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Best piping practices Open Boiler

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Solid_Fuel_Man
Solid_Fuel_Man Member Posts: 2,646
I got a call to diagnose a boil over in an open wood boiler. This is a Greenwood (very low water content water tube HX) and an open tank above the boiler. There is a short run of insulated 1.25" PEX to a generously sized FPHX. 

Boiler has a flow switch wired to close air damper, which I think is about pointless for this situation. 

Circulator runs 24/7 on a switch. 

The circulator has an eroded impeller from what I believe to be worst case piping. Pumping directly toward an open vessel. Total elevation of water level and circ is 4 feet. 

If I move the circulator up top I pump away but would have about 6" of head into the suction side. I am tempted to just reverse the direction of the pump and leave it on the return as low as possible like it is. And do away with the flow switch, or reverse it as well. 

I'm not familiar with doing open systems like this, other than Centeral Boiler where supply and return are both down low and the boiler has high water content and is open to atmosphere. 

I didn't get any pictures as I was in a small Boiler building and nothing was able to be made out as I was too close. 

Serving Northern Maine HVAC & Controls. I burn wood, it smells good!

Comments

  • GroundUp
    GroundUp Member Posts: 1,941
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    If I'm not mistaken, all Greenwood models were designed to be a pressurized system so the fact that they switched it to atmospheric is an issue in itself. They were notorious for overheating, so storage tanks were almost always necessary on systems without a constant load. In all wood boilers, the proper piping practice (Central included but they still do it backwards) is to supply from the lower port and return to the upper port in order to maintain a properly mixed water jacket. Reversing the direction of flow would most certainly help what you're seeing, but then the flow through the HX is backwards which may or may not be of concern depending on the actual load and emitter type.
    Solid_Fuel_Man
  • hot_rod
    hot_rod Member Posts: 22,382
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    You are correct, get the pump as low as possible. Three piece of air cooled pumps are best for those boiling hot systems

    is there heat emitters above the level of the water?  Is so you can flash to steam up in the system
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Solid_Fuel_Man
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    Yeah, I'm not a fan of the Seaton/Greenwood design. Very poor HX and high flue temps. 

    @hot_rod boiler is just a small open loop, no radiation at all. Connected to conventional closed loop system a short distance away. Very large heat load, indoor pool, large home etc. 

    This was put in 10 years ago and rarely used. New owner has used it for 3 years, circulator 0011 is 2 years old. Sprins freely, just eroded impeller from flash boil I think. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    This has glycol in it, and I'm going to flush everything and see if the boiler flows well. I was able to isolate the FPHX inside and flush it and lots of mud came out. I may run a good cleaner before refilling with new glycol. 

    I am not a fan of open systems, this wouldn't be difficult to pressurize. That is in the cards. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • hot_rod
    hot_rod Member Posts: 22,382
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    Yeah, I'm not a fan of the Seaton/Greenwood design. Very poor HX and high flue temps. 

    @hot_rod boiler is just a small open loop, no radiation at all. Connected to conventional closed loop system a short distance away. Very large heat load, indoor pool, large home etc. 

    This was put in 10 years ago and rarely used. New owner has used it for 3 years, circulator 0011 is 2 years old. Sprins freely, just eroded impeller from flash boil I think. 

    With little to no static pressure on that pump, no doubt you are cavitation the impeller to death. The
    high temperature make the water more aggressive also. So you may be sandblasting and cavitation that impeller and volute to an early grave😳
    The higher the temperature the more pressure you need to suppress cavitation. One pump manufacturer shows 4 psi at 190F required. So you would want 8 psi minimum at the pump location.

    Like many open systems, I’d bet it gets to boiling temperature. Whatever that may be at your altitude?

    Deck is stacked against you on this rehab.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    edited December 2022
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    I may just put a pressure relief, air purger, and expansion tank on it. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    hot_rod
  • hot_rod
    hot_rod Member Posts: 22,382
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    I think the open systems were for two reasons, get around ASME pressure vessel regs, and the least painful, or dangerous way to deal with over heating.
    A gravity over heat loop may be necessary, depending on the owners management of the fire within🔥
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream