Beelbeebub

I would suggest that headroom is less important for such a large house. Say we have a very cold snap and your 12kw heatpump isn't able to keep the entire house at 21C (or whatever the design conditions are) Just turn down a few of the unused rooms to 15C for that period. If you have a 2 bed flat and all rooms in use all the time you don't have that ability. But for such a big house the spare rooms can be hibernated for a bit.

5 years is a bit on the short side but I have had multiple ones fail, it's one of the most common things to fail in the heating systems I manage. Prob have one or two a year out of 35 or so.

Which part of the system is that leaking tundish linked to? Is it the domestic hot water or the system loop? Is there a pressure gauge for the DHW tank? There are 2 reasons water is ending up in a tundish. 1) the valve is faulty, the seal may have hardened ornvt some debris on the seal. 2) the expansion vessels are faulty allowing the pressure to rise too much and triggering the relief valves (as they should). It would seem to be very unluck to have both loops (system and DHW) faulty at the same time So the first thing is to note the cold pressures of both the system loop and the dhw tank. The system is typically 1-1.5bar cold and maybe 0.5bar higher when hot. The relief valves are often 3bar (but not always - it should say on the valve) The dhw cold pressure will be your mains pressure, that can be anything up to 5bar, though often 2bar or so. Again, when hot it should be 0. 5 to 1 bar more and the pressure relief will typically be 5bar or more. If the pressure on either system is rising by more than 0. 5-1bar it might be the expansion vessel. If the pressure isn't rising above the value marked on the valve but water is passing then you prob have an iffy valve - they are designed to fail safe i.e. leak or open early rather than open late. If valves are faulty (activating before design pressure) they need replacing by a g3 certified plumber. Very occasionally they can be cleared by activating them *if* the issue is debris on thr valve seat. If the vessels are faulty (big pressure swings) it could be thry are simply out of gas - tyre pump to refill or they could have ruptured the membrane, in which case new vessel.

One possibility is that - for whatever reason - the system was drained down during the servicing and air entered the pipes. The plumber will repressurise but as some of that trapped air gets circulated around it will eventually find it's way to an automatic air vent, either in your pipework or in the heatpump. As that bleeds out you can get a drop in system pressure. Some systems can take quite a while to purge themselves and require a couple of top up cycles. My own system takes ages to get rid of all the air, but that's because it's crap.

Yes but if the vessel has never been topped up from new this shouldn't be a possibility as they are generally precharged with dry nitrogen - strictly speaking they should only be topped up with dry nitrogen, but nobody bothers. There would only be enough water from condensation to squirt out if it had been topped up a few times, which would indicate an issue somewhere (eg leaking prv) anyway.

If the system pressure target is 1.5bar when cold, then your pressure vessel should be at a little bit below (say 1.4bar) when empty. This allows a little water into the vessel when at system pressure and the maximum volume change when hot. So if your water pressure gauge shows 0.2bar your vessel should show 1.4bar. Of it shows lower then the pressure is wrong. You can top it up but the fact the air has been lost points to a failure in either the valve or the internal membrane. If some water comes out of the valve, your membrane is almost certainly shot. Swapping the vessel is the easiest.

2 possibilities You have a leak somewhere You may have an issue with the expansion vessel (red tank) First check the expansion vessel When the pressure is near zero, like in the first picture, get a tyre pressure gauge and read the pressure in the red cylinder. There should be a valve exactly like a car tyre one on the top. It should be something like 0.7bar, 1 bar or similar. If it has pressure eg 0. 7bar but the gauge on the water reads near zero, it is possible you have a leak somewhere. If it reads near zero your expansion vessel has lost has pressure (you may still ha e a leak but it's a bit less likely) If the latter you can pump it up with a.tyre or bicycle pump. Then top it up with the flexi hose. You need to turn the. Slotted screwdriver thing so it's. Inline withe the pipe, then turn the black handle at the other end. There should be a whooshing sound and the pressure creep up. Once at desired. Level (1 - 1.5bar) turn both valves off and disconnect the pipe (get a saucepan under as some water will come out. Beware just topping up the pressure continuously. The water is going somewhere!

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.