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General: Accumulator stat wiring

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    Hope this is easclear and y

    I have plumbed in the new accumulator fed by an oil boiler and wood burning stove with pumped feeds to UFH and rads.

    I have a standard cylinder stat 2/3 way down the accumulator which switches the boiler off when it comes to temp.

    Id like another stat, 1/3 down which prevents it starting if there is some heat in the top (i.e. the fire is running). Can it be done? I thought a low temp stat (usually pipe for frost protection) engaged when the temp droped past the set temp but wont disengage if subsequently rising past it.

    If it cant be done its OK for the boiler to fire, just a shame if the fire is enough.

    Sparky is confused but am I barking.

    thanks
  2.  
    Andrew,

    You need to check for stats that close or open when the set temp is exceeded, or wire it through a relay.

    The upper stat should close when the temp falls below say 50deg. This is when the hot/cold boundary in the tank rises up – because you are pulling hot water from the top and returning it cooled to the bottom so when in use the hot/cold boundary rises.
    For simplicity lets assume there was no replenishment of hot water by the boiler whilst the tank is heeding the heating system.
    Therefore, when that cooler layer hits the top stat it’s less then the set temp so the contact closes and the boiler fires.
    Primary water from the boiler is pumped into the tank from the top and cooler water is drawn from the bottom, thus the boundary layer is now going down wards.
    The bottom stat would be set to the max tank temps (say 75 deg). The boiler delivery temp must be set to more than that say 80 deg. Usually the boiler is just set to max as its the tank that is in control. It must however be greater than the tank set temp.
    When the hotter layer hits the bottom stat (it must have normally closed – NC contacts) these will open shutting off the boiler.
    Thus
    Top stat = min temp setting, Normally Open NO contact. Boiler on.
    Bot stat = max temp setting. Normally Closed NC contact. Boiler off.

    Boiler on = top closed + bottom closed
    Boiler off = top open + bottom open

    The actual temps are fairly arbitrary so long as the top is less than the bottom by 10-20 deg its fine as it relative.

    The problem is when on start up if the tank is half hot/ half cold the boiler wont start until there has been some usage to tripper the top stat. To avoid this you can time switch the boiler out at say 10:30, then continue with the heating / DHW until the tank is pretty much depleted. This will ensure that the following morning that the top stat is closed i.e. boiler on (when timer clock allows).

    Failing that a relay with a one shot contact can be wired to override the top stat on the first occasion this means that regardless of where the hot / cold boundary actually is it will always fire until the bottom stat is satisfied. This means you always start the day with a full tank.

    Hope that help a bit.

    For ref what tank? What oil boiler? What loading valve? I can’t find your previous post – I think there was one??

    Cheers

    Mike up North
  3.  
    Hi Mike

    Thanks I think I was over complicating things.

    FYI
    I did post a while ago, its been a long year.
    Its a 270l copper accumulator tank with various connections:
    Feeds, via 28mm thermostatic, a DHW tank directly above, this is vented.
    Oil boiler, (cant remember what, say 90k btu) pumped in 22mm (cheapskate),
    28mm thermostatic from 14kw hunter wood burner,

    22mm pumped to UFH.
    22mm pumped to upstairs rads.
    I have an unused 22 mm coil which will be a solar dump circuit if\when we replace the DHW tank for solar.

    I may attempt photos once all done done, its nice to see them but nicer to have a warm house first.

    Andrew
  4.  
    Sorry Mike back again,

    Do the two stats need to be different types, not just wired differently? I now think the top stat is a low limit stat (Ill use a pipe stat Honeywell L641B1004, and my wife gets some fetching spring earings) where as the lower stat is a regular high limit cylinder stat (Honeywell L641A1039 ). Otherwise the boiler would kicks in when top stat senses heat below 50, but once the hot water reaches it, it will cut out, so only 1/3 of the tank ever heats up.

    I think with the low limit stat it makes on a falling temp but stays made even if the temp rises above the setting. The break comes from the upper stat.

    If these specific parts are no good can you suggest others.

    thanks, again

    Andrew :bigsmile:
  5.  
    Hi,

    Only got the chance to have a quick look. Ideally you should be using probe stats into pockets on the tank. Although there are many that do clip the sensor into a tab at the tank surface. The distance between the stats dictates the buffered volume.

    The A1039 is ok at the bottom as this is NC and opens when the 40-80 deg set point is passed. Therefore, it switches off when the temp is up and beyond the set point. Set this to 70 or 75deg. You could use the B1012 or B1020 as the principal is the same. This set point must be less (by say 5deg) than the boiler max deliver temp.

    For the top the B1004 has too lower set range but the principal is correct in that is NO and closes to fire something up when the temp falls below the set point.

    If you use this you will not fire up the boiler unless the tank water is below 40 deg, which might be too much for you boiler to haul up to 70 or 75 deg without running the whole tank through in more than one pass. Remember the boiler can probably only raise the temp by up to 20 deg in one go. Therefore, if its say 38deg at the top the boiler will come on, but the delivered water will only be say 58deg, this won’t satisfy the lower stat so the whole lot will go round again.

    You can get a differential controller to do that that acts like a solar controller. You set the top temp and the bottom temp and it does it all for you. Have a look at the ACASO Termomat 2 or 3.

    Cheers

    Mike up North

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