@jamespa I don't understand any point in an air test, if that fails, then the pipes are then broken, and the system cannot be refilled without digging up concrete and ruining finishes. Am I missing something?

Also, will the secondary circuit behind the PHE count towards the system volume of the primary circuit with the heat pump on it, or will we also need a volumizer?  Why don't hot water circuits need minimum volumes?

Posted by: @scalextrix

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@jamespa I don't understand any point in an air test, if that fails, then the pipes are then broken, and the system cannot be refilled without digging up concrete and ruining finishes. Am I missing something?

If it blows open a joint or a pipe you are correct of course.  Perhaps more likely it will detect an existing slow leak due eg to a pinhole in a solder joint or even a pipe and the leak will be detectable in the air pressure but not much water leaks out.  Most likely it will detect nothing, but in principle the risk you identify does exist.

Posted by: @scalextrix

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Also, will the secondary circuit behind the PHE count towards the system volume of the primary circuit with the heat pump on it, or will we also need a volumizer?  Why don't hot water circuits need minimum volumes?

If the pump speeds are correctly matched and the PHE correctly plumbed then yes, but if, for example, the secondary pump is switched on and off by a thermostat (which some installers do) then no, because there is no guarantee that the secondary will be in circuit during defrost.

DHW circuits dont need min volumes because either (a) the heat pump switches to heating mode when doing defrost or (b) the dhw in the tank has sufficient energy which is transferred via the coil.  I dont actually know which, or whether some heat pumps switch and others dont.

Posted by: @jamespa

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there is no guarantee that the secondary will be in circuit during defrost

Thanks, this is what I was thinking about, so as long as heat energy can transfer back through the PHE into the primary circuit during defrost, then that's fine.  I suppose the secondary pump needs to run off of the same circuit as the primary pump so they are matched.

I suppose the other issue is cycling, and I can't quite get that straight in my head. Suppose as long as the PHE is doing it's job well, then the return flow temperature drop to the HP, will keep it running?

Posted by: @scalextrix

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I suppose the secondary pump needs to run off of the same circuit as the primary pump so they are matched.

They need to be switched on/off together.  The pump rates may differ however.  There is a bit of an science to setting up PHEs which I haven't fully understood yet; your installer should of course do this correctly but we all know what installers are like

Posted by: @scalextrix

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I suppose the other issue is cycling, and I can't quite get that straight in my head. Suppose as long as the PHE is doing it's job well, then the return flow temperature drop to the HP, will keep it running?

Depends on how the water pumps work.  Some heat pumps leave the water pump running 24x7, others allow it to switch off and then switch it back on again from time to time to sample the current conditions.  This is something again the installer needs to think about and understand properly.  Unfortunately this is another reason to deprecate the use of PHEs (but I understand why you are proposing to have one).

Incidentally an acquaintance of mine is currently proposing to run his HP on a vented system without a PHE.  Not sure what installers will make of that.

Posted by: @jamespa

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Incidentally an acquaintance of mine is currently proposing to run his HP on a vented system without a PHE.  Not sure what installers will make of that.

As far as I can tell my vented system would have a peak pressure of about 0.5 Bar, which technically does meet the absolute minimum pressure of the ASHP (also 0.5Bar) specifications, but you are then hoping the sensor is very accurate and if it isn't, fault codes and no heating I suppose.

I wish your acquaintance all the best!