Using your hearth to preheat DHW?

[Wet1]

I have to redo the base of my hearth this year. I've also been thinking about possibly preheating DWH with the wood stove. Although I've been entertaining the idea of doing a solar DWH system, which would reduce the need/benefit of preheating the DHW with the wood stove, let's assume the solar system wasn't in the picture and that I didn't want to run a HX in or directly attached to the wood stove (too risky IMO).

Since I'm working with a clean slate here, I was entertaining the idea of integrating a HX of some type into the hearth since I have to rebuild anyway (on a concrete slab). Would it be worth the expense of sinking say a 150' loop of 1" PEX into the hearth for preheating DHW? What I was thinking about doing is laying down 2" of rigid insulation over the concrete slab, then pour say 12" of concrete (with the HX submerged in), and finishing the top surface with slate or some other similar substrate. It looks like 12" of concrete would have an R value of 2 or less, so I would think the majority of the heat would be transferred to the HX over time.

I realize I wouldn't be able to fully heat my DHW by doing this, but I would think it would significantly reduce my DWH costs. If my math is correct, a 1" x 150' tube should hold about 6 gallons of water. If this mass (the base of the hearth) was warmed to say even 100° F, having 6 gallons of 100° water on tap to feed into the traditional DHW system should drastically reduce DHW expenses. And after the initial 6 gallons of water in the PEX is used, there would still be some additional heat transfer into the HX from the warm hearth, granted it would not be 100%, but even 50% would be significant as the delta T is greatly reduced... especially since my incoming water is in the mid 40's during the cold months.

With that said, how warm does your hearth get? Thoughts on doing something like this?

 

[Jags]

With that said, how warm does your hearth get?

Thats exactly what I was going to ask. Under my stove barely gets warm to the touch. Not enough to make it worth while. But I know that some stoves DO get warmer thats why different stoves, require different r value hearths.

I think the idea is pretty neat. I would make dang sure that the surface of your hearth never even gets close to 212F though. I doubt it would ever get that high, but I would double check. Ya don't wanna be making a steam bomb in the house.

 

[Wet1]

I bought a new stove for this season (a BKK), so unfortunately I don't know what to expect for hearth temps with my stove. I guess I was just playing with the idea since I don't think adding the HX would add any more than $200 to the price of the hearth... but even that is a poor ROI if the hearth never gets over say 75°. I guess I should be asking some BK owners how warm their hearth gets, but I also wanted to toss the idea around for thoughts as well.

BTW, If I were to do this, I would certainly install a PRV in the HX circuit to make sure no bombs could ever go off. Then again, installing the HX within the hearth seems like a relatively safe way of preheating DHW as far as heating water with a wood stove goes.

 

[karri0n]

As Jags mentioned, that would really depend on how hot your floor gets from the radiant heat from the stove. The bricks under my stove stay cool to the touch, and never get really warm at all. I'm not certain, but I had thought that the BK stoves only reccomend floor protection from sparks, and don't require any R value for the floor protection, which would lead me to believe very little heat comes from the bottom. You can easily check your manual, and if the manual actually calls for huge R value in floor protection, then a HX underneath may very well be viable.

Depending on your setup/decor/how much work you want to put into an "experimental" project, it might be better to put the HX behind the stove, embedded in concrete with brick or tile facing. Much more radiant heat will be coming out the back than the bottom. I can definitely attest to this with my stove as the entire area surrounding my stove is an old huge brick/concrete decorative fireplace, and the brick/concrete behind, to the sides, and above pick up and transfer heat really well, while the bricks starting from about 1/4 the height of the stove and below are cool to the touch.

I was going to say the actual gain for this may be well below the cost and labor involved, but since you mention that it's a BKK, I think if ANY stove is going to complete this purpose, it's definitely that one.

 

[Jags]

As Highbeam mentioned.....

Hmmm....never saw his post in this thread?? :coolsmirk:

 

[Wet1]

Thanks for your thoughts KO. Looking at the inside of my stove, it looks like there are 2 layers of fire brick in the base, plus the legs on the stove (it's a parlor model) add some distance to the hearth/stove. You guys are probably right, I suspect the area under the stove probably wont get warm enough to bother.

I'd much rather put a HX behind the stove, but that's not an option in this case. A couple feet of to the side might be an option, but it would be tough to seamlessly integrate it into the hearth like I was considering doing with it underneath, which is what made the idea so attractive to me. Then again, I'm pretty much sold on building my own solar system to heat the DHW, but time always seems to be any issue when it comes to projects like that.

Again, this is just something I'm throwing out there for discussion.

 

[Wet1]

makes me wonder what happens when a sudden change in temp [in waterpipe] affects a hearth

Cracked concrete?

 

[karri0n]

As Highbeam mentioned.....

Hmmm....never saw his post in this thread?? :coolsmirk:

hmmmm.... You somehow transformed into jags, HB!

 

[Jags]

makes me wonder what happens when a sudden change in temp [in waterpipe] affects a hearth

Cracked concrete?

You are talking about a temp differential that is probably not going to have any great contraction/expansion effect on hearth products. If the water comes in at 45F, sits and heats up to 100F before being replaced, it is a temp diff of 55 degrees. Just a guess, but that probably won't hurt much.

 

[Jags]

As Highbeam mentioned.....

Hmmm....never saw his post in this thread?? :coolsmirk:

hmmmm.... You somehow transformed into jags, HB!

Oh, you Isle Royal guys are all the same.....hey.... wait a minute.....

 

[Highbeam]

It's me, really, this time. The BKK while being a large stove is not a high btu stove. It is rated for long burns with low average btus which is perfect for many applications. The ash capacity of the BKK is like 9" deep to further reduce heat from the bottom of the stove. If anything, the BKK will put off less heat to the hearth than other stoves.

After a long time with my stove pumping out heat, my raised hearth is still easily touchable by hand in the hottest part which is right in front of the glass. Beneath the stove is cool.

I think it is a no go. Conisider also that you would have a 45 degree hearth in your room causing condensation issues and dehumidification of the air.

 

[karl]

I would install some sort of radiant heat barrier under the hearth if I were going to do this. The heat from the stove goes right through the brick and into the basement, that's why they don't get very hot.

Also, under my stove and immeditately in front of it (the first 6 inches) doesn't get very warm. However, my hearth extention gets very hot the last 18 inches or so. You can barely stand on it in bare feet.

 

[Highbeam]

I would install some sort of radiant heat barrier under the hearth if I were going to do this. The heat from the stove goes right through the brick and into the basement, that's why they don't get very hot.

Also, under my stove and immeditately in front of it (the first 6 inches) doesn't get very warm. However, my hearth extention gets very hot the last 18 inches or so. You can barely stand on it in bare feet.

I have a fully insulated floor beneath my hearth and the hearth surface is elevated above a dead air space. I don't think that the heat is going down to the basement. The stoves are simply designed such that little to no heat makes it straight down. This is done with firebrick, the ash layer, and the multiple false floors made up by ash pans, air channels, and built in heat shields. The hearth surface in front of the glass receives mucho heat from that glass.

 

[Todd]

Hmmm, you got me thinking now. I have run my stove without the bottom heat shield and the brick hearth underneath gets so hot you can't touch it. The clearance is only 4". I wonder if I were to use some copper tubing coiled underneath and run it to my water heater if it would do anything?

 

[karri0n]

I'm picturing a few copper rods fastened to the back of the unit, under the heat shield, going down into the concrete slab. Pour the concrete around them, and embed the PEX in the concrete around the area the rods are embedded.
The copper rods would need to be flattened starting where they enter the concrete for better heat distribution. This would be like putting a HX that touches the stove, but it would work much more gradually and you wouldn't risk boiling or damaging the PEX.

The issue with this is that you mentioned you have a parlor and not a pedestal model, and therefore you would be able to see the copper from the front of the stove.