newbie know-nothing wants to make his floors warm with a detached wood burning stove

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hatlandous

New Member
Oct 8, 2021
1
northwest iowa
hello everyone!
this is my first post here in a earnest attempt to learn about and deploy my own in floor radiant hydronic heating system. i would very much appreciate any and all input as well as any suggestions or links to resources with even more information. i do not know much about these systems (in fact, very little), but am very confident in my ability to learn and execute, once well-informed.

so what am i trying to do? ok. i’ve got a large, home-made wood burner in the shop/garage next to (not attached) my house. this thing is built like a brick and has literally taken everything we’ve ever thrown at it. we use it to heat the shop, so it’s always going and i think it would be nice to take some of this heat and pump it into the floors of my house. in my house, i’ve got three loops (with intentions of adding a fourth, once the master bedroom is done) that i’ve run whilst remodeling the floors. it’s a single story, no basement ranch and the loops run in the concrete floor. each loop is roughly 200 to 300 ft of oxygen barrier pex. so i’ve got a stove in a separate building and tubing in my floors.

now here’s the questions:

what’s the best way to capture the heat from the stove? my first thought was just wrapping it in 1/2 copper tubing? would it be better for a copper loop to live inside the stove? or is there another way i’m unaware of?

generally, what else do i need? i’m sure i’ll need a tank or reservoir next to the stove to hold the hot water and help with temperature flucuations. a pump to send the water from the stove to the house. manifolds for the loops. a pump inside the house to pump through the loops. a thermostat to call the pump(s?). expansion tank? heat exchanger? mixing valves?

as i’m sure is obvious, i’m quite green to the whole process. again, i would be ecstatic to get some feedback as to if this is possible and to be set in the direction of any links, guides, examples of people attempting similar setups. even some basic guides/books of understanding a system like this would be helpful (forum searches on multiple sites yield very specific questions with similarly specific answers).

cheers and thanks,
seth
 

gthomas785

Minister of Fire
Feb 8, 2020
530
Central MA
Show us some pictures of this wood burning rig if you can so we know what you're working with.

Capturing the heat into the water efficiently is going to be a challenge. Take a look at some commercial wood boiler designs for inspiration. Some use a water jacket, others use more traditional tube type heat exchangers.
One thing I'm pretty sure about is that wrapping a stove in copper tubing probably won't get you the results you're hoping for.

Thermal storage is key for a hydronic wood fired system. Look to get 500-1000 gallons of storage and insulate it well. Some people just use an old propane tank. The idea being that you fire the boiler wide open once or twice a day to heat up the tank, and let the hydronic system draw from storage the rest of the time.
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,348
NE PA
It takes pipe inside the firebox. This is known as a coil, even if it is a U shape. Stainless 3/4 pipe Is usually used in a cook stove with hydronic heat added. Just like black iron, pipe and elbows are screwed together above or on the sides of the fire.

Any sealed water heating system requires a pressure expansion tank, and safety relief valve. This type system uses a low pressure feed water regulator to keep the system pressurized at about 10 psi. It uses the same principals as a low pressure boiler. An air scoop with auto purging valve is used to remove oxygen separated in water in coil. An open system uses a cistern open to the atmosphere and can over humidify the area the tank is in. A float can be used for automatic feed to maintain water level. Since you mentioned oxygen barrier, you are probably referring to building a sealed pressure system.

If the stove is lower than the heated floor or slab it can use gravity (hot water expands and is lighter than cold water so it rises to the highest point in system and circulates without pump).
There are many factors such as pipe sizing for proper circulation, (Oversize tubing for gravity circulation) temperature required (radiant is lowest, below 100 compared to baseboard that is more efficient the hotter the water 200 and above) and an insulated storage tank if the coil produces too much hot water. Surface area of coil due to firebox size, circulation speed, are factors and control valves in the system can increase or decrease circulation as needed.

A simple loop gets hotter where close to the stove, cooling as it circulates being returned. A bypass loop is much better for more even heat, along with flow valves to adjust output of each radiant loop getting closer to the same temperature at each loop. The flow valve goes on the bypass, not the radiant loop, it doesn’t slow by closing off what goes through the slab, so opening it allows more hot water to bypass the radiant slab going to the next loop of radiant.

The size of the firebox and internal temperature make a big difference of coil size and how much heat you can remove before becoming a creosote producing hazard. This will cool the flue gas removing more heat from the firebox, so a stack thermometer is needed and you should record what the flue temps are without hydronic added first.

When I know what type system you are building I can refer you to some publications with tons of info.