Electric combi boilers / solar panels / eco heating solutions

How's everyone's heat pump doing in this cold snap?

We hit -2C outdoor temp overnight, which is the design temperature which our heat loss calcs were based on, with the rads and UFH specified to 45C flow temp at -2C outdoor temp.

In reality, the heat pump only hit 38C max flow temp overnight, and the entire house remained at 21.5C throughout.

One thing which isn't really talked about often is the defrost cycle; my wife was working from home yesterday and was freaked out when she saw a massive plume of steam billowing out of the heat pump, thinking it was about to detonate...

It's defrosting itself pretty often right now, which makes sense as UK british weather is so clammy the whole time, but this is rarely factored in to manufacturers' specs, system design calcs or running cost estimations (defrost cycles harvest energy from inside your house, meaning a negative CoP). Different story on the continent where it's generally drier...

How is it affecting CoP? Defrost cycles are pretty power hungry, right?

CoP is negative during a defrost cycle, as it uses heat from the water in your rads/UFH.

CoP for the last 2 days is 4.0, compared to 6.1 last week.

I'm guessing the 21.5 felt pretty comfortable? What's the plan if it drops to -10 later in winter.

4.0 is still pretty great. I assume you're running basically 24/7 at that temp? What's it pulling per day in kW?

I'm expecting a SCoP for the year of near 5, based on the stats so far.

Comparison with gas boiler running costs: if you take the average condensing boiler, which rarely actually runs at condensing temps, you get an efficiency of maybe 80%, meaning a heat pump with a SCoP of 4.8 is six times more efficient in this scenario.

If you run this heat pump using a tariff such as OVO's ASHP one which gives you all ASHP use at 15p/kWh, divide that by 4.8, you get an effective heat pump running cost of 3.15p/kWh of useful energy produced, relative to a gas boiler in the same house which would cost 6.25p/kWh of useful energy produced on a cheap Octopus Tracker tariff. So, ASHP in this case would be half the cost of a gas boiler to run in an identical house.

Sounds great.

What's your actual daily kW usage for it though? Asking as I have the 20kWh battery and wondering how much will get eaten by that.

Yes, running 24/7 using pure weather compensation (no thermostats at all, no timers or schedules, it's just fully on the entire time). The whole house, every room, stays at a constant 21.5-22C. This is too hot, but the house needs to dry out for a while after loads of building work.

Yesterday over 24 hours, average outdoor temp was 2C, heat pump used 28kWh to keep the house at 22C the whole time, which cost £4.20.

Last week, average outdoor temp was 10C, heat pump used an average of 10kWh/day to maintain average indoor temp of 22.5C, cost £1.50/day.

That's some great numbers

When was the last time it dropped that low in London? That's a once a decade phenomenon no?

Usage will drop once I reduce the weather compensation curve to keep the house at a more sensible 20.5C instead of 22C, after the house dries out in a while.

Thanks - that's great info.

I've got 19.9kWh (18 usable really with 10% SoC reserved) which I top up at 7p overnight. On average over the winter, I'm using about 30-40% of the battery, leaving about 11kWh so I could get away with just using the battery on most days which would mean a daily cost to heat of 70p.

Octopus is 28p per kWh in peak (5.30-23.30) so I would assume I would probably use, on really bad days, maybe 20kWh peak which would be £5.80 but I assume less as there's time overnight which I'd get the lower rate.

There are plenty of examples of people with big houses and ASHP/PV/battery who end up with negative yearly energy bills, after exporting all PV at 12p/kWh, charging at 7p, dumping excess battery capacity in summer at 12p etc.

You'd need to dork out to configure and optimise all of this, but free/negative energy bills aren't to be sniffed at.

I’m up north (Leicestershire) so was that cold last year briefly if I remember correctly. But yes a bit of an extreme example

Yeah, I have two EVs so it's never going to be negative but, so far this year, I'm on £480 electric used and £611 exported with £350 in gas, or -£219 all in.

That's with about 18,000 miles driven and wfh for two people most days.

Daikin Altherma 3 heatpump based on R32 refrigerant, which in colder weather will need to use the immersion element to assist with getting your hot water up to temp

Our Samsung uses R32 and I've never used the immersion (apart from bug killing cycle)

Saw a defrost cycle for the first time yesterday, was quite cool!

Nice! Which hot water target temp have you set?

45 degrees. The cylinder is a bit small at 180l but for two of us it’s just about right.

My wife had a long shower this morning then I got a couple of degrees less than piping hot one, so we’re just on the edge, but optimised for our use case. Cylinder was at 16 degrees after both showers, but I guess the water at the top is still around 35

Ah, that makes sense.

Our 250l cylinder with storage at 50C would not be able to be fully heated via an R32 heat pump within the 2hr 14:00-16:00 window without using the immersion.

Ours seems to heat up really quick, think it has a big coil relative to volume. 16º to 46º in an hour at 10:30 this morning.

In other news, since the cold snap I've been reminded how loud our HP is. I suspect it's vibrating through the house due to being wall mounted. Right now it's louder inside than it is outside.

Going to mount it on four of these fun looking springy guys. If data sheets are to be believed it should make a significant difference.

Check how level it is; the bearings in most ASHP fan motors are extremely sensitive to operating position and will wear very quickly if not mounted perfectly.

Good call. I'll re-level after installing these anyway

They look fancy