ASHP - A2A Controls and Consumption

Hi there - are there any members running air-to-air heat pumps (rather than air-to-water) who have had any issues with COP/SCOP that’s lower than expected?

I have a new 7.2kw system that’s running well below the expected 4.07 SCOP.

A CC member on the Agile forum posted the below so I guess i’m not alone in this…


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We have less experience here with air to air but I maybe able to point you in right direction. Could you provide more information on the make/model and any further operational data i.e. how have you determined the rapid cycling is taking place (is it similar to what was reported in other forum)?

I will note that the example given on the other forum speaks to the sad current situation of widespread oversizing of these systems by some installers. A heat pump may not have even been appropriate at all in the case of small well insulated flats (but hard to say without knowing a lot more!).

Thanks Ben.

First off, I don’t know if my condenser is rapid cycling or not. It was a comment from a forum member (Agile) after I raised my question/observation on approximate SCOP achieved so far.

Here’s an overview of my situation and equipment:

We replaced Rointe K Series rads (inherited when we moved in) with A2A. We’re in a converted school c. 150yrs old and have an unusual layout over 166m2. The lounge is big and has a double height ceiling that opens via a gallery to the upstairs (can close off). We’re pretty well insulated (have booked the thermal camera for a check).

We’ve gone with a Mitsubishi multi-split that can take 4 indoor units. Initially going with one in the lounge and one on the upstairs landing. We know from running a log burner (12kW rated output but operated at much lower level) in the lounge that we can heat the whole house to a comfortable temp from a single heat source. The appeal of the upstairs unit is cooling on warmer summer evenings (we lived in Hong Kong for a long time and have used an inverter system there for years).


7.2kW - SCOP 4.07 / SEER 8.13 (MXZ-4F72VF)


Lounge: 5.8kW (MSZ-AP50VG-E1)

Upstairs Landing: 2.5kW (MSZ-AP20VG-E1)

4 days in and it looks like we are using around 22kw/day on the A2A which are replacing 6.2kw Rointe elec heaters that did the space heating (steady 19c in 166m2 space).

We have other Rointe rads (c. 5kw) that are coming out; these are only ever used when the weather is very cold, and I can’t be bothered to light the log burner

The company that did the installation have asked Mitsubishi for their thoughts.

Any help or guidance would be gratefully received. Thank you!

Is this 22kWh / day as measured approx from the house elec meter? Or do you have a separate meter for the units?

Taken from SMET2 IHD. We burn circa 18kWh for hot water, cooking, lighting and base load (av summer usage plus a bit for winter lights etc).

First two days after installation we were burning 40kWh/day, this has now dropped to 35kWh/day as I have got to grips with MELCloud (the Mitsi wi-fi interface) so the SCOP had edged up and the A2A is burning c. 17kWh.

I think you will need more detailed tracking of the energy input and heat load over a couple of weeks to get a better handle on the effective COP you are getting from your A2A heat pump system.

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Yes, more data points required for sure. This was my first impression based on simple consumption calc relative to the sort of consumption envisaged for a SCOP >4.

The MELCloud software that runs the system has no energy consumption monitoring…

Considering a clamp-on meter to get a handle on what the condenser is pulling. Any other suggestions would be appreciated.

Hi Pwshk,

I was the CC bod who said cycling can greatly reduce COP.

Cycling is common as most HP systems are over-sized. Also getting a HP system to perform to its potential SCOP is difficult. If your system is rated at an SCOP of 4, be happy if you can get an SCOP of 3 in use.

Your current system is possibly oversized (I don’t know what insulation you have in your house) but, if it is, it’s not enough to make it perform as poorly as yours is.

For comparison I have a 5kW ASHP in a fairly well insulated, detached, 270 year old house with floor area 115 sq metres. Over the last month my energy use has been 6kWh per day. Thus I would reckon your energy use should currently be less than 10kWH per day. So you have a problem.

Generally Mitsubishi HP hardware (designed, and usually built in Japan) is extremely reliable. I would look at the control and monitoring systems . The USA designed Honeywell controller can be problematic and the UK monitoring software can be poor . I am assuming you have a Honeywell controller, but I am not familiar with Mitsubishi A2A HP controllers.

However I am familiar with early versions of MELCloud - awful software. My MELCLoud system was wildly inaccurate when first installed and I had a long running correspondence with the UK designers of the system, pointing out inherent problems in the software. Annoyingly is based on a previous system that worked well. So it collects useful data that you can’t use!


  1. Ask Trystan Lea to instal one of his excellent monitoring systems, then you will know what your system is doing.
  2. You can stop the system cycling by running it “by the clock”, rather than “on the thermostat”.

My system is currently set to run for two 3 hour sessions per day - when running it averages 1kWhr per hour, 6 kWhr per day total.

Thus I suggest you set the thermostat to a high temperature ( 25 degrees?) so that it doesn’t operate. Find out how long it takes for your HP to achieve a reasonable room temperature, after it is turned on, and set the controller to turn it on at appropriate times, and for it to stay on (2, 3, or 4 hours?) until the house is warm. E.g. in the morning, so it is warm when you get up, and in the afternoon when the HP will run most efficiently as the air is warm outside. Fortunately you have an old house with tons of brickwork in it, so, with your thermal inertia, the temperature will only fall a little when the HP is not on. You may need 3 or more heating sessions during the day when the weather gets colder. With a good HP monitor you can see clearly when your HP is achieving a high COP, and you can adjust the system accordingly.

I hope the above is useful for you. If it is, I would like your help.

I want to learn more about how A2A systems perform, as they are new to the UK, and appear to have potential as a cost effective retrofit solution.

Could you give me a detailed account of your system?
Costs for installation and in use , performance, convenience, problems etc?
Would you be prepared to present the details at a CC HP webinar?

Best Wishes. Andy Hamilton


Hi sorry for delayed reply . I have two suggestions for moving forward beyond what Andy has suggested above (which is all sage advice):

  1. It would be good , if only for my own curiosity (and maybe others) to understand better how you have determined what the current heating output is. It seems like you have based this on your previous heating load with the boiler. But perhaps I have misunderstood.

  2. I have been speaking with Karolis Petruskevicius ( at Evergreen (and formerly Homely Energy) and they make a heat pump monitoring/management device which you may be able to use to get more detailed monitoring on your system (you may need to purchase some add ons to make it work). I would encourage you to get in contact with him.

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Ben makes a good first point - when calculating heat load for Gas boilers the tendency is to over estimate as it makes little difference to performance (10-15%), and this practice has been taken over to HP design where it can make a 50% difference.

On the second point, do compare Karolis and Trystan’s monitoring systems. Trystan works closely with CC, installing monitoring systems for members, and has delivered a good webinar on the well engineered HP system he has recently installed.

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Hi Andy

Thanks for that, it’s all very useful and I’ll work through your suggestions to see how things improve.

Monitoring energy input and heat output is what I need to get a handle on. At the moment I have my current daily burn from the IHD (and from Agile and related apps for past days). This hits on the points @ben makes below.

Yes, I’d be happy to provide hardware spec and costs. Do you want this via DM or dropped in here?


A 50% reduction in performance would be a bit of a disaster. I’m not sure I’m in that league but need to get a handle on actual consumption/output to know one way or the other. 7.2kw for 166m2 doesn’t seem unreasonable.

I know of Homely (via Agile forum) and contacted Karolis last week. He has my model details and is looking at what can be done if anything. That said, I’m not sure that A2A is suited to Homely where folk want to flex consumption but as @andyham says I’m lucky to have a lot of brick to store heat so it might work.

I don’t know Trystan but will have a look at and contact him as suggested.

Thanks @andyham and thanks @ben


You wrote:-

Could you email hardware and Melcloud specs to ?

I can then review and discuss with you.

Best Wishes Andy

Thanks Andy - just dropped you an email.

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