I'm sorry you're having trouble with it, I hope it can be made to work better.  Apologies in advance for a flood of questions, which hopefully will help with the efficiency!

What model is the ASHP?  What else is in the system  - buffer tank? How many radiators?  What controls are there, and how were they setup? Is weather compensation set up, or is it set to constant flow temperature?  What temperature is the heatpump outlet pipe, when it's running (lower is better), and what temp is your first radiator?   How many hours a day do you allow it to run (more is better, as it will allow lower temperatures to be used)?  There is an immersion "boost" heater in a lot of heatpumps - is this allowed to be on (best not)?  Assuming it's a monobloc system, does it have glycol in it? (it will by default keep the unit warm in case it hasn't got glycol in it, so an energy saving turning this off if there is glycol).

How do you know the energy use - is there a meter uniquely attached, or are you looking at overall house consumption?  Do you have anything to compare with, like a years energy usage from a previous gas boiler?  It would be easier by the way to discuss kWh of energy use, rather than £/day, as energy prices are so variable these days.  Energy bills have all gone up recently.  While it should be possible to get an ASHP to operate at a COP of 3, that may well still result in higher bills than gas.

As well as making the HP efficient, there's the issue of tariff - Octopus Go - I think that's 7.5p/kWh at night, 30p during the day?  Clearly advantageous to run the heatpump full whack at night then!  Is this done?  That is, I would expect it to run initially at 8kW, and the DHW to be "full" by 4am or so.  200l will rise by 5C per kWh of heat, so around 10kWh of heat would raise a tank from 10C to 60C(ok, less due to pipe losses, lets say 50C).  This would use maybe 10/2=5kWh of elec, costing 37p/night during the cheap time.  That night time rate is so low, you want to get as much heating in during that time as possible!  

Welcome to the forums @howiewsl. In addition to the relevant questions raised by @robl, can you please let us know the approximate square footage of your property. Also, the installer should have provided you with heating data in their quotation – this should show how much energy your home will require to get heated to 21C – I think that's usually the target temperature they aim for – and how they intend to get you there. 

The fact you're struggling to get over 18.5C indicates there are problems somewhere in the system. On that note, have you at any point just left the ASHP to run for say a week and did it ever get you over 20C, or has it always struggled to get you over 18C? Heat emitters can sometimes be a problem here – this was an issue for us. Did the installers give you a heat emitter guide and do the rads conform to what the guide set out?

On the subject of HW, what temperature are you heating your water too? Most manufacturers set HW to 50-55C by default, which is pointless in my opinion because at those temperatures its scolding and you need to add cold water to get it cooler. We heat our HW to 45C for this very reason. It provides hot showers and baths, and is also in the optimal range for heat pumps to run efficiently. I don't know enough about the stratification of water in HW cylinders, and how quickly water "cools" when hot water drains. I suspect @derek-m might be able to offer some insights on this subject.

In terms of running costs, to put things in context for you, when we run our ASHP it's costing us in the range of £7-10 a day (CH and HW) at present tariffs (22p/kWh with E.ON) but the entire house is at 21C. 

This is just a starting point Howard – hopefully we can work through your system and figure out why it's not doing what it's supposed to.

Were the installers MCS-registered and did they provide a room-by-room analysis of heat requirement, radiator output at given flow temperature, overall heat requirement etc? 

It’s a bit concerning that the original quotes were for 12kW, yet an 8kW model was fitted.

@HowieWSL,

Hello and welcome.

before you start tinkering with settings, it's worth assessing roughly whether your ASHP is undersized. 8kW does sound as if it might be for a 3 bed Victorian semi but it's not possible to say without some details on your home.  A couple of simple questions:

(1) What is the space heating and DHW annual requirement on your EPC - it will be something like 15,000/3000 kWh?

(2) What is the heat loss of your house at its design temperature?  This should be on your MCS documentation and will be  5-10kW at -2 degrees

My numbers are (1) 19000/3000 and (2) 7500@ -3.4.  I have a 14kW ASHP and it's a bit oversized; 10 or 11 would have been OK.  It works well enough though.  

@howiewsl - sadly 'utter nightmares' are all too common, both for the design stage and the installation stage. Even MCS registered installers are perfectly capable of coming up with poor designs. Two of the four original quotes I had last year, all four were from MCS registered installers, were based on design proposals that could never have reached MCS temperature requirements (21/18 degrees inside, -2 outside). No new radiators, inadequate sized heat pump for heat losses etc. A fifth installer made a complete b*lls up of the heat loss calculations - its 'surveyor' would have struggled to survey an empty cardboard box. As I have said elsewhere, my impression is that MCS is a toothless regulator, given how common these problems are. 

Another problem is the manufacturers are allowed to market their heat pumps using nominal outputs under average/ideal conditions, which ignores that fact that heat pump performance falls off when you most need it, ie in cold weather. It's like being able to sell a car as capable of say a top speed of 80mph which it can just about achieve under average/ideal conditions, but give it a head wind and then a hill to climb, and it will only achieve 55mph. My Midea heat pump is nominally 14kW unit, but when it is freezing outside it's output drops to less than 12kW, and my heat loss at -2 outside is just over 12kW. A 14kW output may sound fine for a 12kW heat loss, but it isn't because output drops off when it is most needed.

That said, there are suggestions that some, maybe all, heat pumps can provide a range of outputs, configured by setting dip switches. The idea is the manufacturers make a single unit, and then market say three different 'models' with different outputs, but the only difference is the dip switch settings. This may provide a solution for me (I need to wait for my installer to return from an extended trip abroad) and it might be worth finding out if your heat pump output can be up-rated in a similar way, if it turns out that it's factory gate settings are too low for your heat loss. If the output is configurable, hopefully 8kW won't be the upper end of the range it can provide. 

@cycleneil not always an issue. I had a range of quotes from 16kw to 12kw. We fitted 12kw and its totally fine, no issues with heating at all.

@batalto - these wide ranges (mine went from 9kw to 16kW) are illustrative of the problems with design. The designer/installer should do a full heat loss calculation (room by room, U-values, air changes etc) and this is all basically physics so given the same input data, they should come to very similar total heat loss figures, to which they then match the heat pump output (taking into account, as I now know, the heat pump's output at low ambients, not the nominal 'model number' rating), so they should all recommend similar outputs. But they don't... so some are over-specifying, some maybe get it right, and others are under-specifying. How do Mr and Mrs Average, who may have only the vaguest notions of the calculations, know which design, if any, is right?

I'm pretty sure my nominal 14kW output heat pump can't reach MCS requirements (21/18 inside, -2 outside), but it's difficult to establish as we move into warmer weather. I currently have the heating on 24 hours/day, bog standard Freedom weather comp and room stat in a room that should be 18 degrees set to 18.5 degrees. When it is warm outside the room reaches its intended temperature (and can even overshoot with solar gain), but when its colder sometimes the room doesn't get above 17 degrees. We haven't has a sustained cold snap with sub zero temps since the heat pump was fitted, so I can't say for sure what will happen, but I have a strong suspicion the room will fall even lower in temperature.

@cathoderay I actually did my own heat loss along side the suppliers. Some didn't even come to my house and did it remotely...The issue is that any heat loss makes a load of assumptions in materials used. I'd say ours actually overestimates the heating needed in the house - we've not struggled except in our biggest room with 6m of bi-folds. Frankly I can't see any way to get that room really warm on very cold nights without more rads or UFH. But even then, it warmed up once the sun came up in the morning. It was the same with the gas central heating.

I adjusted my weather compensation from the standard Freedom settings over a few days, but since then I've not touched it in months - it's set to keep us at 21° and I leave the thermostat at 21.5°

@batalto - I absolutely agree about the effects of assumptions. While heat loss calculations in theory can be accurate, all sorts of assumptions do get made. One of my quotes for example was based on a whole house heat loss, ie treating the whole building as one big single room. I have also never seen a heat loss calculation take into account fireplace losses (I have an inglenook, albeit with a small throat) or drafts, except indirectly through air changes. Like you, I had done my own heat loss calcs, in my case back in the 1980s for the first CH installation, so I know the principles, and had a baseline to assess the the figures the installers came up with. Some were absurd eg one had the kitchen, with two and a half outside walls, and a 50% larger floor area, and three windows, had a lower heat loss than the dining room with only two outside walls and two smaller windows. Another had the external wall length of the kitchen as 2.3m (it is actually nearer 12.3m, so probably a typo, 2.3 instead of 12.3) I continue to be concerned that the vast majority of home owners are not so clued up on heat loss calcs, and so are not in a position to spot installer heat loss errors, and subsequent heat pump/emitter sizing errors.

On the plus side, doing your own heat loss calcs means you get an insight into where the heat goes, and what the effects of changes in say loft insulation or adding more insulating glazing is. Most of my heat loss is through the solid stone walls, and it will stay that way because the building is listed, and neither internal nor external insulation is an option. The windows have very high U values (single glazed metal frame), but mostly they are small, and the losses can be reduced by adding secondary glazing. In the 1980s I had to set up my own calculator/spreadsheet, but now there are a number of spreadsheets available online, including of course the one you generously link to, which I think is based on the Freedom calculator. Anyone who knows how to use a spreadsheet can do the calcs, all it needs is a tape measure, pencil and paper to note down the measurements, and a few spare hours.          

It’s a lot simpler to spec a gas boiler than an ashp.  While there are similarities - modern examples of both have huge modulation ranges, so don’t just switch on/off like earlier versions of each.  Oversizing either type makes minimal efficiency difference.  The significant difference though is that high power gas boilers are cheap, so they would routinely get fitted, to no detriment - who would risk fitting one with ‘just enough’ power?.  It’s normal to have 24kW or more gas boiler in a uk property, that subsequently gets a far lower power ashp, as ashp prices and fitting issues noticeably increase with their power rating.  
As well as the sizing issue, installers and users have been lulled into bad habits with the huge power previously available.  Many gas boilers in the uk are only turned on briefly twice a day, to ‘save energy’, while actually this does the reverse - it forces the unit to run at high temperature inefficiently, and is a terrible habit to bring over to an ashp where this matters even more.  Those new fancy aftermarket controllers - they’re just on/off things, I think you have to get the manufacturers controllers to get weather compensation.

Hi everyone! Thanks very much for all your helpful replies. OK, a LOT of questions here and an epic reply below - but all good because it helps refresh my memory on a few areas - especially what we were originally quoted for, which was a 10kW ASHP, not 12kW - apologies for that.

Anyway, I'll tackle these in the order they appeared...

@robl - Hi, thanks very much for your help - my answers are...

Model of ASHP?
It’s an ‘Oxford’ 8kW ASHP by Global Energy Systems - we were originally quoted for the 'Thurso' (10kW).
What else is in system?
System includes: 200L HW tank, buffer and 8 radiators (2x 140cm, 4x 120cm, 2x 60cm - all double-panels).
What controls?
A fixed Global Energy Systems control panel where you can adjust most of the ASHP settings manually. We also have a remote Hive thermostat.
Weather compensation (?) or is it set to a constant flow temp?
Constant temp. (I’ve checked with GES and there is no weather compensation setting for our model).
What temp is the heat pump outlet pipe when running?
Hard to decipher because of the dizzying array of codes and instructions in the world’s most un-user-friendly controls and manual. The ‘water out temperature for heating’ is currently set to 48C. The ‘Max heat pump water out temp (without e-heater)’ is set to 50C.
What temp is out first radiator?
Unknown, we don’t have this monitoring, other than getting a thermometer out when the system is running.
How many hours a day do you run it?
It’s technically on 24/7 if the house needs it, but generally it’s spends a total of 5hrs per day heating our home in mild/warm March weather, and 7(ish) hours in colder winter weather… those are some very approximate figures based on averages from our Hive app.
Is the immersion ‘boost’ heater allowed to be on?
Yes, though I believe this is only activated at -6C.
Does the system have glycol in it?
Yes.
Is there a meter uniquely attached? Or are you monitoring more generally?
There is a separate meter attached and we have a SMETS2 smart meter. As the ASHP meter is in a rather cluttered outbuilding, we’ve not been checking it closely recently. However, during some close monitoring in January we worked out that the ASHP was about 85% of our daily electricity usage. Though that’s probably meaningless for the rest of the year.
Do you have anything to compare usage with?
Nope. We this is out first home and only moved in in June last year. The ASHP has been here since the start of December 2021. Previous to that, the house was on oil-fired heating (we're out in the sticks).

...Re. Octopus, yes we're on it and our rates are currently 24p daytime, and 5p nighttime (00:30 to 04:30am).

@editor - Hello! Hope all is well?

My answers to you are...

Have you left the system running (for say a week) to see if it does get up to 20C?
Yes - a number of times over the winter and it’s always struggled to hit 20C consistently. That’s before, during and after Global Energy Systems tried to optimise the settings after we asked for help.
Did the installers give you an emitter guide and do these rads conform to what the guide set out?
Yes, they did provide an emitter guide very early on. Good question about rads now conforming - they’ve all been replaced, but one of the many ridiculous installation issues was that no one had ordered the rads (and there was no immediate availability for some sizes), so it was a massive last minute rush that delayed everything by weeks (and we’re talking late November/early Dec here)… so whether the correct rads were actually installed in the end, I can’t tell without further calculation… in fact, I wouldn’t know where to start without knowing where they got them from. Certainly one of the small 60cm in my little office room is undersized as that space has never been a comfortable temp.
What temp are you heating your HW to?
The ’T-water tank’ setting is 50C - this was what GES set it to after we’d complained about the HW running out fairly quickly.

@cycleneil - Hi there, yes - the installers are MCS-registered and they did provide a room-by-room analysis of the heat requirements. In fact, this may have happened a couple of times.

Hiya @kev-m - thanks for the reply. Hope these answers are useful...
What is the space heating and DHW annual requirement on your EPC (it will be something like 15,000/3000 kWh)?
Space heating: 11487.0 kWh per year / Water heating: 2135.0 kWh per year
What is the heat loss of your house at its design temperature?  This should be on your MCS documentation and will be  5-10kW at -2 degrees
This is 5.3kW at -2.2C (continuous heating).

Finally @cathoderay  Cripes! Your ordeal sounds even worse than ours. Very sorry to read all that. It's a complete 'Wild West' situation out there at the moment, isn't it?!

Hope that's useful - thanks again everyone - it's been interesting reading your comments.
H