Posted by: @hr_3

Hi All,

 

I thought my unit was in weather compensation mode, but was in fact in fixed temperature mode (at 55c, getting about 3.65 cop for life of unit since October 2021). I had recently turned the fixed temp down to 50c from 55c and saw it move to 3.9 cop daily with this change and was fairly happy with this.

 

However, have now found out how to turn this off and change to weather compensation. It is much, much slower at bringing the house up to temperature, I assume I need to tweak the length of time it is on for? 

 

E.g. before, it would come on at 7am, be off by 12-2pm on most days, maybe come back on at 5pm before going off again at 7pm for the night (I ran the cycle for 12 hours)

 

Now I think it could be on much longer, but I assume it would be using less energy? How do others run their units with weather compensation? Should I run it for say 5am to 9pm? We aren't keen on really hot bedroom etc, so maybe turn the TRV down in those rooms?

 

Oh also could someone post their flow temps for the following temps with their weather compensation on?

 

10c    32.8c

5c      36.7c

0c      40.7c

-5c     43c

-10c    43c

My WC curve is 43 at -3 and 25 at 20.  This gives values as above.  I haven't tested it below ab out -2c.

The most important thing is that you are comfortable and the house is at the temperatures throughout the day and night that you want.  The next most important thing is to do this using as little energy as possible. Usually, using as low a flow temperature as you can get away will satisfy this second point.

If you are using weather compensation and you let the house temperature drop, it will take longer to warm up the house than if you have a higher set temperature. So if you want a constant(ish) temperature with weather compensation you will have to have the heating on for longer.  Whether this will use less energy is difficult to say.  It might because the COP will probably be higher but it might not because the heating will be running longer.  It's a bit of a balancing act.  

If I were you I’d want to try weather compensation.  We run our ASHP from 5am to midnight by using the timer to switch the heating off.  The temp drops 2 or 3 degrees overnight and recovers through the morning.  If the bedrooms get too hot we turn the TRVs down.  

Your COP sounds good anyway.  What ASHP do you have?  I assume you have radiators with a flow of 50-55?  

Hi Kev,

thanks for your input (I had been re-reading your thread).

Heat pump is a Steibel Eltron WPL17 8.5KW unit, with rads all fitted at the time with the ASHP. Yes flow 50-55 on them. 

I'm on my second day of weather compensation and the readout from the ashp is saying 5.3 COP (12kw in / 65kw out) but it did take most of the day to get to the desired temp whereas like I said before it didn't. Though the usage is quite a bit lower so from an energy saving side this looks promising.

I've tweaked the system to be on now from 5am - 10pm and then it goes into an 'eco' state overnight so only kicks back in if temp drops below 16.5c in the house (which it typically never does unless very cold outside).

Posted by: @hr_3

thanks Alec and hpn for the info,

 

The heat pump unit, well I have a number of units but I am guessing the main controller is what you speak of? I have a wall thermostat or wpm which is in the hallway upstairs / the main controller unit is in the loft

here is pdf of what I have

STIEBEL-ELTRON.CO.UK "https://www.stiebel-eltron.co.uk/en/products-solutions/renewables/controller_energymanagement/wpm_system/fet/technical-data.product.pdf"

it seems to be a thermostat/ controller unit?

pg. 13 of below is in English

https://www.stiebel-eltron.co.uk/content/dam/ste/cdbassets/current/bedienungs-_u_installationsanleitungen/FET__e1064f33-a10e-4db2-ad4a-8c8b54b588f1.pdf

Alec - are you saying it is better I do not use wc

 

 

Heat pumps and any other controller that is not from the same manufacturer is an unhappy mix, and best avoided. Zoning with heat pumps compromises performance of heat pumps. Only use heat pump controller with weather compensation. the lomger the runtime, the lower the curve and the higher the CoP

Posted by: @alec-morrow

Posted by: @hr_3

thanks Alec and hpn for the info,

 

The heat pump unit, well I have a number of units but I am guessing the main controller is what you speak of? I have a wall thermostat or wpm which is in the hallway upstairs / the main controller unit is in the loft

here is pdf of what I have

STIEBEL-ELTRON.CO.UK "https://www.stiebel-eltron.co.uk/en/products-solutions/renewables/controller_energymanagement/wpm_system/fet/technical-data.product.pdf"

it seems to be a thermostat/ controller unit?

pg. 13 of below is in English

STIEBEL-ELTRON.CO.UK "https://www.stiebel-eltron.co.uk/content/dam/ste/cdbassets/current/bedienungs-_u_installationsanleitungen/FET__e1064f33-a10e-4db2-ad4a-8c8b54b588f1.pdf"

Alec - are you saying it is better I do not use wc

 

 

Heat pumps and any other controller that is not from the same manufacturer is an unhappy mix, and best avoided. Zoning with heat pumps compromises performance of heat pumps. Only use heat pump controller with weather compensation. the lomger the runtime, the lower the curve and the higher the CoP

Hi Alec,

sorry not sure I follow

my unit is Steibel Eltron (all of it) including linked above - sorry if I was unclear.

only use the steiner Eltron controllers..

if there are more room control units  set them to be on permemntly at a high temperature. They do not feedback to the heat pump, thus enabling optimum efficiencies.

@alec-morrow Are you familiar with Daikin Altherma 3 controllers ? 

Jump to 6:00 (if you can't be bothered to watch the whole thing - its worth a watch if you are bored).

Fit and forget seems to be the approach for some installers that just want the system to work and not have any call backs regardless of running efficiency and costs.

We sat down with Dr. Karolis Petruskevicius, founder of Homely, and he explained the importance of weather compensation and the role it can play in making heat pumps efficient.

Really the benefit of all compensation controls is the force heat generators to operate at higher and highest efficiency. It’s the temperature stable state of the building that enables this, you don’t need the full output of a radiator when the room is at target temperature.

Stabke state is when thermodynamically interesting things happen in the building as it becomes a thermal store. The building mass always trying to equalise with the air temperature.

where you have both indoor and outdoor sensor on a heat pump or a boiler, the flow temp is calculated by looking at both temperatures.

Outdoor temperature drops and the flow temp rises before the thermal mass cools, as surely as it must

Anecdotal research shows gas savings of 10-15% on condensing boilers and 20-25% on heat pumps

@editor

Apparently our present government don't think that it is important for people to reduce their energy consumption, just to use it at a different part of the day. I suppose that philosophy will help keep up gas consumption and hence wholesale prices, thereby not only feeding more into the coffers of the oil and gas companies, but also the renewable electricity generators. Probably good for tory party funds.

Its more sinister than that.. the people who actually keep standards low are the manufacturers themselves. The civil service defer to experts who come from said manufacturers who are all outside the UK. They don't want the training cost and dont mind if the UK misintstall their better technology and keep buying outdated controls like honeywell and heat miser.

Any way our "collective" stupidity has enabled the R&D of heat pumps and all other technology...so its not all bad (unless you are trying to make sense of a new heat pump)

Posted by: @alec-morrow

Stabke state is when thermodynamically interesting things happen in the building as it becomes a thermal store. The building mass always trying to equalise with the air temperature.

where you have both indoor and outdoor sensor on a heat pump or a boiler, the flow temp is calculated by looking at both temperatures.

I still feel that weather compensation as usually implemented is only a first step, a first-order approach looking at only the outdoor temperature and trying to maintain a target indoor air temperature. A flaw with the approach of raising flow temperature as the outdoor temperature falls is that air-source heat pump efficiency falls with both increasing flow temperatures and decreasing outdoor temperatures: a double whammy.

More savings could be made by looking at the forecast and "charging up" the thermal stores of a building now if it's going to get colder later, and by depleting the thermal stores if it's going to get warmer later which will make it easier to recharge the thermal store and warm the air temperature back up. A sort of second-order weather compensation which considers the rate/direction of change of temperature as well as the current temperature.

Then even more savings may be possible by considering the expected cost of the energy driving the heat pump, whether through something like an Agile tariff, predicting solar PV output or even good old Economy 7.

I'm not sure much work is being done on this yet. The adaptive/"smart" learning controls I've seen seem aimed at learning more precisely how fast the building loses heat and warms back up, with the aim of more precise target temperature tracking, rather than minimising the cost of keeping the temperature in some acceptable range.

So many homes are still on simplistic on/off all-or-nothing controls, I suspect it's probably more commercially beneficial to spread the simple weather compensation controls first, even if imperfect. I shouldn't let perfect be the enemy of good, I guess!