Homemade Wood Stove Hydronic Radiant Heat Setup

rbs12

Let me start off by saying that I am not a contractor but a DIY homeowner/fabricator. I live in New England, needed an inexpensive way to heat my new living room so I built this heat exchanger for my wood stove last year to heat the radiant heat loop in my garage-turned living room. I couldn't find anything like this on the internet but used some information that I found on this site so figured I'd return the favor and put this out there for others to see.

The exchanger was soldered in place on the stove to make sure that it fit snug and is connected to the rest of the system in the basement via two unions. This way I can disconnect the whole system and move the wood stove if necessary. There is also a high point air vent at the top of the exchanger.






It's a big room (420 Sq/Ft) with 7" of concrete over the pex loop and a thick carpet and pad. I was nervous that this set up wouldn't be able to heat that much floor but it worked fantastic all winter long, keeping the room at 74 degrees.






Two relays control the whole system. There is a aquastat on the back of the stove that starts the circulator pump when the water directly around the stove is up to temp. The aquastat in the basement checks the overall temp of the heat loop. When that temp is at 120 degrees and the room calls for heat the relay open the 3-way directional valve which then loops the water to the floor until the water goes back down to 80 degrees at which point the loop closes and the cycle continues. There is a check valve just to the left of the "T" on the bottom pipe that stops the pump from pushing water back through the floor loop when the room is not calling for heat. The 3-way directional valves "off" state is set so that if I lose power the loop to the floor is always open. This should hopefully alleviate any over temp/over pressure issues when I lose power. There is also a pressure tank and release valve incase that happens. In the future I may upgrade to a back up battery for the circulator pump. When dealing with high temp water/steam one can never be too careful. The two on/off valves with drain allow me to isolate the heat exchanger, drain it and disconnect it without having to drain the entire system or make a mess of my floor when I do.






The reheat rate for the heat loop couldn't always keep up with the request for heat in the room so I would manually set the system to just loop the whole floor, then at times I had excess hot water available so I will be making a few upgrades/changes for the upcoming season. One will be to jacket the copper pipes with cement board under sheet metal to make it even more efficient. My plan is then to add a 3-4' length of radiant baseboard to the heat loop in the basement with some computer fans behind it. When the room is up to temp and I have excess hot water in the loop the fans will kick on to heat my basement. I'll update pics and info as I make them and hopefully it will not take me a year again. Overall I am very happy with the system and it cost me roughly $700 to build. I also did not notice any significant change in the amount of wood that I burned this year or a drastic change in my electric bill due to the circulator pump. Any questions or concerns feel free to comment.

rbs12

I forgot to add that the two led lights installed on the aquastat on the stove simply tell me what is going on in the basement without me having to go down there. One tells me the circulator pump is on and the other tells me that the floor is calling for heat.

Jamie Hall

Nice. Very nice. Especially that it is simple...

ChrisJ

What happens if the system leaks and runs out of water or if the pump dies and you have one hell of a fire going?

I'm betting that stove can easily melt that solder.

I'm asking because my boss built something similar and it really scared him one night when he was in bed and the pump died. Pex melted..... It was starved of water and in his words almost exploded. Whether or not that was true or even possible I don't know. But it really scared him and woke him up.


He then built an outdoor wood boiler.

JUGHNE

Very inventive....looks good!
I do see 30 PSI on the gauge, what is the pressure relief valve rated at?

Things like this were attempted in the 1970's. Back then one thing I noticed was excessive creosote build up in the chimney
do to lowered stack temp causing condensation of wood gases.

Has your chimney build up changed from the past??
As a 42 year volunteer fireman I have attended many chimney fires......just about always at the worst time of day.
3 times we carried the still smoldering stove out of the house and put it in the yard......the wife would not let it back in.

In 1977 you could have been published in The Mother Earth News.....I still have the early copies.

Jellis

Cool unorthodox way to move heat around. I was happy to read you had a relief valve and took power outage precautions.
I once did a service call for an "Engineer" who thought it wise to put a coil of copper in his fire place and pipe it to a radiator at the back of the room, no relief valve, no expansion tank.
I wonder if a storage tank would be a good addition to your system?

rbs12

@ChrisJ I have also thought about all of this and this system does require some monitoring but for the reasonably low cost I'll be extra vigillant. There is a make-up valve attached to the system to account for any leaks that may occur and stop a "dry" situtation from happening. I check the water pressure gauge a couple times a week to check for changes. It is constantly at that 27-28 PSI range. If there was a leak the make-up valve only replaces 15 PSI so I'd see a difference. I've thought about adding another pressure gauge upstairs to save me the trip down but this also forces me to check the rest of the system as well. As far as the pump my house is only 1600 sq/ft so my stove is normally only turned up twice a day, in the morning when I get up and at night when I get home. The return line to the stove exchanger can be grabbed by hand and I always check it to make sure I can feel the water flowing through the pipe. If the pump were to quit between the pressure tank and pressure release valve I would hope they would control things until I got home and noticed.

@JUGHNE The valve is rated at 30 PSI and the system runs just below that. In my research I did notice that people with the creosote issue mostly had that problem due to wrapping the pipe itself. I did update my chimney to a 6" insulated liner and extended it four additional feet at the beginning of last season as well. I did not notice any excessive build up this year.

@Jellis I have also thought about getting some old H/W heater tanks from the local scrap yard to store the hot water but for now I'll just plug along with what I have. Maybe that will be my next upgrade.

GroundUp

That's pretty cool, very inventive. Something I'd like to mention is that there is no reason to be running 27-28 PSI especially with only a 30 PSI relief. The Extrol tanks are precharged to 12 PSI at the factory, did you pump the bladder up to 27 PSI to match your water pressure or what was the deciding factor on running such a high pressure when 12 was more than sufficient?

rbs12

@GroundUp I did not pump up the precharge, the system just "settled" to that PSI on its own. I never really thought too much about the running pressure. I guess I should put a gauge on the tank to check its precharge. Thanks.

GroundUp

@rbs12 it's not as simple as just checking the air pressure when there is 27 psi of water on the other side. You need to check the air pressure when the water pressure is at 0, then match the water pressure to what the air pressure is. There is absolutely no reason to run it above 12 psi, and the bladder will live a much longer life as well as the relief valve. Running that close to relief pressure will eventually create a leaker if it hasn't already

hot_rod

how will it circulate in a power outage? Maybe add a UPS to run the circulate in event of power out

Any way to turn the fire off in the event of power out,

Add ing a flue temperature gauge is a good way to monitor cold flue operation

Solid_Fuel_Man

I would highly, highly recommend redundant circulators. Wire the second circulator off a second aquastat set to a higher temperature. This way if your first aquastat or circulator fails you have backup.

I've burned wood for 100% heat and DHW for a few decades now. And trust me, time has a way of making things not work as planned or originally installed. Water in an enclosed vessel with an uncontrolled fire in very close proximity has a way of doing bad things if something fails. Power outages and circulation failures are your worst nightmare.

A few years ago I had just finished my redundant dump zone (large unit heater in garage) for my gasification boiler. The first dump zone was fooling my radiant injection controller that the supply was 20 degrees cooler than it was making it ramp up radiant temps. This worked very well, until that very night when I finished my unit heater in the garage. Had a big power surge which blew up my radiant injection controller rendering it (ane my dump) useless. Well I learned that very night that it was worth my time to pipe up that big unit heater. Ran it for a couple of hours until that fire was out with no harm/no foul to the boiler.

rbs12

@hot_rod I do plan on hooking up a UPS to the pump. My generator has this as one of its circuits so I won't have to put the fire out but if power goes out while I'm not home the UPS will cover me.

@Solid_Fuel_Man Another good suggestion, redundancy can mitigage a lot of the risks with this type of system. You've got me making plans for modifications in my head as I type.

Thanks for the input!

ChrisJ

Is there a way to cut air to the fire in case of power outage?
Such as feeding it via a blower and or using a spring loaded door that shuts if it loses power?

EYoder

The wood boilers I install use a solenoid and flap, with a loss of power it drops shut.
https://www.amazon.com/Dormeyer-2005-M-1-Laminated-Solenoid-Replacement/dp/B000CCNGMU

rbs12

Appreciate the input from everyone and now looking for some more suggestions. I'm about the make my upgrades (close in the heat exhanger) and I'm not sure if cement board is the way to go. It looks like it has a mesh fabric intertwined in the outside of it which might melt that close to the stove. I was going to use hardibacker but it looks like that has too much fiber in it and isn't rated for heat applications. Any other suggestions other than regular cement board or steel?

Solid_Fuel_Man

Steel would be your best bet. You are looking at some temperatures above 600F at times. I wouldn't use anything which will offgas, you could use aluminum as it's easy to work with.

SuperJ

You'll kill that circulator prematurely by hanging the motor downwards. The shaft always needs to be horizontal.

hot_rod

If you ever build another a few suggestions, silver, high temperature solder. Ues return bends or bend the tube and eliminate a lot of solder joints.

Good that it is outside the fire box

I'd run the tube loops horizontally, you will trap air on top of every return bend should it ever flash or loose fill pressure and get air in the system.

Some thoughts from building a handful of them over the years.

Fly

Love this.  I’m doing same thing. Have 50 foot copper coil on rear of stove.  It’s not enough heat. Gonna move it to top. It feed a electric water heater.  Circulator pump with expansion tank   Hoping I can get enough heat from it to not run water heater. It increased my bill $200 a month. 

Viper9087

rbs12

I know this is old and I'm not reading through the entire post, but soldered fittings against the stove are illegal.

For this to be a safe system, obviously you need a blow off, but most importantly the copper pipe cannot make any direct contact to the steel of the stove, second of all there can be no soldered connections on the stove. You have the right idea, but you need to be using flexible copper pipe and bending it into that zigzag shape, not using elbows and solder.

If the system run dry, the air pressure will increase with extremely hot steam, the solder will melt, and the piping will literally explode creating shrapnel and burns to anyone in the area not to mention burn your house down.   DIY systems like this are exactly why it is considered banned.

Best of all if you read your homeowner insurance policy, you will probably find a clause where they will not cover you for any damage resulting from a system like this.

I have a similar system in my wood stove, but it uses a single flexible copper run and is consistently fed off of the main home water line so it is impossible for it to run dry, and the leak will be evident.   I also have an expansion tank and blow off valve connected to the system, a sprinkler system installed above it, and numerous safeties in place.

I would seriously reconsider the piping that you have on the stove

ArachnoGeek

Viper9087, this is really helpful! This might be a dumb question, but in your system, you mention your line is consistently fed with your main; if I do this, how do I ensure that the cold water from the main isn't always mixing with the heated water? If I put a check valve, will it remain a closed system? Thanks!

Cappy88

Adding a simple water meter might end all the concern about if you have run out of water in your loop. They can be purchased relatively inexpensively and installed directly inline. I see them with 1/2", 3/4" & 1" connections. The system you have built looks very interesting. I like that it cost you under $1,000 and it was all done by using common sense thinking. Be interested to see the upgrades, if you have made them, as you had stated. Respectfully.

rbs12

@Viper9087 Thanks for the input. I have not read anything regarding solder being illegal against the stove. Is this a state or federal law? Do you have a reference? Obviously running dry was a concern I had when I first built this so to answer @ArachnoGeek s question this is why I put a make up valve in the system. It is tied to my main but only feeds if the pressure gets below 15psi (most likely indicating a leak). As the system runs higher than 15 psi the main is not feeding it constantly. I did add a UPS backup to ensure that if power goes out the system still circulates. I have tested it and it gives me about four hours of backup power. I also have a generator that runs my whole house for longer outages. This was my third winter running the system and it is still performing great. The room stays at 70-75 degrees and I have not seen a noticeable increase in wood consumption. Overall I am very satisfied with my decisions and have saved a ton of money in heating expenses.

ChrisJ

If that piping gets hot enough to melt the solder the water pressure will be 250 PSI+.

Do you have a pressure relief installed? If so, I don't think it'll never get hot enough to melt solder as long as there's water in the piping.

I don't know who viper is but it looks like that's their only post here.

rbs12

@ChrisJ yes I do have a pressure relief valve and expansion tank installed. I agree that it would take a lot of "things gone wrong" for me to get to the meltdown or explosion point. Again I'm pretty happy and confident in the system.

EdTheHeaterMan

I believe you would be wise to place a 30 PSI relief valve on the fitting at the wood stove. You can place a Tee fitting there and use the top for the air vent, then use the branch for the relief valve. Be sure to connect a safety discharge tube on the valve outlet that ends within 6" of the floor or over a drain in the floor.

If you ever make another one, use the horizontal tube idea. Get a tubing bender to make some tight bends around the wood stove on smaller tubing and connect them to a header at either end. 5/8" or 3/8" OD soft copper tubing bends nicely.

Mr. Ed

ChrisJ

You can also use a turbo torch to braze no need for oxygen especially if it's something not in a tight space or right next to a txv

Dave Carpentier

Cool thread.
Is it okay to have that air vent so close to the hot surface ? Do they have a max environmental temperature ?

hot_rod

Dave Carpentier said:

Cool thread.
Is it okay to have that air vent so close to the hot surface ? Do they have a max environmental temperature ?

Usually 230- 250F fluid temperature. Some brands have plastic caps, so that could be an issue next to a very hot wood stove surface.

If it is a HX for DHW the air vent needs to be NSF listed, low lead.

rbs12

Update: It's almost time to fire up the system for the fourth winter. This has worked amazing every year, has kept the room at 70-74 degrees and needed no maintenance other than replacing the water every year.

random12345

I think wood smoke sickens and kills people, but your design is ingenious.

Solid_Fuel_Man

@rbs12 may I ask why you replace the water every year?

ChrisJ

random12345 said:

I think wood smoke sickens and kills people, but your design is ingenious.

Can you explain why you think this?

Personally I think wood smoke produces awesome food.

Not to mention technically you can burn wood with zero smoke 

random12345

I never should have posted that comment. It's none of my concern, and it was irrelevant to the discussion anyway, so if the OP is reading this, I apologize. I meant no offense.

rbs12

@Solid_Fuel_Man I replace the water just to cut down on any bacteria growth in the lines. It's a closed loop system but is the makeup valve is fed by my domestic hot water so I figured instead of adding glycol to the system and never knowing the exact mix it's just as easy to change it annually.

Solid_Fuel_Man

All you need is a double check valve (required by code) and you never have to worry about heating water contaminating your domestic water. 

By changing yhe water you will get mineral buildup in the heating side which is not good. 

MikeinMaine

Thank you all. I have found this post AND the comments very helpful as I contemplate hooking up my parlor wood stove (sole heat for my 2500 sqft well insulated home) to my floor slab radiant tubing that I installed when I built the house 10 years ago but never used. I am in a situation where power outages are common, especially in the winter. I was thinking about using an old style vertical hot water tank next to the wood stove and simply having a thermosiphon loop between the stove and the tank. I used to see these in older folk's houses here in Maine back in the 70s hooked to their kitchen wood cook stove. They were typically made from copper. There would be relief valve at the top of the tank. Then a circulator pump driven circuit take water off the tank and run it thru the floor whenever the tank temperature was high enough. The tank would operate as a heat buffer in case of power outage. The system would be pressurized with an expansion tank, automatic make up valve, etc. However I would want to run glycol thru it. My wood stove is fairly small and I typically don't feed it enough wood to run more than 4 hours. It can also burn coal and that would run all night. Thoughts?

hot_rod

The tank needs to be above the boiler to get a good thermo-siphon. A thermo-siphon dump zone might be needed also, if the tank cannot absorb all the boiler output. If it runs for day perhaps.

A 12VDC circulator in series, would assure the radiant could run in power outages, and could also use the floor as a dump zone if you have a DC circ, or a large battery/ computer backup pack to run a 120V circ.

Solid_Fuel_Man

I'd recommend a UPS and an ECM circulator over DC pumps.

Where in Maine are you located? I'm in Aroostook County 

rbs12

@MikeinMaine Hi Mike, I am not too familiar with thermosiphon loops but simply used a UPS system like @Solid_Fuel_Man suggested. The one that I have will run the system for about 4 hours, plenty of time for me to hook up my generator or get home from work. With all the other fail safes I incorporated I am not too concerned about beyond 4 hours as the stove will not be hot enough at that point to super heat the water. I am in Southern NE but have property in Maine near Sunday River as well. More than happy to answer any questions you might have.