Kerr parallel boiler troubles

Foz682 Posted By Foz682, Jan 28, 2017 at 7:46 PM

  1. Foz682

    Foz682
    Member 2.
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    Jan 9, 2015
    11
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    Loc:
    Nova Scotia
    Hey all,

    Recently finished connecting my Kerr Tw2000 wood boiler to my Kerr saturn oil fired boiler, Setup is in my garage and feeds to the house about 20 ft away. Went with supply on each to the return of the other, with a circ between the oil supply and wood return. Plan is to solely use wood unless I'm away for a day or two. Wood should keep the water in the oil boiler hot, and the oil boiler should behave normally, supplying the house with heat with it's own circ. Only wiring connecting the two is the end switch on the N/O valve to TT on oil boiler aquastat, and of course common power supply.
    On the wood boiler:
    Single acting aquastat controls that circ, have it set at 175°F.
    Low setting on dual aquastat is at 180° to close the air damper
    High is at 205° to open safety zone

    On the oil Boiler
    High is set at 190
    Low is set at 170
    Diff is 15

    The problem I'm having is that the wood boiler gets to 180 and stays there...circ feeds to the oil boiler and takes a long time to rise water temp high enough to start the circ feeding the house...when it finally does, it runs for 2 minutes tops and shut down because water temp has dropped to 160°. Return pipe from house gets too hot to touch for more than a few seconds quickly.
    My baseboard rads get warm, but only for a short time and are not heating the house. System works fine when I run just the oil fired boiler.
    The temp in the wood boiler doesn't really change much, it may drop below the circ cutoff briefly near the end of the burn cycle, but it's taking roughly 25 minutes for the water temp in the oil boiler to recover.

    I'll try attaching a piping schematic almost identical to my setup. All piping is 1-1/4", except for a short piece of 1" copper between wood supply and oil return(which I'm very suspicious of being the culprit of the slow rise)
    Thanks
    Boiler Diagram.png
     
  2. TCaldwell

    TCaldwell
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    Oct 26, 2007
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    Loc:
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    You might consider a injection loop, both boilers share a common pipe with closely shared supply/return tees. This will stop your short cycling however this will change your current strategy of series piping to parallel. A set point controller will then toggle between boilers, a set point of 130 for example will run the wood circ above that temp and the oil circ below that temp.
     
  3. stratton

    stratton
    Member 2.
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    Mar 13, 2012
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    Loc:
    southern ct
    Foz, I had the same boiler, same problem, huge temp swings.... PITA. After two years i ended up replacing with a D.S. MACHINE AQUAGEM 3200. I added a 80 gal buffer tank to my system which brought me to 120 gal cap. That was the ticket for me. Hate to say it but your 27 gallon cap and 180 degree water which is need for baseboard, will be super hard to achieve.
     
  4. maple1

    maple1
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    Sep 15, 2011
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    If the Kerr is getting to 180 and staying there (which is what I read), then you likely have something plumbed or controlled/wired wrong. Are the circs pointing the correct direction? Check valve? Not much more to say from here. You should be able to find where things are going wrong, by following the diagram at the same time you follow the pies with your hand & see where the heat is or isn't flowing. Sounds like it is not getting from the Kerr to the oil unit. Which kinda points to the check valve to me. But without being there & seeing it, hard to tell. And you say the diagram is 'almost identical' to your setup - more fuzziness. The Kerr manual gets pretty detailed re. piping & control diagrams, you should be able to follow yours & map out the differences & see what's wrong.

    Re. the wood aquatstat & 175 - does that have a diff setting? What is it at? Are you sure the wood circ is actually running? Or is the heat/flow you're feeling just convection? First thought there is lower that to 160 or so & see what happens.

    Yes, that Kerr is not that efficient, but if it is holding 180 while the oil is not holding much & your baseboards aren't getting very warm, there would seem to be a simple problem happening. And 180 is not needed for baseboard, or it shouldn't be. If it is, your baseboard is undersized.

    (I would not set my oil settings above 160/180).
     
  5. Foz682

    Foz682
    Member 2.
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    Jan 9, 2015
    11
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    Loc:
    Nova Scotia
    Thanks all,
    I believe we found the culprits. My uncle came over today to check it out, he installed the oil boiler years ago. He determined that the 1" pipe between the two boilers was restricting the flow, swapped that out for 1 1/4"pipe and we also replaced the circ between the boilers with a 3 speed one and set it on 'high'. He lowered the aquastat controlling the circ between the boilers to 140 and the low on the oil boiler to 155.
    At the moment the wood boiler is sitting at around 175, oil boiler at 170 and the house is plenty warm at 24°C (75F) While it's -8C (18F) outside.
     
  6. maple1

    maple1
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    Sep 15, 2011
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    I'd likely try switching that circ to speed 1 & see how it does. Good that you got the heat to where it should be!
     
  7. Letsburnwoodnotoil

    Letsburnwoodnotoil
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    Oct 1, 2013
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    Did you get this all set. I see the piping issue on your drawing.
     
  8. leon

    leon
    Feeling the Heat 2.
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    Feb 3, 2013
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    Loc:
    Southern Finger Lakes Region of New York
    The minute I saw your drawing I knew exactly what the problem was!!!

    You have a spirovent AND a steel expansion tank.
    You cannot use a spirovent AND a steel expansion tank EVER.
    What has happened my friend is that the spirovent has allowed all the air cushion
    in the steel expansion tank to be replaced with water and that is NOT GOOD.

    The other thing that needs to be corrected is the wood boiler steam chest tapping is not connected in series to the steel expansion tank as it should be with the oil boiler and that is a huge problem as you may have a lot of trapped air in the steam chest of the wood boiler that cannot escape.

    Your circulator has no point of pressure change so it can never work properly and would not work properly EVER.

    No amount of plumbing replacement piping will solve this for you you need to remove the spirovent and drain the excess water out of the steel expansion tank regain the air cushion that was lost due to the spirovent.

    I have a steel expansion tank used with my coal stoker and it has worked very well and I will never have a bladder tank again.

    If the boilers were piped in series your plumbing would be so much simpler to take care of and easier to do any repairs. If the boilers were piped in series you could use just one circulator for both boilers and not two with the circulator coming off the steam chest tapping on the first boiler in line.

    Its better to pull the water through the boiler to get the air out rather than pushing water and air back into the base of the boiler causing cavitation corrosion.
     
  9. maple1

    maple1
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    Sep 15, 2011
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    What part of his post implies his cushion is gone? I don't see any mention of pressures?

    I don't think it's his drawing - 'almost identical'.

    And sounded like he got things working back in January - so not sure now why I'm even posting this....
     
  10. leon

    leon
    Feeling the Heat 2.
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    Feb 3, 2013
    415
    17
    Loc:
    Southern Finger Lakes Region of New York
    The Spirovent eliminated all the air cushion in the steel expansion tank and if he has an autofill bingo!
     
  11. maple1

    maple1
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    Sep 15, 2011
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    That would lead to pressure spikes and losing water out the relief valve. Don't think I read any mention of losing or leaking water or any mention of pressure issues or pressures at all.
     
  12. Fred61

    Fred61
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    Nov 26, 2008
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    When I first read this it was nap time at Fred's place and I was in a vegetative state and only read down to the diagram. My first and only thought was air lock.-------sssnoooore<>
     

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