Welcome to the forums @barry-the-bear

Your experience does seem a bit unusual, but I noticed that the unit in question is a 4kW Ecodan QUHZ. What makes this model stand out is that it uses CO2 (R744) as its refrigerant, which is quite different from other Ecodan models that use traditional refrigerants like R32. While I’m not a refrigerant specialist, CO2-based systems tend to operate differently, especially with regard to temperature management.

One key aspect is that CO2 systems generally prefer lower return temperatures, and this can impact the delta T. With a CO2 system like the QUHZ, a delta T of around 30°C isn’t uncommon, which can explain why you’re noticing specific temperature behaviours. It’s possible that the system is designed to prioritise certain flow temperatures during mode changes (for example, between heating and defrost) to optimise refrigerant performance.

It might be that when the unit switches modes, it needs to hit a specific flow temperature - like the 32°C you’ve observed - before resuming normal operation. This is likely tied to the system’s internal logic for efficiently managing CO2 refrigerant, which behaves differently compared to more common refrigerants.

It could be helpful to have a chat with a Mitsubishi support to get some insight into how the QUHZ manages these transitions. They should be able to explain the logic behind the temperature regulation more clearly.

Let us know.

Thank you for your welcome and reply.

You are right that the QUHZ use CO2 (R744) refrigerant. CO2 does have special properties and may operate more efficiently closer to its critical point (31°C), but I have no idea if that plays a role in the delay transitioning from DHW mode to heating mode - and I'm not sure Mitsubishi do either or at least it's not something they've bothered to explain to me. I have talked to them several times, but not once was the refrigerant mentioned.

About the specific (transitional) UFH flow temperature, there seems nothing special about 32C. That's just what it was on the day. What seems to matter is that the post DHW flow temperature matches the required UFH flow temperature. If I fixed the UFH flow at 40C for example the UFH would start fairly promptly, at 38C it take a little longer and so on. The UFH delta T is typically around 7C.

Cheers!

So I suppose the real question is you house cooling down to much or are temps ok?

How is the circulation pump operated on all the time?

The house isn't cooling while for heat mode - the fabric of the house and the MVHR and so on ensures that doesn't happen, but that doesn't mean it isn't desirable to have a shorter delay. The pump doesn't run all of the time. 

Posted by: @barry-the-bear

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The pump doesn't run all of the time.

That may be your issue. You have hot water sitting in, I assume is an insulated pipe, so it will not cool quickly. If your pump runs when ever there is a demand for heat, the hot water in the pipe is flushed away when the diverter valve moves to heating system and cools really quickly.

The other issue could be if you have mixer valve on the UFH, it will just recycle 90% of its return water and only let a small amount of hot water through. So you see an issue, but in reality you UFH is getting hot water.

Today even without having to do DHW heating, my ASHP compressor went off 8 hrs ago, water is just circulating, sensing the return temp, if it needs the compressor to start it will.

I don't know if you are right or not!. I am sure though that the UFH doesn't receive any hot water until QUHZ sends it and UFH pump, housed in the packaged thermal store won't run until the flow temperature reduces to the required level. My thought was that the rapid cooling from minute 11 to 15 ideally should continue a little longer.