# Wood Stove Coil system design planning



## woobs420 (Dec 2, 2013)

Hello everyone this is my first post here. I just moved into a large house that is very tricky to heat. It is a geodesic dome with two additions one is on a slab 12" thick with a 3 foot rock bed below. This is like a post and beam barn skinned with 4" rigid foam unsure of the r-value as of yet. The other is a 3 story monster addition that we call the "dorms". Currently we heat with a small hand fired coal stove in the dome and a very large Hearthstone I soapstone wood stove that sits on the slab in the post and beam addition. There is also a 118kbtu oil hot air furnace in the dome basement. Hot water is via a 40gal electric in the basement as well. The house is L shaped and so bringing heat the rear part "the dorms" is a challenge. 

Current plan is to have at least wood fired hot water set up in the next few weeks and at some point build a few small hydronic unit heaters into the colder rooms made from used ac parts heater cores etc etc.

I am struggling with a few issues on system design and I will start with the water coil. I am trying to go recycled and as low budget as possible and while the pre made stove coils look nice they are very pricey. I had a few thoughts on making my own water coil and have heard conflicting stories on metal usage. My thought was to build one out of 1/2 or 3/4 copper with five or six loops and use high temp solder. 20 bucks should about cover the coil that way. It would be placed behind a baffle and about an inch from the top of the stove so would not often see direct flame mostly flue gas as the damper just before it is closed. The exterior of the top about 3 inches above the coil is regularly about 300 to 500 deg. The stove runs 24/7. I have heard this would be fine but also have heard that the copper will pinhole in a few weeks. I am fine if I can get a few years out of it I have also thought of using black pipe but have no bender so I would have to use npt elbows and such and would be worried about leaks. Anyone have any thoughts on the durability of the copper? 

Storage, I picked up a free used older superstor SS-60 indirect tank this weekend. I had planned to run the stove coil into that (it will be below the stove so no thermosiphon) with a 007 taco pump. I also have good shape 150gal propane tank I could use for more storage in the future. Now here are the questions I have. I am sure that the stove will be ok heating the 60 gal tank in fact im worried it might do to well and I will need a heat dump or the bigger tank. Obviously I cant heat the entire house with this system but I was thinking it might be enough to keep a small amount of warm air moving through a unit heater or two? Also we almost never use the oil furnace only when its super cold and only then just a boost if we get lazy with the wood and coal like early mornings. So its only on for 20to 30 mins every now and then. I can pick up a 200kbtu water to air exchanger for about 200 bucks and put it in my furnace no prob. I was wondering if this set up with the 150 gal tank would be able to provide enough hot water for short booster runs on the furnace? I am not to worried about recharge times as it is only on for maybe an hour a day when its really cold. Thanks!!


----------



## dirtman (Sep 15, 2011)

I don't have any experience with copper inside my stove. I have three thermosiphoning grids of black iron in there with just threaded elbows and high temp thread sealant. they haven't leaked yet in four years. They do sell weldable fittings which would be better but I couldn't locate any. I thermosiphon the hot water into two 100 lb propane tanks. I tried a grate type grid once without thermosiphon and with the first power outage I pulled that out. If you can't keep that water pumping through the pipes you won't enjoy the noise even if nothing else bad happens.


----------



## TnAndy (Sep 15, 2005)

I think you better be hunting more storage or a good radiator/heat exchange system. 

I built a fireplace log grate out of 3/8" black iron pipe bent ( use a standard 1/2" conduit bender ) in the shape of a "C", then used 2" square tubing as a manifold on the open ends of the C (top and bottom) as a supply and return....about 15 rows of C's on the manifolds.....then mounted this in my open fireplace. Ran a pump on a simple thermostat (aquastat) that clicked on anytime the heat in the fireplace was high enough, and circulated water into a *500* gallon old propane tank....and I could get the tank up to 200 degrees without too much effort.

Make *SURE* you have a couple ( one for backup in case one fails.....and they do ) T&P valves in the system for relief ( and a way to take the water out if they do open )....you're building a boiler here.

AND you'll need an expansion tank in the system unless it is an open system, or the T&P valves will open quite regular. I used another old water heater tank (20gal) mounted at the highest point in the system. This lets the water in a closed system expand as it heats, and compress air in the expansion tank. Those little ones they sell ( couple gallons ) for water heaters is NOT big enough.

By the way, the smaller the pipe you use ( notice I used 3/8".....the 2nd time around....ahahahaa ), the more surface area you can pack in a given space, and up your exchange at the stove.


----------



## woobs420 (Dec 2, 2013)

Thanks Dirtman, What was the max temp you had the tanks set to? Was that just for DHW or heating as well? Not to worried about power loss the entire system will be running through a 2500w sine wave inverter with a 30 millisecond transfer time. I am also installing a diesel backup I am in far northern VT and multi day power loss is a common thing here. Another thought on copper coils would be that I could get more surface area compared to black pipe. But I am leaning to the black pipe route.


----------



## TnAndy (Sep 15, 2005)

dirtman said:


> I If you can't keep that water pumping through the pipes you won't enjoy the noise even if nothing else bad happens.




ahahahaaaaa...AMEN. 

Power quit on us once, and my T&P valve was located in the "wood shaft" to the side of the fireplace ( dumb waiter system for bringing wood up from the basement ).....sounded like an Atlas Rocket lifting off out of a silo, as steam rolled out of the wood chute door !

It was an 'interesting' system......I did it for 2 reasons....one to make my big ole fireplace something less than a heat sucking up the chimney machine.....and also I wrote my master's thesis on the design and implementation of it (MS in industrial technology)....so it was a "two-fer"..... 

I've since pulled it all out, put in a good gasifer wood stove, use 1/2 the wood, and wouldn't go back to that water system again.


----------



## woobs420 (Dec 2, 2013)

TnAndy said:


> I think you better be hunting more storage or a good radiator/heat exchange system.
> 
> I built a fireplace log grate out of 3/8" black iron pipe bent ( use a standard 1/2" conduit bender ) in the shape of a "C", then used 2" square tubing as a manifold on the open ends of the C (top and bottom) as a supply and return....about 15 rows of C's on the manifolds.....then mounted this in my open fireplace. Ran a pump on a simple thermostat (aquastat) that clicked on anytime the heat in the fireplace was high enough, and circulated water into a *500* gallon old propane tank....and I could get the tank up to 200 degrees without too much effort.
> 
> ...


Yes there will be plenty of safety valves and I have an old pressure tank for the well I was thinking of using for an expansion tank. 200 degrees on a 500 gal tank I like the sound of that I have looked into making a huge tank myself with rubber wood etc. Was your tank insulated?


----------



## TnAndy (Sep 15, 2005)

Yeah, wrapped in fiberglass batt, and then a box of 2" styrofoam built around it.


----------



## woobs420 (Dec 2, 2013)

TnAndy said:


> Yeah, wrapped in fiberglass batt, and then a box of 2" styrofoam built around it.


What was the hot water used for DHW, space heating etc? Due to my small tank size I was planning to use the furnace exchanger as a heat dump set at 185 or so


----------



## farminghandyman (Mar 4, 2005)

If any one cares, this is how I help set my DD house, 

the coil is a piece of black pipe, bent in a U, 
the storage tank is a old electric how water heater, no elements
the coil thermo siphons, there is a PTV and the shut offs are drain back to the coil so if it is shut off from the system it is open to the air via the drain back.
on the tank, on top there is a (car thermostat) custom housing, that will open up if the water get to hot and it will reverse thermo siphon again down the section of hot water heater to the bottom of the tank, (it works), this system is set up to work on a non pressure system and by the pitcher pumps, there is a two way valve under the counters to select hot or cold water, to the pumps, 

there is a tempering vale on the hot water pipe so it remains at a pre set max. hot when water is pulled from the tank, so one does not get scalding hot water, 

one pump in the kitchen and one in the bath (no toilet out house yet), with an adapter pipe on the spout they can fill the tub, 

the drums is there current water storage, and it is filled from out side and there is a over flow on it so it does not flood the basement area if over filled, there is a pipe on the out side of the house with a cap on it they can fill it with, 

currently this is the system they are using, 

_would have preferred to _set it up in there attic but the attic is not heated so it would have been gravity feed, and work of there kitchen cook stove as it is used more consistently than the drum stove in the basement, 

but they do have water at the kitchen and a bathing area, in the summer they use a solar shower out side, Black Drum on a stand and a shower head, they fill in the morning and is hot by evening, some time they have to add some water to cool it down a little,

When they moved in there was no water in the house or bathing area, the old lady never used any thing but a bucket, and a bucket under the kitchen sink for a drain, the bath room was filled with boxes of old books and newspapers, and junk, no plumbing of any kind, and virtly nothing had been done to the Building since it had been built in 1929, (including painting the walls,


----------



## TnAndy (Sep 15, 2005)

Mine was space heating only.


----------



## nosqrls (Jun 9, 2012)

The reason store bought coils are expensive is they are generally made of steel, copper melts at like 1900 degrees F. steel 2800 degrees F. And they both become pliable at lower temps. A piece of steel conduit rolled into a loop should work. I have seen wood stoves with a tank built in both sides and back. plumbed in bottom one side out top on other side. If you can bring fresh air from outside to the fire It Will save allot on heating. I have seen thermal probes in the tank so if the fire goes out and water cools pump shuts off. Also with this type of heating you can feed hot water heater. When fitting expansion tanks always put then on the cold leg of system especially if using one with a rubber bladder, rubber bladders do not like hot water. If heater is in a basement and exp. tank is in the attic is the best setup less chance of vapor bubbles.


----------



## Virgil (Sep 29, 2009)

I second Dirtman and the other safety comments above... If possible get rid of the circulator pump for the coils within the fire box... Design the water circuit so that it is a natural thermosiphon upward flow for the heated water exiting the stove... Don't use copper in the fire box.... Threaded or welded black iron pipe can take the heat in the fire box... However, very good thermosiphon systems place the pipe coils or capacity tank outside of the firebox on the top or near the rear area of the stove... It's a real experience when the electricity is cut off to the circulator pump feeding water to the coils in a hot fire box....Been there done that.... Think NUCLEAR REACTION !!!! When it blows... I had three T&P valves on such a pump system... None of Which reacted quick enough to prevent the Big Bang theory to occur.... Absolutely heed the warnings above about placing many safety relief and pressure valves through out the water system.... Also you may look at a previous tread: Wood Stove/ Burner Water Heating.... 

Virgil....


----------



## nosqrls (Jun 9, 2012)

If going without pump insulate hot leg piping and not the cold leg insures max. delta T (temperature differential) and max flow. You will also need a larger pipe diameter. Also no down hill runs on hot leg. You can also build for Natural Circulation and use a pump Just put a spring loaded check valve(normally open till pressure from pump shuts it) in parallel with the pump (flow arrow same direction as pump) so if pump shut off for any reason the check valve will open and natural circulation takes over.


----------



## dirtman (Sep 15, 2011)

I keep the valves on my storage tanks open to the air. Since my water is always circulating through the floors it never gets hot enough to build any steam pressure. When I first built the system I used one tank and a tempering valve but later added another tank and eliminated the valve so I could feed the first and second floors separately. I have seen 200 degree water fed into the pex that is exposed in my boiler room without out bothering it at all. My pumps are running as long as there is fire in the box. I had to replace one this year. jI don't know if that is a reasonable service life or not. I've seen old boiler systems so corroded that you wondered how they would still work and the pumps still pushed water.


----------



## ET1 SS (Oct 22, 2005)

woobs420 said:


> ... with 4" rigid foam unsure of the r-value as of yet.


Normally they say around R-8 per inch.

4" = R-32





> ... Current plan is to have at least wood fired hot water set up in the next few weeks and at some point build a few small hydronic unit heaters into the colder rooms made from used ac parts heater cores etc etc.


Sounds like fun 





> ... I had a few thoughts on making my own water coil and have heard conflicting stories on metal usage. My thought was to build one out of 1/2 or 3/4 copper with five or six loops and use high temp solder.


I would be hesitant using solder.







> ... It would be placed behind a baffle and about an inch from the top of the stove so would not often see direct flame mostly flue gas as the damper just before it is closed. The exterior of the top about 3 inches above the coil is regularly about 300 to 500 deg. The stove runs 24/7. I have heard this would be fine but also have heard that the copper will pinhole in a few weeks. I am fine if I can get a few years out of it I have also thought of using black pipe but have no bender so I would have to use npt elbows and such and would be worried about leaks. Anyone have any thoughts on the durability of the copper?


Our woodstove goes directly to a steel drum, and then to stovepipe. Inside that drum I placed 50' of 3/4" copper tubing bent into coils. I used regular flange fittings [no solder] to connect them to black pipe [outside of the drum].





> ... Storage, I picked up a free used older superstor SS-60 indirect tank this weekend. I had planned to run the stove coil into that (it will be below the stove so no thermosiphon) with a 007 taco pump.


Taco #7 is a good pump 





> ... I am sure that the stove will be ok heating the 60 gal tank in fact im worried it might do to well and I will need a heat dump or the bigger tank. Obviously I cant heat the entire house with this system but I was thinking it might be enough to keep a small amount of warm air moving through a unit heater or two?


Our thermal bank is 3 dead water heaters. 40-gallons each, so about 120-gallons total.

Our woodstove is our primary heat source for our house.




Install a pressure-relief valve [or two], and your good.


Our home has 600' of 3/4 PEX tubing mounted underneath the flooring. That is how our floor gets the heat.


----------

