Hans

Add one thing that is not about the unit at all: an oversized heat pump is a loud heat pump.

If the capacity is well above the actual heat loss of the house, it cannot modulate low enough to match a mild day, so it runs hard, satisfies the demand, stops, and starts again — over and over. Every start and stop is audible, the compressor spends its time at the noisy end of its range, and it is worse in the shoulder seasons when the house barely needs anything.

A correctly sized unit on a low flow temperature runs long and quiet, which is the behaviour you want anyway for efficiency.

You can see this from the outside without any special equipment: watch the cycling. If it starts and stops repeatedly over an hour on a mild day, sizing or controls are the story and the noise is a symptom. Turning down the weather compensation curve so it runs longer and gentler often reduces the noise as a side effect.

Strong agreement on the emitters, and there is a cheap diagnostic that separates the two cases in one evening.

On a cold day, deliberately raise the flow temperature well above your normal curve — as high as the unit will go. If the house then warms up properly, the heat pump can deliver and your emitters are undersized for the flow temperature you normally run. If it still cannot hold temperature at maximum flow, the unit or the flow rate is the limit.

It is inefficient and you would not run like that, but it is an hour and it tells you which half of the system to spend money on. Note the electrical consumption while you do it, because that is also a good illustration of why you do not run like that.

One caution: do not conclude anything from a single mild day. The test only means something near your design condition.

Worth adding what the efficiency actually looks like, because "we already own the equipment" is doing some heavy lifting in the plan.

Cooling with a heat pump is not free. You are running the compressor and the circulation pump, and you are rejecting the heat outdoors, so the outdoor unit runs and makes its noise on exactly the still summer evenings when you would rather it did not.

The efficiency is generally decent — moving heat is cheaper than making it — but the modest output from a floor means you are running for long periods for a small effect. Have a look at what that does to your summer consumption before assuming it is close to free.

Also check your export or time-of-use arrangements if you have solar. Cooling in the middle of a sunny day is close to ideal for self-consumption and changes the sums considerably in its favour.