Hot Water From Wood Furnace

[thecontrolguy]

As the discussion regarding the expense / benefit ratio of gassers versus OWB's rages on, I had a brain fart the other morning that went something like ...

If I want to heat a conventional ducted air system with hot water by adding a large (ish) hydronic coil into the ducting, why then can't I get hot water FROM the heated air via the same hydronic coil. So, for those that already have a good, efficient, primary air heat source (like a propane furnace) and could benefit from an alternate hot-water source, install an hydronic coil sized for the new load (such as side-arm heat exchanger) and get hot water any time the furnace is running to heat the house. I understand that you would be taking BTU's out of the air stream, but that could be accounted for by a variable-speed pump and supply air temperature sensor to ensure a minimum down-stream air temp.

Further, as I already have an(aging) oil furnace ducted to an RSF-100 wood furnace (modified by me for greater efficiency) that is then ducted to the house ducting, I could install the hydronic coil to use all (excess) heat that the big wood furnace is cranking out without have to damper it down or otherwise overheat the house. (frequently, as i don't like dampering combustion air and creosoting up the old chimney) I would use this heat to preheat domestic hot water (sidearm xchanger) Also, to increase actual BTU-HR capability, I could damper the suplly air immediately back to the return duct to create a short loop where the air goes through the oil furnace (and fan), through the wood furnace, through the hydronic coil and damper back to return air to oil furnace, etc. Damper would auto-open on call for heat from house and pump would NOT run if oil was heating the air (electric hot water tank).

Any thoughts as to the efficiency of this scenario and / or if it is started in the wrong list? Cheers!

 

[stee6043]

Are you suggesting only doing this when you're heating with propane and not wood? I guess I'm confused. But if you're heating with propane I'm not sure it makes much sense to rob your system of the BTU's. Any heat you remove from the air is obviously not going to your home. Thus, you're going to spend more to heat your home. I'd bet my wifes paycheck that it would cost half the money to heat your hot water directly with propane than it would to try to reclaim the heat from your propane furnace, BTU for BTU.

 

[smokinj]

The coil in the fire box works best

 

[thecontrolguy]

In response to stee6043;

I would ONLY take heat from the wood furnace while it was fired, and if the house thermostat is satisfied. That way, I can continue to burn ALL the wood in the furnace firebox at the full rate and avoid overheating the house and coking up the chimney. I could also store the heat in a large storage tank, and re-use this heat for the next time the house t-stat calls for heat and the wood has burned out. That way I could store heat from a wood-furnace (much cheaper than a good boiler) and be able to recover it for space heat and dhw, as needed.

I would also NOT consider a coil-in-the-flue heat exchanger as the intense heat from the wood gasses (>600F) and the very frequent requirement for cleaning out all the soot that would build up. Also, problems with cooling the flue gasses to condensing temperatures, and/or overheating the in-flue coil and making a steam bomb. AND, my tall masonry chimney needs a good blast of heat initially to get a substantial draft going, so and obstruction would reduce that and cause me some fire-starting issues.

 

[sdrobertson]

In response to stee6043;

I would ONLY take heat from the wood furnace while it was fired, and if the house thermostat is satisfied. That way, I can continue to burn ALL the wood in the furnace firebox at the full rate and avoid overheating the house and coking up the chimney. I could also store the heat in a large storage tank, and re-use this heat for the next time the house t-stat calls for heat and the wood has burned out. That way I could store heat from a wood-furnace (much cheaper than a good boiler) and be able to recover it for space heat and dhw, as needed.

This is interesting...would you be running another set of duct work to make a second loop because I would think the airhandler wouldn't remove enough of the heat so the house would still get some heating. With a large enough airhandler heat exchanger it should work but I'm not sure how hot the water would get. Interesting indeed but I don't know if it would result in enough stored btus to make it worth the set-up costs.

 

[smokinj]

In response to stee6043;

I would ONLY take heat from the wood furnace while it was fired, and if the house thermostat is satisfied. That way, I can continue to burn ALL the wood in the furnace firebox at the full rate and avoid overheating the house and coking up the chimney. I could also store the heat in a large storage tank, and re-use this heat for the next time the house t-stat calls for heat and the wood has burned out. That way I could store heat from a wood-furnace (much cheaper than a good boiler) and be able to recover it for space heat and dhw, as needed.

This is interesting...would you be running another set of duct work to make a second loop because I would think the airhandler wouldn't remove enough of the heat so the house would still get some heating. With a large enough airhandler heat exchanger it should work but I'm not sure how hot the water would get. Interesting indeed but I don't know if it would result in enough stored btus to make it worth the set-up costs.

sorry wrong post

 

[CrappieKeith]

We make a Hot Rod capable of pre heating your domestic hot water by an 80 degree rise at 18 gallons per hour.