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Weather compensation and air source heat pumps

Mars · 2022-01-12T15:37:14Z

There have been a number of posts on the forums about weather compensation and I've been getting a lot of comments and emails on the subject. I thought it would be a useful idea to have a thread dedicated entirely to weather compensation and I'd like to divert homeowners here. If anyone has the time and the inclination, it would be very helpful to briefly explain, in the simple terms, how the curves works and what they set about accomplishing in ASHP systems.

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Air Source Heat Pumps (ASHPs)

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Alec Morrow

(@alec-morrow)

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16/10/2022 10:20 pm

Posted by: @editor

I think, on paper, this sounds like a great and useful system. For me, I don’t understand why all big make heat pump manufacturers don’t have intelligent and highly effective weather compensation built in as standard.

They do…at least the more comprehensive manufacturers do.

Nithing new about weather compensation either, it was mandated on all heat generators in 1985 in Germany..

Too many interests exist to keep it out of the U.K. though!

Professional installer

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Mars

(@editor)

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Topic starter 18/10/2022 8:56 am

@derek-m, can you please refresh my memory which heat curve you’d suggested for us..

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Alec Morrow

(@alec-morrow)

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18/10/2022 9:11 am

there are no hard and fast answers..

as it’s depends on emitter size and type, thermal performance of the buidling as well as how long at the higher temerature time period of the controller

essentially the bigger the area of the emitter the lower the curve, that’s why floors have curves as low as 0.5, and convectors curves as high as 4.5

The curve is the rate of change of the flow temperature, a radiator which ha smaller heated area than a heated floor needs a faster rate of change to put the same amount of heat in the room..

A longer on period enables a lower curve, and lower flow temperatures too

Id start at 1.4. the controller will have micro adjustments, it may even auto adapt.

What brand is the controller, and what other control are there?

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Derek M

(@derek-m)

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18/10/2022 3:07 pm

@editor

Probably try 1 or 1.2 first and see how your system performs.

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Mars

(@editor)

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Topic starter 19/10/2022 9:42 am

@derek-m, the one aspect of weather compensation I’m grappling with is whether it’s just a heat curve that adjusts itself on outdoor temperatures only, or does it also take note of the indoor temperature.

In other words, for weather compensation to work on a system, does it need an accurate outdoor and indoor temperature reading?

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Alec Morrow

(@alec-morrow)

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19/10/2022 10:08 am

@editor some do some dont

gernan manufactures generally have it sorted with no indoor sensing, as the outdoor sensor is mandated in germany

people may like to ask how and why it works with only an outdoor sensor!

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batalto

(@batalto)

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19/10/2022 11:26 am

@alec-morrow I have tested this by logging my power use over last winter and also logging the average weather temperature outdoors. The line is more or less straight, with a slight curve at very low temps. You can see it in my signature. At the end of this month i'll start logging again. Currently the weather isn't cold enough to give anything meaningful - cold nights and warm days really mess with the averages

12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE

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Derek M

(@derek-m)

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19/10/2022 3:51 pm

Posted by: @editor

@derek-m, the one aspect of weather compensation I’m grappling with is whether it’s just a heat curve that adjusts itself on outdoor temperatures only, or does it also take note of the indoor temperature.

In other words, for weather compensation to work on a system, does it need an accurate outdoor and indoor temperature reading?

Hi Mars,

It is my understanding that WC sets the WFT dependent upon the programmed curve and the outside temperature. You may therefore get variations in the indoor temperature caused by solar gain, wind chill, rain effect and even human activity

Also having an indoor temperature sensor helps mitigate these factors by sensing any variation from the setpoint and slightly adjusting the required WFT. Obviously, it only accurately controls the temperature in the room in which the indoor sensor is located, so you may find South facing rooms are warmer due to solar gain and North facing rooms are cooler. This is where balancing of the heat emitters may be needed to even out some of the temperature differences, or use of TRV's.

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Benseb

(@benseb)

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19/10/2022 6:05 pm

Quick Q

I’ve tweaked our WC curve. It’s been running at between 25-39c this week.

today for example it’s 11c outside and running at 26c flow. 5c delta.

however the heating is only running for about 60% of the time.

Is that expected for a “low and slow” run? Or should it be running more?

Is that just down to it still being fairly warm-ish outside?

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.

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Alec Morrow

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19/10/2022 6:17 pm

@benseb

assuming there are no other controls, the thing to do is to monitor room temperatures, not what the heat pump is doing...afterall thats why you have heating.. to have warm rooms when you want them...

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Derek M

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19/10/2022 6:28 pm

Hi Benseb,

Just as a car engine has a minimum speed, so does a heat pump. What is probably happening is that your thermostat, if you have one installed, is switching off your heat pump, or if you are using weather compensation, the controller is switching off the heat pump because there is insufficient load, even at minimum speed, and the return water temperature is increasing and reducing the Delta-T.

As Alec suggests, monitor the actual indoor temperature along with the other temperature readings.

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Mars

(@editor)

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Topic starter 23/10/2022 10:22 pm

@derek-m, you’ve read through a lot manuals and looked at weather compensation graphs from different manufacturers. Are the heat curves usually quite similar?

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