I bought a Heat Pump

shinytop

Reed_Richards said:

I have been simultaneously massively impressed and a little horrified by @shinytop's method of controlling their heat pump using finely-tuned weather compensation and abandoning the use of a room thermostat.  This is not something that Mr and Mrs Average would either want or need to do ( @Cardew ) but it's probably the right way to eke out every last drop of efficiency that a heat pump can provide.

It has inspired me to be more ambitious with my weather compensation settings.  I now have

Outdoor temperature:           Min 1 max 19
Leaving Water Temperature: Max 50 Min 26

50 C is the maximum LWT specified on my MCS certificate and this target temperature will be used if the outside temperature is 1 C or lower.
If the outside temperature is 19 C or higher the target LWT would be 26 C.
Otherwise there is a linear relationship so if the outside temperature is 10 C (half way between 1 & 19) then the target LWT would be 38 C (half way between 50 & 26).   So far this has proven to be sufficient to keep the house warm with the outside temperature at 8 C or below.  It means that the target Leaving Water Temperature is lower than it used to be which, in principle, should give me greater efficiency.  It's hard to be sure if this is working because the outside temperature is never constant.  But the peak power per cycle of the heat pump seems to have dropped.       
      

I've been experimenting with running my ASHP more like a boiler and the results are in.  With weather compensation, I switch the thermostats up to 35C and let the automatically adjusted flow temperature control the house temperature.  I have a curve that keeps the house at 21C or so between (so far) 0 and 15C.  With the boiler method, I set the room thermostats to 21C and the flow to a constant 45C.  This is what happened.  Red is boiler-like, blue is weather compensation. The trend lines are exponential, both with R2 >90. 



For those interested, here is the detail on two days, one boiler and one weather compensation. In both cases, the heating is off 12-5am.  Although not shown, as well as using a bit less energy, the weather compensation method keeps the room temperature more constant.

Meatballs

I think the "boiler" approach would be to leave the house for a few hours or working day with the heating completely off and compare the total energy usage.

Even a boiler should be run heavily weather compensated and as low as possible if you are maintaining temperature all day.

shinytop

Meatballs said:

I think the "boiler" approach would be to leave the house for a few hours or working day with the heating completely off and compare the total energy usage.

Even a boiler should be run heavily weather compensated and as low as possible if you are maintaining temperature all day.

I did. It's off for 5 hours 12-5 am.  

Verdigris

I know I shall be designing my system to run at 35 degrees. Just looking at those graphs the "little and often" approach is far more efficient and kinder to the heat-pump, too. It will also increase the amount of battery-stored electricity you can use, becauser the load is more likely to stay withing the battery's discharge limits.

Meatballs

shinytop said:

Meatballs said:

I think the "boiler" approach would be to leave the house for a few hours or working day with the heating completely off and compare the total energy usage.

Even a boiler should be run heavily weather compensated and as low as possible if you are maintaining temperature all day.

I did. It's off for 5 hours 12-5 am.  

Well both examples are off...

What's was the total energy usage in both scenarios from 5-6am (the recovery periods - assuming they recovered the temperature in an hour?) and what was the energy usage for a standard period of 5 hours when they are on? 

shinytop

Meatballs said:

shinytop said:

Meatballs said:

I think the "boiler" approach would be to leave the house for a few hours or working day with the heating completely off and compare the total energy usage.

Even a boiler should be run heavily weather compensated and as low as possible if you are maintaining temperature all day.

I did. It's off for 5 hours 12-5 am.  

Well both examples are off...

What's was the total energy usage in both scenarios from 5-6am (the recovery periods - assuming they recovered the temperature in an hour?) and what was the energy usage for a standard period of 5 hours when they are on? 

They are both off because I'm comparing like for like.  But this is adding some more weather compensation numbers (in yellow) when I was running it 24/7. Still uses less energy even when running for an extra 5 hours.  

For first 3 hours of each of these days shown in the previous post, the thermostat approach was 10.1 kWh; the slow and steady 5.5. It takes a lot of energy to heat all the water from about 17C to 45C. The house does warm up a bit quicker with the thermostats when the heating has been off all night.

The last 8 hours are 15.6 thermostat and 12.3 slow and steady.  

Reed_Richards

Thanks to @shinytop, great information, very interesting.  The thing I don't understand is that at some low outside temperature shouldn't 45 C be the temperature that the weather compensation algorithm chooses?  The chosen temperature has got to go up as the outside temperature goes down so I would have expected the two trend lines to converge at low temperatures whereas they seem to diverge across the range.  

shinytop

Yes but the highest flow temp reached on weather compensation above was 38C, so nowhere near 45.  My house won't need 45C constantly to maintain 21C inside until it's -7C outside. The ASHP will then be using about 4kWh per hour so I hope that doesn't happen very often.  

I think the divergence seen is at least partly because the ASHP is less efficient when it's working hard, which it does when its target flow is a long way below the actual flow.  

Reed_Richards

I think all ASHP owners fervently hope that spells of really cold weather are few and far between, not because their heat pump could not cope but because coping is disproportionately more costly than it would be with a boiler.

But @shinytop, did you chose 45 C for your "boiler mode" test because that is your MCS certified maximum temperature?  Given that you know that you only need 38 C to maintain a constant temperature (when its above freezing outside) but "boiler mode" may require you to raise the temperature rather than keep it steady I would have been tempted to set something like 40 C, i.e just a little bit more than 38 C.  Obviously you would need to increase that if it did get below freezing outside but it would not be too terrible. 

What I am doing at the moment is tweaking down the low end of my weather compensation line to see:

1. If I can still control the temperature enough.
2. If I can perceive any benefit in terms of power used for heating.   

shinytop

Reed_Richards said:

I think all ASHP owners fervently hope that spells of really cold weather are few and far between, not because their heat pump could not cope but because coping is disproportionately more costly than it would be with a boiler.

But @shinytop, did you chose 45 C for your "boiler mode" test because that is your MCS certified maximum temperature?  Given that you know that you only need 38 C to maintain a constant temperature (when its above freezing outside) but "boiler mode" may require you to raise the temperature rather than keep it steady I would have been tempted to set something like 40 C, i.e just a little bit more than 38 C.  Obviously you would need to increase that if it did get below freezing outside but it would not be too terrible. 

What I am doing at the moment is tweaking down the low end of my weather compensation line to see:

1. If I can still control the temperature enough.
2. If I can perceive any benefit in terms of power used for heating.   

50C is the design temperature at -3.2C; I chose 45 because I thought 50C would be too expensive!!  What you suggest may be my next experiment; to run the system with the thermostats and with the flow temperature a little bit more than it needs to be.  I can do what you suggest and go for a constant 40C or go with weather compensation and offset the curve a few degrees upwards (very easy to do with my controller)