Heat pump one year stats (2023 Update)

[Theo]

Just over a year ago we had a 13kW Grant air source heat pump installed. It's in a 1960s chalet bungalow with a very leaky loft conversion, 1990s cavity wall insulation, 100mm loft insulation, 2021 EPC D (the 2011 EPC was E with nothing changed since then).

The ASHP is doing all heating and hot water. It's separately metered, and the readings on the 1 year anniversary were:

ASHP 2867.5 kWh Immersion boost for legionella 46.56kWh [1] Total 2914 kWh

The ASHP doesn't have a heat meter, but in previous years we'd have used about 1200 litres of oil:

10.35kWh per litre => 1200*10.35 = 12420 kWh Assume oil boiler 85% efficient, 10557 kWh of heat

Effective SCOP: 10557/2914 = 3.62

I'm pretty happy with that.

Elec tariff: 35.06p/kWh Gas tariff: 10.31p/kWh (we aren't on mains gas)

Annual cost of elec at current rates: £1021 Hypothetical cost of gas boiler at 93% efficiency:

10557/0.93*10.31 = £1170

Oil: At ~80p/litre (currently): 1200*0.8 = £960 At 90p/litre (past months): £1080

So fairly competitive with both oil and mains gas, even on current tariffs.

In the current temperatures (-3.5C outside) the smart meter says the house is using about 45kWh per day. That includes a load of washing (hot water down the drain), tumble drying (in the garage) and 3D printing (garage) - on a summer's day like that we'd use 15kWh, so the net heating and hot water load is about 30kWh. The house is perfectly warm - the heat pump is managing it quite happily.

Theo

[1] They've installed the most annoying timer for legionella cycles and for various reasons often they don't run. I'm in the process of rewiring this, but for the meantime we haven't been doing a full weekly 60C boost for sanitisation.

[Tim+]

Interesting info about your consumption. Ta.

Regarding the HW, is the legionella cycle done using an immersion heater or by the heat pump? If the former, a simple Wi-Fi switch that lets you set schedules would seem a simple solution.

Tim

[Theo]

It's an immersion, but ideally it should be synchronised with the HP. So the HP brings the water up to the max temp it can do (ours is 55C) and then the immersion bumps it up to 60C. That way the HP is doing most of the work and the expensive immersion is only used for a short while.

Unfortunately the tiny DIN rail timer they've installed at the inaccessible back of the airing cupboard is interlocked with the HP (don't run the immersion unless the HP is also enabled), but we tend to run hot water on an ad hoc basis and often miss the timer slots. The HP can itself trigger its own legionella cycles, so I'm rewiring it to do that (via an extra contactor, but I also need to put in a tank thermistor so it can tell the temperature) - I'm not going to mess with it until the weather warms up and it can be off for a while.

Theo

[charles]

you don't want to get Legionella. I had it - not nice nor good for one.

[John J]

Doesn't the Grant control system permit you to schedule hot water cylinder charging during the early hours while you're tucked up in bed asleep. It's a simple matter to programme the immersion heater to boost the water temperature at the end of the heat pump hot water cycle. My Vaillant 15kW along with increased size radiators was installed a couple of years ago by a so called professional company (to get the RHI payment). Turned out they were far from it! They insisted the cylinder had to be boosted by an immersion heater once a week which was no problem as I had previously installed a 7 day time control and immersion heater as backup when I installed the twin coil cylinder in conjunction with a solar collector about ten years ago. During most of lockdown and beyond we suffered abysmal heating performance with radiators rarely exceeding 35 degrees and not getting any sense at all out of the installers. In disgust I recently contacted Vaillant directly and booked a service by them. Their Engineer found a myriad of incorrect installer settings which explained the 35 degree radiators (design temp for correct output was 45 degrees) anti legionella set for tea time on Mondays instead of 2am which meant the hot water preference meant the radiators were off for an hour at tea time on Mondays, Then to cap it all he asked why the heat pump legionella cycle temperature was only 50 degrees. Apparently the unit is capable of 60+ Suffice it to say my radiators now emit sufficient heat to keep the house comfortable and the anti legionella cycle is controlled by the AHSP system controls. I'm not expecting the extra heat into the house to be free but it will be interesting to see how the next twelve months of operation goes now the system is performing as required.

[Andrew]

13Kw Input power or nominal output power ?.

Dividing your total HP electricity consumption by 13 gives 220 hours of use, which is difficult to believe for an entire year. Since the efficiency varies according to the weather, and the 13Kw figure is output power, how do they calculate it ?.

If your loft has been converted then surely loft insulation is no longer appropriate since the conversion should have been insulated to the standards applicable at the time (not stated), but if it was built as a chalet bungalow then there would not be a 'loft' to convert anyway. Something is missing here.

[Andy Burns]

heat pump manufacturers (all?) quote in terms of output

you need to also divide by the annualised CoP aka SCoP

e.g. vaillant show a table of CoP for their 12kW ASHP, as you'd expect it varies depending on outdoor temp and flow temp required

between 3.6 and 4.8

[Theo]

Output power. Input is rated at 21A, but in reality is typically 3-4kW when running, although it modulates down based on flow temp. So a COP of about 3-4.

It was done in 1970s but only about half is living space. There are crawl spaces at front and back, having the rear of the upstairs walls and the ceiling of the downstairs ones. The insulation isn't good here (100-120mm horizontal rockwool between joists, 80mm vertical rockwool batts held in with string) but access is a PITA to improve matters.

Theo

[Theo]

There's no Grant control system, just a standard central heating timer (Honeywell Lyric, allegedly 'smart' but very dumb). Which is unsuitable for a heatpump, hence I'm building my own. It doesn't control the immersion, that's on the DIN rail timer.

(I prefer to run hot water in the daytime when there's more ambient heat, rather than the coldest part of the night)

That's roughly my experience: installer a bunch of numpties, did lots of things wrong. Good thing I had done my homework and knew what the heatpump is capable of, even if Grant disable most of the features. The unit (made by Chofu of Japan) is very good, Grant just cripple it.

I did consider a DIY install, but think it would have been too big a job for me. There's still been quite a bit to clear up after them.

Theo

[Andrew]

"Why R290? R290 is a natural refrigerant with a very low GWP* of three. This offers the following advantages: • future-proof, as not affected by the F-Gas Regulation" <--- what does this mean ?. Can they be diy'd ???.

[Andy Burns]

sounds like it, cheaper too?

but presumably you can't get the Boiler Ugrade Scheme grant if you don't use an installer who's a member of the right "club"?

[Theo]

R290 is propane, which is not a fluorinated refrigerant and hence isn't covered by the regulations covering fluorinated gases (F-Gas). That means you don't need an F-Gas ticket to install, and could DIY. They're also easier to repair should there be a leak.

Many heatpumps, though, are monobloc which means you only need to do water and not refrigerant pipework. They are precharged with refrigerant at the factory, much like a fridge is.

The downside with propane is it's flammable, which makes installations more awkward (in particular, in split aircon systems where there's an indoor unit, you can't install in small rooms because of the dangers of flammable gas buildup if there's a leak).

Theo

[Andy Burns]

It's probably in your fridge? And it seems that refrigerant-grade propane has no oderant added.

[Theo]

Depends on the fridge, but a heatpump or a/c unit has rather more refrigerant than a fridge I'd have thought?

Hence the greater volume of room you need to dilute it if it escapes.

Not R290, but this doc has an example of a 100W fridge having 0.05kg of R134a, and suggests a crude metric is the compressor kW = refrigerant kg, which would make a 10kW heatpump with a COP=4 have 2.5kg of refrigerant.

An A/C unit would be a similar kind of calculation, depending on how much cooling it provides.

Theo

[Fredxx]

I'm not sure how things scale. Less than 0.15kg of propane is the threshold, after which room size, and protection mechanisms then come into play.

It's also on a per system basis, so if you have multiple compressors driving the same room, refrigerant volume is based on a single unit. On the basis you don't normally have simultaneous multiple failures.

[John J]

Are you suggesting other refrigerants are not flammable?

[Fredxx]

Actually most aren't. Freons have been discredited but some are still allowed, so we resort to hydrocarbons.

[Theo]

Correct. I think it's mostly propane (R290), butane (R600) and pentane (R601) that are flammable, most popular chlroinated/fluorinated gases aren't flammable.

In this table:

the ones in safety group A3 or B3 are particularly flammable.

Theo

[Alex Leon]

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[Steve Jones]

Just a question, Did you have to remove the Radiators and use underfloor heating or where the current radiators up to the job?