@scalextrix I cant help with the CH side but you might look at the plate heat exchanger used with a mixergy cylinder to get a handle on the DHW. By all accounts it works very efficiently. see here

ENERGY-STATS.UK "https://energy-stats.uk/mixergy-heat-pump-performance/"

Mixergy Heat Pump Performance | Energy Stats UK

In this article we do a deep dive into Mixergy Heat Pump Performance. How does it compare to other cylinders? Should you get a Mixergy cylinder?

Posted by: @scalextrix

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suspect the copper pipe in the concrete floors downstairs is as old as the house (~60 years), and I see no evidence of there being separation between the concrete and copper.  We did have a minor pinhole leak in the loft when an old pipe had corroded probably due to flux corrosion.

You are sitting on a ticking bomb, which isn't going to do you any favours at some point. Good chance the pipe is too small as well (?).

I would bite the bullet and do a proper job, especially as the tax payer is likely to pick-up most the cost.

Posted by: @scalextrix

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how hot would the ASHP have to run, 46, 47?

Depends how big the PHE is. At low temperature they need to be huge to get minimal differences in flow temperature.

Posted by: @Anonymous

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Good chance the pipe is too small as well (?).

No, the pipe sizing is very adequate, 15mm to rads and 22mm primary for each floor in the building.  None of it needs changing from a sizing perspective.

Posted by: @Anonymous

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Depends how big the PHE is.

Ok so how big does it need to be?

@bontwoody thanks, I see they are supposed to be 90% efficient or more.

So maybe if I use that as a rule of thumb, a 45C flow from the ASHP would give 40.5C into the radiators.

Posted by: @scalextrix

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@bontwoody thanks, I see they are supposed to be 90% efficient or more.

 

So maybe if I use that as a rule of thumb, a 45C flow from the ASHP would give 40.5C into the radiators.

There are sizing calculators online. The basics are the the higher the temperature the smaller the PHE can be. The PHE in a combi boiler is pretty small. But your max temperature will in the 40s, assuming you run weather compensation that is your max temperature on a typical 7 Deg day your nearer 35, so that will affect PHE size. Get it wrong you could have a heat pump cycling at lowish flow temperature.

Do the job correctly - replace the dodgy pipes, route them elsewhere to get from A to B.

@scalextrix As @JohnMo says though at lower temperatures they need to be bigger so that will probably be the worse performing one in the CH. Also they arent cheap, the Mixergy one is about £300.

@bontwoody £300 on a £7k ASHP project isn't really a major issue.  The designer will have to size it anyway, my real question is how much efficiency impact it might have, plus any other unexpected issues.

@johnmo not clear the pipes are dodgy, but they could be, nobody can know until after they are pressurized. Right? 

The only routing is the current routing through the concrete slab, not digging that up for an ASHP, if that's he only way then we have to remain on gas.

Posted by: @scalextrix

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they could be, nobody can know until after they are pressurized. Right? 

Do an air test on them then you will know. Passes test they are fine. If it fails it would have done pretty soon anyway. A pressurised system will only be running at less than 1.5 bar.

Our plate exchanger (working in a hybrid system) doing 36 degs is 460mm tall and 80mm wide and has 40 plates. Bigger would be better.

Posted by: @Anonymous

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Our plate exchanger (working in a hybrid system) doing 36 degs is 460mm tall and 80mm wide and has 40 plates. Bigger would be better.

Useful information thanks, we have a cupboard where all that can go, seems pretty small.

Posted by: @scalextrix

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@bontwoody thanks, I see they are supposed to be 90% efficient or more.

 

So maybe if I use that as a rule of thumb, a 45C flow from the ASHP would give 40.5C into the radiators.

Work on 2-3 percent per degree.  This is a reasonable estimate of the reduction in COP because you will need to run the heat pump  at a higher temperature to compensate for temp drop across the phe.

I converted my heating from unpressurised to pressurised 15 years ago without a thought, when I changed my boiler.  It now has an ashp instead.  I don't know if the boiler installer bothered air testing the pipes before the switch.

1.5bar isn't high, it turns out that my pipework, originally for an unoressurised system, is good for 9bar ( I found thus out by accident, a radiator blew fortunately before the pipework failed).

Knowing what I know now I'd air pressure test the system in your situation, with the intention of going pressurised if it passes, maybe a phe if it doesn't unless you can find the leak and it's accessible.  This could be decided at instal time or in advance if there is a convenient time to drain the system.