Hi Matt,

It is always difficult trying to give definitive advice when one does not fully know how your system is installed and configured, but I will try to interpret what I think is happening from the information provided.

In both instances your system is cycling, which is probably to be expected at this time of year, though not necessarily desirable. As you are no doubt aware, the heat loss from your home is dependent upon various factors, the main one being the temperature difference between indoor and outdoor, but solar gain, wind chill and rain effect can also increase or decrease the overall heat loss. Looking at the specification data for a unit of your type, even when operating at minimum compressor speed, the unit is capable of producing 3.8kW of heat energy, with an outside air temperature of 12C, and a required Water Flow Temperature (WFT) of 40C. If the heat demand of your home is less than this value under these operating conditions, then the system will cycle.

The upper graph would appear to show that the heat pump is starting, and continuing to run for a number of minutes after having reached the required WFT, and is then being switched off by either the thermostat or the heat pump controller.

In the lower graph it would appear that the heat pump is being started and running until the WFT reaches the now lower required temperature, it then appears to be being switched off by I suspect the heat pump controller. Whilst still cycling, I suspect the heat pump is running for a shorter period of time, and probably using less electrical energy.

When carrying out tests and observations on one's heating system, try to only make one adjustment at a time, and then observe the affect over a lengthy period of time. Dependent upon the thermal mass of your home, it can take several hours for the indoor temperature to fall by just 1C with the heating switched off, particularly when the weather is mild.

Looking at the manual for the Salus RT510TX Thermostat, I cannot see any mention of ITLC capability. What is the setting of the two DIP Switches? If DIP Switch 1 is set to 6CPH and DIP Switch 2 is set to TPI, then this could be the reason why your system is cycling 6 times per hour. Set DIP Switch 2 to Span +/- 0.25C, increase the thermostat setpoint by 2C, and monitor the operation of your system for several hours. If possible record any changes in indoor temperature.

https://library.mitsubishielectric.co.uk/pdf/book/Ecodan_FTC6_PAC-IF071-3B-E_Installation_Manual_BH79D843H02#page-38-39  

For your flow temps, one thing you may want to check is how the flow and return temp probes are installed.  Mine were installed badly and under read the actual flow temp by 4-5 degrees.  Let me know if you need any help to check these and I can tell you what to look for and where to look. 

@derek-m Thanks Derek,

That's really helpful of you.  I had wondered whether cycling at this time of year was to be expected, so that's reassuring.  Point noted about make one change at a time, my plan is to leave it as it is for at least a few days, probably a week and then make some comparisons in energy consumption.  I'll make one tweak at a time going forward.

Thanks for taking the time to look up the manual for the thermostat.  It's actually a RT510TX+, that '+' makes all the difference as that has the ITLC.  I had set DIP switch 2 to Span and increased the setpoint by 2C already to try at let the heat pump do it's own thing.  What intrigues me is why the first graph I showed wasn't reaching a WFT of 40C, which is what the minimum on the weather compensation curve was set to.  I may email Salus to find out a bit more about what ITLC is actually doing.

@kev-m Thanks Kev,

I've got an FTC5 but looking at the manual there is a simialr section.  Are you referring to the lower and upper limit for "Heat pump thermo diff.adjust"?  I'll take a look to see what it is currently set at and maybe consider returning them to the deafult if different.  Am I correct in thinking that if you have a target flow temp of say 4C and a upwer limit of +5 and lower limit of -5, then the intended behaviour would be for the heat pump continue to run until the WFT reached 45C before switching off and then not starting up again until the WFT dropped to 35C?

Would be interested in knowing what to look for with the temp probes.  Did that effectively mean it was running hotter than it needed to and using more energy in the process?

@matt_l

Thanks for pointing out the +, (I must pay more attention to detail). Unfortunately Salus have not yet made a manual available detailing how ILTC actually operates.

I suspect it may operate in the following manner.

If there is a large difference between the setpoint and the actual temperature, it will switch on the heating more frequently, say up to 6 or even 9 times per hour. As the difference reduces, as the actual temperature gets closer to the setpoint, it will reduce the number of on cycles each hour.

This method of control may be useful with electric heating or gas or oil boilers, but it is far from ideal with a heat pump.

As a test you could try putting the thermostat into 'manual' control, so that it is always calling for heat, and see how your heat pump performs when operating in weather compensation mode.

Posted by: @matt_l

@kev-m Thanks Kev,

I've got an FTC5 but looking at the manual there is a simialr section.  Are you referring to the lower and upper limit for "Heat pump thermo diff.adjust"?  I'll take a look to see what it is currently set at and maybe consider returning them to the deafult if different.  Am I correct in thinking that if you have a target flow temp of say 4C and a upwer limit of +5 and lower limit of -5, then the intended behaviour would be for the heat pump continue to run until the WFT reached 45C before switching off and then not starting up again until the WFT dropped to 35C?

Would be interested in knowing what to look for with the temp probes.  Did that effectively mean it was running hotter than it needed to and using more energy in the process?

In my case, when the WC was set at, say 40 deg, the flow was reading low so it had to get to 44 deg before stopping heating.  So it was running hotter than it was supposed to. I checked with some cheap aquarium thermometers. 

The probes are two cables that come out of the controller each with a copper bullet on the end.  They should either be in purpose made 'pockets' or taped to copper piping underneath insulation.  Mine were taped to plastic piping in the open air.     

For the 'Heat Pump Thermo diff I assume that's how it is supposed to work.  Mine is set at -5 and +5.   However, I don't see anything like that difference between cycles so I'm not so sure. 

There is a 'Thermistor Setting' section in the Service Menu where an offset can be applied to the thermistor readings.  It might be worth checking that. 

Thanks both for responding and apologies for not coming back sooner, not been too well lately.

So the changes that I described in the first post have improved the COP significantly, I'm getting between 5 and 6 when it's been averaging between 12-14 C outside, previously I was getting 3.5-4 at similar outdoor temperatures, so a good start. 

The only downside is that it's heating the house to about 22C, when we're targeting 20C. The FTC5 controller doesn't appear to be able to set a flow temperature less than 25C on the WC, so the controller really needs to know what the indoor temperature to avoid overheating the house. 

I tried setting the third-party thermostat to take control again to see if the changes i had made to the WC curve made a difference to the COP there.  Looks like it is better than it was but not as good as WC only.  Difficult to tell as the outdoor temperature has dropped off in the last week or so. 

I've asked the installer if they could install the wirless controller from Mitsubishi so that I can run it on room temp (auto adaptation) mode, which I understand should be the most efficient.

@matt_l

The fact that the indoor temperature is slightly higher than that desired would indicate that your home is quite well insulated, and your WC curve may require a slight adjustment.

Try setting the warmer end of the curve at 25C for an ambient temperature of 15C, and the cooler end of the curve at 50C for an ambient temperature of -10C. That should produce a slightly cooler WFT when the outdoor temperature is 12C. You can try different settings until you achieve the desired indoor temperature.

When operating in WC mode the FTC5 should show a large +0C on the display, this is the offset to the WFT. Please watch the following video which explains how to change the offset towards the end of the video.

https://www.google.com/search?q=weather+compensation+of+FTC5+controller&rlz=1C1OPRB_enGB514GB552&oq=weather+compensation+of+FTC5+controller&aqs=chrome..69i57j33i160l2.31210j1j7&sourceid=chrome&ie=UTF-8#fpstate=ive&vld=cid:a02b8ba7,vid:uC3jydJydRQ

I hope that you find this helpful.

Posted by: @matt_l

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Having thread other posts on here, I’m now aware of the advice not to use third-party thermostats so I decided to look at changing it so it was working on the compensation curve on to see if I can get it working a bit better.

I think the advice against third-party on/off thermostats is a bit extreme, but the basic idea is correct: weather compensation will almost always work better if correctly set up. I use an external thermostat to stop the heat pump activating when either the house or outside is too warm, and at the times when people are usually trying to get to sleep (while not noisy, the indoors pumps are not silent, nor are TRVs if they close) and to make a few other tweaks.

Posted by: @matt_l

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When I looked at the compensation curve, the installers had set it at 50C @ 0C and 40C @ 15C, so I’m not quite sure how the above was achieving lower flow temps.  Any thoughts?  I was told that third-party thermostats couldn’t control the flow temp on Ecodan HPs.

Was the controller in compensation mode (+0 or + or - followed by a number on the display)? The above looks like it could be auto-adaptation (target indoor temperature on the display) doing its usual ramping-up-and-backing-off pattern.

Third-party thermostats definitely can control the flow temp, either over MELCloud or by directly plugging into the Ecodan's control connector. I'm not convinced of the benefits of doing so, though.

Posted by: @matt_l

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I've asked the installer if they could install the wirless controller from Mitsubishi so that I can run it on room temp (auto adaptation) mode, which I understand should be the most efficient.

Good luck with that: it completely failed for me, far too aggressive and costing far more than weather compensation, and I can't tell if there's a fault or not because there's no precise-enough description of how it should work!

By the way, I think "room temperature" and "auto adaptation" are different modes on the Ecodan and only one is available on any system, chosen by DIP switches inside the main control unit (usually installed indoors near the water pumps).