We have an 11.2KW Ecodan with an FTC6, installed 2022 with a 300l HW tank and 10 circuits of Wundasmart underfloor heating in around 120 sqm, installer (who was generally great) left every circuit thermostatically controlled.

We spent a lot of time learning about the system over the last year, and one quirk of the Ecodan is a minimum run time which it imposes on itself and the impact on flow temperatures. After conversations with Mitsubishi, this is called the Compressor On Guard Strategy, which enforces a 20 minute minimum runtime, unless the Superheat has been >10C for 10 minutes. It took us a long time to find out from Mitsubishi that this existed.

It doesn't matter a lot what the above means, but the effect - a minimum run time - is interesting. Normally if the pump's minimum heat delivery is higher than the demand, then during low flow conditions (e.g. only a few thermostats calling for heat) the flow temp rises and the pump will turn itself off when flow exceeds 5C above the compensation curve set temperature. In our case, with flow around 27C that might be when flow hits 32C, cunsuming around 1KW of energy.

However the Ecodan will prioritise the importance of keeping on running over halting when exceeding the flow temperature, and will keep trying to heat the water while it keeps running. The compressor runs harder and harder as the temp spirals and we were recording 55C flows at which point it turned off. This incurred very high energy consumption (3KW or more). 

This had a huge impact on the COP, dropped below 3 during mild weather. During cold weather it's never a problem, there's always enough flow because so many circuits are calling for heat and minimum runtimes are over 20mins anyway.

We contemplated adding a buffer (cost around £1K installed) but instead trialled removing all thermostatic control in all rooms except the coldest one and using full system volume all the time with the compensation curve driving the system. The result in mild weather is dramatic: we can keep flow temps at 23C above 13C outside, the pump cycles twice an hour with max flow never exceeding 5C above set flow temp, Melcloud-reported COP is regularly over 5 and often over 6, it appears to be phenomenally efficient, borne out by much lower daily energy usage in mild weather.

We do have some small challenges with some warmer rooms getting a couple of degrees C above what we'd prefer but it is cheaper to just live with the extra heat, cost goes up if we re-introduce any thermostatically control rooms, because the volume reduced and pump works harder as above. Our Wundasmart manifolds use automatic balancing valves which set the circuit flow dependant on flow/return temps and can't be manually adjusted down, which is an unfortunate side-effect of an automated system.

The lesson learned is that Ecodans and lots of thermostats really don't work well in mild weather and it's much cheaper (and generally nicer) to go to the effort of working out the right compensation curve and running low flow temps against your full system volume.

Might be useful to anyone with an Ecodan, lots of thermostats and seeing big spikes in flow temps and energy consumption several times an hour during mild weather.

Rod