You could just modify your curve down to -5 or beyond? I've also got an AC unit which can heat, so worst case I can use that for a blast of warm air

Posted by: @benseb

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most people setting Weather Comp stop at the design temp. Eg 45c at -2c then leave it flat so it’s 45c so matter how cold it gets. 

Surely we should let it increase as it gets colder? Eg 50c at -5c

Not everyone has the headroom to do this. If a system is designed quite tightly such that at -2C ambient and 45C flow is almost as much as a heat pump can output, then increasing the flow temperature decreases output at the same time as decreasing ambient temperature decreases the available heat, so the heat pump would actually put out less heat (or possibly throw an error?). My system should reach this point at about -5C I think, although I haven't calculated it exactly.

I wonder if its worth flipping the dips on my unit to upgrade it to a 14 or 16kw unit. Not sure how much this might impact efficiency if its still running at low flow temps

Posted by: @batalto

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Not sure how much this might impact efficiency if its still running at low flow temps

My hope, if I ever need to do this, is that it will increase energy use and so costs (obvs, 14 => 16kW), but it should not adversely affect COP (more energy in, but also more energy out), so COP stays the same. What I am less sure about is how that extra energy gets delivered, if it means by way of a higher LWT, ie we need to increase the LWT to get the extra heat, then COP will go down. I think. 

Weather Compensation set to OEM standard, and ASHP left running 24/7. First really cold test for the Grant Aerona 10kW. -4degC air = +45degC water. A bit of 'suck it and see'. One of my first lessons learned was to leave it running overnight on a low heat demand. I tried a cold start with system volume at room temperature and outside air at freezing on a foggy morning. The ASHP spent all morning in multiple defrost cycles as the large initial system heat demand was sending the evaporator temperature too low. That was causing the coil to ice up much quicker than if the ASHP had been steadily chugging away overnight on a low output. Secondary heat is via a 5kW log burner if I need to fairly quickly boost the indoor temperature. Last resort is the 3kW back-up immersion heater. That's only really intended for emergency system frost protection plus bare minimum space heating if the ASHP ever fails. It's not useable space heating.

I am giving set back a go tonight. As I have 7.5p/kwh from 00:30 - 04:30 I will leave the heating on as normal. Then I want to set back for a small period to save some battery to extend into the day. Currently when its cloudy my batteries are dying around 12. Today I am still on battery due to the sun, so might make it most of the day without touching peak grid. Dropping that 1.5hrs should save me 3 or 4kw of battery storage.

00:00 - 04:30 - 21 degrees

04:30 - 06:00 - 19 degrees

06:00 - 24:00 - 21 degrees

If this works I might reduce down to 20 degrees at 10pm or something to lower my off peak demand. Its always warmer upstairs anyway

Well, I guess I'll be getting to see whether the ecodan's databook claim of still producing 55°c water at -10°c ambient is anything like true. It appears that I'd already set the curve to cover this depth of cold!

Posted by: @batalto

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I am giving set back a go tonight. As I have 7.5p/kwh from 00:30 - 04:30 I will leave the heating on as normal. Then I want to set back for a small period to save some battery to extend into the day. Currently when its cloudy my batteries are dying around 12. Today I am still on battery due to the sun, so might make it most of the day without touching peak grid. Dropping that 1.5hrs should save me 3 or 4kw of battery storage.

00:00 - 04:30 - 21 degrees

04:30 - 06:00 - 19 degrees

06:00 - 24:00 - 21 degrees

If this works I might reduce down to 20 degrees at 10pm or something to lower my off peak demand. Its always warmer upstairs anyway

Have you considered running your heat pump from the mains supply, rather than the batteries, during the warmer part of the day when the heat pump should be more efficient, and saving some battery power to help run your heat pump during the colder peak period, when the heat pump is not as efficient.

In doing so it may save a little money and also help out the grid.