I've built a 3 zone C/H system: Z1=upstairs, z2=downstairs and z3=towel rails (that heat shower and bathroom). Z1 and Z2 are controlled by timer thermostats. Z3 is controlled by a dedicated timer and is logically OR-ed with Z1 so that it also runs when Z1 demands heat.
When only Z3 timer is on but there's no demand from the other zones (during the summer, for example), once the TRVs on the towel rails have shut off the boiler keeps going (via the auto bypass) to keep the primary temperature at the boiler set point. This is a waste, a little bit noisy and presumably is slightly excessive for the ABV. It's a system boiler so I don't have separate control of the pump. How can I can improve this? I'd wondered about a flow switch in series with the timer to detect when both Z3 TRVs were off, but this would need a timer so that Z3 demand was only inhibited for 10-20 minutes before the pump runs again to see whether the TRVs have opened or not. Is there a better way? Pressure change maybe?
Dave