Mitsubishi Ecodan Heat Pump - Flow Temperatures

[kunalg57]

Hi all,

 

Posting regarding some questions I have with my Mitsubishi heat pump setup, which has been in place for the last 6 months or so. 
 

We have 2 heat pumps installed - a 14kW system to service the ground floor underfloor heating (~200sqm) and a 8.5kW system to service the first floor and loft fan assisted radiators. 
 

The underfloor heating on GF is split into 8 zones, and I have app controlled TRVs in each zone on first floor / loft (5 zones) to give me individual heating control for each room. 
 

The reason for this setup was to create (in principle) an energy efficient system where we would only use energy to heat the rooms where required. This was my first foray into heat pumps so please excuse the amateur approach based on some videos I’ve seen and reading about HPs online.
 

The 14kW system is set to a fixed flow temperature of 35 degrees to service the underfloor heating. The 8.5kW system is on a compensation curve from 52-42 degrees based on external weather temperature. DHW is set to 50 degrees for both (2 x 300L Mitsubishi pre-plumber cylinders). 
 

Over the last 2 weeks I’ve observed an odd fluctuation in the 8.5kW flow temperatures (image attached). Every hour the flow temperature falls and then picks up again back to the set level. This is happening every hour, particularly overnight and early evening when external temperatures are lower. Call for heating from 3 of the 5 zones on first floor/loft radiators is consistently on during these times - mainly because the rooms are not reaching their target set temperature (19 degrees). 

 

Temps have been extremely cold recently (-5 overnight) so this is the first time the heat pump is being “pushed” I guess.

 

Is this normal? Are the hourly dips in flow temp caused by the heat pump kicking into defrost mode or does this point to an underlying issue with the heat pump, or indeed the way that I’ve set up my system? 
 

Any guidance would be much appreciated.

Thank you so much!   

Edited December 11, 2022 by kunalg57

[PhilT]

It's the defrost cycle, lasts around 6 minutes

[kunalg57]

40 minutes ago, PhilT said:

It's the defrost cycle, lasts around 6 minutes

 

Thanks Phil that's super reassuring.

 

Another question, and again apologies if this is basic.

 

The flow temperature of the 14kW heat pump is set to 35 degrees. The space heating pump on the internal DHW cylinder is set to Speed 2, which is required to generate sufficient flow for the UFH zones. There is a 5-6 degree drop-off in flow temperature once the heated water flows through the space heating pump into the UFH manifold (flow temperature drops by 5-6 degrees, not return temperature), and I can't figure out why. 

 

When I set it to Speed 1, this drop-off in flow temperature goes away. 

 

Do you know what could be causing that?

[PhilT]

5 hours ago, kunalg57 said:

There is a 5-6 degree drop-off in flow temperature once the heated water flows through the space heating pump into the UFH manifold (flow temperature drops by 5-6 degrees, not return temperature), and I can't figure out why. 

 

When I set it to Speed 1, this drop-off in flow temperature goes away. 

Not sure I follow. Do you mean the drop you see on the MELcloud graph? This always happens when the HP pauses for a mode change, heating to hot water then back again. If you try pump on speed 1 throughout it should still drop, maybe to a lesser extent