hi
I have evohome with opentherm controlling viessman 100wb1b heat-only boiler. switched live output of evohome hot water kit BDR is also connected to the hot water "go hot now" input on the boiler, this makes the boiler run at 77 in order to re-heat the cylinder (which works great now).All rads have HR92's, and there is an ABV in the system.
Once thing I have noticed in the warmer weather now (all CH off), is that in DHW demand times, just after the boiler has finished reheating the cylinder (which takes about 15 mins for a 10 degree recovery, I have offset of 10 degrees set), it then overruns (pump on, burner off) for about 30 mins.the pump is controlled by the boiler directly not evohome (has to be with opentherm IFAIU). I presume this is the boiler heat exchanger protection kicking in and keeping the pump on. After finishing heating the cylinder the flow temp will be about 80, watching the temp dial on the boiler it then climbs to about 90 just after the cylinder demand has shut down, then slow cools to 75 ish over the 30 minutes of overrun, then the overrun stops. During this overrun period the ABV has opened and the water is definitely going round the ABV's loop (I can feel the high temp in it). there is an overrun time parameter in the evohome hot water settings as well this is set to 0 mins in my case, it doesn't appear to have any effect in my system.
The loop of pipe that is purely used by the bypass is pretty short (in the airing cupboard ) - about 50 cm at most. total amount of uninsulated primary pipe in the airing cupboard that it passes through maybe 1.5 metres including zone valves , pump etc. there than another 5m or so of Uninsulated under floor primary pipe, and then another 7m or so of Insulated (through a loft), comprising the rest of the primary circuit back to the boiler. whole thing times 2 flow for flow + return.
I am thinking there isn't enough uninsulated capacity in the bypass loop to shed the heat quickly enough in this situation, hence the overrun lasting 30 mins.
I suspect this is only happening in the summer when DHW demand is running the boiler in "very hot" mode. in the winter , the remains of the very hot water from DHW mode will be consumed by the CH demand, and/or if its goes into bypass after CH demand has finished, the temps at that point will be a lot lower so the overrun time needed a lot shorter.
So.. wondering if I need to make the ABV bypass loop uninsulated part longer , by putting an extra tiny rad , in series with the ABV, in order to throw the heat away more quickly when DHW heatup has finished? Possibly with some diverting/isolating valves so that I can move it into the loop in summer and out of the loop in winter when its probably not needed? the small rad would go in the airing cupboard near to our clothes airer so heat would not be totally wasted.
thoughts?
I have evohome with opentherm controlling viessman 100wb1b heat-only boiler. switched live output of evohome hot water kit BDR is also connected to the hot water "go hot now" input on the boiler, this makes the boiler run at 77 in order to re-heat the cylinder (which works great now).All rads have HR92's, and there is an ABV in the system.
Once thing I have noticed in the warmer weather now (all CH off), is that in DHW demand times, just after the boiler has finished reheating the cylinder (which takes about 15 mins for a 10 degree recovery, I have offset of 10 degrees set), it then overruns (pump on, burner off) for about 30 mins.the pump is controlled by the boiler directly not evohome (has to be with opentherm IFAIU). I presume this is the boiler heat exchanger protection kicking in and keeping the pump on. After finishing heating the cylinder the flow temp will be about 80, watching the temp dial on the boiler it then climbs to about 90 just after the cylinder demand has shut down, then slow cools to 75 ish over the 30 minutes of overrun, then the overrun stops. During this overrun period the ABV has opened and the water is definitely going round the ABV's loop (I can feel the high temp in it). there is an overrun time parameter in the evohome hot water settings as well this is set to 0 mins in my case, it doesn't appear to have any effect in my system.
The loop of pipe that is purely used by the bypass is pretty short (in the airing cupboard ) - about 50 cm at most. total amount of uninsulated primary pipe in the airing cupboard that it passes through maybe 1.5 metres including zone valves , pump etc. there than another 5m or so of Uninsulated under floor primary pipe, and then another 7m or so of Insulated (through a loft), comprising the rest of the primary circuit back to the boiler. whole thing times 2 flow for flow + return.
I am thinking there isn't enough uninsulated capacity in the bypass loop to shed the heat quickly enough in this situation, hence the overrun lasting 30 mins.
I suspect this is only happening in the summer when DHW demand is running the boiler in "very hot" mode. in the winter , the remains of the very hot water from DHW mode will be consumed by the CH demand, and/or if its goes into bypass after CH demand has finished, the temps at that point will be a lot lower so the overrun time needed a lot shorter.
So.. wondering if I need to make the ABV bypass loop uninsulated part longer , by putting an extra tiny rad , in series with the ABV, in order to throw the heat away more quickly when DHW heatup has finished? Possibly with some diverting/isolating valves so that I can move it into the loop in summer and out of the loop in winter when its probably not needed? the small rad would go in the airing cupboard near to our clothes airer so heat would not be totally wasted.
thoughts?
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