Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Piping a wood boiler with backup

rick in Alaska
rick in Alaska Member Posts: 1,458
I have a job I have inherited because no one else will do it, and I hate turning people down. There is currently an indoor wood boiler installed, that is tied into a 330 gallon storage tank, which according to the manufacturer of the boiler is 500 gallons too small, and from there to a plate heat exchanger, which goes to a radiant floor. There is also a Toyo oil fired boiler connected up to the plate exchanger. Currently what happens is if they want to use the wood boiler, they valve off the lines to the oil boiler so the piping goes right to the plate heat exchanger, and when they want to use the oil boiler, they close down the valves form the wood stove and open up the lines to the oil boiler. There is also an indirect water heater tank after this set of isolation valves. ( Now that I write this I am wondering what exactly it supplies as there are two water heater tanks in a different location.)
The storage tank was supposed to be an open system tank, but after taking off the insulation, it is actually closed. It is just basically an oversized fuel oil tank with multiple tappings for inlet and outlet lines. There are two 3/4 tappings about 12" and 20" down that currently have a vinyl tube tied between them that they say they use to set the water level. There is no way for this tank to vent as it is currently piped, and there is no relief valve on it.
The company who makes this tank says it was probably a custom made tank by them, but they do not have records for it. However, they did say that they only pressure test this type of tank to 5psi. So, the only way I can see to use this tank is to have it as an open system, no pressure, tank. I am thinking that if I took the vinyl hose off at the top tapping, and just strapped the tube up on the side of the tank, that I could still have a visual on the water level, but that the top 3/4 hole could be used as an air vent. Just not sure about that one though.
Also, the oil boiler is the typical grossly oversized boiler that when I was there was running 53 seconds on, and two minutes off. So, I also need to have some kind of buffer tank for that unit.
My question is how to pipe all this so that the system could have automatic changeover depending on which heat source is being used, and if the storage tank is heated already.
Supposedly there is inhibitor in the storage tank, so that is a bonus anyway.
This house has other concerns also, but this will be a good start.


  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    I had a Toyo oil miser 180 boiler as well! The thing short cycles so bad.

    In my opinion the best storage is pressurized, propane tanks are the cheapest option. A 500 gallon in a good starting point. That way, you can avoid open systems and plate exchangers.

    Each boiler needs it's own circulator with flow check. The circ is controlled by the boiler it serves. You can put a switch or aquastat in the storage tank to enable the Toyo boiler when storage is too cold for use.

    I've found that the Toyotomi boilers do quite well heating storage with excellent output and one nice long run time. You must have an external means of enabling it though. There is a yellow "timer" jumper which I use at the TT on it. You have to cut the jumper and splice in you external limit. Set the dip switches to the 36degree dead deadband position.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • hot_rod
    hot_rod Member Posts: 22,476
    It would be best to try and keep it as a closed loop, for corrosion issues, even if 5 psi is the max pressure. IF the distribution needs a higher pressure, for the location of heat emitters above the boiler? Use the HX to separate the low pressure tank and wood boiler form the higher pressure oil boiler side.

    You will need an expansion tank sized to that tank capacity if you run it as a sealed system. On my 500 gallon buffer I leave about 18" up top as an air bubble for expansion. Basically it works as a compression tank air management system that is built in to the tank. the air bubble will cut down on the buffer fluid volume.

    The key to solid fuel boilers is trying to fire the boiler to the changing load, so the buffer capacity is less critical. Depending on the load 330 gallons will not store a lot of energy for no burn day. If you have the components, pulling the load off via an ODR will extend the tank drawdown.

    I use a simple differential control to toggle my two systems. I have an indoor wood boiler with a Lochinvar LP Nobel as back up. When the tank drops to 120, the LP fires. I maintain 120 as the tank also supplies my DHW with a plate HX as an instantanous.

    You don't really want the LP boiler to heat the 330 tank, just the wood fired. Possibly the LP heats upper portion of the tank if you want DHW from the buffer via a plate hx.

    These schematics, both wiring and plumbing are pretty close to what you want.

    I've spent years perfecting my wood boiler system, it is pretty hands off now for controls. Other than keeping the fire burning :)

    Idronics 10 has a few different piping and wiring options.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rick in Alaska
  • GroundUp
    GroundUp Member Posts: 1,956
    Generally speaking, a simple aquastat on the wood boiler supply line that interrupts the TT to the oil burner works like a charm. If the wood burner is hot, the oil stays off. If the wood burner goes cold, the oil automatically takes over. Pretty simple system but I've done dozens like that and it works like a charm. Unless I'm not understanding the layout, that is