This has been going around my head all night. So I’ve tried to use my system to put numbers to things.
My Ecodan R32 8kW (PUZ-WA85VAA) seems to be able to be able to go down to a minimum output of about 3.3kW.
I’m looking at the morning of the 17th of October to see an example of a mild time when we are on the edge of needing any heating on Emoncms - app view
The MyHeatPump app suggests that my system volume is somewhere around 80L-100L. This probably does vary because of the TRVs set up to limit temperatures in bedrooms, these are most likely to be closed down when it’s milder.
My heat pump seems to be set up to allow a drop of about 10K in flow temperature before starting the next cycle.
On the 17th the flow temperature target seems to be around 31C with the Auto Adaption mode allowing an overshoot of about 5K. This matches the 5K overshoot and 9K undershoot settings on the controller.
So assuming the flow temperature needs to be increased 10K again until the target is met again, just heating the volume of water would take (using this calculator):
16.9 mins - 80L
21 mins - 100L
Obviously there will also the time added because of the heat emitted from radiators and lost through pipework outside the thermal envelope. There will also be the heat taken to warm up the thermal mass of the pipework and the actual radiators.
If I were to add a 50L volumiser this would give:
27 mins - 130L
31 mins - 150L
In between cycles this heat is still being pumped around the system and will eventually be emitted (ignoring any heat loss) so I guess it could be thought of as a distributed thermal store.
As you say @matt-drummer it’s better to add volume by adding more emitters as this would allow the flow temperature to be reduced and might stop the cycling altogether if the emitted output matches or exceeds the input heat.
However in circumstances where this isn’t the case (oversized unit, undersized emitters, mild weather) isn’t there benefit to a volumiser allowing longer cycles, allowing the heat pump to operate at lower temperatures for longer? And then stay off for longer as the flow temperature would drop slower while the heat in the system volume is emitted.
It would appear that this would at least prolong the life of the compressor, I just can’t work out if it would increase efficiency.
All of this might also depend on the behaviour of the Auto Adapt mode where it allows a loose flow temperature. I think in Weather Compensation mode on the Ecodan the heat pump doesn’t allow much over and undershoot so maybe there would be reduced benefit.
I may also have completely misunderstood or miscalculated something.