Custom Draft for Woodstove

[Adabiviak]

Has anyone tried adding a vent, or otherwise the equivalent of an outside air kit that exits onto the heat sink of one's refrigerator?

The thought I had today was that if I did this (with enough throughput to feed the stove normally) that incoming outside cold air (which is required here, since the stove isn't fitted for an OAK itself) would keep the refrigerator run time lower and otherwise be slightly warmed as it did that job. The air has to come from somewhere... might as well have it doing some work on the way in?

 

[blades]

That would be similar in operation to my Heat recovery air exchanger

 

[Corey]

That would be similar in operation to my Heat recovery air exchanger

Sort of the exact opposite isn't it? ... a heat recovery air exchanger is recovering heat from air you are sending outside and returning it to air you're bringing in.

This contraption would be taking heat you're putting into the room from the refrigerator and sending it up the stack of the wood stove?

Not sure how you'd gain from taking the 100% efficient heat produced from the refrigerator, and ducting that into your wood stove with an 'ideal' efficiency of 50,60,70%... but in reality likely 40,50 or maybe 60%. ...and that is assuming the whole thing is 100% insulated, so picks up no other heat from the room. In reality it will be picking up some heat and in probability it will pick up a fair amount of heat.

Seems like your best bet would be to let the 100% efficient heat from the refrigerator flow into the room as it is doing, and make your OAK as short and as well insulated as possible. Any 'warmth' the OAK picks up from the room is going into the wood stove and only returned at a max 70% efficiency.

 

[heat seeker]

This may not apply, but many years ago, when I was in the tropics, I aimed a fan at a refrigerator's coils, thinking it would help. The result was a total loss of cooling to the box, which returned when I removed the fan. I don't know much about refrigeration, but figure the extra cooling messed up the pressures in the sealed system. YMMV of course.

 

[SeanBB]

My understanding is that the woodstove is more efficient when it is fed with cold air. When burning wood we must have some air coming into our homes from outside so that air should go directly to the woodstove.

 

[Adabiviak]

That seems weird... why would stove manufacturers try to pre-heat the air on its way to the combustion chamber in their designs?

 

[blades]

Tube stoves all use preheated ( actually becomes super heated ) air for the secondaries which takes place in the fire box or around the outside of it ( various designs) before the air is introduced at the top of the box. If you were to just blast cold air up there likely get little to no secondary ignition. Additionally pumping cold air in the box would lower the temp resulting in lower efficiency unless you are trying to create a blast furnace by feeding air under the fire. Course that would significantly reduce the burn duration.

 

[begreen]

That seems weird... why would stove manufacturers try to pre-heat the air on its way to the combustion chamber in their designs?

Some, but not all stove mfg. do this with the secondary air. Other stoves have a more direct shot to the secondary ports. In a few stoves it's almost a straight shot. Both ways seem to work though efficiencies may vary.

 

[Corey]

Depends on where you measure the efficiency. The stove burns more efficient by bringing in warmer air. If you consider the extreme - the stove bringing in 1500F air would have a pretty complete and very efficient burn.

The problem is - if you're taking that heat from the room, then you're essentially burning wood at ~60% efficiency, heating the room, pulling that heat back into the stove in an effort to make more efficient combustion. It's like trying to run an electric car by generating power with a wind turbine bolted on the roof...various laws of thermodynamics stop that from working.

So overall...yes the stove is more efficient with hot incoming air, but it you take that heat from the room, you're loosing efficiency overall. The best you can do is grab heat right from the firebox. Or if there is a way to recover truly 'waste' heat, that's another option...but again - heat in the living space isn't waste heat.

 

[festerw]

Do they make a larger double walled pipe that the outer wall serves as the combustion air? I know Selkirk makes a pellet vent version like that, but it's obviously to small for a wood burner.

 

[Corey]

I don't know of any double walled pipe for a wood stove. But here again - heat in the flue isn't really waste heat. (at least for a wood stove*) You need heat in the flue to drive the draft. If there is more heat than needed for draft, you should damper the stove down. Some heat may escape through the flue wall, but in that instance, insulation would be the best bet to keep the heat in the pipe and produce the needed draft. If you keep the heat in the flue, you burn less wood for the same amount of draft.

Unfortunately, sending extra heat up the flue with the goal to recover some portion of it, bring it back to the firebox and 'reburn' it at half the original efficiency still can't produce any actual gain.

*note - if you have a forced draft like a pellet stove, then the heat in the flue is somewhat 'wasted' as the draft fan is doing all the work. So in that case, the double wall pipe can recover some actual efficiency.