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
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
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