I initially posted last winter and thanks to both the information provided by the community here and in liaison with Grant support my setup has moved on somewhat and seems to be largely running ok when the outside temp is more than around +4C

Original thread is here :

Forums | Renewable Heating Hub

As there's a change in focus on the advice I'm looking for, I have decided to start a new thread rather than continue the old one.

In summary, the house is 1960’s large bungalow which is being fully refurbished in stages with a full basement below, only a small part of which is going to become habitable, the rest houses utilities and storage and of coruse acts as a good buffer to the outside temperatures.  Half the ground floor is now finished and we are living this part now with the ASHP’s providing HW and CH via UFH to this.   Roof is a "Warm Roof" but it only 90% finished and should be fully completed in Feb, so currently the thermal effeciency of the roof is somewhat compromised.

The ASHP have still not been formally commissioned and cannot be until the whole of the house is finished as until then insulation is not finished and half the UFH has not even been installed yet.

Having read the SAP calcs with the benefit of hindsight there are so many mistakes in them I’m not sure they are worth the paper they are written on.  I assume they were used to calculate the sizing of twin 10kw ASHP’s which on face value seems quite a lot for a house that’s going to be fully refurbished with new windows, roof and insulation.

Nevertheless the finished rooms are keeping warm the majority of the time on a single master only ASHP and the master uses what appears to me to be a modest amount of power to do so until the outside temp falls to a level where defrost cycles starts to kick in.  At this point the power usage goes up considerably.  Instead of sitting steady anywhere from 1.3-2.0kw usage, the power starts to spike considerably shooting up to near max of a single ASHP around 3.5kw, then after 15mins plunging to 0.2-0.4kw for the next 15mins.  Overall heating starts to slide slowly as the inefficiency of this constant defrost-max-defrost cycling takes its toll on the heating loop temp.  At was at this point I bean to investigate more about my cascade setup and how it was supposed to work.

The ASHP have been installed in a cascade setup which seems quite uncommon and I have struggled to find much information on this other than from Grant who finally got the slave running for me about 2 months ago by talking me through a couple of missing config params.  In summer of course it didnt really matter as the master alone was more than covering our demands.

Setup is as follows:

• 2x 10kw Grant Aerona ASHP in a cascade setup as master & slave with just one EP-001 and one EP-002.   Slave is currently setup via param 46-13 to kick in when the outside temp is 3.5C or lower.  Only the master ever heats the HW regardless of outside temp.  I have confirmed that this is an approved setup by Grant.
• Both ASHP have only the non-smart old style Chofu controllers.  The controllers are in the unheated basement which is typically around 10C in winter and 15C in summer.
• Both ASHP have a grant 30l volumiser installed on the plinth next to the ASHP unit and as a result we do not have a central combined volumisers/low loss header tank.  Both volumisers have the immersion disabled as enabling it seems to cause each one to pull an additional 3kw when the defrost cycles kick in which is even worse. Again this is an approved Grant setup although I don’t really know why we ended up with two volumisers rather than the low loss header tank.
• Both ASHP/volumisers are plumbed in parallel into the main loop.   The main loop feeds either the HW tank when the relay is enabled, or two UFH manifolds each running roughly half the house.   Each manifold has zone valves each of which have WiFi thermostat controllers in the rooms.
• Both ASHP have had their pump speeds set to minimum via DIP switches on the pcb. The flow regulator valves have not been set and tbh I’ve never seen a ball int he window which makes me suspect they are still on too fast and the ball is flying off the end.  I have no idea what the correct setting should be however even if I was to try adjust the valves.
• 3 UFH zones completed in the house so far.
  • Kitchen/lviing/dining - large room 980 sq ft. Off Manifold 1.  Room is stable around 20c - thermostat is set to 22C. 
  • Main bedroom - 305 sq ft off Manifold 2 room fluctuates between 20-21c - thermostat is set to 21c
  • Main ensuite - 110 sq ft off manifold 2.  Room is stable around 20-20.5C - thermostat is set to 21C
  • These 3 zones represent a total estimated heat loss of around 8kw and 45% of the house floor area. These areas all have high spec double glazing and a new MVHR installed.
  • The remainder of the house has no CH or any heating at all as not refurbished yet and old gas boiler was ripped out. Old windows are double glazed but some have failed due to age, all have trickle vents and some rooms have wall vents for additional airflow due to humidity.
  • Remainder of house not yet refurbished is 1400 sqft ground floor with UFH plus a further 340 sqft in the basement with planned new radiators (as limited ceiling height and wall foundations prevent fitting UFH).
• We have installed a new 10kwp solar array and a new 10kw inverter with 16.1kwh of batteries.  We are on Octopus Cosy tariff so top up the batteries 3x a day between 4-7am, 1-4pm and 10-12pm. 
• HW is set to run at 55C and kicks in at 1pm (as off peak at this time) until the 240l tank is up to temp and the thermostat turns it off at around 48C which typically takes around 90-105mins.  Otherwise CH is on for the full 24hrs when not overridden by the HW cycle.
• We are in West Yorks.   Climatic curve has been set mostly by trial and error to 32c@20c / 45c@-6c
• Following is a list of the only params which have been changed from the defaults. Note all params are identical on both master and slave units.
  • 21-02  WC changed to 45C
  • 21-04  WC changed to -6C
  • 21-05  WC changed to 23C
  • 31-11  DHW changed to 55C
  • 31-21  changed to 150mins
  • 43-01  frost protection room air temp changed to 6C
  • 43-11  frost protection outside air temp changed to 2C
  • 46-00 enable heater (backup slave ASHP as wired) to 3
  • 46-01 changed to 55C
  • 46-4 changed to 0
  • 46-05 changed to 2
  • 46-11 changed to -20C
  • 46-14 changed to 2
  • 51-46 heater type (backup = slave) changed to 1
  • 21-41 hysteresis of heating water set point changed to 4C
  • 46-13 outside temp for backup heater (slave) to enable = 3.5C

So the problem starts to be when the temp drops to around 4C or less and the defrost cycles kick in with increasing regularity. These cause big spikes in either the master alone or both master and slave power use. The colder it gets the more pronounced the saw tooth use of electricity gets. There is a gradual slow drop in room temps too, which I suspect is due to the loss of loop and floor temp from the defrost cycles taking their toll. This appears to be quite a common issue from what I’ve read here and on the FB group.

From what I had read having a slave enabled should substantially mitigate the defrost cycle issue as in theory only one ashp is allowed to defrost at once with the other picking up the heating whilst the defrost cycle completes.   In reality however when the slave kicks in at whatever temp is set, broadly speaking the power usage just doubles! Twice as many defrost cycles and overall no real increase in room temp.  This seems contract to my expectations I would have thought two ASHP’s would each modulate their power down to use half the current loading compared to the master alone before the switch over, yet it doesn’t seem to do that!  I also expected the slave to efficient wipe out the losses from the defrost cycles. 

So what should I change to:

1. Get both ASHP’s modulating together when both master & slave are active
2. Reduce the impact of the defrost cycles on master and master/slave
3. Should I change the set point at which the slave kicks in?  Currently set to 3.5C.   Debating dropping that even lower to 3C or even 2.5C.
4. Should I attempt to reduce the flow rate further by adjusting the valves (with a spanner?) and if so to what flow rate?
5. Any suggested tweaks to the WC curve?  By some people’s advice my temps are set quite high, but low by others.  It does feel like we have what has been described as the defrost doom loop so reducing temps in that case would make sense.  I can see a case for either turning down the temp at the bottom end (to reduce defrost frequency) but also for turning it up (to try drive more heat into the loop between defrosts).

So having typed all that over a couple of days off at xmas and reading up even more whilst doing so I have decided to make a change to my WC curve in an attempt to avoid what I suspect is likely the root cause - the "defrost doom loop"!   As a result I’ve reduced the bottom end curve from 45C@-6c to 38C@-2c.   This should reduce the demand and loop temp by around 4C at the point at which the doom loop has been starting.   The week ahead is forecast quite cold so I figured it was a now or never opportunity to test out quite a radical change….

• • 21-02 WC changed from 45C to 38C
  • 21-04 WC changed from -6C to -2C

I am of course still very much open to other any suggested changes as per above…

Adam