for the wood heat content. For example 2 cubic feet of white oak burned at a 40% efficiency=??

A no power situation can be a problem with heat exchangers on or in wood stoves. without circulation you could flash to steam rather quickly on a small HX.

You also need to be sure you don't cool down the stove to much and encourage creasote formation in the stove or flue piping.

I would think the heat off the woodstove itself would be the safest way. maybe one of those fans that spin via the heat from the stove.

hr

TommyMc

Woodstove to preheat water for hydronic system

I live in a 2 story cape heated with an oil fired boiler and baseboard radiators. Each floor is a seperate zone. I also have a woodstove in the livingroom which runs pretty constantly during the coldest months of winter in VT. Much of the wood heat goes up the stairwell and keeps the upstairs warm. Downstairs, the wood heat doesn't distribute well, so we need to use the central heat to keep the more distant rooms warm. (I've tried fans, but they only make it feel drafty.)

I've been wondering if I could use the woodstove to give the oil burner a boost. The woodstove sits directly over the boiler which is in the basement. My idea is to tap into the return line in the downstairs baseboard loop and build a simple heat exchanger to preheat the water before it circulates back through the boiler. I'm thinking of a coil or series of U's made of 3/4" copper and placed very close to the back of the woodstove. I don't know how efficient it would be, but it would certainly add some heat back into the system.
My concern is that there are times when the boiler will be shut off, and the woodstove will be burning. Power outages are a good example...and fairly frequent around here. In this case, the water in the pipe behind the stove will not be circulating, and will get hot. Will the pressure relief valve just below in the basement (and the expansion tank) be enough to handle this? Also, will all the U bends add too much drag on the circulator? Or is this just a wacky idea and a waste of time? Thanks for any ideas.

Bob Gagnon plumbing and heating

Wood Stove Recirculation

I ran some radiant with aluminum plates, in the walls and ceilings near and above my wood stove, and I have radiant in the walls and ceilings and floors in the cooler part of my house. I have a pump that circulates the warm water, up to 120 degrees, into the cooler part of the house, where the water is 60 degrees, it cools off the wood stove room and warms up the rest of the house, and it also stores the heat in the walls and floors, so even when the stove dies out the house stays warm for hours.

Thanks, Bob Gagnon

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TommyMc

Radiant wood heat

That sounds interesting, although a lot of work to retrofit. My outer walls are logs and the rooms I want to heat don't have much available inner wall...lots of closets and doors,etc. I could run radiant in the floors though. So basically you aren't so much heating the rooms,(60 degree water) as preventing them from losing heat as fast as they normally would?

Bob Gagnon plumbing and heating

60 degree water

No, I'm heating the rooms. When I first turn on the circulator pump the wood stove room is warm, about 75 degrees, but the rest of the house is cold, around 60. The water in the radiant system below the cold part of the house may be 55 degrees. I simply circulate that water with the heated water near my wood stove, the ceiling is about 100 to 120 degrees. Trying to circulate this heat with a fan makes you feel cooler, and it cost more to operate a fan than a circulator pump. Ther is no problem with cresote, because the heat exchanger is external, the flue gasses remain the same temp. I'm simply moving the excess heat, to the cold part of the house using my radiant system, it works great.

Thanks, Bob Gagnon

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John Penwarden_3

Was about to do that very thing

Had an older modular house (built in the 50's) that had a huge box stove in the basement that had a coil inside. Was told that the P.O. (ex father-in-law), had the coil connected to a HW loop that ran around the house with a small circulator before he installed an oil boiler. When I bought the house, the coil was busted (was designed with a union inside the stove!). Anyway, there is a company in NY that makes the very same wood-stove with coils already installed. If I had a hydronic system, this is the way I would go!