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Working a wood stove into my hot water system

Hi all,

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?

thanks,

Jay

Asked by Jay Hersh
Posted Tue, 08/09/2011 - 12:29
Edited Tue, 08/09/2011 - 13:38

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15 Answers

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1.
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Jay,
Too bad about the septic pump. That will be a long-term maintenance headache. I guess gravity septic didn't work for your site, right?

I strongly urge you to keep your systems as simple as possible. Don't depend on a circulation pump to move water through your wood stove.

Install your wood stove on your main floor, and your storage tank on the floor above. A simple thermosyphon loop between a stainless-steel coil in your woodstove and the tank will create plenty of hot water when the stove is in operation. Don't forget to install two T&P relief valves -- one near the stove, and one at the tank -- and test them regularly. No ball valves or stop-and-waste valves are allowed in the thermosyphon loop, for obvious reasons: you are building a potential bomb.

Both P&T relief valves should discharge safely into drains, with an air gap between the discharge and the drain.

If you don't understand the plumbing, don't do it. These systems are great, but they are dangerous if you build it the wrong way. I've had such a system in my house for 30 years, and it's worked flawlessly.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 08/09/2011 - 13:34
Edited Tue, 08/09/2011 - 13:36.

2.
Helpful? 0

Hi Martin,

The leach field that was approved with the sub-division is uphill of our lot in a right of way on the neighbor's lot. The top part of our lot where the house is located is an open field that gets a bit soggy. I don't know if they ever tested it for perking or not. There's a lot of forested lot downhill from there so someday we may have some testing done to see if we could relocate the leach field downhill and eliminate the pumping station.

Thanks for the comments on the thermo-siphon. Everything I've read about them agrees that care must be taken due to the potential pressure build up. That's why I was considering either some type of heat collection outside the firebox or a thermo siphon tank with a heat exchanger in it if I was going to try to draw the heat produced by the stove to the solar thermal mass storage tank.

Assuming I skip that level of indirection, I'm still a little fuzzy on how the DHW supply connects into the therma siphon system. I've seen diagrams that seem to show that the inbound DHW is plumbed into the thermo siphon loop and that the hot water drawn comes directly from the thermo siphon tank and is replaced with inbound DHW water. I've seen others that seem to show the thermo siphon is a closed system that collects heat and that the DHW goes through a heat exchanger in the thermo siphon tank.

How is yours plumbed up?

thanks,

Jay

Answered by Jay Hersh
Posted Tue, 08/09/2011 - 18:59


4.
Helpful? 0

Hi Martin,

thanks for the links. I had seen the 1st one but couldn't tell from the diagram there (or even from the diagram at the 2nd one) whether this was a standard tank or one with a heat exchanger built into it. The FAQ section of the 3rd one seems to answer my though:

Basically in answer to the question "How do I connect my stove to the DHW tank" (this question is paraphrased) what they say is that, in addition to the cold going in and the hot coming out, a standard tank has 2 more places to connect pipes. The drain spigot at the bottom and the pressure relief valve at the top.

It hadn't occurred to me that the pressure relief valve outlet and drain valve would get co-opted to adapt the tank for this approach. So the short answer to one of my questions is that the typical way this gets plumbed up is to have the water going through the firebox integrated into the DHW supply system.

This answers that question but still leaves me needing to decide how to integrate this with the drain water heat recovery and solar thermal into an overall system. In the winter, when the stove is running the configuration shown in these links should, utilizing a standard 40+ gallon DHW tank (electric or gas), provide plenty of hot water. But at times of the year when the stove isn't going the configurations shown assume it's going to use electricity or gas to provide the DHW.

While the solar thermal systems, working in combo with the drain water heat recovery, probably won't provide much hot water in the winter, at those times of year when I'm not using the stove it should be able to do a pretty good job of this. I'm not sure whether, for this to work best, having a holding tank of any appreciable size (such as 40+ gal) inline has a positive or negative impact.

The drawbacks I see are:

1) if I use a standard electric or gas DHW heater tank with the heating portion of it de-activated, when the stove isn't in use I'll have a large amount of water standing at room temperature for several weeks at a time when I'm not using the house on vacations. That's something I wanted to try to avoid.

2) If I use a smaller tank (say 10 gal) with a heat exchanger in it, one where the tank contents are hooked up to circulate through the stove's firebox and the DHW supply is separated and runs through the heat exchanger, then the standing water won't be part of the DHW line. If this heat exchanger were after the drain water heat recovery, but before the solar thermal tank then the water coming to it would be pre-heated and it wouldn't end up sending any heat collected "down the drain" by inadvertently heating the outbound waste water. However, the first problem I see is that if the water coming through the heat exchanger were hotter than that in the solar thermal mass tank (and there's a good chance of that in the winter) it would then end up losing heat to that tank which, depending on the temperature of the solar thermal mass tank and the water as it exits the heat exchanger, might then have to be replaced by the point source on demand heaters.

The other problem I see with this approach is that when no hot water is being drawn there is nothing to take heat out of that 10 gal tank and it could build up a bit of heat and pressure.

This leads me back to the other approaches.
A) Use the smaller tank connected up to the firebox loop as in #2 above and have a pump controlled by a DTC circulate water from the larger solar thermal mass storage tank through the heat exchanger in the smaller tank to pull the heat generated by the stove out of this system and dump it into that tank

B) Use some type of heat collection external to the stove's firebox that would be inline on the DHW after the drain water heat recovery and solar thermal mass store.

Maybe I'm over thinking this but, while I want to be careful I'm not building a boiler bomb, I do want to try to connect up the system in a manner that doesn't leave large amounts of water standing around for weeks at a time and which integrates the ability to heat the DHW with the appropriate non fossil fuel source (any electricity for the point source heaters or other backup heater would originate from coal so I consider it in the same class as propane) for the circumstances.

That's proving fairly tricky but I can't imagine I'm the first person to try to put something like this together..

thanks again for the pointers,

Jay

Answered by Jay Hersh
Posted Wed, 08/10/2011 - 09:43

5.
Helpful? 0

Jay,
There are a zillion ways to provide domestic hot water. In my own home, I have an 80-gallon stainless-steel Superstor tank. The water in the tank is the same water that circulates through the coil in my woodstove. The separate heat-exchange coil in the bottom of the Superstor tank circulates a glycol solution from my solar collectors. An on-demand propane water heater provides backup.

Here's my advice: for a vacation home, you don't want any of this. Buy an inexpensive on-demand water heater and you're done. That's all you need.

When you decide to live there full-time, you might want a different system.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 08/10/2011 - 09:57

6.
Helpful? 0

Hi Martin,

I already have the 3 banks of solar thermal tubes and a good sized SS tank to use with it as the drainback tank in storage. I got the tank for free and the tubes at great price from a Western MA reseller closing up his business.

While I'm sure you're right that for a vacation home the amount of cost that would be offset by integrating these systems isn't large. However, it's much easier to put certain things in when the house is being built than to retrofit them later.

This is certainly true of the drain water heat recovery system.

If I did want the water heater built into the stove's firebox it's also true of that too since the factory in Brattleboro can install it for a very nominal fee over the price of the actual hardware itself. It's also true of trying to create a place for any tank that would be used in a thermo siphon system.

While it might be a little more hassle I could skip the factory installation of the heating element inside the firebox and retrofit one into it later. Then I could try to plan out the plumbing in a manner which would allow that and the other elements of the alternative hot water heating systems to be cut into the DHW supply lines by putting in some strategically located Ts and 1/4 turn ball valves and then decide.

It's probably worth looking into some of the zillion ways between now and when I start installing pipes so if you have any suggestions of where else to research and read up on what others might have done they'd be appreciated.

thanks again,

Jay

Answered by Jay Hersh
Posted Wed, 08/10/2011 - 11:42

7.
Helpful? 0

Hello Jay,

I am also in Vermont in the Burlington area. I am getting ready in the next week or so to do almost the same set up on my wood stove. I would be interested in seeing what you are doing for a set up.

Thank you,
Brian

Answered by brian letourneau
Posted Thu, 09/22/2011 - 18:16

8.
Helpful? 0

Hi Brian,

the stove and chimney are supposed to get installed on Oct 5th. So I should get a better look at some of the guts of the unit than I was able to in the store. Surprisingly they didn't really want to let me start removing pieces of the one they had on the show room floor :-). Then I'll be able to brainstorm about how best to try to capture some heat for use as an auxilliary DHW heating source.

I did end up punting on the integrated thermo siphon coil. Although after going over locations with the stove installer and revising the installation location slightly such that the stove would actually be located in a place where a 10-15 gal thermo siphon tank could have been positioned to work with it going with that approach would have ended up taking up about 1/4 of the space in the kitchen pantry closet and I still wasn't sure about how to plumb that all in with the rest of the system. So I put it off for now.

Instead I've moved a small temporary storage tank up to the place to use as a drainback test tank in conjunction with a home made solar plate DHW collector I built that will also get moved up there shortly. Once the windows and doors are on the house and a few more things get done (my contractor is very behind schedule and I've been having to do a lot more work myself just to get the weather tight shell completed than I'd originally planned on and am therefore behind on interior build out) I'll have the ability to play around with the stove in conjunction with this small solar thermal drainback system to test out some of my ideas for integrating the two systems.

At least this way I'll have a source of hot water for my jury rigged shower from either the wood stove or the plate collector that doesn't involve heating the water up on the propane camp stove.

I'll try to remember to update this page with some of my experimental results if I get the chance.

Jay

Answered by Jay Hersh
Posted Wed, 09/28/2011 - 10:03

9.
Helpful? 0

Jay,
If you are sizing a tank for a thermosyphon coil in a wood stove, 10 to 15 gallons is way to small. You want an 80-gallon tank -- minimum. Even then, you'll probably be blowing your pressure/temperature relief valve every now and then.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 09/28/2011 - 10:06

10.
Helpful? 0

Just curious. Is there a set up where heating the water on a woodstove can be turned off so to speak? Like high temperature fluid in the circulation loop or a heating element that moves out of the heat or is built to stand temps when not filled with fluid????

Basically what solutions exist to be able to switch on and off water heating via a woodstove?

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 09/28/2011 - 10:37

11.
Helpful? 0

AJ,
If you have a hot-water coil hooked up to a thermosyphon loop and a storage tank, the only way to turn it off is to put out the fire. If you try to install a ball valve in the loop, and you experiment with shutting it off, you will have built yourself a powerful bomb.

If the water tank gets too hot, the P/T relief valve will blow. It's also possible to install an aquastat that controls a circulator connected to a "dump loop" -- a loop of baseboard radiation that dumps excess heat.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 09/28/2011 - 10:43
Edited Wed, 09/28/2011 - 10:50.

12.
Helpful? 0

That's interesting Martin. I'd seen set up diagrams showing much smaller tanks but I know you have actual 1st hand experience with these things so I'd trust your recommendation. In this case then it makes me more secure in my decision to forego this approach because an 80 gallon tank would take up 1/2 or more of the kitchen pantry closet, something I'd rather not do.

I think I'm going to experiment some with my idea about using an external heat exchanger built in between a surround (probably soapstone) on the outside of the stove with water circulating through it drawn from the solar DHW drainback tank. It might not draw a whole lot of heat and thus might not end up being a practical approach, but it will be easy enough to test and from what I can tell a lot safer than a thermo siphon approach even if the thermo-siphon was coupled with a way to draw excess heat off of the thermo siphon tank.

thanks again for all the advice on this Martin.

Jay

Answered by Jay Hersh
Posted Wed, 09/28/2011 - 11:05

13.
Helpful? 0

I understand the bomb concepts. Have plumbed every home I built. Maybe my idea of a moveable firebox loop is not out there yet. Withdraw the loop and no bomb. Simple enough.

Jay, sounds like you are close to a working plan, keep us updated.

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 09/28/2011 - 12:02

14.
Helpful? 0

I bought one of these heat exchangers from ChimneyHeaters.com and it is Stainless Steel. The rest of the pipes in my house are Copper and some plastic. I get a tremendous amount of heat from this thing and I am worried that it will melt the copper or plastic tubing.

The heat coming out of the Chimney Heater is about 190f does anyone know if this is to hot for the copper or Plastic pipes in my House?

Answered by William Laurence
Posted Thu, 05/31/2012 - 17:18

15.
Helpful? 0

William,
There are several types of plastic pipe out there, including PEX and CPVC, so I'm not going to comment on the upper temperature limits for plastic pipe. One online source says that PEX is rated up to 180 degrees; another says 200 degrees.

You won't have any problems with copper. Don't forget to install two pressure/temperature relief valves.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 06/01/2012 - 03:57

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