Posted By jonr on 09 Jul 2012 10:11 PM


I like the idea of a wood burning boiler and a several thousand gallon water storage tank much better.

Interesting proposal. Can you explain this more? It seems that you are exchanging the thermal mass of the concrete for that of the water, which is a fair trade. You can then control the rate of usage of the heated water, which cannot be done with heated masonry mass (unless you open a window). In essence, your "controllable comfort" is that you can turn it off, correct? But how much heat is lost from the storage tank, likely located in a basement due to its significant size and weight? Is it easy to heat 2000 gallons at once? What kind of maintenance does the storage tank require? Has it been successfully done before?

Correct. Best to put some insulation around it to be able to really shut it off (unlike the masonry). A search will show numerous people using tanks (but I don't have one).

Any polyethylene tank. I'd stay under 110F although most say 120F or 130F.

One can do similar btu storage calculations on masonry, but the numbers aren't impressive.

I've been buying polypro tanks for unpressurized solar tanks. I regularly heat them up to 65C (150F or so). They are a bit more expensive than PE tanks but more stable with heat. You can also do a cement tank and get a polyurea spray for the inside.

Most people would want more than a 25-degree temperature difference, the reason being that a 2000 gallon tank would be 6'x6'x10' allowing for expansion and insulation. Builditsolar.com has numerous DIY efforts mostly using structural wood and plastic liners. EPDM is good for about 150. PVC is good for about 170. You can get PVC welded into a custom fit liner, but the expense for a small tank like my 200-gallon heat bank is not a whole lot less than getting one welded up from stainless steel. Mine cost $1,000 and its good for 211 degrees, or more like 180 to provide a safety margin.

On the sweat side of the equation, if you need 50k btu/hr to get you through the night and, say, 30kbtu/hr for the other 16 hours, you'd be burning a cord of wood every 28 days. That's more work than I want to do. You might want to work on the building envelope before you look at storage.

Finally, heat banks are vented and radiant floors are pressurized so you will need a heat exchanger to connect them. Figure $300 for DIY copper or about $1k at retail.

50KBTU/hr overnight would imply an average overnight temp of something like -25F even for a code-min house. Averaging 30KBTU/hr during the DAY would imply you left a couple of windows open, or a high for the day that was also below 0F, and all windows are shaded by site factors.

If that's the 28 day mid-winter average, I'd DEFINITELY be looking at lowering the load via envelope upgrades just on comfort issues alone! (I also might consider moving to a warmer climate- YMMV. :-) )

50KBTU/hr overnight would imply an average overnight temp of something like -25F

Or 0F - depends on the house size and age and how much you value more than 8 hours (under milder conditions) of heat without the hassle of building another fire.

No need for a heat exchanger but you do need space.

My plumber told me that five radiant zones on two floors would be unworkable except as a pressurized system. Something about the relationship of the tank and the floors, pump sizes and startup pressures.

Actually, I have two heat exchangers in my heat bank: wood stove boiler in and radiant floor out. The first is a safety feature. I have a header and expansion tank 20 feet up that gins up about 8 psi in the open stove loop, which includes a 4kbtu/hr baseboard heating element as heat dump. Should I lose power, the header tank takes over as a thermosiphon (gravity) loop so the stove can idle safely until it goes out. You can use a universal power supply to keep the pump circulating water to the heat bank as well. I have both. Of course, you would care a great deal less if the stove was outside.

Yes, the Brits use header tanks to pressurize hydronic loops, but to do so the heat bank must be closed and small. My stove calls for a header tank large enough to hold expansion equal to 7 percent of the water in the stove loop. I think you can see where a 2k gallon heat bank would take some significant engineering.

Again, I'd attack a large old house with caulk and foam guns before I turned to heat storage.

Well, this topic has changed from masonry heaters to wood fired heat stored in heated water. I happen to like water as a heat storage medium. I'm thinking of combining the concepts. My house design heat loss is about 12,000 BTU/hr. So if I stored 24 hours worth in 500 gallons of water I'd need to heat it about 70 degrees f. The Build it Solar site has plans for a suitable tank using an EDPM liner in a foam insulated plywood block. My thought is to essentially wrap the refractory core of a masonry heater kit with a hard soldered copper coil with mineral wool insulation between the tubes then build the brick exterior around the coil & insulation. The effect is to add the thermal mass of 500 gallons of water to the masonry's thermal mass. But it allows heating areas remote from the fireplace with simple radiators. Is it worth the trouble?

Adding coils to a masonry heater works for domestic hot water but I wonder if you will be able to pull off enough heat for hydronic storage. My stove uses a cross flow integral water jacket to pick up about 70 percent of output as hydronic. I need a loading valve to keep the stove burning efficiently. If I just flipped on the pump, the stove would cool too much. While I doubt that you could get enough coils in a masonry heater to send up a plume of black smoke, I have to wonder how many degrees you could add to 500 gallons of water without continuous firing.
If you're planning a radiant slab, there's no reason for the fireplace, save esthetics. The slab has more mass and a greater reach than any masonry heater. It's the same principle: you're adding heat slowly to a mass which will then release it slowly. There are a few European stoves sold here: http://www.ebay.com/itm/NEW-Indoor-wood-stove-GASIFICATION-BOILER-with-top-cooking-plate-/170853364027?pt=LH_DefaultDomain_0&hash=item27c7a7713b

Todd,
    Thanks for the link but that is way way more heat output than I need.  My design heat loss at 0f is 12,000BTU/hr.
    In North America the usual placement of water heating coils in masonry heaters has been directly in the fire which hinders a clean hot burn.  While in Denmark the water heating coils are placed outside the firebox and have much less effect on burn temperature.  What I'm proposing, as far as I know, hasn't been tried.  What I want to try is a lot more tubing than is normally used and all on the outside of the firebrick core.  With more tubing and insulation between the firebox and exterior masonry I'm hoping to store, in the heated water, most of the heat that would normally heat the exterior masonry.  But the interior firebrick would still be very hot for a clean burn.  It still would be a masonry heater and still store much of the heat in the masonry, firebrick mostly.  The heat in the firebrick would continue heating the water long after the fire is out.  The big unknown is how much heat can be captured in the water loop. 

Well, not the first hydronic masonry heater: http://heatkit.com/html/lopezs.htm (You got me curious.)

Sorry I missed the DD temp, the point of which is that 97 percent of the time your weather will be at 10 degrees OR WARMER. In other words your heating demand will be well under 12K btu/hr most of the time. Two points: a masonry heater may well be overkill on all but a few nights a year. That is almost certain to be true if you're heating 500 gallons of water. In the link above, a guy heating 200 gallons of water in Ohio reports that radiant heat alone kept his shop in the mid-70s. Secondly, payback on masonry heater plus storage is going to take a long long time compared to a heat pump.

I am not one to talk, mind you. The secret to saving money with DIY is knowing when to quit. I'm not good at it yet.

Posted By Liebler on 31 Jul 2012 09:39 PM
Todd,
    Thanks for the link but that is way way more heat output than I need.  My design heat loss at 0f is 12,000BTU/hr.
    In North America the usual placement of water heating coils in masonry heaters has been directly in the fire which hinders a clean hot burn.  While in Denmark the water heating coils are placed outside the firebox and have much less effect on burn temperature.  What I'm proposing, as far as I know, hasn't been tried.  What I want to try is a lot more tubing than is normally used and all on the outside of the firebrick core.  With more tubing and insulation between the firebox and exterior masonry I'm hoping to store, in the heated water, most of the heat that would normally heat the exterior masonry.  But the interior firebrick would still be very hot for a clean burn.  It still would be a masonry heater and still store much of the heat in the masonry, firebrick mostly.  The heat in the firebrick would continue heating the water long after the fire is out.  The big unknown is how much heat can be captured in the water loop. 

We have done some masonry heaters in the 90s where the SS piping was embedded in the cement but the HX was always custom made. I like it better than having it in the firebox. 2000deg is not necessary to transfer heat to water and keeping it a bit lower is easier on the rest of the components, not to mention metal expansion and fatigue.