1970's 4 bed detached house with loft and cavity insulation in central southern England heated by a 7kW Vaillant Arotherm+. 508kWh used in January according to the Vaillant app, but it was helped by lighting the woodburner on 6 evenings for a few hours (more for the comfort of the heat source than an inadequate heat pump). Almost all the electricity on the Octoput Go tariff either directly or via battery so the approx energy cost is about £50 for the month.

January 2025 was our first month with ASHP. 

In 2024 we spent £3675 on LPG and electricity for the whole year. In January we spent £600 with ASHP.  I suppose starting in a cold month is a good test of the system.. but at this rate I can’t see us getting any return on our £15k investment! 
a conservative 7 year payback would mean our annual energy spend should not exceed £1500 …we have spent 39% of that in January alone!

@carpenterstation can you tell us a bit more?

Location?

Heat loss calcs?

Are you using Weather Compensation (make sure you do!! Get your head round different way of working)

Any batteries? 

What tariff - check you're on best for you. Very much case by case

@tim441 East Midlands Uk. Yes to weather compensation. 15kwh of batteries / 10kw solar. Octopus cosy tariff. (Ovo heat pump no longer available).

not sure hat you mean by “heat loss calcs” but the design said 17kw ASHP so we have 2 8.5kw Mitsubishi Ecodan cascade.  House is heated to 19 degrees. Water to 45 degrees. Ground floor is UFH (250m2) walls pretty good for insulation. Full double glazing.

set to not run during peak octopus cosy 4-7 pm - bur every day the system runs in that window despite no heat being called and hot water being timed off. 

@carpenterstation £600 sounds a lot - how many Kwh? Is that the ashp alone or whole house?

Maybe check your schedules to make sure cheap slots are maximised by filling batteries and heating home? Probably best to heat at night without setback. And discharge of batteries is used properly at other times to avoid higher cost imports.

In Jan I used about 1650kwh in total inc ev, ashp etc at cost £350 but was on Agile which did not work well. Last year Agile averaged 12p/kWh for me. Now moved to Intelligent Go which I think will average under 15p.

Jan ashp usage was 1050kwh but included a day of power cuts. Likely cost around say £220 or 230

My annual ashp usage is around 6000kwh

Heat loss calculations are important and useful to understand. Your installer will have done them for the grant and to decide system.

1. They are used to correctly size the ashp, radiators, pipework etc

2. Give you something to base expectations correctly. 

So installer almost certainly gave you them. Heat loss for year? With/without hot water?

To be clear.. I took no notice of my heat loss calcs until I was getting my head around usage  weather compensation, tariffs etc. Installed 2020. Moved to weather comp end 2022. 

My heat loss info follows

My heatpump has a meter on it which means i can track exact monthly usage. Meter installed as part of original installation I think it was a MCS requirement at that time. Installed 2020 with RHI grant.

Not sure of current MCS requirements or best practices.

My LG controller is software 3.05.5a which does not provide any useful data.

More recent software has some data I believe and other people may be extracting through ThinQ modbus or 3rd party apps.

Here is my usage for last 2 years (was not tracking previously). Readings started during early 2023 and after moving to AI (LG term for weather compensation) but should be pretty accurate guide. I gradually lowered AI curve during 2023. 2024 made very few changes - if any.

Extrapolating the start reading from early 2023 it's fair to assume the previous 2 years on fixed flow temps had a usage just over 8000kw pa.

AI savings circa 28% and now 5800kwh last 2 years in 2023 and 2024

16kw Therma V

200 sq m house (half uninsulated, old and half modern)

MCS heatloss calcs suggested:

Space heating 23776kwh

Hot water 3821kwh

Total energy usage 27597kwh pa 

Flow temp 55deg

Hot water 50 deg

Main rooms heated to 21deg 

Bedrooms cool (did not uprate rads) but fine.

Actual usage means we heat some unused rooms less than MCS. 

20024 used AI and Hot water is heated to 47deg overnight with daytime schedule to 40 deg and off 4pm to 7pm (Agile)

2025 moved to Intelligent Go and changing schedules to maximise. Including heating overnight with no setback

SCOP unknown but estimate 3.50 to give actual Total Energy Usage circa 20000kwh pa. Heat pump does not cycle so seems "right sized" considering old, uninsulated main house + modern extension

I'm not sure what it tells us!!! But overall feel happy enough with setup and costs now. Hopefully useful if nothing else to confirm importance of moving to AI/weather compensation, using 24x7 and reduce DHW. 

@tim441 I don’t know exactly how many kWh because my cascade system is unable to tell me that.

I don’t think a meter can be an MCS requirement because we have the certificate - but no way of measuring actual usage (due to cascade systems being unable to do that without buying additional metering.  I don’t know why they can’t)

So £600 is whole house. (As was the comparison to 2024 actuals). I need to buy some additional kit to measure actual usage - but I can get a good feel for usage and see the cycling from the solar app.

Our actual usage each year for the last 5 years has been a consistent 29,000 kwh. That was from LPG with a boiler installed in 2020. (So 90%+ efficient). With a SCOP of 3.5 I should be using 7500 kWh of electricity per annum for the  same level of heating/hot water. I know Jan 2025 was 18% cooler than Jan 2024 - I don’t think that necessarily means 18% extra energy - it’s probably a lot more than that… but there is an decrease in energy usage 2024 to 2025 in January (2024 was c. 3500kWh LPG and 950kWh house/car. 2025 was the same 950 kwh for House/car and 2150kWh for heating - so without adjusting for weather that is an increase in efficiency of 61% - at least 87% with the weather impact. But that is a not very impressive conservative SCOP estimate of 1.9. 

Cosy lowest price is now 12p so I won’t be averaging that this year (I too was averaging 7p last year with agile). And yes I am cycling the 15kwh batteries 3 times a day making best use of the tariffs, and the bedrooms are backed off at night.

january was the first month, and the installation was problematic with the house on occasion dropping to less than 5 degrees. Obviously we used a lot of energy to get the house and the 250m2 ground floor slab backup to 19degrees - so that is at least part of the problem. But I also think the random non heating cycles that run for an hour each day burning 4kw when no heat is being called is not helping.

🤞🤞 things will settle down. Others on the forum.may have more advice or ideas. As you say the Jan requirement probably required a lot of heating outside of cheapest slots but will not be repeated with weather compensation and steady house temps. 

As mentioned setbacks might be better used at times of expensive tariff slots - less so at night.

29000kwh sounds a lot but I'm sure you're doing what you can to reduce.

I can't measure my SCOP but maybe with hot water an overall 3.0 over a year might be reasonable?

Obviously too late now but can't help but wonder if cascade was necessary/right way. System design is clearly critical. But hey ho.. its now about getting best from what you have.

Posted by: @tim441

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Obviously too late now but can't help but wonder if cascade was necessary/right way. System design is clearly critical. But hey ho.. its now about getting best from what you have.

Quite likely not but...with two pumps you have the opportunity to get a very good turndown/modulation ratio and a level of redundancy against failure.  Most of the time running one should provide enough energy and when its very mild one only will still be able to turn down to the required output, so much less need for cycling.  

I dont know if its plumbed this way or the extent to which the controller supports it, but the opportunity is there to use them singly or together as the load demands.

I am sure you know that the trick (to get high efficiency) is to get the flow temperature down as low as possible, which generally means 24*7 operation with weather compensation tuned as low as possible and no external controls (which can play havoc with the efficiency - particularly 'smart' ones which can be an efficiency disaster).   Cosy is a complicated tarrif to use - avoiding the peak almost certainly is sensible, whether it makes sense trying to take advantage of the troughs Im not sure.  This is where Homely/Havenwise or other tariff optimisers come in.  Has the system got a buffer tank and if so is it 2, 3 or 4 port.  Hopefully not or if there is one its 2 port (ie a volumiser).

January 2025 was cold (~16% colder than 2024) but all the same you should be getting a heating only COP of well above 3.  Mine was 3.7 and I have radiators running at at a max FT of 45C; with your UFH, presumably running at a max FT of perhaps 35C, you should be doing better.  Im a bit south of you but East midlands isnt that much different weather wise to where I am, unless you get the cold wind from the Urals like they do in Cambridge!

@jamespa I didn’t want either cascade or a low loss header. But the installers refused to install a single ASHP system due to property size (450m2) despite all the historic evidence I had indicating what kWh we normally need for heat and Water (which suggested a 12kwh system - not the 17 kWh cascade that has been installed) The EPC (required for MCS accreditation) is also a work of fiction that justifies a higher heat loss calculation. The cynic in me thinks I was done over.

i don’t know whether the black thing in the photo is a buffer tank, an optimiser, or a low loss headers- and I have no idea how many ports it has!  Sorry I just don’t understand the terminology and wouldn’t know what one of those even is? I am told it is a low loss header - required as part of the cascade install probably to protect the compressors. But I don’t know.  The only other tanks in the install are 250 litre hot water tank and 2 expansion vessels

Black thing looks like (probably) a low loss header combining the output of the two pumps.  You will get complete mixing in that which could in principle cost you up to 15% in efficiency, but actually probably quite a bit less if you get the flow temperature well down.  Thats going to be key.

Your cynicism may well be justified but as you say you now need to make what you have work.  I dont think you can do much about the LLH without a major replumb.  Have you done the basics ie

• ensured that there are no external devices turning the heat pump on/off (smart/dumb thermostats, Hives , Honeywell evohomes etc)
• ensured that any TRVs are set well above the desired temperature
• balanced the radiators/UFH (if you have any adjustment possible)
• ensured the WC curve is as low as possible when system operated ~24x7 (you can certainly miss out the peak cosy period)

Given the complexity of the system your installer really should provide a system diagram.  If they haven't and you have any remaining leverage I would demand it and retain the leverage until you have it.

You have some excellent kit (possibly rather too much); the trick is going to be to get it working well.  Without a system diagram you are operating with both hands tied behind your back.

I have no leverage, no system design, no hand over details. Weather compensation is”automatic” for Mitsubishi Ecodan - and I have been told not to meddle….

the UFH valves are sticky. Some need replacing - but if stuck they remain open and the pump stops firing when everything is up to temp. But there are 9 valves all independently controlled by room thermostats which could be causing me some inefficiency.  But all are set to 19degrees all the time - and the stat for the radiator part of the system (upstairs) is set to 17.5. TRVs on all rads to balance the upstairs rooms. Most upstairs rooms are not in use. 

i have timed out the expensive Cosy window for both water and heating. (But I fill the batteries before that window starts anyway - 15kwh should be enough to cover those 3 hours)