I'm struggling to understand the "logic" used by the FTC6 to run the compressor and pumps.

Graphs from today, 8 degrees OAT, WC curve target is 40 degrees

Graph shows flow temp and power consumption for the complete setup over a couple of 20 minute cycles.

The first part of each cycle makes sense - compressor starts up, flow temp increases. Compressor runs harder, flow temp increases further. Reaches target temp, compressor backs off a bit.

Flow temp stabilises for a bit, then everything shuts off - including both pumps. Flow temp is steady, because the water isn't moving. 

After 2 minutes, the pumps start up, flow temp drops rapidly. 

3 minutes later, the compressor starts up again.

I don't understand why everything switches off when the flow temp is stable?

I don't understand why both pumps switch off the instant that the compressor stops?

The reason why the pumps were stopping is because the heating was switching off.

The Evohome controller starts "modulating" the BDR91 boiler relay as all the TRVs get to within 1 degree of target temperature. Since it can only modulate by switching off, it starts cycling every 20 minutes (the minimum number of cycles, set in the system configuration). 

After a few minutes, the Evohome boiler relay switches on again, the pumps start running, flow & return temps start to drop, and the compressor switches on.

This wasn't happening before, because it was cold outside, and a couple of rooms weren't getting close to target temperature. Evohome was continuously calling for heat.

Main culprit was the bathroom, losing a lot of heat through the extractor fan ducting. The electrician had removed the baffle flaps, because they fouled the flexible ducting. I've refitted the flaps, modified the ducting, and they now work. Much less heat loss, bathroom gets up to temperature, so now the Evohome is cycling.

Not sure what to do about this?

I guess I could disconnect the boiler relay, and just use the Mitsubishi wireless remote as a the controlling thermostat, and the Evohome as adjustable TRVs (without any "call for heat" function)?

I'd have to find a suitable location for the Mitsubushi wireless remote though - somewhere that it actually has some control over the heatpump, but doesn't switch it off too soon.

@clockworks That all makes perfect sense and highlights that heat pumps don't need external control from on/off boiler thermostats they just hamper performance.

Sounds like moving the wireless thermostat is the correct option for you.  Hope it works out for you.

Rather than disconnecting the Evohome boiler relay, I've added a switch across the relay contacts. This way, I can either have it on continuously,  or flip it back and give Evohome control.

The boiler relay was just wired up as a "zero volt" thermostat to the FTC6 - closed circuit for heat, open circuit for no heat.

With the switch closed, Evohome will just be controlling a few TRVs as limiters, while the Mitsubishi remote is doing the "call for heat" and controlling the heat pump. Basically the same as if I was just using a few wax TRVs, but programmable.

Now that I'm back in pure WC mode, I don't think the wireless remote will be having any influence, apart from "on or off"?