ReedRichards

There is a school of thought that says leaving some rooms unheated does not necessarily save money with a heat pump. The argument goes like this: If the unheated rooms are not well-insulated from the heated rooms (which would normally be true) then the heated rooms will lose more heat than otherwise so will require a higher water temperature (this assumes you are using Weather Compensation) to maintain their desired temperature. The heat pump will therefore perform less efficiently and the reduction in efficiency outweighs the reduction in heating load. Personally I think this could well be true in some circumstances, not necessarily in all circumstances. But it's certainly worth being aware of this possibility.

Thinking about this some more, your actual electricity consumption is close to what it seems you were given to expect. Your most fundamental problem is that most of the time you kept some or all of your house colder than was (probably) assumed in the heat loss calculations but did not reap any benefit. Short cycling has a terrible reputation with gas and oi boilers and this bad reputation has been carried over to heat pumps. How bad it really is for a heat pump will depend on how your particular model works and how short the actual cycles are. My heat pump is limited to a maximum of four cycles per hour and seems to do fine with that.

Yes, although these are calculated figures for a year with average (winter ) temperatures. I would have thought that 20305 kWh is a big heat requirement for a modern well-insulated house but the self-builders here are better placed to comment than a retro-fitter like myself. If I were you I would first figure out how your zones are controlled now and how your upstairs zone manages to call for heat when required. Do you actually have the motorised valves necessary to control both zones independently?

In which case you should have got a big pack of documents after installation. Your MCS Installation Certificate should state what SCOP you are expected to achieve and what your calculated space and water heating demands are (as per your EPC).

Within reach of Halifax?

If the house (2019) was a new build then it's possible that it did not qualify for RHI in which case the OP may not have used an MCS-accredited installer.

Try setting the tank temperature to 50 C and see if that produces adverse effects (i.e. you run out of hot water when too many people take baths or showers in quick succession). If there are no adverse effects then your 250 l tank is not too small and you can improve efficiency by setting the temperature lower.

There are some fundamentalists who advocate that you rely entirely on weather compensation to keep your house warm and don't use zones or thermostats. This means that you have to keep your house at the same temperature 24/7 (or you have some pretty sophisticated control) and it might not work with a mix of UFH and radiators unless set-up to work this way from the outset. I have a two zones and I have thermostats and weather compensation gives me significantly greater efficiency than using a fixed output flow temperature, as I did initially. And the right weather compensation curve gives me significantly greater efficiency than the over-conservative one I inherited from the installer.

I cannot measure the COP of my heat pump but comparing running costs with the oil boiler it replaced makes me believe I achieve somewhere like the SCOP value of around 3 that was promised. I'm running the house at the same temperatures that I used to. Your 200 litre hot water tank is too small for a household of four people so you have to keep it at 55 C to get enough hot water; my 300 litre cylinder is kept at 50 C. Your yearly power consumption of hot water is a lot larger than 12 times the two monthly figures (of about 80 kWh) that you gave; any idea why? The wrong Ecodan model might be costing you in standby power consumption (your summer heating power consumption?). But even so your COP figures are very poor. You have a big house but if it is well-insulated I'm surprised that you needed a heat pump with an output of 11.5 kW. I live in a 4 bedroom bungalow, timber frame built in 1980, so not insulated to modern standards but I do fine with a 12 kW heat pump. I used about 6500 kWh for heating and hot water last year, which is not that much more than your 5400 kWh total (and my house is kept a tad warmer). Is your heat pump smart enough to use a lower output water temperature when the upstairs radiators are not calling for heat? Could the output from your radiators be too small, forcing the heat pump to use higher output water temperatures? The most common cause of heat pumps performing badly is when they are retrofitted to an existing heating system without changing the radiators.

You don't mention whether you use a fixed output water temperature for your heating or if you use Weather Compensation. The latter should give you a significant improvement in efficiency.

The people saying no heating is needed yet are talking entirely from their own perspectives. It's your house and you know if it needs to be a touch warmer. It's probable your heat pump is set to do Weather Compensation. This will reduce your maximum water flow temperature if it is warmer outside. It is possible that the high end settings are wrong so when it is relatively warm outside, as it is now, then the target flow temperature is not hot enough to give you 20 C. Perhaps the previous occupants were content with 19 C at this time of year? So check if Weather Compensation is on and what the settings are.

Is your oil boiler on all day? If you can keep your output water temperature for heating down to 40 C then you might get a SCoP of around 3.5. So you could aspire to use less than 1/3.5 (2/7) of your current energy demand for heating. Work out what that is and you can work out what your ASHP would cost to run. Except that you can't because nobody know the extent to which the government will intervene to control electricity prices.

Also, there is along thread on the LG Therma V here:

I agree, you don't need it at 55 C do you? But to answer the question: Are you sure you're not fighting a programmed schedule? If you have any DHW heating program scheduled you can only change the temperature (or turn off the DHW heating) for the duration of the scheduled period.

If you have exclusively UFH (I presume wet UFH supplied by your oil boiler) then that should work equally well with an ASHP. But if you have a standard HW cylinder then the surface area of the heat transfer element (the coil) is likely to be too small so your cylinder will be slow to heat up. Maybe with UFH that's not so important? Or if you continue to use your immersion heater it wouldn't matter (but would be more expensive than using the ASHP).

Under the old RHI scheme you used to get extra payments if you fitted heat metering. The only pro I can see now is that you can better assess your SCoP to tell if your heat pump is performing as specified.

A2A heat pumps can also be used very economically for heating in spring and autumn when it's not too cold outside. A heat pump with cooling is not a permitted development and would require planning permission. I presume this also applies to A2A.

I have no way of measuring my delivered energy but comparison with my previous oil consumption suggests I achieve a SCoP of around 3 with my ASHP. And that is with all radiators, UFH should do better.

It seems to me that the formula is only scientific if the input parameters are derived scientifically. Is there a scientific basis for a minimum cycle time of 10 minutes or could it just as easily be 5 minutes or 20 minutes?

ASHPs don't generally have the modulation range of a gas boiler (but are better than oil boilers which typically don't modulate at all). So when the demand for heat is low they will tend to cycle. Whether these cycles are "short" depends on the sophistication of the controller, I have a third party controller which only allows one cycle per 20 minutes. Anyway, the more over-powered your ASHP the more times during the year it will spend cycling. Short cycling is bad for conventional boilers and the mentality that is is bad has been carried over to heat pumps. In reality I think it will depend on what actual time constitutes "short" and how the heat pump performs a cycle, does it start at full output and modulate down or start at a low output and modulate up? Mine seems to do the latter.

There is an inverse square law that applies to infra-red heaters and other forms of radiant heating. If they give you X amount of heat when you stand 1 m away than you get X/4 heat at 2 m and X/9 at 3m etc. The radiation that misses you heats the patio flooring and that, in turn, heats the air.

There is a myth, oft perpetuated that an ASHP requires a well-insulated house. ASHP are not magically different from boilers, stoves and other forms of heating so the only circumstance where an ASHP would not be suitable is if its heat output could not match the requirements of the house. You can easily get ASHPs with up to 18 kW of heat output; I don't doubt there are bigger ones for the commercial market. And if the heat requirements of your house are large then it will be expensive to heat whatever form of heating you choose.

I have no way of knowing my CoP. But based on a comparison with my former expenditure on oil, my SCoP must be approximately 3.

My heating controller imposes a minimum cycle time of 20 minutes. So I don't think I have to worry about short cycling - and it might be the same for the OP. In any case, for any heating cycle, long or short, my heat pump starts at around 2 kW of power draw and works its way up as the temperature of the output water increases. If this water temperature never needs to get very high to satisfy whichever thermostat is calling for heat then it ought to be operating very efficiently.

If you take the volume of the inside of your house and divide by the volume of the inside of your fridge then multiply that by the cost of your fridge to make a volume to volume comparison then the question might become "Why are heat pumps so cheap?"