Posted by: @editor

@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?

Hi Mars,

The required heat curve is mainly dependent upon the heat loss of the property, and the type and size of the heat emitters. It is basically trying to match the heat energy supply, to the heat energy demand or heat loss, at varying outdoor air temperatures, and thereby maintaining a reasonably constant indoor temperature.

I would suspect that most properties in the UK would fall in the 0.8 to 1.2 range, in that for each 1C change in outdoor air temperature, the WFT needs to be increased or decreased by somewhere in the region of 0.8C to 1.2C for your particular home. A good starting point would probably be to set the curve to a WFT of 25C at 20C ambient and 50C WFT at -5C ambient. That gives a slope of 1, 1C change in WFT for each 1C change in outdoor temperature.

Auto adaptation or modulation control has now been incorporated in some systems, to add to, or subtract from, the weather compensation curve, if the indoor temperature deviates from the desired setpoint. But these only work correctly if the system is correctly installed, configured and optimised.

I have not researched Homely in detail, but I do believe that it also utilises weather forecasts, and Time Of Use (TOU) tariffs, to help minimise costs, but not necessarily use, since it could be encouraging the heat pump to run at times when it is not as efficient.

Whilst I am a firm believer in utilising modern technology to help minimise our energy consumption, I am not yet convinced that adding additional complexity to replace a capability that is already installed within the system is the way forward. A further concern would be what happens to your heating system if you lose your internet connection?

@derek-m, that’s a terrific explanation. Thank you. We’ve put ours up to 1.8, and that’s been very comfortable throughout the property. Not cold. Not hot. I’ll drop it to 1.2 tomorrow and see the difference.

At 1.8, our flow temperature has been between 32-37C.

Posted by: @editor

@derek-m, that’s a terrific explanation. Thank you. We’ve put ours up to 1.8, and that’s been very comfortable throughout the property. Not cold. Not hot. I’ll drop it to 1.2 tomorrow and see the difference.

At 1.8, our flow temperature has been between 32-37C.

Hi Mars, what controller is this, where in the house is it, and what other controls are there on the heat pump?

@alec-morrow, this is precisely where our weather compensation issues stem from. The installers never activated weather compensation for us when the pump was put in, so we ran our ASHP for years at set point temperatures (40 or 45C depending on how cold it was). The more we learned, we realised weather compensation was essential from an efficiency perspective so we contacted the installers and asked them why they hadn't activated the weather compensation – part of the issue was the lazy install.

They slapped the main controller for the heat pump next to the hot water cylinder in our "heating cupboard". At any given point in time it's over 27C in that cupboard when the heating is on. So not ideal.

In addition to the main controller, we have a smart Salus thermostat to switch on the radiator circuit and wired in Heatmiser thermostats to switch on the UFH circuit. Both are usually set to 23C, as we know that with out set flow temperature of 40 or 45C, rooms will typically get to around 21C which is what we like. This also prevents the heat pump from cycling as we never come to temperature for rads or UFH in winter. 

It sounds like you have found a sweet spot

the numbers on a heat curve are broadly* the same across all manufacturers

think this:

Start point would be 20c outside 20c inside flow temp 20c

for every drop of one degree outside the flow temp rises 1c at a curve of 1.00

at a curve of 0.5, 0.5c

at a curve of 1.8c flow temp rises at 1.8c

Etc

* anti cycling measures mean you may not always be following the curve, and some curves are gently bent to over come thermal inertia in the system and house

no hard and fast rule 

@alec-morrow, it's a sweet-ish point, but probably still not as efficient as it should be. But that's something we'll have to live with.

I've dropped it to 1.2 today, just to see the difference over 24 hours.

We decided to let our colder rooms be colder (they need insulating) rather than battle to heat them. 

Our curve is now 1.0 so our flow has been about 25-29 last week. The main rooms are 19c which is perfect for us. 

previously we probably ran the flow +10c hitter to just get two rooms up to temp. So they are closed off this winter then deep retrofit planned for spring!

@editor

Hi Mars,

Because your system has a range of fixed weather compensation curves it is not possible to fine tune the system to match your home's heat loss characteristics, by adjusting the slope with the end parameters. It may therefore be necessary to select different curves for different periods throughout the year.

I would suggest that you initially set your system for weather compensation and select curve 1. Increase any thermostats 1C or 2C above the desired indoor temperature, and monitor the operation of your system over several days, noting indoor temperature variations. If you find that you are not achieving the desired indoor temperature then select the next higher curve. Repeat the above until you achieve reasonably stable control.

I would be interested to see the results.

Posted by: @derek-m

A further concern would be what happens to your heating system if you lose your internet connection?

+1 to this. It is very often ignored by salespeople. A related question is: what happens to the heating system if the additional controller fails?

As you know, I'm happy having an external controller tweaking the Mitsubishi one's curve based on other factors (including weather and electricity price), but if it fails for any reason, the heat pump's own controller is always in a state which wouldn't get uncomfortable quickly if left to run. The house could get a bit nippy the morning after a failure, but there's never a risk of ice on the windows or people roasting hot in their beds! I'm a bit concerned that these on/off controllers might leave the heating off if it fails.

if internet connections fail the system runs to the transmitted parameters.. internet failures are rarely  excessively long.

And on the interface/room controller adjustments can be made…

Im always amazed by how people in the U.k. would rather find fault with what’s on offer rather than try and understand how things actually work.

@derek-m, that's the conclusion I've come to. It looks like 1.5 works for this time of year (8-15C ambient temperature). When we start to drop below 8C, that's when we'll switch to set points.

@grahamh has just posted this article on weather compensation and the Lizzie curve. It's very interesting, and we're finding that the 36-37C mark at 10-15C ambient is comfortable for us.

https://renewableheatinghub.co.uk/weather-compensation-the-lizzie-curve