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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorNoodle
    • CommentTimeJan 19th 2019
    Hi All,

    Hopefully you can help troubleshoot my buffer tank for me.

    I've got a pellet boiler direct linked to 1000l buffer tank. dhw/ CH takeoff at top, ufh on second take off as per photo.

    Engineer says the stats are supposed to work as follows:
    "The top and bottom stats are controlling the pellet boiler. The top one can be set to 50-55 degrees and the bottom to 65-70 degrees. The top stat brings the boiler on and the bottom switches it off." the middle stat is a divertor and for testing purposes ive set so it has no effect anything.

    Which sounds right but I don't think is how they are working. What my testing has found is:

    - The top stat (A) turns on the pump when tank temp drops below setting, but also turns the pump off when that temp is reached at the top stat. ( proved by me turning the stat down when its working) If was working as per engineer surely me turning the temp down shouldn't cut the pump out?

    - The bottom stat(B) overrides the top stat, so if buffer water temp is below the stat setting it allows the top to operate, visa versa if water temp is above the setting it stops the top stat from doing anything/ turns off boiler to buffer pump.

    The effect of this is that bottom stat doesn't seem to be doing anything in reality and the tank is just being controlled by this top stat?

    It also means the ufh takeoff flow can go down to 40/45 degrees before the top of the store drops enough to turn on the boiler pump again. Ive been told/ read that i want ufh flow at 55/60 degrees so the ufh mainfold can mix it down to be efficient? + also the bypass is a little open on the manifold so 45 flow to it actually results in 40 degree water around the circuit.

    To me it appears its almost working properly but that the top stat is turning off the pump when it shouldn't be doing so, unless manually changing the stat overrides what would otherwise happen!?

    I thought about swapping the stats round as a simple fix but not sure thats how it should be working, as top stat would then be stat (B) and turn off the pump when top reached x degrees, and the bottom would be stat A and turn on the pump when bottom cooled below X AND top was below Y? Would this work but perhaps put the stats in top and middle rather than bottom where all the cold returns come?

    Also thought to solve the ufh flow issue to bring the top stat down to the middle pocket so whatever the stats are doing is to top half of buffer instead of top 1/3?

    Hopefully this all makes sense!?

    Any thoughts would be very welcome!


    Do you have a 'control package' 'black box' or something that the stats feed into?
    In which case it is probably wired up incorrectly.

    Did the engineer put it together for you as a service for which you paid?
    In which case get him back to fix it.

    If neither of the above then what you are trying to do is entirely reasonable. The top stat senses low temp and turns on the boiler and the bottom stat turns the boiler off when the bottom of the buffer tank is up to temp. But you can't do this with just 2 stats, you need a relay as well - is there one in the circuitry?
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