Working a wood stove into my hot water system
I'm looking for thoughts on how to tie a wood stove into my DHW system in a new house I'm building. Some background is needed though...
I'm building up in VT north of Stowe and my foundation is now in and my main deck is on. Timber frame is due to show up soon and the SIPs to start going on the week after the frame work starts. To refresh folks memory the house is about 1440sf footprint, 2.5 stories with the 1.5 stories above grade finished. Of below grade area about 1/2 of it has walk out walls to the south & west and that portion will eventually get finished. The design is passive solar with a thermal mass slab of 2.5in concrete topped with VT slate as the flooring for the cathedral area that makes up 1/2 the 1st floor and which gets the lions share of the passive solar that comes in the south facing wall (which runs dead E to W). Roof is R-53 and walls are R-40. Thermal modeling via REM Design shows a -6F heat need of 26,000 BTU/hr.
I'm now revisiting my DHW strategies. I've been all over the map on this. In the short term the home will be a vacation home but in the long term it will be a retirement residence. The optimal approaches for heat and DHW differ based on these usages because when we're at the place we plan to use the 30,000 BTU Vermont Bun Baker wood stove, but when we're not there the heating system and passive solar will have to keep it warm. Likewise since it's a vacation home it doesn't seem to make a lot of sense to use a DHW system based on maintaining a volume of water at a fixed temperature in something like a Superstor versus some type of on demand system.
The other reason an on demand system seems better suited is that I intend to put in a drain water heat recovery system and a drain back solar thermal system (comprised of a 40-60 gal tank connected to 3 banks of 10 evacuated tube collectors) to pre-heat the inbound DHW.
I haven't decided on whether to go with a whole house DHW heater based on electric or propane, or smaller mini tanks like the Bosch Aristons. Our house has a fairly long electric run from the road and so we have 100 amp service with only 30 breaker slots. Any whole house electric DHW solution needs 240V and it turns out that the septic pump is 240V. Aside from the amperage draw that starts to cut into the available number of breaker slots. The down side of propane for whole house DHW is it adds another through wall penetration for intake and exhaust and these are likely to already prove a little trick to locate. So I'm leaning to the mini tank 120V units with one in the kitchen and each of the 2 baths as they're cheap enough.
Furthermore, being Vermont where it is cloudy a lot in the winter the solar thermal can only provide so many BTUs. So while it can help pre-heat the inbound water in the winter it's not likely to be able to provide all the heat needed for DHW then.
In the short term, when we're using the house as a vacation home in winter (and will therefore need DHW) the wood stove will be going a lot. In the long term when we live there the wood stove will be going most of the time in winter. Outside of winter when the solar thermal system is likely to be able to gather more heat, and the house itself will be able to derive most or all of its heat from the passive solar, we're unlikely to use the stove.
It seems to me that when the stove is in use there will be some extra heat available from it that could be collected and banked into the solar thermal mass tank to supplement whatever heat the solar tubes can collect in winter for pre-heating DHW ahead of the on demand units. Maybe there's a better way to do this but my initial thought was to figure out some way to use a differential temperature controller (DTC) to turn a pump on that would circulate the water from the solar thermal storage tank through some means of passing heat into it that comes out of the stowe.
The stove can take a thermo-siphon heating element in the firebox, so this is one approach. Alternately I can try some type of external heat exchanger mounted on the exterior of the stove or it's exhaust pipe.
If I went with the heating element in the firebox my understanding is that it's probably not a good idea to try to pump the water out of the solar thermal mass tank directly through it because even though that would be an open system (which reduces danger of pressure buildup) if the DTC or pump failed at the very least not having any water in the heating element would probably damage it and they're several hundred $.
So if I want to do the heating element in the firebox it's probably better off to set things up with a thermo-siphon approach using a tank located close to and higher than the stove. If this tank had a heat exchanger in it then a DTC could control a pump to circulate the water from the solar thermal tank through that heat exchanger.
One alternate approach is to use a heat exchanger exterior to the stove with a DTC, or even something as simple as an attic fan controller, to activate the recirculation pump.
It's also possible that the same way the the DHW simply moves passively through the drain heat recovery unit and the heat exchanger inside the solar thermal mass tank it could also move through an exchanger in thermo-siphon tank thus avoiding the need for a pump and controller. My concern with this approach was that it there was heat in the solar thermal tank and the stove isn't in use and the thermo-siphon tank was cold that I could actually lose heat to it.
Additionally I've seen several different ways to plumb things up so that the DHW water is drawn right out of the thermo-sihpon tank.
Given the profusion of potential approaches I was wondering if anyone reading this forum has any practical experiences they can share as to what approaches are viable and of those which might work best for our circumstances?
Posted Aug 9, 2011 1:29 PM ET
Edited Aug 9, 2011 2:38 PM ET
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