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Plumbing A Wood Boiler

I'm adding a wood boiler to my existing system that is an updated gravity flow system. My current circulator is on the return just before the boiler. The wood boiler manufacturer recommends that I tee in between the existing pump and the boiler return with a additional pump and direct flow to the wood boiler. He also says to tee in another pump at the outlet of the existing boiler between the boiler and radiators (a push-pull arrangement). This pump will pull water from the wood boiler and either put it into the radiators or circulate it backwards through the boiler when the original pump isn't running. The original pump will be controlled by the thermostat. What I can't figure out is how will water ever flow through the existing boiler if the wood fire dies down? The wood boiler is in the garage and the lines are exposed to freezing temperatures (although they are insulated) so I have the keep the 2 new circulators running to keep the lines from freezing.

Another question - will Taco 007 pumps block flow if they are not running? If so I might be able to create a bypass circuit using the pumps to direct and restrict flow.

Thanks in advance.



  • Mark WolffMark Wolff Member Posts: 256

    Totally off tangent, but without seeing the piping schematic from the manufacturer, I would tend to want to install the boilers in series, with the wood fired boiler first (receiving the return water first) and the supply from the wood fired boiler feeding the return on the regular boiler. This will allow you to use the wood fired boiler at will and utilize its heat at your leisure without buying lots more controls.

    The second option is to buy more pumps, valves and controls to allow individual isolation of each boiler, and would require manual shutoff of one boiler to use the other. This sounds like what the wood boiler manufacturer originally intended. If this doesn't appeal to you, either use the first way, or the next way.

    The last way I can see this working would be to utilize a Tekmar 2 stage (2 boiler) control box. You don't even need to use the outdoor reset feature, though it would be a nice addition and save you some fuel. Using the 2 stage control would allow you to run your wood fired boiler and if it isn't being used, the second stage would kick in, turning your other boiler on and heating your home.

    As for the Taco 007, it doesn't have an integral check valve system unless you buy it with it, but in normal environments, it does greatly impede the counterflow of water. In most situations, you can get by without check valves in the system if you install your piping with heat traps.
  • greggreg Member Posts: 71

    primary-secondary pumping. pipe the system in a loop and pump it continously.

    Use two different thermostats. Either shut off the gas unit or set it for a minimum temp to be safe.

    And why not use anti-freeze?
  • Mark MathysMark Mathys Member Posts: 30

    Thanks for the advice Mark & Greg.

    I was thinking about primary secondary and am doing some some research. This would let me run both at the same time when it gets cold (-40). I would like to use antifreeze but am concerned about developing leaks as the house is about 100 years old and there are a lot of connections (31 rooms). I am also considering a heat exchanger and putting the antifreeze in the wood system only.

  • BillBill Member Posts: 26
    Wood boiler

    Is the woob boiler a closed system? Some of the ones I've seen in my area are an open system, i.e. having no relief valve, just an open port on the back. If this is the case with yours I recommend a heat exchanger, and definitely antifreeze on the wood boiler.

  • Mark WolffMark Wolff Member Posts: 256
    Heat exchanger

    Remember to tell your supplier to account for the glycol in the system when they size the heat exchanger, the transfer rates are slightly less and you don't want an undersided exchanger. Good luck.
  • Mark EathertonMark Eatherton Member Posts: 2,542
    Ain't nothin free...

    'cept maybe gravity, and theres lots of folks charging for that (Ski resorts).

    It won't cost you anyting to get down hill, but it'll cost you and arm and leg to get back up the hill...

    Most solid fuel boilers I see are open vessel/loop type. You've got to pay attention to the inhibitor levels in the water, ot the boiler will dissolve. It's made of mild steel. And being open, it is sheer hell on glycol. Any of them. Most of the manufacturers I've seen generally recommend circulation for freeze protection, but if they're being used correctly, you won't need freeze protection.

    Watchout for power failures though, I hear that can be a scarry proposition. Seems like maybe a solar backed UPS migth be in order if you are regularly exposed to long term power failures.

  • hrhr Member Posts: 6,106
    If it's

    a non pressurized type of wood burner, skip the glycol, as ME noted, it will not last in an open system.

    I use a flat plate HX to isolate the open type of wood burners. Less hassle than having to run a pump if you distribution is above the top of the wood burner.

    Better yet go with an Aqua-Therm. They are pressurized, and a delight to plumb in with exisiting boiler and plumbing!

    Primary/ secondary is, by far the best way to make this work out and allow input options. It is a lot more work, and fitting, however.

    This job has an open type outdoor wood burner feeding into the primary loop via the HX, Loads for the DHW and some radiant, and a backup water heater for the wood burner.

    hot rod

  • MattMatt Member Posts: 10
    wood boiler

    I had my wood boiler installed the same way your manufacturer said. It is a quick and easy way - but, not the best. I found that out after running mine for a couple of years on my converted gravity system.
    This year I gutted all my gravity mains in the basement and repiped my system into multiple reverse return zones. it used to be one large zone. I ripped out my wood boiler and kept my oil boiler installed. Everything started and ran great. I put a Tekmar 262 unit in and an indirect WH. I put the 2 stage unit in in case I ever put a second boiler in. Well... I wasn't originally going to put the wood boiler back in but it started getting cold out and I thought why not. After racking my mind for awhile trying to make the 262 unit work on paper with a wood boiler I gave up. Reason being is that the wood burns out and the 262 will not shut the wood stage off if it burns out. So... I took the triple honeywell aquastat that I had left over from my oil boiler and put it on my wood boiler. I piped the wood boiler in primary-secondary on the supply of the oil and before the indirect. This setup has worked absolutely flawlessly! I can heat my indirect and when the wood boiler temp dies the stat shuts off the wood circulator. It took me awhile to figure that setup out but it is simple and it works.
    BTW you need a second high limit aquastat to dump heat. On mine I start my primary pump and 2 zones and it works. The triple aquastat is the ticket to thesse setups in my opinion. Sorry for the length of this post.
    P.S. The TeKmar products are very good and easy to work with. I am running my oil boiler on reset and it has already saved us money.
  • Mark MathysMark Mathys Member Posts: 30

    Thanks again for the information. The unit I'm installing is a pressurized wood gasification boiler made by Alternative Heating Systems (

    In our old house in MN I installed an Aqua-Therm boiler and it is indeed a good unit and is still going strong after 13 years. In that system I had a Slant-Fin propane boiler and both were treated with ethylene-glycol. It was all new plumbing with a water-to-air heat exchanger and the system was pretty simple because I could shut down one system without have to worry about it freezing. The old system I'm hooking to now has 16 radiators on 3 stories plus a basement and is a heck of a lot more complicated. The existing boiler is in the basement but the new boiler is in a detached garage.

    As for backup I do have a generator and also a gas space heater in the basement so if all else fails and I run out of gasoline I can drain most of the system and keep the basement warm enough to protect the parts that might not drain. I really don't want to use glycol, mainly for efficiency reasons, but also because I would probably need several hundred gallons and the heat exchanger. The wood boiler holds 80 gallons alone and much of the existing system is plumbed with 4" pipe. Right now I'm waiting for Dan's book on Primary Secondary systems and will be doing some more studying. I just finished reading the "Pumping Away" book and several other of Dan's books so now I have to make some more revisions in my design. I'm sure I don't have to tell you guys this that I'm not a heating professional but I do like to do things myself whenever I can and I like to have as much information as possible. I also enjoy a challenging problem especially when I can learn a lot along the way.

    As for glycol problems I am pretty familiar with them because I work with glycol everyday, although my experience is with heavy duty diesel engine cooling systems. I'm a chemist and manage the fluids analysis lab for the local Caterpillar dealer. One thing I'm curious about, can anyone tell me what is the typical charge for a glycol test that you send to the lab? We do 2 levels of tests that run about $10 and $25 and I was just wondering how that compares?

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