ReedRichards

Using your immersion heater for 15 minutes you believe it should have used 0.75 kWh but you calculate that the 3 degree change you achieved requires 0.6 kWh. So is your immersion heater 0.6/0.75 = 80% efficient? Or is the problem that you can only measure temperature to the nearest degree, so working with a 3 degree change does not achieve good accuracy?

I have a third party controller for my LG Therma V. This restricts the number of cycles per hour (to 3 cycles). It can happen (although not very often) that my room thermostat is calling for heat but the controller is waiting for the allotted time to start another cycle so the fans and the heat pump internal water pump are off. I never get bangs or pops.

Storm Arwen caused power cuts lasting 24 hours and much longer in my area. But the weather was not too cold at that time so you would probably have been okay.

I actually have a 12 kW LG heat pump and (replacement) radiators. It was designed for a flow temperature of 50 C and I was promised an SCOP of 3.2 for heating. Now using Weather Compensation I get a SCOP of somewhere around 3 overall (heating and hot water). I don't have a high temperature model, mine can get up to about 55 C. But unless LG have made vast improvements to their efficiency in the two and a half year since I got my heat pump then the idea that I could achieve an SCOP even approaching 3 whilst heating the water to 65 C seems wildly overoptimistic. But I'm in the far north of England where outdoor temperatures are lower so I'm not going to achieve an SCOP as good as I could in the south. If what you are proposing really would work in the south then it's just yet another north]south divide.

I would love to see a picture of the finished FCU. As a work-in-progress it looks quite ugly but I'm hoping it will look nice when finished.

After the first few days My LG Therma V has been trouble-free for me in over two years. It has achieved broadly the SCOP that I was promised. I suspect that there are subtle differences between different makes of heat pump that we don't fully appreciate. It doesn't have a "vampire load" i.e. it uses very little power when in standby. It can modulate down to around 20% of its nominal full output. It starts up at low power and works its way upwards, rather than starting at full power and working down as I believe some other heat pumps do. Some of the control options, like Load Compensation, are badly documented. So yes, I would recommend the Therma V in general terms, but it depends a bit on exactly what heating system it will need to supply.

I have a 12 kW monobloc unit from late 2020. I got a third party controller for my heating, which I rather regret as it means I can't use the Load Compensation feature (called Air + Water control). I've never seen specs for modulation capability. Unlike some heat pumps it starts off at low power and works its way up. The minimum stable power seems to be about 2.5 kW, possibly less in warm weather. I don't know the minimum flow rate. You can control the heat pump by flow temperature or a combination of flow and room temperature if you use the LG controller. Although you can manually adjust the weather-compensated flow temperature by +/- 5oC, I don't think you can program that. Programming a set-back room temperature might have the same effect.

Whether or not you actually get ill, regulations require that all components that come into contact with potable water (which includes the hot water you will wash with) are WRAS Approved for use as such. For example, I have a recirculated hot water system that takes hot water from my cylinder round a loop so that the hot taps run hot quickly. This requires a special circulation pump that is WRAS Approved for potable water (and uses brass components in contact with the water). I would quite like to adapt this system using a motorised valve. Motorised valves are commonplace items in central heating systems but I have not been able to find one approved for use with potable water (at any reasonable cost).

Finding a way to retrofit a longer coil (or something with an equivalent heat transfer capability) to an existing hot water cylinder would reduce the cost of a heat pump installation. I had to fit new radiators but that was relatively cheap and painless, it seems more of an obstacle than it really is.

Planning forgiveness is called "retrospective planning permission" - and you cannot assume that you will be forgiven.

I'm not familiar with the Octopus offerings; I don't have a smart meter so they're not available to me. Presumably you need 20 kWh or so to recharge your batteries; does octopus give you long enough on their cheap rate to achieve this? My old oil boiler used to use approximately 20,000 kWh per year and on the few coldest days of last winter it used over 40 kWh in a day so I suspect there may be a few days when you have to draw some electricity at peak rate. But the economics of heat pumps is that they are cheap to run most of the time which compensates for the few expensive days in unusually cold weather.

My, that's a lot of battery storage - and not so much need for it without the heat pump, I would have thought. Or do you have some sort of V2H system where a car battery doubles up as a home battery?

The reason the air is too dry is because the starting point is outside air that is much colder than is typical in the UK so with a much lower moisture content in the first place. It's not the fault of the heating system, except insofar as the usually don't add deliberate humidification.

Have you looked at the power output of such boilers? They seem to start at about 24 kW and that's not because your house needs 24 kW of heat, its because you need a lot of power to provide instant hot water. But a "high power" gas boiler like this can still be made relatively compact and seemingly increasing the power output does not greatly increase the cost. Heat pumps are much less compact that gas boilers (although my old external oil combi boiler was a bit of a monstrosity). And to increase their power output they need to get a lot bigger. So get the heat pump equivalent of a combi boiler you would need to find a way of making the heat pump much more compact (without blowing you off your feet as you walk past) and without increasing the cost. You would also need a much wider modulation range, but I imagine that is relatively easy compared to the other challenges.

No No. Any heat pump that boasts about being capable of heating water to high temperatures rings alarm bells for me.

Surely it has got to be one or the other? Either the BUS is popular and keeping all the MCS installers busy or it's not popular and you should find MCS installers kicking their heels?

Personally, I find Heat Geek a bit OTT but you could try one of their "assured" installers: https://www.heatgeek.com/find-a-heat-geek/

1. Keeping a hot water tank at temperature is the least favourite task of a heat pump because it's the least efficient thing it has to do. If the water in the tank goes a bit tepid then at least it can be more efficient whilst heating the the water from tepid to warm. So there might be an efficiency saving turning off the tank heat pump when you don't need hot water (e.g. overnight). Or you could offset the tank temperature at night if you are able to do that. 2. UFH tends to be very slow to respond, particularly when it's buried in a big slab of concrete, as is likely in a new build. You keep the slab at temperature, that warms the rooms above it. You really have to keep it on all the time. I'm surprised you can get away without a buffer tank.

But what's in parallel is the mains supply, a switch, and then a feed to the immersion heater. The switch is controlled by the heat pump but the immersion heater has a dedicated circuit. The iBoost control is the mains supply, then a switch, then a 3 kW dimmer, then a feed to the immersion heater. The iBoost controls the switch and the dimmer. So what could go wrong with what I describe? It's never happened again. If it did happen again I now know exactly what the error code means. I would call my installer as a first resort and try to tackle it myself only as a last resort.

That's not true, it depends how the wiring is done. About a day after my heat pump was installed it stopped working with an error code (the cause was a blocked filter). Because of the error code I had no control over the heat pump and could not turn on my immersion heater to maintain a supply of hot water until the installer came back (which fortunately he did very promptly). But the lack of an override was unacceptable and I wanted to make sure that this could never happen again. So asked for the switch for the immersion heater to be wired in parallel with the heat pump controller. At the moment I have a WiFi controlled switch but it could equally well be an iBoost or similar.

Okay, I'll buy that. With an ASHP we know it is trying to achieve a small delta T, typically 5 C. So you can size your radiators accordingly and if Delta T is actually 3 C or 7 C it won't make much odds. With a gas boiler, the object is to achieve a much larger delta T. If this is fixed and you know what it is then you can still size your radiators accordingly. But you indicated that it is variable and the difference between 20 C and 10 C could make a much more significant difference to the radiator output. So what value do you use? If, for example, the gas boiler uses 10 C delta T to heat the room up but drops to 20 C to maintain the room temperature then you would size your radiators based on the 10 C delta T. But that mode of operation requires that the gas boiler knows the room temperature and how far that differs from the set temperature and can therefore do a form of Load Compensation. So my thinking is that variable (and large) delta T is only viable in a gas boiler that does Load Compensation.

Without a doubt it is possible. If your house is badly insulated it will be expensive to heat whatever heat source you use. With either a low temperature gas boiler or an ASHP you would need radiators with a large surface area, increasingly large as the rate of heat loss from your house is increasingly large. At today's prices an ASHP achieving a SCOP of 3 (which it should) would have about the same running costs as a gas boiler. My heat pump installer is based in Blythe, is that north-east enough for you? He installed my heat pump in December 2020, seemed to know what he was doing then and has over 2 years more experience now. Or you could try one of the installers that have paid HeatGeek for some sort of certification/verification https://www.heatgeek.com/find-a-heat-geek/ (althoug personally I think HeatGeek tends to be a bit OTT, you might not get the set-and-forget that you want). You need to be careful mixing UFH and radiators. UFH can typically be run at a lower water temperature than radiators need but responds much more slowly. Get the control wrong and you lose any economic benefits that UFH might bring you (because you have to make the flow water hot enough to satisfy the requirements of the radiators).

As the room gets closer to the radiator temperature (i.e. hotter) the radiator has more work to do, because the work it has to do increases with the difference between the outside temperature and the room temperature.

If the boiler sets its own delta T it must be able to read the room temperature. You would have difficulty matching the radiator heat output to the heating demand if delta T is not fixed so the boiler must compensate for that..

Whereas for a gas boiler the opposite is true, the lower the return water temperature the better the efficiency so for a gas boiler 52.5/32.5 is more efficient than 47.5/37.5 which is more efficient than 45/40. Correct me if I'm wrong.