My central heating used to consist of a Potterton Profile 60e boiler with radiators on 2 floors and a hot water cylinder upstairs. The system is vented with a circulation pump. There was a three port valve that sent the boiler's output to either the hot water cylinder or the rads as requested. It worked fine.
Then I retrofitted underfloor heating on the ground floor. The three port valve was replaced with 3 two port valves (1 each for the UFH, rads upstairs and the hot water cylinder) and under the control of a Heatmiser wiring centre and thermostats. The whole system has generally worked fine for the last 3-4 months except that the overtemp "breaker" on the boiler occasionally trips out and requires a reset with a fingertip. This has happened perhaps 4 or 5 times since October.
I think I have identified the circumstances under which the boiler trips out. When the system is demanding heat but the heating period ends (such as at 07:15 on a weekday morning when I plan on leaving the house and reduce the daytime temp to 14 rather than 18) the Heatmiser system tells the boiler to turn off and it closes all of the 2 ports valves. If the boiler was still going full steam at that time its heat output has nowhere to go and the boiler trips out. The circulation pump does continue to run for perhaps 10 minutes but only the 2 towel radiators are on the bypass circuit and if they're good and hot there's a good possibility there's nowhere for enough heat to dissipate fast enough. I had this happen yesterday; I heard the Heatmiser relays click off and the boiler stop and perhaps 10 or 15 seconds later I heard the boiler trip.
I suspect this issue never happened with the old system as when the boiler shuts off the "excess" heat went to either the rads or the hot water cylinder as the three port valve was always open on one side or the other. But now the 2 port valves all shut off and then the boiler might trip.
I've asked Heatmiser if the wiring centre can be configured to delay the closing of the 2 port valves but they say no. I have turned turn the boiler's output temperature which should help but I would welcome any other suggestions on how to further reduce the odds of the boiler tripping out.
Then I retrofitted underfloor heating on the ground floor. The three port valve was replaced with 3 two port valves (1 each for the UFH, rads upstairs and the hot water cylinder) and under the control of a Heatmiser wiring centre and thermostats. The whole system has generally worked fine for the last 3-4 months except that the overtemp "breaker" on the boiler occasionally trips out and requires a reset with a fingertip. This has happened perhaps 4 or 5 times since October.
I think I have identified the circumstances under which the boiler trips out. When the system is demanding heat but the heating period ends (such as at 07:15 on a weekday morning when I plan on leaving the house and reduce the daytime temp to 14 rather than 18) the Heatmiser system tells the boiler to turn off and it closes all of the 2 ports valves. If the boiler was still going full steam at that time its heat output has nowhere to go and the boiler trips out. The circulation pump does continue to run for perhaps 10 minutes but only the 2 towel radiators are on the bypass circuit and if they're good and hot there's a good possibility there's nowhere for enough heat to dissipate fast enough. I had this happen yesterday; I heard the Heatmiser relays click off and the boiler stop and perhaps 10 or 15 seconds later I heard the boiler trip.
I suspect this issue never happened with the old system as when the boiler shuts off the "excess" heat went to either the rads or the hot water cylinder as the three port valve was always open on one side or the other. But now the 2 port valves all shut off and then the boiler might trip.
I've asked Heatmiser if the wiring centre can be configured to delay the closing of the 2 port valves but they say no. I have turned turn the boiler's output temperature which should help but I would welcome any other suggestions on how to further reduce the odds of the boiler tripping out.