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Thread: Boiler cycling with Thermal Store problem

  1. #1
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    Default Boiler cycling with Thermal Store problem

    Hi, I'm here to see if anyone can recommend a way to solve my boiler cycling issue.

    I have a conventional gas central heating system, Fully pumped, indirect DHW,with Wall Hung condensing boiler, one pump, 2 x 2 way motorised valves, manual Room Stat, TRV's, Boiler bypass controlled by a non TRV rad for CH and a gate valve between boiler flow and return for DHW. The thermal store has a pocket for the cylinder thermostat and is set at the recommended 75c.

    I have recently installed this Thermal Store tank (170 litres, indirect) in my loft so the total run of the primary circuit is 24m (12m flow,12m return, 8m of each run is in 28mm, the remaining 4m is in 22mm) the height of the top of the primary coil is around 6.5m above the boiler outlet - I have a 6m head Grunfoss pump set to max - level 3. (the supplier of the pump was absolutely adamant that the 6m grundfoss would cope with 6.5m)

    From 30c the tank takes 26mins to ramp up to 60c then it stops (the store tank thermostat is set to 75c - as recommended) it stays off for 9 mins then comes on for 2 minutes, temp now 65c, then off for 4 mins then on for 2 mins and sits around 67c for ages, I gave up monitoring after an hour when the tank had still only reached 67c, and the boiler was still cycling in the above manner. The recovery time is therefore very poor, and my CH/DHW pump never switches off.

    After a couple of hours, the cycling rate changes to 4mins off, 30 seconds on and it does this continuously.

    I don't know if the problem is with my existing system design or with the Thermal Store itself, irrespective, I like the idea of the thermal store if i can get it to work as it should.

    I have had several suggestions, which seems to revolve around a hit and miss approach. Can anyone give me any concrete reasons why cycling occurs and what I should be addressing to eliminate possible causes in a logical sequence?

    For example, how many turns should my gate valve have on the flow and return by pass? Doesn't my boiler already have a built in bypass (Baxi barcelona, wall hung condensing 10 yrs old)?

    An engineer suggested I change my return in the primary circuit to 15mm, this would also give a better temperature differential so my boiler condenses more efficiently he says. What should the differential temperature be? 11c, 20c?

    I am pretty well snookered on this problem and looking for a simple solution, would a 'proportional' cylinder stat cure this problem - I dont know anything about them, nor even if they exist!

    Thanks

  2. #2
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    I think you need to explain better how this thermal store is plumbed in. How are you actually using it? You say you have a conventional system with indirect DHW. Normally a thermal store would be plumbed in such that it replaces the DHW cylinder and the boiler feeds the thermal store and nothing else. You then draw your central heating flow and return from the thermal store and your hot water is heated via a heat exchanger also fed from the thermal store. Also a thermal store in the loft does seem odd as not only do you get really long pipe runs, but any heat leaking from the store is lost into the roof. The store ought to be within the house where the DHW cylinder was. As for what your current problem might be are you sure that everything is venting correctly? With a store in the loft how much head have you got to your feed tanks?

  3. #3
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    Try lowering the temperature setting from 75 to 65 and see if it does the same thing. If it doesn't then the boiler might be having trouble getting it to temp. If that's OK then try 70.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.
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  4. #4
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    The behavior of the boiler suggests that the boiler set temperature is too low , as the water around the coil heats up to close to boiler flow temperature the amount of energy the coil can transfer is reduced , hence the heat from the boiler cannot be dissipated leading to cycling .
    The boiler set temperature needs to be well above the store temperature to avoid the above scenario and to ensure that the boiler is controlled from the store thermostat not its own .
    As a matter of interest why an indirect store ? Normal practice with thermal stores is to take the boiler flow directly into the top of the tank with the return from the base , which , coupled with a pair of thermostats and a latched relay gives no cycling , low return temperatures and rapid recovery times.

  5. #5
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    Default Boiler thermostat

    I think that this is a limitation with the boiler, it could be that the thermostat is set to low or there is an overheat cut out tripping out. However quite a few system boilers top out at around 65C in order to prevent excessive scaling. Therefore you would be best advised to limit the cylinder temp to 60C.

  6. #6

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    Does you boiler have a digital display rather than simple dial gauge? If so, there may be installer settings that allow you to set the maximum flow temps etc which is acting as a high limit. This is certainly the case with some boilers I have recently had to control.

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