Hi Scott,

I have this problem on my UFH and have repeatedly mentioned this to my installers but have had no satisfactory investigation into it (I'm the only one measuring flow and return temps). I believe my consumption to be too high also, especially for the quoted COP so am very interested in seeing what responses you get to your post.

The ASHP can heat water in two ways: via the compressor/inverter process like a reverse fridge (when you get high SCOP) and efficient. Or using a back up heater like an immersion heater in your hot water cylinder. This is used in some systems only during very cold periods. 
I was wondering if your back up heater is being used a lot of the time?  I have mine switched off. Full stop. 
Worth checking. 
Also try gradually reducing your flow temperature 1C every week until it’s too cold. And then switch it back up 2C

A third thing struck me - are you constantly heating hot water?  Switch that to only heat at night (if you are on a two rate tariff) and don’t use gallons of hot water. Else the ASHP had to keep switching between hot water and home heating which is less efficient 

Posted by: @scottfen

Hi all

My heat pump is sapping electricity like there is no tomorrow.  For reference, my heat loss calculator at install was 13,500 (i understand an average house uses 12,500 in elec/gas, so not too far off - i live in a three bed mid terrace house) - the SCoP is 3.86, so estimates i should use 4000 Kwh to heat the home inc DHW.

i've had it in three years now, each year i've done closer to 6000Kwh foe heating and hot water.  Didn't really notice until i got my smart meter and saw how much elec i was using, investigated it with the data form the heat pump and here i am.

I contacted Mitsibushi for help and they said data shows my return temp is consistently higher than the flow temp - would that mean the unit is having to work unnecessarily hard constantly?  The data was showing efficiency at 7% so they suggested the thermistors were on the wrong pipes - installer has been back out and swapped them around and the data is now showing efficiency as you would expect with correct CoP, however, the amount of consumption hasn't decreased and the return/flow is still showing the same readings so my elec is still sky high.

I usually have the timer on (but not drastic changes) - 18-19 during the day and then 17-18 overnight with a gradual increase in the morning, so no major spikes up and down.  Last night i tried a constant 19 but this morning woke up and before even getting up for the day, the smart meter was showing nearly £5 :-/ !

Any help/guidance appreciated!

Hi Scottfen,

I think that there may be some confusion in parameters. I assume that the 13500 that you mention is kWh's, and I suspect is the annual heat demand for your property. This is not heat loss, which is a measure of how much energy leaks out of your home each hour under specified conditions, normally 20C to 21C indoor to -2C or -3C outdoor.

If the 13500kWh's is correct and your heat pump is consuming 6000kWh's, then your SCOP is just over 2.

Is your heat pump operating in weather compensation mode? Do you have thermostats or TRV's affecting the operation of the heat pump controller? Are you heat emitters correctly sized and has the system been balanced?

Thanks for the responses so far - to be honest, it's been installed three years ago and i've very much left it to it - i have an Ecodan system with a large 200L water tank in the loft, as far as i'm aware nothing else other than some TRVs on the living room rads and a couple of others.

@JulianC As far as i'm aware, the setup is simply to heat in the compressor/invertor manner

Re the 13,500, yes this was the annual heat demand needed before ASHP was installed and our previous heating system (just dug it out, slightly more overall - 13,455 for space heating, 2291 for Water heating) - the workings out for what the RPI payments were based on this/3.86 - essentially being told that is what it would need to produce to match what our heat demand previously was - essentially using 4079 khw of energy to match it for the year (based on overall SCoP of 3.86) - and this then made it comparable to what we would spend (clearly not the case if we are touching 6000KwH)

I use the wireless Mitsubishi controller, have mixed it up for where it sits (downstairs/upstairs/in different rooms).

Mitsubishi advised not to use weather compensation because i was using the wireless thermo, although not sure why that made a difference?  Looking at the commissioning checklist, it states the weather compensation settings is 45degree flow at -3 and 35degree at 15 and the heating temperature is down as 45 flow and 42 return (but that's not happening)

Re the heating of water, this only kicks in when needed and overrides the heating so i leave that to do what it needs to do, so i don't have that on a timer at all.

It's been serviced every year since install so assume it is correctly balanced at that point?

Posted by: @scottfen

Thanks for the responses so far - to be honest, it's been installed three years ago and i've very much left it to it - i have an Ecodan system with a large 200L water tank in the loft, as far as i'm aware nothing else other than some TRVs on the living room rads and a couple of others.

@JulianC As far as i'm aware, the setup is simply to heat in the compressor/invertor manner

Re the 13,500, yes this was the annual heat demand needed before ASHP was installed and our previous heating system (just dug it out, slightly more overall - 13,455 for space heating, 2291 for Water heating) - the workings out for what the RPI payments were based on this/3.86 - essentially being told that is what it would need to produce to match what our heat demand previously was - essentially using 4079 khw of energy to match it for the year (based on overall SCoP of 3.86) - and this then made it comparable to what we would spend (clearly not the case if we are touching 6000KwH)

I use the wireless Mitsubishi controller, have mixed it up for where it sits (downstairs/upstairs/in different rooms).

Mitsubishi advised not to use weather compensation because i was using the wireless thermo, although not sure why that made a difference?  Looking at the commissioning checklist, it states the weather compensation settings is 45degree flow at -3 and 35degree at 15 and the heating temperature is down as 45 flow and 42 return (but that's not happening)

Re the heating of water, this only kicks in when needed and overrides the heating so i leave that to do what it needs to do, so i don't have that on a timer at all.

It's been serviced every year since install so assume it is correctly balanced at that point?

 

A couple of points from a fellow Ecodan owner:

- the thermistors have to be positioned properly, on the the correct copper pipes and covered with insulation. If they are on plastic pipes or not insulated, they can be several degrees out.  Mine were originally installed on plastic pipes and read 5-6 deg low. At least they were both installed badly the same way so the delta looked OK.  If you want to check, you can buy some very cheap aquarium thermometers off Amazon that are quite accurate. Attach them to the flow and return on a radiator close to the ASHP and you'll get a good idea. I can't think of any obvious way a correctly plumbed ASHP would have flow lower that the return. 

- If the wireless controller is in room temp mode I don't think it uses weather compensation even if it's set up. You could try changing it to run off the weather compensation curve and see what happens (I assume you know how to do this; shout if not). You'll find out if your rads are sized correctly if you do.   

- servicing the ASHP won't include balancing the radiators

- You will be doing very well to get a SCOP of 3.86.  I have an Ecodan and after a lot of tinkering I'm getting around 3.5 over the (quite mild) winter for heating.

FYI my heating requirement is 10-15% more than yours and I seem to use 25-35 kWh/day average over the winter.

Some thoughts from a heating expert that sent me a message relating to your issue: 

Could be one of a few things, one of the sensors knackered, sensor cables have been inadvertently swapped round, an alternative downstream heat source is turned on, three port valve to cylinder is stuck open so the DHW temp is heating up the return are some of them.