Plumbing - proper balancing/bypass

[Spedley]

22mm is through the heat exhanger - low flow rate in parallel with 28mm.

I still have recieved my accurate thermometer yet but I think there is an alternative solution as the boiler circulation loop is much more tunable. I'll set the heat exchanger to take enough heat out and then fine tune with the boiler but I may have to invest in a boiler protection valve to be sure.

 

[Spedley]

Grrr, darn thing! I've attached a temperature probe to the boiler return feed and adjusted the system to a 65'C return. After about 5 minutes the boiler shuts off.

I suspect this means the pump is too small (grundfos 15/50). Looking at the chart, a head of about 3.5m would cause flow rates to drop below 0.5 L/s which would put the flow temperature at 85'C causing the boiler to shut off. Does this sound right or is there another explanation?