Beelbeebub

You usually need to buy the camera too, I use thr "top Don" one, it just plugs into your phone and away you go.

The ufh should be able to get those rooms warm if it did before. The flow temp shouldn't be very high ie a HP should be able to achive the temps. My suspicion is too. Much flow of going via the radiators, which is shutting down your HP, the UFH then doesn't have a supply of warm water to "top up" the loop temp via the mixer valve and then they underperform.

I had a cottage where the radiator in the hall was absolutely scorching and all the other rads were like warm. The boiler short cycled and lots of gas was burnt. We closed the valve on that rad right down and all the rads became warm, the boiler stayed on for long continuous burns and the cottage was warm for not much more money

As I said the other option is one rad being "too open" and starving the other ones.

Either your emitters are very undersized - thr ufh loops should be OK, the whole. Point of ufh is it being a big emitter. Maybe your rads are too small? What type are they? Single panel, finned? Double panel double fin?. How big are they? Rads for HP should be bigger than what is typical in a boiler heated house. Your plumber should be able to tell just looking - 'they look bigger than I wound expect" or "nah, looks. like I. Would expect for a gas boiler house" (ie too small!)

Potentially the Vito boiler was supplying higher flow temps and each emitter was outputting more. This is why radiators and pipes sometimes need to be swapped for bigger ones when a HP is installed. Get an IR thermometer (better yet an IR camera for your phone - can use it to track down heat leaks in the walls too) Check your rads, your (heated) floors. Is there a rad that is especially hot, in particular if it's flow and return are very close - they should be 5-7C different. It's possible one of the rads is "short circuiting" the flow, which cause the HP to shut down (as the flow and return are too close) and your overall heat output to buikding is lower plus less efficient because of start stopping.

Link to the manual. https://trade.centralheating.co.nz/assets/resources/Chofu-Operating-Installation-Manual.pdf

What are your radiator sizes? Are the ufh heated rooms warm enough? What is your floor temp in those rooms(cheap laser thermometer will do) Look at what the return temp (parameter 00) and output temp are (09). Ideally they should be 5-7C apart. If the return is climbing closer to the output then your emitters aren't getting rid of the heat quick enough. It could conceivably be a radiator loop that is too open and the flow is all rushing thorough there, hardly losing any heat.

This may not be a good idea, temps that high may cause issues with the slab and floor coverings - the mixer and pump system is there to run the UFH at a lower temp than the rad system. IIRC thr max floor temp is supposed to be 28C, running at those flow temps will almost certainly result in parts of the floor getting hotter than this.

The suction temp is the temp the refrigerant gas is when it leaves the outside coil and enters the compressor, it is then compressed and leaves as a hot gas (discharge temp) So a suction temp of 4C means the air moving over the coil outside was probably at about 4C Then 88C gas was leaving the compressor. Which seems a bit high for heating mode, I would expect it to be more like 50-55C for a 45C (water) flow temp. That suggests something might not be quite right.

The first thing to note is that this sub zero weather will cost alot to heat especially with a HP. For example, my gas consumption is about £8 a day in these conditions. You say parameter 21-00 is set to 0. This is fixed flow temp. Look at parameter 20-01, that should be the temp your HP is outputting. The default is 45C, and maybe that is too low? It will also have been too high for the milder weather. The default for 21-02 is 45C, so the fact it's been set to 50C implies someone has changed it - possibly because your heating system needs more than 45C in these conditions.

Could you install something between the cylinder sensor and HP that basically tricks the HP that the cylinder is at temperature after a power cut? Depending on the sensor it could be as simple as a relay and resistor.

If you aren't bothered about claiming the grant then you could install a muktisplit A2A heatpump This would give you cooling in the summer and leave the gas boiler for water and backup heating in winter. (summer water could be immersion heater from PV?) You haven't mentioned if you have a gas hob - factor that cost in if you want to get rid of gas altogether.

JohnMo's solution sounds nice and simple. If you do decide to go for mechanical ventilation, I have used the ventaxia sentinel kentic series a few times now and been happy with the price/performance. Build quality seems pretty good and (hopefully) spares & support should be OK given they are a UK company. 🤞 I use the radial 75mm smoothbore ducting with manifolds for ease of installation and reduced noise between rooms. For some properties I have put a supply and extract in opposite corners of the room to get around the issue of undercutting doors etc.

That's good, thought they ought to include that in the main table figures rather than as an extra. That way they could include the cop. I have seen various ways of showing the performance of HPs but the best (IMHO) is to have a outside temp / flow temp table with each "cell" giving power, input power and cop. It then had 3 tables giving those figures at the maximum, most efficient and lowest outputs. There was some shading on the cells where output was being affected by defrost. That would seem to give all the info needed.