@bontwoody:

I have upped the heatmisers - they are calling for heat constantly but I am still getting the below feedback: 

There was no other hot water demand from 8-9am, any ideas what might be causing the flow rate fluctuations? 

@jaket If you have all the circuits in the house calling for heat then unfortunately the heat pump can’t modulate down any further so it has to start turning off till the flow temp drops then it turns back on again.

if you turn the flow temp up this will stop you just need to work out what temp you need to stop the cycling 

@jaket It could be that your emitters cant dump enough heat into the building quickly enough and so the heat pump turns off and waits. You could try opening all your radiators valves fully and see if that makes a difference. There is a tricky relationship here with flow temp. If you raise flow temp you increase the difference in temp between the emitters and the room and so dump heat more quickly, but you also put more heat into the house potentially making it too hot.

I notice that you flow temp at the moment is 34C some heat pumps struggle to work reliably as low as that so I doubt you can go lower and might want to increase it. Its a bit of trial and error really and it really helps if you have a good monitoring system

Posted by: @gary

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@jaket If you have all the circuits in the house calling for heat then unfortunately the heat pump can’t modulate down any further so it has to start turning off till the flow temp drops then it turns back on again.

if you turn the flow temp up this will stop you just need to work out what temp you need to stop the cycling 

Ah so you are saying my WC curve is too low? Make sense, I will shift the curve up. 

The house didn't feel particularly warm after 8 hours of having the heatmisers turned up, so perhaps the increase in the outside temp over night (from -2 to about 6) has caused the heat pump to try down regulate despite the heatmisers calling for more heat... 

@jaket yes curve too low there are plus and minus symbols on the front screen turn it up a few degrees you don’t need to adjust the whole curve this will move the whole curve up temporarily.  Monitor it for another hour and see what you get.

You may find that to stop cycling you will need to run it hot enough that you reach your limit on your thermostats where you don’t want the house any warmer so it all turns off till the house cools down and it starts again.

Firstly, a bit good news regarding COP - it’s not as bad as you might think! Mitsubishi reporting of electric consumption is known to be overdone – for example my January consumption so far is reported as 500 whereas it is measured as 450. So your Jan COP could be 902/340 = 2.7, which is not terrible for January. So those tweaks to the curve you made may already have had a positive outcome.

I ran WC mode last winter, but am running Room Temp mode this winter and I can see that the cycles, when they occur, are longer and more variable in temperature but overall are more efficient. It might be worth trying a day or two using Room mode and see what kind of temperatures your unit settles on and use those as a basis for your curve settings.

The flow rate range for the 6kW unit is 8.6 to 17.2 lpm, so a setting 3 was at the lower end of this but ok, but 5 might be too high if it is resulting in 19 lpm. Not sure what the implication of this is, it might be that the water is flowing too fast to extract the heat from the heat exchanger in the external unit effectively.

Ideally, to get an efficient system it seems to be best that the flow is balanced between the primary and secondary sides – this is difficult if you have a changeable load. You have two pumped zones, one of which covers two thirds of the house, and the other one third using two loops – so there are a number of quite different combinations of demand that may be on or off. Setting the flow temperature by the external temperature in WC mode is only half the equation – if you only had one loop in the UFH open for example then very little heat would be transferred so the RWT would rise and then the pump switch off. There may be a buffer tank of some size in your installation, I can’t tell.

If the weather forecast is for one of those days when the outside temperature remains quite flat it is ideal to use as a guide to the heat loss from the house as it won’t be varying much across a the day, and if you can find a flow temp that holds the IAT level then you have a definite benchmark. Not sure about the idea of opening the radiator valves fully though, ideally the radiators should have been balanced so that the resulting heat output from the first to the last radiator on the loop is consistent - not a process I feel comfortable with trying change myself.

How much energy is going into the HW tank on a cycle? Check where the thermistor pockets in your HW tanks are – there are probably a couple at least. The thermistor screen viewable in the settings menu will tell you what they are reading – though you only get one temperature on the MelCloud screen I think the second one determines when the controller stops the heating cycle.

@harriup Thanks for your comments! 

I am going to give the current tweaks a few days to run and see what the outcome is. 

The outside temp change in the last 24 hours has been all over the place - so not ideal for trouble shooting, but if the COP doesn't improve I'll give the other modes a try. 

@gary & @bontwoody 

Okay, so heatimisers are not limiting anymore - I set them to 23c, and the temp achieved maxed out at 21c, this is with the higher WC curve. 

But... I am still getting some aggressive cycling. 

The rads and UFH are fully open.

There is quite a high delta temp, but I am not sure if it's the return temp triggering the cycle to go off? (Not sure why this would happen as there is nothing limiting the set up from dumping more heat into the house given that everything is set to 23c, and nothing has hit that threshold? 

@jaket Hi- your getting there its cycling less than it was, increase the temp further and see if you can get rid of it.

@jaket @gary I concur. Well done. Once you have stopped it cycling you can set about rebalancing the rads to get your individual room temperatures right. Delta t looks fine, between 3-5 degrees is pretty usual

@jaket as the unit is cycling so much you wont get a stable dT from the brief periods its stable it looks like it would be 35C flow 32 return so a dT of 3C that is exactly what I would expect at those flow temps, you won't see a dT of 5 until you get into the 40's.

The heat pump is trying to supply a flow temp of around 35C, the rads and UFH get rid of 3C of that heat leaving the return temp to the HP of 32C.

Then heat pump transfers in more heat to account for the loss but its working at its lowest possible power. 

Over a period of 15 minutes this causes the flow temp to increase beyond its set point so the heat pump has to shut down.  Once the flow cools its starts back up again and the cycle repeats.

The only way to counteract the cycling is to increase the flow temp to such a level that the heat pump can operate at its lowest power without causing the flow temp to increase beyond its set point.

Posted by: @gary

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@jaket yes leave at 5 19 litres per min is fine mine runs at a similar rate

apologies. I have to make a comment on flow rate. Jake’s HP is a 6kw it is not an 8.5 model and it’s flow rate recommendation is around 11 LPM. I also note that the first post from Jake showed the HP trying to maintain a DT5c however on the recent hourly temperature chart it was down to 3 LPM. In order to accommodate all the modulation settings that the HP can provide I have found it’s important to keep Farley close to the recommended flow rate for your model of HP. Mitsubishi have changed the data book recently which now shows the recommended flow rate for each model. This is new because earlier copies of the data book only presented a range of flow rates. See below. 

I hope this helps rule out one possible cause. It clearly is cycling which suggests it’s not able to dump the heat fast enough. Do you know what the emitter value of the whole system. One side issue - is your flushing valve fully closed?