We’ve just moved from a Grant oil boiler to a Mitsubishi Zubadan ASHP. We have Polypipe underfloor heating throughout the whole house. The only radiator is a towel rail in the bathroom.

With the old boiler we’d have a setback temperature 2 or 3 degrees lower than the desired temp and just have the rooms heating when we wanted them to be warm. The manifold temp was set to 50 upstairs and 53 downstairs (concrete pad and takes longer to heat). 

I understand that the recommended setting for ASHP is just to have them on continually so we’ve now just set a target temperature for each room with a 0.5 degree setback.

However, since the HP was installed we’ve seen electric consumption pretty much quadruple and I’m worried we’re not setting something correctly. This week has been cold so maybe this is causing extra pressure.

The installers set the flow temp to 55 degrees. I immediately lowered this to 50 as I understood that an air pump needs to be lower to benefit from the efficiency. I’m now thinking it’s just running a lot of the time and electricity usage is therefore rocketing.

Is it better to set it to a 55 degree flow temperature and let it heat up rooms a little quicker and not be heating as continually. Or is it better to be running more at a lower temperature?

Does that make sense? If there’s any other information I can supply, I’d happily do this.

you need to measure whats going on really before you make loads of changes. If you can measure the temp of the flow and return to and from each manifold when the pump is running in steady state and see if it’s different from the primary flows on the mitshi controls.

if the delta t of the primary is still 2-3, but the delta t at the manifolds is more, then you need to open up the secondary flows more. But if it’s the same, the you probably need to play with pump speeds and slow the water down so it can transfer some heat.  But, and it’s a big bit. You don’t know if you’ve you 10 or 25 litres a minute going through the secondary side.  The problem can be of you say only have 15 litres a minute going through the house, then 7-10 litres just go straight back to the heat pump via the buffer. This narrows the delta t and shuts the pump off. 

You ideally need the same flow on the secondary side as the primary, which is a pain to know with a buffer.  I’ve got a low loss header which is a pain in the ass and I’d love the installer to bypass it 

I remember you saying all the pumps were on max, I doubt you need them all on max, but it’s best to try to measure whats going on.

Thanks. I still need to check this out. It's taken all my mental capacity to wrap my head around the heat pump. So it should be as simple as just adjusting the flow rate on each circuit on the manifold to lower it..? Is it best to try this a bit at a time to see how it goes? And then how do I judge if it's helping things? Just from the COP?

Flow rate according to the Mitsubishi panel currently is between 21 and 23.

Honestly I don't think there's much point in me questioning this with the people that installed the pump. They seem to have a very limited, narrow field of knowledge and no overall understanding. One actually admitted he'd never worked on an underfloor heating system before. 

How large is your tank you have reheat set at 5Cish depending on how much water you use you may not need to reheat.  Ignore what the tank temp is reading there could still be plenty of hot water available.  If you run a tap for a bit it could cause a reheat that will be very inefficient to raise it 5C.

We only heat overnight for 90 mins at 00.00 lasts all day even if the tank says it’s 20C there is still plenty of hot water for bath at 6.30

We only have the hot water schedule set for 2 hours 00.00 to 02.00 so it can’t reheat.

I must say Im also coming to the conclusion that there is a problem of this nature.  OP has optimised the initially high flow temperature and its now reasonably low (given that there is, sadly, an almost certainly unnecessary buffer).  Thus, like you, I suspect that water flows are wrong.  I suggested he putting some temp sensors on the buffer ports to understand whats going on, for me thats the most likely next step.  Unfortunately I think installers see buffers as the solution to all problems involving potential mismatches between heat pump and heating system, as opposed to the cause of a high proportion of the problems reported on this forum.