ReedRichards

@RonnyIf the outdoor temperature is near your max setting of 21 C then the leaving water temperature would be near your min setting of 34 C if AI=0. I think that setting AI=-5 reduces the LWT by 5 so you are setting it to 29 C, or a bit higher on colder days.

I don't know how, or even if it is possible, to get the heat pump to charge up the buffer tank on demand. Every once in a blue moon there is a big short spike in my electricity usage, it happened this afternoon when nobody was at home. I presume this is heating up the buffer tank because there wasn't any other power requirement that I can think of at the time. Running something that consumes 3 kW with 100% efficiency is a better match to the output from my solar panels than something that consumes 6 kW with 300% efficiency. If the power demand is not too high I can cover it with free solar electricity, for higher demand I have to import from the grid.

I noticed that at the bottom of the cylinder there is a unconnected immersion heater, the same thing that I have. In my case the bottom section is the buffer tank. At the moment I have solar electricity to spare and I was wondering what would happen if I used it to heat the water in the buffer tank via its immersion heater.

Ah, different company but also made in Manchester. It was the bee logo I recognised as my cylinder also has one - but I now think that to be a Manchester branding.

This is how it works, as @dpmillerstated. @Ronnyis that a Telford cylinder (like mine)?

As I now understand the AI mode, if the outdoor temperature is between the min and max settings then the LWT is decreased linearly from the max to the min setting. For example, In your case @Ronnyif the outdoor temperature was half way between -7 and 21, which is +7, then the LWT would be 40, which is half way between your min and max settings. Since it is very rarely as cold as -7 or less, your LWT would very rarely get as high as 46.

I have been looking at the settings that control the AI mode; I have not changed these myself so they are either defaults or input by my installer. These are: Outdoor temperature for auto mode: Min 5 Max 19 LWT for auto mode Min 34 Max 50 My heating is all radiators hence the 50 C max LWT but do other set-ups use similar parameters?

@Ronny, don't you have a meter measuring just your heat pump usage? I had to have one fitted as a necessary condition to get RHI payments but I find it pretty useful, particularly as I have solar panels so my main meter reading does not tell me how much electricity I have used. I would be concerned about noise from fan-assisted radiators.

So are we saying that when the radiators are cold they won't give out as much heat so the delta T between flow and return will be less? The heat pump cares about this so modulates its power down to control delta T whereas an oil boiler would go at full power until the return water gets too hot? These are the first three power cycles my ASHP goes through, starting when it comes on at 7 am and showing the first hour until 8 am. Thereafter most cycles are like the last one in shape. The power consumption certainly rises to a peak and then goes down.

Okay, 30 kW until the flow temperature reaches the set point of 80 C (say) versus 12 kW until the flow temperature reaches the set point of 50 C (say). Which gets to the set point the fastest?

Yes, but no. If you turn it on, it runs at 30 kW for X minutes and then has to turn off for a while to prevent overheating so on average you don't achieve the full output. It's like driving through a big city, you may always drive at 30 mph but you won't travel 10 miles in 20 minutes because you will keep encountering traffic lights and other obstacles.

I have just found the calculation I made comparing the heat output from my old radiators assuming delta T = 50 C with the MCS calculation for my new radiators. My old ones gave 30% more heat output and that should certainly raise the house temperature faster. I don't know if they were actually operating at delta T =50 but my condensing boiler produced copious quantities of vapour so it can't have been too high.

I throttle back on the temperature overnight, setting it at 17 C after being at 20.5 C (target) from 18:00 to 22:30. I have solar panels and my inverter keeps a log of the house energy consumption. Once in a blue moon there is a blip or two of usage in the middle of the night but the norm is no activity (hot water is also off). I wonder why I am different to @Chris Bottomley who experiences hourly activity to keep the buffer tank at temperature? I appreciate that a 30 kW oil boiler (mine was 25 kW) is capable of a lot more power that a 12 kW ASHP but under what circumstances does it get to use all this power? It may use it to heat the domestic hot water; my old cylinder used to have a coil with a smaller surface area or it might be a combi boiler. But the calculated heat requirement of my house to maintain temperature is 10.5 kW . If the rooms are colder than their specified temperature they will derive more heat from the radiators than when the rooms are at temperature. But the radiator output depends on the temperature difference so its' never going to be that much more. So I would expect a 30/25 kW boiler to rapidly get to the point where it has to cycle in order not to overheat the central heating water. I don't doubt that @Chris Bottomley is right that his ASHP heats the house more slowly that his oil boiler used to but I'm not convinced that comparing maximum heat output capacities provides the explanation.

Gas pressures???

Any "boiler" has to raise the central heating water to the specified operating temperature and then keep it there. Unless it is ridiculously cold outside my ASHP has enough power to keep the water at temperature. It may do this at the expense of a very poor CoP when its very cold outside but using my Therma V in the basic (non-AI) mode it managed to achieve this throughout the last winter. Once the heating water is at temperature then the rate with which it can heat the house will depend on how much heat the radiators give out compared to how fast the house is losing heat. That's true of any radiator-based heating system. So one reason an ASHP could be slower to heat the house than an oil boiler was is that the radiators give a smaller output at their specified operating temperature. That would most likely be disastrously true if you did not replace the old radiators when you got the ASHP but might still be true anyway. Once you reach the specified operating temperature for any boiler then it must be all down to what the radiators can achieve. So then the other aspect of response speed it how fast the "boiler" can increase the temperature of the heating water. I did a back-of-the-envelope calculation comparing my ASHP with my old oil boiler. My oil boiler could heat the water faster but had further to go to reach the operating temperature so my conclusion is that there is not much in it. In the real world of my house I have not noticed any difference in speed of response, except in the kitchen where I kept the old radiator and paid the price. Its output is 30% below the calculated requirement and it is indeed slow to warm up in the morning. But I knew that would happen and I know the reason why so I get very puzzled when people assert that there is some fundamental difference in response speed between an ASHP and other types of "boiler".

I'm curious about this as there is no obvious reason I can think of why it should be true. Or rather the obvious reason is that your old radiators were oversized for the water temperature of your oil boiler whilst your new radiators are well-matched to heat pump. So the response speed has nothing to do with the heat source and everything to do with the radiators. Could this be true?

Say 15p per kWh so £150 per MWh. 5.2 x £150 = £780 used since early December. Before the ASHP I had an oil boiler that cost me £910 per year in oil. We've just had a cold winter and spring but I won't be using nearly so much electricity in summer. I reckon I am on target to match the oil boiler for running costs.

Hello and welcome. My Therma V was installed a week or two earlier than yours. So far it has used about 5.2 MWh of electricity. I expect the running costs to be broadly in line with what I was paying for my oil combi boiler but you'll do well to switch from gas and not pay more. I have just gone back to trying the AI setting as I could not get that to work in winter (the room did not reach the set temperature).

The gas boiler in my old house was pretty smart. It could do weather compensation and also load compensation. I have only just discovered the technical term for this but the way it worked was to modulate the boiler to run harder when there was a big difference between the temperature measured by the room thermostat and the set temperature. Conversely the boiler would modulate down when the room was at temperature. Having lived with this for 20 years I think of it as the way any heating system should operate. Now the LG Therma V can supposedly do weather compensation but is there any load compensation capability? Given its fondness for cyclic operation I suspect not - which would be a pity because it is a missed opportunity for greater efficiency.

Remind me how to access the logged data and I will check what mine says. Oh, found it. My air temperature seems to vary around 100 degrees, from 92.5 up to 107.5. Obviously this is wrong but not always the same amount of wrong so it must be measuring something.

My hot water timer is off at night, 50 C from 06:30 to 10:00, 40 C from 10:00 until 17:00 then back to 50 C until it goes off at 22:00. If it was reasonably sunny the previous day then that should lead to no demand on the heat pump to heat the hot water (we don't typically use a lot of hot water in the evening so the tank will still be above 50C on the morning following a sunny afternoon). However if not then the water will be heated hot enough for morning showers etc. My buffer tank has an unconnected immersion heater. Would it be sensible to heat that with spare solar PV electricity once the hot water cylinder is at temperature?

I don't know exactly what my hot tap temperature is set at because the TMV is not calibrated - but it is about 40 C. The TMV is not 100% accurate, the taps run notably hotter if I have heated the tank to its maximum temperature of about 78 C (using the immersion heater and "spare" electricity from my solar panels). I know from experience in my previous house that if the tap/tank temperature was 50 C (no TMV) then my wife would complain that the hot water was too hot. So we compromise on a temperature that is borderline hot enough for washing-up but which she does not complain about. Ideally the hot water tank will not reheat after we shower in the morning so I can make maximum use of my solar electricity when it becomes sufficient around mid-morning. In winter you are probably correct and I should reduce the hysteresis setting.

My hysteresis setting (which I did not set myself) is either 8 or 10 C; I don't remember which. But that's okay, my hot water is set at 50 C but on leaving the cylinder it is fed through a Thermostatic Mixing Valve which brings it down to about 40 C. Hot water has priority. I don't think it is really a secret that the code is the four digits of the software version, which you can view under settings. Once you know the code: Menu - Setting - Press and hold the up arrow - Enter code.

So it's a Building Regulations requirement to guard against the unlikely eventuality that the Heat Pump managed to heat the water in the cylinder to 75 C (or whatever temperature causes the trip)? Edit: It looks like a Reliance – Single High Limit Cut-off Thermostat With Pocket (82°C)

Inside this little box is a temperature sensor which trips out if the water in the cylinder gets too hot. The consequence of the trip is that the valve which directs water around the heating coil in the tank does not activate so you cannot reheat your DHW. Under the black cover is a button which you can press to reset; you also need to restart the system, it seems. I can see that you do not want to overheat the water in your cylinder but surely it is a physical impossibility for the heat pump to do this so I am struggling to understand what useful purpose this trip serves.