ASHP compensation curve, UFH and wood burner

Hi, just new to this forum. I was directed here by someone on the green building forum. This was my post there:

I have a 14kw Ecodan ASHP. Had a lot of issues with the original installation, but has worked ok over the years. However, I am wondering if I can run it more efficiently. I currently have 5 zones with individual thermostats and ufh and then the upstairs which has radiators on a separate circuit. These are all fed into a heatmiser control unit and then into the Ecodan unit.

I have adjusted the compensation curve over the last year or so, but I have read with interest those who manage their house temperature fully from the compensation curve (heat geeks etc). If I understand correctly, they do away with the individual room thermostats and have the whole system open, trying to balance input with heat loss. I suppose my question is - how low a temperature have some of you gone on the compensation curve. Mine currently runs at 37 at zero degrees or below. Wondering if I can go a good bit lower, set all thermostats to a high temp so they are constantly on and see what happens?

The other thing I wondered is how a wood burner would influence this kind of setup. I have a wood burner in the lounge. Use it during the day and evening if we are in. Some days not at all depending on movements. How do you accommodate this input into the system setup?

New thoughts appreciated - trying to get electricity bill down to a manageable level!

Happy to learn from anyone’s experience!


Probably me!

Those with HPs will surely be along soon :slight_smile:

Hello @1980scotland and welcome.

With this cold weather it’s interesting looking through the shared dashboards. You can get an idea from there of the kind of temperatures folk are running at at different outside temperatures and the resulting performance. It is quite complicated, there are a lot of different factors at play.

37 is a good temperature if that works for you at zero or below. I think many are closer to 40C. Heat geek is at 40C at -2C outside:

If the thermostats are set higher than the temperature the woodstove will heat the room too, and the control is weather comp curve on the ecodan, it should keep running. I have my own custom controller so Im not 100% on what the standard controller does but Im pretty sure that’s right.

Does your ecodan controller display the consumed and produced energy figures? That should give you the running total COP for this month, and then you can drill down to recent months and years. It’s not always very accurate but its a good indication.

I live in a far colder climate ,yesterday it was -35C tomorrow it suppose to be -1C, what the minimum temperature are you trying to achieve in the house. that would be the first question I would ask. that has the greatest bearing on most answers. the second is do you know your air infiltration levels … that has the next greatest bearings … lastly the R factor of your house… in your climate, in the UK my home in Canada would not even need heating as the trace from appliances and hotwater tank would be enough. but based on your climate for the home to feel comfortable the biggest concern for me would be the humidity with in the building and air infiltration. you have zones, leave them zoned that will save you the most energy that way . leaving bedrooms cool and low use area much cooler and only living area warmer ( if you have can isolate the living area from the cooler areas even better yet -doors, bifolds curtains etc… ). pull down the humidity to 50% or less ( this will also make cooler area feel warmer as it humidity levels drop). try to get air infiltration to as low as possible. wood stoves inside a house are really only good if the are burning constantly if they are not they are pulling heat out your house constantly as well ( generally equal up to ~30% heat loss) . if you have a wood stove, hopefully it draws it air from outside. otherwise the draft is just wasting tonnes of energy when it not in use, as heat just goes straight up the chimney and outside. . timed thermostats save quite a bit of energy.- ie cooling area and warming area when occupancy can be expected – example my upstairs cools down to 17c during the day when no one is around up there and then an hour before my son comes home ( 5pm) it warms up to 20 c till 10pm then drops to 17. generally the sun during the day will warm it to +20c during the day and it only maintains it for that 5 hours in the evening before it cools down for the next 18hrs … the same can be said for my bedroom it is warmed up in the evening before I go to bed ~20C and then allowed to cool at night to 17c ( I am under blanket so it feels warm anyways) and stays at that temp until the following night cycle to warm up again

Thanks for the comments and links! I am aiming for background temp of 19 degrees. I have heat meter and electric meter for the heat pump. Last year I averaged 2.6 COP across the whole year. Trying to inch that up If I can! My house is clearly not as insulated as it needs to be. I have bought a Flir camera, so will chase drafts and focus on removing air leaks to start with!

My meters are just manual read. Will take some readings each day over the next few weeks and see what’s what. Expecting -5 tonight, so its a robust test of the equipment!

What are they (make/model)? You might find they are quite easy to monitor without the expense of the other OEM kit. You can feed that data into Emoncms on any computer - e.g. Pi or VM.

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sorry for the delayed reply - some travel out the country.

The model number is PUHZ-HW140VHA2-BS
It is linked to an FC4 controller.

If you think that can be linked to a computer directly, that would be interesting!

Hi again,
Been looking at the open energy monitoring options for monitoring. The level 3 system is quite pricey at 639, so was wondering if I can do for less!

I have a raspberry pi3 unused. I already have a Sontex supercal 531 fitted which has an mbus output I believe.

Wondering if I can buy the mbus to usb unit they sell, an optical sensor for my electric meter and go from there? What would I connect the electric sensor to?

Hope I’m not barking up the wrong tree with this more simplified approach.

If the heat and electric meters are already hooked into the EcoDan, then you should be able to see all the data you need on MELCloud. The interface is horrid, but it’s better than nothing.

With MELCloud, you can possibly scrape the data from it, without any additional hardware. That’s what I and a couple others do. There’s no easy, off-the-shelf solution for that, but these projects might help get you started.

Unfortunately not Tim! They are both stand alone units not linked to the Ecodan unit in anyway! That’s why I think the Emon setup is the way forward. But as ever, trying to do it as economically as possible!

On the MBUS, yes that will be fine.

The issue with an optical reader is that it is not instantaneous power readings, however, it may be ‘good enough’ and connecting that to a Pi is an easy task. You should then be able to use Emonhub to read both and emoncms to do the analysis (just use and EmonSD image).

I have a PiZero connected directly to a Pulse sensor and this has worked for years :slight_smile:

@TrystanLea #pleaseaddtothedocs

I use an RJ45 to serial adapter ( and made up a connector to the RPi. The pins chosen are purely for convenience (all next to each other for the connector I made up). I also use 3V3 rather than 5V.


RJ45 → RPi Pin
3 → 15 (GPIO 22)
4 → 9 (Ground)
7 → 17 (3V3)

Don’t get your Pin Number and GPIO Number mixed up!


    Type = EmonHubPulseCounterInterfacer
        pulse_pin = 15
        bouncetime = 5
        # bouncetime = 1
        # Rate_limit is the rate at which the interfacer will pass data
        # to emonhub for sending on. Too short and pulses will be missed.
        # rate_limit is minimum number of seconds between data output.
        # pulses are accumulated in this period.
        # rate_limit default to 2.
        rate_limit = 5
        pubchannels = ToEmonCMS,
        nodeoffset = 3

Thanks Borpin. This is terrific, appreciate the insight. I think I will buy the SDM 120 modbus meter. I will just add it into the system for ease at this stage. Given I have majority of other components, this is not too much of an outlay! I am a bit of a novice so will take step by step. I am sure I will get more into this as I progress.

So, now running for a day or two with the emoncms.

I have opened all zones and set thermostats high so they stay continually open. For the first 5 or 6 hours, I was getting amazing COP and the return temperature gradually increased. Ran well overnight. But for the last hour, I get this kind of cycling, because I assume the return temp is too close to the flow temp because the floor slab is fully at temperature. Is this what you would expect to see? Now that I am getting data, obviously need to dial the system in better, but wondering how the heat pump should behave with an optimal setup?

Looks like you might have some short cycling going on here, with 4 cycles occurring in the last half-hour. Are you sure there are no valves closed during this time?

It’s odd that the first 3 cycles are super short, yet the 4th runs for a more reasonable 25 minutes, which suggests to me that there is capacity in the emitters to absorb that energy.

This could be the classic Ecodan problem, where it races to get up to temperature as fast as possible, overshooting and turning off. Fiddling with the flow temps / weather comp might help here. Alternatively, there’s something to be said to just turning off the heatpump when the house is warm enough.

For more about cycling, @johncantor has just done a video about it:


Thanks Tim - will take a look. Now I have had it going for a few days, it seems to have settled in a pretty consistent pattern as follows:

Pretty mild today, but house is staying constant at about 20 degrees, so I think the compensation curve is not too far away. Think there is a bit of balancing to be done on flow rates, but I kind of expected to see much less short cycling than this!

Will take a look at the video for inspiration.

Could the room thermostat delta be part of the problem (if you have one) or part of the solution (if you don’t)?

As you are probably aware, thermostats have a gap gap (delta) between on and off (sometimes referred to as hysteresis incorrectly).

Most thermostats have a fixed delta. If this delta is quite small or the thermostat is in a location where is gets a cold draft when the door is opened, it could be the cause of the cycling.

I use Home Assistant with an independent temperature and a relay to call for heat (on my UFH system). I can set the delta to whatever I want (it is actually set to 0°).

During the cycling, is the thermostat calling for heat all the time? If so, are the TRVs set to a higher temperature than the room stat? Or are you running without a room stat? If the latter, perhaps you need one to “Turn off the HP”.

Thanks Borpin. I don’t think this is a thermostat issue since I have set them all to a high temp to keep them open, and trying to keep the desired house temperature on the compensation curve. Feels like a modulation issue - that the unit can’t run low enough just to maintain the temperature.

I may see if I can set the timer to run for a couple of hours, off for a couple of hours etc, see if that helps!

I wished I had added the monitoring years ago - shows how high electricity prices have focussed the mind!

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