I have just discovered that HeatpumpMonitor now shows 'compressor starts':

HeatpumpMonitor.org starts & starts per hour - Hardware / Heatpump - OpenEnergyMonitor Community

We are not on HeatpumpMonitor.  I was interested in knowing how our system compares to others.  What does good look like? 

We have 1,817 starts/year, or 0.21 starts/hour, measured over the 6/12 'heating on' period.  Mean flow temp is 33.4degC.  That puts us very close to the middle of the pack.  From that, I conclude that 'we are probably OK'.

How is everyone else using data to conclude whether or not their compressor cycling is OK?

Are there other parameters/measurements to consider?

Posted by: @clockworks

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I had a 6kW Ecodan installed 6 weeks ago, replacing an old oil boiler.

It's working, house is warm, I'm tweaking it to get the best economy.

I've added some external temperature and electricity usage monitoring - Shelly EM clamp on the supply cable from the CU in the garage, and three Shelly temperature probes to the flow, return, and radiator circuit.

I'm running in weather compensation mode.

Looking at the data from the Shelly monitors, the heat pump seems to run pretty consistently when bringing the rooms up from the "overnight" setback, or when the outside temperature drops rapidly, but once the house is relatively stable, it is "cycling" roughly every 20 minutes.

The flow (LWT) gets up to where it should be according to the compensation curve, stays there for a couple of minutes, the power consumption drops to 30 watts (just the 2 pumps running), all temps drop rapidly in about 2 minutes, and the cycle starts again.

The LWT is varying by about 6 degrees, with the radiator flow temp tracking it very closely (rad sensor fitted just past the secondary pump).

The return sensor shows the temp varying by 2 or 3 degrees.

 

I've tried using the plus and minus buttons on the Mitsubishi control panel. I've made sure all the TRVs are fully open. Each time I change something, the cycling may stop for up to 2 hours, but as soon as everything stabilises,  it's back to the 20 minute cycles.

Running costs are reasonable (compared to the oil boiler) and the house is warmer.

I'm just concerned about the cycling, as most people say "low, slow, and always on" is best.

 

Edited to add:

4 bed detached dormer bungalow, built 1966, Cornwall

Low-loss header fitted.

Honeywell Evohome smart TRVs

Can you post some plots so we can have a look please?

At present many heat pumps will be defrosting (which can look like cycling) because current temps are in that zone at least in much of the UK (maybe not in Cornwall).

All heat pumps will cycle when the OAT is such that they cant modulate down far enough.  Penzance today is apparently 8C, which actually should be in the middle of the 'steady zone' if the capacity of your pump is well matched to your house (which is, sadly, relatively rare).

Whats the LLH for?

If you post some plots I am sure several people will be able to comment and/or point you at additional data you need to collect to diagnose.

First pic is the power pulled by the ASHP, pumps, valves, and controller.

Second shows the temps using "bullet) sensors wedged onto the pipework under the insulation.

Flow and return are inside the garage, about 2 feet from the heat pump. Radiator sensor is on the pipework disappearing up into the loft, after the secondary pump.

The LLH was fitted by the installer because " we always fit them now, keeps customers from messing it up by closing all their TRVs".

I now know that a lot of people don't like them, and they can waste a bit of energy. It does seem to be working OK though, as the primary and secondary temps track each other extremely closely.

I'm near the coast, halfway between Penzance and Helston. RNAS Culdrose weather station matches very closely to the actual temps we get- generally about 1 degree lower than the BBC, Openweather, etc.

Edit to add:

Although I'm using Evohome to "call for heat", not the Mitsubishi wireless thermostat, the Evohome boiler relay is constantly on, as there are always at least 2 rooms not up to temperature, plus one big radiator that's fitted with a normal TRV that's fully open.

I did try setting all the Evohome TRVs to 26 degrees, and running with a more negative offset on the Mitsubishi control panel. None of the rooms actually came close to 26 degrees, so all the rads were in the circuit and open. The heatpump still went back to "cycling" after 2.5 hours.

As for sizing, heatloss calcs suggested just under 6kW at minus 2 degrees.

Going on our coldest day so far, zero degrees plus or minus 1 for 24 hours, the ASHP got through 29kWH. Specs say COP of around 3.2, so that's 93kWH of heat in 24 hours, or average 3.9kW per hour.

Unfortunately I don't have temperature plots for that day, just the total power consumption.

I guess that means the heatpump is a little bigger than absolutely necessary? But not massively out.

Im not an Ecodan exert but I would say that the first picture confirms its cycling, roughly every 15 mins

If its about 8C outside, which the BBC suggests it is, then this indicates either a slightly oversized heat pump or that the LLH is causing mischief. You are apparently consuming 1kW and your FT is about 36, assuming it is about 7-8 outside the Mitsubishi databook indicates a COP of over 4, which suggests a 4kW output, which is above the minimum you would expect it to modulate down to.  Im therefore unclear why it appears to be cycling and also note it never reaches a stable point.

To me this feels a bit odd but (perhaps) not horribly so.  This needs someone familiar with Mitsubish to advise whether this performance, where the FT doesn't really stabilise during a cycle, is normal for this pump or whether you have something amiss.  If running costs are good then it maybe nothing to worry about.

Sorry I cant be more helpful.

Can you post a picture of your LLH how much volume does it have?

What is the flow speed in your primary circuit, go to service menu, running information, code 540, the value it displays is in Litres/min.

A 6KW Ecodan should ideally run around 11L/min if its more than that turn the primary pump speed down.  

If your primary pump speed is controlled by the main controller then that's in the same service menu in the screen above the running information menu named pump speed it ranges from 1-5 you can change it and recheck the flow rate.

The heat pump needs to be running and in heating mode to check all this.

You said the TRV's are all open but are the lockshields on the other side of the rads all open or have these been adjusted since the heat pump was installed.

LLH is something the make up themselves apparently. Basically a length of 32mm copper, about a metre long. I make that about 0.8 litre?

Both pumps are Evosta DAB 3 units, 3 speeds on each of 3 profiles.

As installed, the primary was set to constant differential flow, lowest speed. Running info 540 says 14lpm.

I've just looked at the instructions for the pump, and maybe it should be set on constant curve mode? I've just switched to this, and code 540 is showing 10lpm.  Would this be better?

The lockshields are set to keep the rads reasonably well balanced. Not tried it with them all fully open

The flow wasn't massively out, I'm no heating engineer but a heat pump needs constant flow, so the differential flow setting doesn't seem right and the constant curve is closer to the required max so I would try that out, it won't do any harm, it probably wasn't causing the cycling you are seeing.

As you say the LLH is just a pipe to allow the heat pump to circulate back on its self in case the TRV's are closed so it has somewhere to flow rather than causing a flow error.  Not the ideal design but I understand the reason they say why they do it, if you had a mitsubishi pre-plumbed HW tank it has exactly the same arrangement.

You measured the same temp at the rads as the primary so you are getting good heat transfer between the circuits.

When you took the temp readings what should the flow temp have been?  I'm guessing 38°C, if so, what's happening is the heat pump reaches the required flow temp, its tries to reduce the input power that drops to 1kw on your plots but before it can react further the flow temp reaches 39C and this causes the heat pump to shut off.  The flow temp drops and the heat pump fires up again and the cycle repeats.

In my experience, the Ecodan will only allow a 1C rise (in weather comp mode) above the set flow temp before it shuts off.

Only my opinion, but to me this sounds like you are heating a very minimal water volume that is too small for the Ecodan to be able to react to in time and that is causing the cycling.

If it were me, I would remove all the TRV's and open up all the lockshield valves so you have the maximum water volume and flow available, if you still have the same issue then you probably need a volumiser tank to increase the available water volume to allow the heat pump time to modulate.

I agree with @gary. 

You need to go back to basics and get it working with weather compensation as the principal/only control and other external controls disabled.  If there is still a problem then as @gary suggests you may need additional system volume.  It sounds like it might be a case of the LLH masking an underlying problem that really should be fixed (which is of course its principal function as far as the installer is concerned!).

That does seem to fit with what is happening - the peak flow temp gets to one and a bit degrees above what I'd expect it to from looking at the WC graph.

Watching the power consumption meter, I can see that the heatpump is capable of modulating down to around 600 watts - it does this at the start of each cycle, then steps up to 900 watts after a few minutes.

Seems odd that it doesn't really modulate down again as it reaches the target flow temp.

I'll try opening all the rads at both ends, and reducing the flow temp by a few degrees.

Presumably I should use the weather comp curve to set the temp in rooms with "open" rads, then try and balance the room temps using the lockshields on rooms that want to run too warm, and use the smart TRVs just in the bedrooms to keep them a couple of degrees cooler?

Posted by: @clockworks

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Presumably I should use the weather comp curve to set the temp in rooms with "open" rads, then try and balance the room temps using the lockshields on rooms that want to run too warm, and use the smart TRVs just in the bedrooms to keep them a couple of degrees cooler?

Yup, or better still turn down the lockshields in the bedrooms so you dont need TRVs there either

If you need time-variable temperature in the bedrooms, then turn down the lockshields there so that, without the TRVs, the bedrooms run at the highest temp you want them, then use the TRVs to reduce it from there for the time periods you want it cooler.

In general heat pumps run best with the fewest possible external controls, ideally none!

@clockworks I wouldn't change the flow temp yet, try and see if its any better at 38C if you drop the flow temp cycling is only going to get worse anyway.