Welcome it sounds like your heatpump is too big for your property and as you have found out over zoned.

It sounds like you have it running as good as you will get it without major changes.

What unit are the Heatmiser thermostats communicating with.  There usually a control unit that controls the manifolds like a UH-8.  Are the Heatmisers wireless or hard wired?

The reason I ask is that the only improvement I managed to make is to get the entire downstairs controlled through 1 of the Heatmiser thermostats so all zones open together so at least I could be sure everything was calling for heat at the same time 

@gary Thank you for your reply. We were already coming to the conclusion that the ASHP might be oversized.

We do have a UH8 Underfloor Heating Control System and the Heatmiser thermostats are hard wired. We have been considering removing the thermostats and having Daikin’s Madoka controller installed. If anyone has knowledge of this perhaps they could indicate the pros and cons before we proceed further or would it be better to do as Gary suggests and control the whole of downstairs with one of the Heatmisers? Would that be a problem if they are hard wired?

Posted by: @boylips

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I have not posted on the forum before, but have been following other posts for some time and have been making tweaks to my system using suggestions by more experienced ASHP owners.

We have the above 8kw system heating a new build 4 bedroom house, insulation very good, total floor area 150 sq m. situated in rural Cambridgeshire. There is underfloor heating downstairs with 7 zones controlled by Heatmiser thermostats. 5 radiators upstairs, 4 of them with TRVs plus 2 ladder radiators in the bathrooms. The one radiator without a TRV is in the main bedroom where the one Honeywell thermostat DT90E is also situated, so not ideal. There is a 300l unvented hot water tank in an upstairs airing cupboard.

We were initially told not to change any settings and let the thermostats switch zones on and off. During our first winter it became apparent that the weather compensation settings were not optimised for efficiency -3,48 removed link The heating would blast away in the morning, it would get too hot and then everything would switch off for the rest of the day.

After a lot of trial and error we now have the thermostats downstairs set to high, so they rarely shut down the individual zones. We operate with one zone keeping the heating on all day, aiming for an IAT of 21 downstairs and 19 upstairs, but still shut the heating off overnight unless the OAT is very low. We find that the rooms get too warm if we keep the heating on 24 hours. We also leave upstairs radiators fully open.

Weather compensation is now set to -5,42,10,30 although during the recent cold spell we offset the WC by -4 as it was getting too warm indoors.

Our main issues now are that cycling starts to occur when OAT gets to 9/10 degrees C and we have noticed that the flow rate can drop to 6.5 or 7.1 l removed link

Flow rate is up at 21 l/min when the heat pump first comes on in the morning but by mid morning it is starting to fall. The Daikin installation manual states minimum required flow rate is 12 l removed link  

On the whole we are happy with our heat pump, especially after the changes, we generally tend to be too warm rather than too cold.

We do not have any monitoring equipment other than the built in MMI screen of the indoor unit and no technical knowledge. It has been a steep learning curve and thanks in part to this forum we have managed to now have a set up that is more comfortable, efficient and economical than it was before.

If any members are able to comment or make further suggestions it would be much appreciated.

We do have a scheduled visit from a Daikin service engineer in February.

The first thing that I noticed is if you had to use a -4C offset when the weather was colder this would indicate that the cold end of the WC curve is too high.

Are you still using the heatmiser to control room temperatures? If so, this is probably why the flowrate falls as UFH loops are closed off.

You will probably get cycling starting in the 9C/10C range because the heat pump cannot run any lower on a continuous basis. How frequent is the cycling?

What size is your heat pump and what was the calculated heat loss for your home?

@derek-m As this was the first prolonged cold spell to test the WC we expected to have to make changes.

The Heatmiser thermostats downstairs are only used to switch the heating on in the morning and off at night. Otherwise they are all calling for heat during the day as set high.

As the OAT today has been a fairly constant 11/12 degrees C with little solar input and no wind, the LWT has remained at 30 degrees C with a flow rate of 7.1 l/min and no cycling. The IAT downstairs has been at a constant 21-21.5 degrees C.

If the heating is interrupted to heat the hot water during the day, when the heating comes back on cycling will start every 6/7 minutes. As a result we have started to set the hot water to come on at 2000 when the heating goes off.

We are starting to learn what works best for our system, but is the flow rate of 7.1 l/min too low or even 6.5 l/min at its lowest?

We have an 8kW heat pump but I cannot find any info regarding heat loss, but as a new build house we are well insulated.