# Supplemental heat with wood and solar



## Kevingr (Mar 10, 2006)

I need to get a handle on my high heat costs and I know many of you use some type of wood and/or solar to accomplish that. From what I've seen though much of it is for radiant in floor heat. Here's my situation and what I'd like to do, please give me some advice and let me know if this will work.

First off, a couple of things you need to know. 
1) Central MN location so winter months are mainly cloudy.
2) We have hot water base board heat fueled by propane.
3) I have a shop that I'm remodeling this summer that needs heat and has a nice south facing wall.
4) The shop is about 200 feet away from the house.
5) The house is an old farm house about 2200 square feet, insulation is ok, but more conservation items will be added this summer (more insulation, sealing more holes, some new windows).
6) I cannot afford an outside wood boiler at $10,000 installed with all the needed parts.
7) I cannot put any type of woodstove inside the house itself.

So, to heat the shop I'm thinking of two things.
1) A barrel stove surrounded by brick to increase it's thermal mass. 
2) Make a solar collector on that south facing wall the directs warm air into the shop. I saw this on Solar Gary's website.
Between those two things my shop will be nicely heated. Early winter and late winter the solar will work just fine, with some benefit deep into the winter months. That's where the barrel stove will help.

So, since I was going to be putting all this in my shop, I was wondering how I could use this as supplemental heat for the house. I've seen a picture here on HT where someone has copper tubing wrapped around the upper barrel on a double barrel wood stove, I assume this is for in floor heat. I was thinking of doing that same thing, but running the piping to the house where there'd be a water exchanger tank that is input into the boiler system. Since the south facing wall will basically be a solar panel, maybe run some pipes through there too.

Would the water from the pipes wrapped around a barrel stove be hot enough to add anything to a boiler system 200 feet away? I'd put in a very well insulated run from the house to the shop of course. I also understand that a double barrel woodstove isn't going to stay burning as long as an outside boiler, but since I'm home almost all day everyday, stoking it isn't an issue. Plus, it'd just be supplemental heat to help offset the propane bill. I have available wood so that's not an issue. 

The big question is how can I supplement the house heat by utilizing what I put into the shop, given a boiler system and a not in floor radiant heating?

And lastly, I won't be making a bomb, all the appropriate expansion tanks, pumps, valves, etc. etc. would be installed


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## tomjones (Dec 22, 2007)

I am thinking about much the same issues, with a few differences.

You can use a lot of this stuff for other than hydronic heat in the floor, it just depends how nifty you want to be. We once took a car radiator and built a frame for it with a squirell cage fan. Hooked the radiator to the hot water from the coil you are talking about, and you can pump a lot of heat. 

Dont discount the cost of insulating that pipe. My run is only going to be about 50 feet, but I expect a significant loss no matter how well I insulate. 

One other thing to think about is heat storage. In my research and experiance I think this is one technique greatly underused by many of us wood burners.Take a 500 gallon tank, which does not take much room. Super insulate it with coiled copper loops in it for heat exchangers to add or extract heat from the "battery".

500 gallons x 8.35 lbs per gallon=4175 lbs of water

Raise that from 90F to 160F and you have a battery holding 292,250 btu's uf heat. Better yet go to 1500 gallons and you can store just shy of a million with a 75degree temperatrue swing. You can add that heat into the system with solar, wood burning, whatever. (May be efficency probelms with high temp solar.) 

What I want to do is install some sort of high efficiency dual stage gasification wood burner ( maybe built on a barrel stove chassis, I know there are guys that have done that) and then build an almost seperate heat exchanger to move heat from the exhaust into water and store it. Put a separate loop in for adding water to the battery, one for heating the domestic hot water, a third for shop heat (probably not used much because when you are in the shop you want to keep the stove running to heat the battery) and finally a loop to run to the house and an exchanger in my forced air system. I really beleive this system could provide all heat used in a home and shop complex with one properly sized stoking per day. Burn it hot, fast, and eficient, pumping it into the battery.

That is my plan anyway. Currently searching for components. Looking for a good design for the stove that I could fabricate myself. Oh well, it seems my life is a search for the right way, with just a little bit of actually doing mixed in.

Tom


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## fishhead (Jul 19, 2006)

A friend of mine hooked up an inside the house car radiator to his outside wood burner. It has a fan behind it to push air through the fins and puts out a lot of heat.


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## SolarGary (Sep 8, 2005)

Hi Kevin,

The first part about using a combination of solar and wood to heat the shop itself sounds completely practical. I use solar to heat my shop and it works fine with full or part sun. But, when its cloudy it would be nice to have wood heat -- I'm also thinking about adding this. If nothing else its a good way to get rid of all the scrap.

On trying to move the heat 200 ft to the house. I think you are pushing the envelope some, but maybe it could work. I have a somewhat similar setup with solar water heating collectors that are a bit over 100 ft from the house and that we use for space heating in the house. All the info on my system is here:
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

The potential problems I see:
- 200 ft is quite a ways
--- Lots of digging. Should be down below frost level.
--- You will lose some of the heat on the trip (see note below)

- Heat transfer from wood stove barrel to pipe coil
--- This does not seem likely to be very efficient. But, if the "wasted" heat that does not get into the pipe coil heats the shop, maybe thats OK.
I'd still be concerned that you put all the effort into the trench and then find you can't get much heat into the coil. 
Maybe a loop inside the firebox would be better? But, be sure to take all the precautions to prevent steam explosions -- pressure relief on the coil side of any shutoff valves.

- Integrating the wood burner heat with the boiler
--- Not sure how exactly you integrate the wood burner heated water with the boiler heated water and the baseboard. 


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On the thermal losses for the hot water going through the trench.
I took some pains to carefully measure the temperature drop in in my system over the length of the 100 ft trench.
Once the initial temp swings were over, it settled down to about a 0.2F drop from the tank to the house, and an about 0.4F drop from the house back to the tank.
In my system, this is about a 5% loss, because the temperature drop inside the floor loops is about 12F (0.6F/12F = 5%).

You also lose the heat left in the pipe when the pump stops circulating water. If you use 3/4 inch pipe, this is 0.023 gal/ft or 9 gallons for 400 ft of pipe. About (9)(8.3lb/gal)(120F-40F) (1 BTU/lb-F) = 6000 BTU 
So, best to let it run for as long as possible at a time.

You would certainly want very good insulation. I like the scheme I used (see link above), but would double the thickness if I had it to do over.
I would also probably use PEX instead of CPVC -- just a bit tougher all around.
Some of the commercial insulation products don't seem on the up and up with their claims on R value to me -- when you look at the thickness they have, the claims just don't add up.
Also, you need to separate the in pipe from the out pipe with some insulation to keep them from exchanging heat with each other.

Gary


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## fishhead (Jul 19, 2006)

Are there any simple tables showing btu output per square foot of the various collectors at different latitudes?

I've been wondering if it might pay to build some stand alone collectors for my house.


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