It seems to be saying that by separating the systems, then glycol usage can be reduced.

I've heard about maintenance issues with anti-freeze valves, so maybe for social housing they prefer glycol... I'm only guessing here.

Certainly from my house I'm looking at a plate heat exchanger as my old pipework probably won't hold 3 bar.

Posted by: @scalextrix

↑

Certainly from my house I'm looking at a plate heat exchanger as my old pipework probably won't hold 3 bar.

I'm not sure that makes any sense at all, so I would question whoever is telling you this properly to examine the alternatives.

Fundamentallly the pressure you need in the system is determined by the radiators/pipework.  You basically need enough to ensure it's never negative.  A phe won't change this!.

Now you might argue that by pumping slower on the secondary and thus having a greater DT on the secondary than on the primary, you can reduce the dynamic pressure drop and hence the max pressure.. However if you are going to do this it will raise the required ft in the secondary and thus necessarily also in the primary.  Given say a 5C drop through the phe added to this, you would almost certainly be bette accepting a higher DT at the heat pump and eliminating the phe.

Bad and Ill conceived decision I suspect, but of course extremely convenient for the installer.

In short, I don't currently believe the argument for a phe in the situation you describe.  I'm open to being convinced, but I think you should be challenging whoever is telling you that you need one to work through the full argument, rather than do the easily thing which will almost certainly increase your running costs by 15%.  I would bet a tenner that they can't but stress I'm happy to be proven wrong.

@jamespa nothing to do with the needed pressure.

Current pipework is likely 60 years old, buried in concrete and as such in an unknown condition.  Current vented system would max out at 0.5 bar in the concrete floor.

As far as I can tell, most heat pumps require a minimum of 0.5 bar, and in normal operation range between 1-1.5 bar, with a pressure relief valve at 3 bar.

As all floor coverings are more valuable than pipework, it's not sensible to hope the pipework may withstand up to 6x pressurization over current expectations.  The financial losses from not having a PHE are greater than having one.

Posted by: @scalextrix

↑

As all floor coverings are more valuable than pipework, it's not sensible to hope the pipework may withstand up to 6x pressurization over current expectations.  The financial losses from not having a PHE are greater than having one.

Understood. 

The only point I am making is that I'm far from sure that a PHE fundamentally reduces the pressure required in any given system for any given level of performance, in fact I can't see how it possibly can do so. 

Of course it's possible, perhaps even likely,  that heat pumps, and indeed boilers, unnecessarly enforce a minimum pressure, and so you need to sheild some or all of them (it will be manufacturer dependent)   from reality, by interposing a phe.  However so far as I can see this would be (a) technology independent,  (b) artificial (IE a function of the control electronics not actually required by any physics) and (c) necessarily performance neutral or more likely detrimental.

From an installer point of view system separation is extremely convenient, which is doubtless why we see it more than perhaps we should even though it almost invariably compromises efficiency.

As long as you are aware of the trade offs and the factors likely to motivate installers then you are making an informed decision, which is perfectly fine and indeed very much yours to make.

Posted by: @jamespa

↑

artificial (IE a function of the control electronics not actually required by any physics)

Yup, I see no reason to artificially pressurize low temperature heating technology, but as far as I can see that's the method most Heat Pumps follow.  Higher temperature systems can kettle, and in that case I see the point of system pressure to keep the water from doing so and reducing efficiency.