Hey guys, I’ve been reading the older threads and they have already answered a lot of my questions. If you don’t mind, at this point I’d like to lay out some of my project thoughts and solicit opinions from you. I’ll begin with a description of the house and some of my thoughts and see where it goes.

First  question is how many cord of wood per year do you burn now?  From this you can figure out how many BTU's per year you need to make up.

Second, What is the ratting in BTU's for the woodstove and do you ever have to run it full bore to keep the place warm?

To me the easiest solution is to replace the wood stove with an Nat gas or propane stove in the same location.  IF you want to keep the wood stove, adding a direct vent, sealed combustion wall furnace would be easy and quick.  You lose some efficiency over a boiler and radiant heat, but if your heat load is small enough, it might not matter to much.

Cleers
eric

 

In Paducah's ~4200HDD climate you can probably heat the place just fine with a propane hot water heater-combi system. Your design-day outdoor temps are probably low single-digits-F, but your "typical" numbers are quite modest, and in record-breaking cold you can fire up the wood stove if a water heater's modest output was being strained.

The thermal mass of the floor isn't a bad thing, but it won't be hard to control with the tubing in an overfloor situtation, WarmBoard (tm) style with thin-aluminum heat spreaders (necessary to avoid warm/cold stripes on the floor, and to get more heat out of the PEX and into the subfloor.) The thermal mass won't "pull the heat down" exactly, but will store and release heat whenever there's a temperature difference between the tubing and the pavers. The mass will give add a moderating affect on the floor temp but unlike embedded-tube slabs, won't much affect the response time of the system (which will be pretty decent.)

You might be able to pull it off with an electric HW heater and a heat exchanger, but that depends a lot on what your heat-loss numbers look like.

Like eric said, if your heat load is low enough the absolute efficiency doesn't much matter, but your peak output requirements still do. Still, even when it's not keeping up, radiant can be pretty comfortable, sitting on the warm floor with a pillow & comforter watching the shrieking winter storm outside your windows. Your high passive gain probably keeps the fuel use low, but the glazing represents a higher peak-load condition when it's a not-so-sunny 5AM and -5F outside. A propane stove or wall furnace where the woodstove now sits would put out more heat than a typical propane water heater, even if the radiant ran the show most of the time. The difference in combustion efficiency would be negligible, but as-operated there will be higher efficiency when there's more thermal mass to work on (as in the bulk water of the tank + the thermal mass of the floor.) On any propane appliance, avoid standing-pilot ignition, which can be a significant standby loss when you're not actively heating.

BTW: I neglected to mention- there's nothing in your radiant heating situation that would call for reflective foils, etc. On a really high-temperature water between the joist staple up configurations there is (sometimes but rarely) a rationale for radiant barrier, but most of the time it's uncalled for. (And never when you can't get at least a 1/2" of air space between the foil and radiating element.) Above the floor tubing delivers the heat with much cooler water, and you'd have to build it up to add the air gap, which would be silly.

It's probably worthwhile to put in strips 3/4" of XPS in the spaces between the tubing & stringers though. That'll add another ~R3.3 to the floor insulation increasing efficiency & enhancing response time. It doesn't have to be a perfect fit to have an effect.

Posted By eric anderson on 11 Feb 2010 11:36 AM

First  question is how many cord of wood per year do you burn now?  From this you can figure out how many BTU's per year you need to make up.

Eric, first, thanks for the reply. Here in Western KY no one sells wood by the cord, they sell it by the rick. A rick is what is also known as a "face cord", 4'x4'x16"-18". So a rick is roughly 1/3 of a cord. In a very cold winter we burn maybe 2-3 cords. In a normal winter, less then 2 cords.

Second, What is the ratting in BTU's for the woodstove and do you ever have to run it full bore to keep the place warm?

Our wood stove is the same one that we installed when we built the house 20 years ago so it's probably not quite as efficient as it once was. New I believe the BTU rating was 40K-42K BTU. That said, if I ever crank the stove up to full bore, it can get uncomfortably warm fairly quickly unless outside temps are in single digits.

To me the easiest solution is to replace the wood stove with an Nat gas or propane stove in the same location.

We REALLY prefer the radiant heat from the cast iron wood stove to any type of blown air heating solution. We have tried a pellet/corn stove and although it kept the house warm, for us, the moving air just wasn't as comfortable as a radiant heat source.

IF you want to keep the wood stove, adding a direct vent, sealed combustion wall furnace would be easy and quick.  You lose some efficiency over a boiler and radiant heat, but if your heat load is small enough, it might not matter to much.

Cleers
eric

 

I'll do some research on sealed combustion wall furnaces. Thanks again for the input and ideas.
Jeff
Paducah, KY

Posted By Dana1 on 11 Feb 2010 03:06 PM
In Paducah's ~4200HDD climate you can probably heat the place just fine with a propane hot water heater-combi system. Your design-day outdoor temps are probably low single-digits-F, but your "typical" numbers are quite modest, and in record-breaking cold you can fire up the wood stove if a water heater's modest output was being strained.

Good point on using the wood stove to supplement the hydronic radiant. That was exactly what I hoped to be able to do; use radiant for most of the time and then fire up the wood stove when it's really cold.

The thermal mass of the floor isn't a bad thing, but it won't be hard to control with the tubing in an overfloor situtation, WarmBoard (tm) style with thin-aluminum heat spreaders (necessary to avoid warm/cold stripes on the floor, and to get more heat out of the PEX and into the subfloor.) The thermal mass won't "pull the heat down" exactly, but will store and release heat whenever there's a temperature difference between the tubing and the pavers. The mass will give add a moderating affect on the floor temp but unlike embedded-tube slabs, won't much affect the response time of the system (which will be pretty decent.)

You might be able to pull it off with an electric HW heater and a heat exchanger, but that depends a lot on what your heat-loss numbers look like.

Like eric said, if your heat load is low enough the absolute efficiency doesn't much matter, but your peak output requirements still do. Still, even when it's not keeping up, radiant can be pretty comfortable, sitting on the warm floor with a pillow & comforter watching the shrieking winter storm outside your windows. Your high passive gain probably keeps the fuel use low, but the glazing represents a higher peak-load condition when it's a not-so-sunny 5AM and -5F outside. A propane stove or wall furnace where the woodstove now sits would put out more heat than a typical propane water heater, even if the radiant ran the show most of the time. The difference in combustion efficiency would be negligible, but as-operated there will be higher efficiency when there's more thermal mass to work on (as in the bulk water of the tank + the thermal mass of the floor.) On any propane appliance, avoid standing-pilot ignition, which can be a significant standby loss when you're not actively heating.

Thanks, dana, very helpful info. I'll have to digest and then finish my heat loss calcs. I am sure that if I decide to proceed with the radiant system I discussed above, I will have many more questions.
Jeff
Paducah, KY