Do you have a Mitsubishi FTC or only the Heatmiser Neostats? I don't know Neostat. If you have both, is there some way to take the Neostats out of the system temporarily?

I've an 8kW Ecodan in a four-bed semi-detached 1990 house. The last few days, it's using 3.5-4kW when raising the flow temperature the last few degrees to a target of 44°c. The average is about 2.2kW. Yesterday the heat pump used 41kWh. The day before that was a record 46kWh. COP is around 2.5. These are scary times for costs, but mercifully it should get warmer soon, ending the COP-trashing defrost cycles. Even so, your 90kWh sounds high to me: a chalet bungalow will cost more to heat, but your 2017 insulation as-built should be much better than my retrofit, so surely not double?

The second thing is that leaving the heatpump on 24x7 may get you more efficiency but most people care more about cost than efficiency and there's little point keeping all the rooms warm if everyone is tucked up in bed, as long as it's warm again when they get up and doesn't cost a bomb. Kev M's started a discussion about that at Heat Pump Truth or Myth#1 - Keeping it running 24/7 uses less energy

@mjr I have the Mitsubishi FTC as well as the Neostats. Which has always confused me as when I'm on the MELCLOUD app, it shows me Zone 1 and a desired temp and an actual temp but just assumed the Neostats over rule that and if the stats call for heat, then the Ecodan reacts.

I suppose the only way to remove the Neostats from the system is I could simply power them off, but then my Ecodan wouldn't do anything as I think it only reacts to the call for heat from the stats. This morning, I put all my Neostats in standby so there was no call for heat. Despite upping the flow temp on the Ecodan, it doesn't respond or do anything as there is no call for heat. So I'm not sure I can over rule the stats.

I had hoped that putting all the stats at 30c, the Ecodan would work up to the flow temp, then just sit in the background in tick over mode circulating the water round the system and keeping the rooms at the desired temp. But it seems not. Seems that if the stats say I want heat, the Ecodan just runs its nuts off using 3kw an hour trying to achieve the 30c despite the flow temp only allowing for an average temp of around 21c.

I accept it has been particularly cold, and that costs are particularly mad. I can sit it out until it warms up but was using this cold period to find ways of making the Ecodan perform its best regardless of the weather.

@mjr In my case I’ve a 14kw Ecodan in a 1971 detached 3 bed bungalow so retrofit with best insulation I could manage. I deliberately have no zoning to use the radiators as one huge array. I had a Hive stat but following advice on here I reduced my flow temp as low as I could to maintain between 19 and 20 degrees. I effectively bypassed the on/off function of the hive by increasing the desired temp on it to 23 degrees so Hive is constantly calling for heat. However I have weather comp  swoon so really that is what is controlling the Ecodan. Yesterday when we were -10 degrees overnight and -3 at best during the day we used just over 4KW with a COP of 3.57! On the warm end of the curve I have it set at 35 at -5 and 25 at 17 degrees which is a very shallow curve. I did originally use rad stats for unused rooms (effectively zoning those rooms) but no longer do that and have all the TRVs open at max.

@brooster What you have done is exactly (I think) what I tried yesterday and what I hoped to achieve.

I set all my Neostats to 30c. Every stat called for heat. I ran my Ecodan with weather compensation on and had a flow rate of around 45c to start and as the rooms warmed up, I lowered the flow temp down until the rooms stopped warming up, but also didn't start cooling down. I think I ended up settling at around 35c, may have been 33c. I then let it continue running and hoped that the Ecodan would at some point, realise the house is at the flow temp and then go on tick over. 

But it didn't. It continued running at the 3KwH usage for every hour of the day until it got to about 10.30pm, I was up to 80 KwH usage for the day and I couldn't bear it no more for fear of the daily cost. So I reverted the stats back to the desired room temperature and backed it all out.

So I did and wanted to achieve everything you have done @Brooster, but I just couldn't stop my Ecodan from chugging through the electric! I could only dream of 3.57 on a cold day, probably nearer 2! I just don't know why my Ecodan continues to run madly when the house has reached the desired temp relative to the flow rate temp I set it at.

PS. I note you have a 14KW Ecodan for a 3 bed bungalow, I have often wondered if mine is undersized at 8KW but it was professionally spec'd out when I built the house, albeit the suppliers have now gone bust.

Posted by: @mcwatson1974

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@brooster What you have done is exactly (I think) what I tried yesterday and what I hoped to achieve.

I set all my Neostats to 30c. Every stat called for heat. I ran my Ecodan with weather compensation on and had a flow rate of around 45c to start and as the rooms warmed up, I lowered the flow temp down until the rooms stopped warming up, but also didn't start cooling down. I think I ended up settling at around 35c, may have been 33c. I then let it continue running and hoped that the Ecodan would at some point, realise the house is at the flow temp and then go on tick over. 

But it didn't. It continued running at the 3KwH usage for every hour of the day until it got to about 10.30pm, I was up to 80 KwH usage for the day and I couldn't bear it no more for fear of the daily cost. So I reverted the stats back to the desired room temperature and backed it all out.

So I did and wanted to achieve everything you have done @Brooster, but I just couldn't stop my Ecodan from chugging through the electric! I could only dream of 3.57 on a cold day, probably nearer 2! I just don't know why my Ecodan continues to run madly when the house has reached the desired temp relative to the flow rate temp I set it at.

 

PS. I note you have a 14KW Ecodan for a 3 bed bungalow, I have often wondered if mine is undersized at 8KW but it was professionally spec'd out when I built the house, albeit the suppliers have now gone bust.

 

In this really cold weather, your ASHP will probably be defrosting every hour or so.  When it does, the flow temp will drop quite a bit.  You might find the ASHP is running flat out to try and recover from the defrosts and barely if ever reaching its target flow. This is my Ecodan defrosting. 

It's a 14kW and it's delivering over 10kW when it's heating the flow from about 25 deg back up to 40.  It's doing this every 60-80 minutes.

@kev-m I think your defrosting point is definitely something that is happening on mine. I didn't realise it was that happening but when watching the power usage during yesterday, I did notice a drop off in usage every so often, not sure how often it was. 

In fact, when the power dropped, I was hoping that the Ecodan had reached its 'time to tick over' point and that it was all going to be low usage from there on. But that drop off only lasted a few minutes and then bam, usage back up to the 3KW.

I just looked at my hourly chart and as you rightly point out, the flow rate drops during specific points.

These were my temperatures during the 'test' yesterday of running it 24/7 against flow rates. 

And this was my energy usage yesterday, albeit my whole house, not just the Ecodan. As you can see, minimum of 2.5k-3kw every house which was the Ecodan, anything above was general house usage. I abandoned trying the 24/7 strategy around 10pm. I have other stuff going on between 2am and 6am such as charging car and heating water as lower tarrif.

@mcwatson1974 Congratulations, you've found the heat loss of your house, it matches whatever heat output your 8kW ecodan was delivering when it was consuming 3kW...

That's why your ecodan didn't throttle down, it's return temps were in spec and it kept delivering heat into the property equal to the heat loss.

@hughf In that case, I don't like heat loss!! 😆 House was built only 5 years ago and insulated to death but I do have about 11 metres of bifolds going across the rear of the house and I don't think the fitting of them was great so think a lot of heat does escape out of them.

I was hoping someone was going to say, click this, or switch that, and your ashp will mellow when you get to your 21c! Ho hum. Never did like the winter!

Posted by: @hughf

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@mcwatson1974 Congratulations, you've found the heat loss of your house, it matches whatever heat output your 8kW ecodan was delivering when it was consuming 3kW...

That's why your ecodan didn't throttle down, it's return temps were in spec and it kept delivering heat into the property equal to the heat loss.

This is my thought exactly. I suspect many of us were running at or below design temps (4 degrees below in my case), so is it any wonder the ASHPs were maxed out? Remember @mcwatson1974 that your design heat loss is the heat that your house is losing constantly, so if your heat loss is 7kW, then your ASHP needs to provide 7kW constantly, and with it being so cold outside, it's quite reasonable that it is using 3kW of electricity to do so. All that means that you'll use 3kWh every hour of electricity.

I suspect if you left it like this then once the weather calms down you'll find it being much more economical given the double whammy with ASHPs of more heat needed for the house and less heat available in the air when it's cold. You might want to consider a bit of a set back flow temp during some of the night if your controller allows this, since given  the heat pump will heat the house back up in the morning and you could save a bit of energy.

Posted by: @brooster

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Yesterday when we were -10 degrees overnight and -3 at best during the day we used just over 4KW with a COP of 3.57! On the warm end of the curve I have it set at 35 at -5 and 25 at 17 degrees which is a very shallow curve. I did originally use rad stats for unused rooms (effectively zoning those rooms) but no longer do that and have all the TRVs open at max.

@brooster When you say you used just over 4kW with a COP of 3.57 I presume your total electricity consumption was around 100kWh for 24 hours, not 4kWh! I wondered if @mcwatson1974 might have misunderstood you. Even so a COP of 3.57 is impressive.

Posted by: @mcwatson1974

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@hughf In that case, I don't like heat loss!! 😆 House was built only 5 years ago and insulated to death but I do have about 11 metres of bifolds going across the rear of the house and I don't think the fitting of them was great so think a lot of heat does escape out of them.

I was hoping someone was going to say, click this, or switch that, and your ashp will mellow when you get to your 21c! Ho hum. Never did like the winter!

Do you have the heat loss calculations for your home? Do you have any form of buffer tank or low loss header in your system?

Try dropping the temperature setting from 21C down to 20C and see what happens.