ReedRichards

In the case of my new Therma V, which is about a year old and, I think, a Series 4, it switches off. This makes sense if it comes on to do cooling at an even higher outside temperature but since the cooling feature is turned off on my heat pump I regard this as a bug. However I think my old Therma V, which must have been series 3, would have maintained the LWT at the minimum figure (25 C in your case) if the air temperature was above the maximum. So unless @ProDavecan tell you, you will need to answer your question by experiment.

I'm now on my second LG Therma V, 12 KW U32. According to my "Owners and Installation Manual" H designates an Air to Water Heat Pump M designates Monobloc 12 is the heating capacity (so 12 kW) 1 indicates 220-240 V ~50 Hz M indicates Leaving Water Temperature Middle Combination (which is the only option given for this designator) U3 indicates a UN3 Chassis (alternative is UN4) 2 indicates the series number So you have a Series 2 heat pump. I can't tell you if that means you have the right controller or if your heat pump would work with the newer model of controller.

@Michael_S, your controller is either very new or very old; it looks nothing like the one I have (which is the same as the one in the picture on the previous page).

But does it cost that much? This company is selling a 15.5 kWh battery for £2,500: https://www.fogstar.co.uk/products/fogstar-energy-15-5kwh-48v-battery

Do you have an EV tariff or similar? That also makes a huge difference to the financial case, but in the opposite sense to the difference @PhilT is referring to.

If you paid for all your electricity then the cost of running the heat pump ought to be less than the cost of running your old lpg boiler (except, perhaps, for the fact that you are making the house warmer with the heat pump).

This seems an excellent methodology but your house will only get too warm if you aren't using a room thermostat and/or TRVs as these will act as an end stop. You could set the room thermostat a few degrees higher than you actually want and set all the TRVs to max but then you would have to spend time balancing all your radiator flows so that each room reached the temperature you want at about the same rate. And then adjust your heat curve settings. Or you just reduce the cold end of the curve until you can't get the house warm enough. Some heat pump owners seem to get a bit obsessed with chasing efficiency. For me, efficiency is good but not at the expense of comfort or convenience.

I have seen post on this before. Although many thermostatic showers say they have a minimum HW flow temperature that is too high for a heat pump, actually most work perfectly well with lower temperature HW.

Yes it can, you need to look at the modulation range and how little power you might need. If it's 1.7 kW, can your 8.5 kW heat pump even supply 1.7 kW continuously or would it have to supply, say, 3.4 kW with a 1:1 on/off duty cycle. And that's when it's -5 out, what if it's 10 out, how little power will you need then? You can't want 45 C flow temp for UFH and the return temp can't possibly be less than your room temperature, can it? Typically for a heat pump the return temperature is about 5 C less than the flow temperature. So maybe you want 35 C flow, 30C return?

That's good data. But after 10 minutes you have now warmed up all the water inside your house to 26.7 C average. So even if everything was turned off at that point you would still have heat going into the house; that doesn't stop. You will still derive benefit from all parts of the heated system that are inside the insulated fabric of the building; that energy spent heating the system was not wasted, as you seem to imply.

That sounds a bit non-physical. If you are using lots of energy then you must be heating something, even if it's with a COP of near-one. And if it's not inside then it must be outside. If it's outside, as you seem to think then what is happening? Is some motor inside the heat pump getting very hot? Do those mechanical parts inside the compressor need more lubricant? Your symptoms "best part of no heat added to the house" sound more like a fault than regular behaviour.

So say it's 3 times an hour for 5 minutes, that would be a duty cycle of 25% On, with an On time of 5 minutes. Why do you think a heat pump would mind that?

That's not wrong but you would have to carefully balance every radiator in the house for it to work. Efficiency is great, it saves you money. But you can save even more money by wearing lots of clothes and not bothering to heat your house. So be aware that you may be sacrificing convenience for the sake of efficiency.

That seems like a perfectly valid observation. But if the heat loss is at least 2800 W, how on earth is it currently being heated by a 1700 W (@ Delta T = 50 C) radiator?

So there are some radiators where it isn't? What differentiates these ones?

Just a thought but I have an open plan lounge and dining room. This was originally heated by three radiators and I had to add a fourth one we we moved from oil to a heat pump. The heat loss of the two rooms combined was calculated to be 3900 W but the calculation split this into 3 "areas" and assigned a heat loss to each. So the difference between Company A and Company B could be down to a difference of opinion as to what they think constitutes the lounge, if it isn't clearly separated from another area by a door.

No, it will be hotter because you are heating it with your UFH. Or even if not you will still be losing heat through the floor but if it's 10% with UFH it must be about 5% without; given the same floor covering. I suppose the worst case would be if the heat that penetrates the insulation can escape laterally to something that is colder than 8 degrees C. So you would have to level the ground, cover that with an appropriate membrane then insulation but around the edge of that insulation you need to dig a trench down to where the ground really is 8 C and fill that trench with insulation.

This is absolutely correct so there are two issues: Why do the two companies quote vastly different values for the heat loss? Why do both companies quote heat loss values far higher than your current radiator is capable of matching with its current heat output?

Surely it depends on how much insulation you have under your slab? And you can probably run your UFH at a lower temperature than radiators which should reduce the running cost? Otherwise why on earth does anyone go to all trouble of installing UFH and the risk of having to dig it up later should anything go wrong?

Not wet UFH, surely?

The Stelrad model of such a radiator has a quoted output of 1672 W at Delta T = 50 C. So the 1732 W figure must be the rated output and not what you will achieve with a heat pump, which will operate at a much lower delta T than 50 C. But it could be roughly what the current radiator produces at your current operating conditions with an oil boiler (80 C flow, 60 C return perhaps?). Either way, if this is the only radiator in the lounge then the calculations suggest it is currently insufficient to keep the lounge warm enough in colder weather. If that's not true then you need to consider why.

Have you tried Octopus?

This is for an earlier version of the software but that might not affect the wiring: https://midsummerwholesale.co.uk/pdfs/homely-installation-manual--mini--en-a5-lg-v1.2.pdf

In winter the field mice like to take up residence in my loft. There isn't much to amuse them up there so they like to chew on any foam pipe insulation that they can find, whereas so far they haven't touched the aluminium-clad stuff. I was concerned that your insulation looked too much like the type that is susceptible to rodent damage.

My feeling is that this feature is a gimmick and it is better to control your DHW recirculation pump independently, as I do.