8 hours ago, Michael_S said:

would assume that it then either gets confused (thermostat says current temp is 30C so why is return so warm or perhaps it sees return at 34C and increases flow to 39C?

 

But basically it needs to either reduce output (modulate down) or accept a bigger difference between the flow temp and the tank temp.

Don't think it gets confused it just monitors return temp and responds to that.

 

But will it modulate down, while doing hot water.  Mine certainly doesn't, it runs full load and full circulation pump speed throughout the whole DHW heating cycle. Some heat pumps allow different settings. 

 

9 hours ago, Michael_S said:

One issue with the PHE+Pump solution is that you don't maintain stratification so well

Not so sure about that. Once heating cycle is complete the cylinder will stratify as normal

 

8 hours ago, Michael_S said:

not sure a new one would go through our loft hatch

I did a slimline cylinder 

 

 

21 hours ago, Michael_S said:

Thanks.  Where is the write up?

 

So what goes wrong if the cylinder coil can't give up heat quickly enough?

 

Link upthread already. This takes you to the conclusions 

Nothing "goes wrong". The flow temp will get very high, but the HP does not object - though the CoP will suffer. As @JamesPa says, you can take quite a hit before that justifies the capital cost of fitting a different cylinder.

And if you go onto something like Octopus Go you could be using 8p/kWh to do the DHW so COP becomes even less relevant. 

22 minutes ago, IGP said:

And if you go onto something like Octopus Go you could be using 8p/kWh to do the DHW so COP becomes even less relevant. 

 

22 minutes ago, IGP said:

And if you go onto something like Octopus Go you could be using 8p/kWh to do the DHW so COP becomes even less relevant. 

Indeed.  Even less reason to throw away a perfectly good cylinder.  For a long time the industry insistence on doing so has annoyed me, it now annoys me even more.