Plate Heat Exchanger in system - issues?

Shedman

Considering replacing open vented gas boiler (23 yo Potterton Suprima) with heat pump. Main issue is most of the pipework is nearing 40 years old and seems to be 22mm backbone with 10mm microbore either teed off or manifold and exiting part way up walls to radiators (ground floor pipes buried in screed and likely to be in direct contact with concrete). Two storey 4 bed detached around 1800sq ft.

Been told that there should be sufficient flow to achieve a COP of 3.5+ but in order to 'guarantee' that then system would be under pressure (around 1- 1.5 bar compared to 0.5 bar for current OV system). This gives me concern that given the age and design of the old pipes then we could get leaks which would be difficult and expensive to sort out especially if under the screed.

A solution could be to put in a Plate Heat Exchanger to keep the system OV but they indicated that, as well as additional cost, there would be loss of efficiency so they couldn't assure what the COP would be and that there could be issues with increased rate of corrosion with risk of more air being introduced into system.

Has anybody got any experience or knowledge on impact of adding Heat Plate Exchanger to heat pump system and/or risks of adding a pressurised heat pump (or even a pressurised system gas boiler) to a similar pipework setup as ours? Thanks

tacpot12

Many old systems have been converted from OV to pressurised and most hold pressure very well. The soldering skills and materials used by plumbers were better 40 years ago. The old joints have typically got a layer of copper oxide on the inside that protects the copper and solder, but if your system hasn't have corrosion inhibitors added every five years or so, contamination in the system could have worn away this layer and be attacking the solder. So leaks are always a concern.

Your options are to:

repipe - this will guarantee optimal performance and should reduce the risk of leaks if the installer is competent and diligent.

go with the suggested plate heat exchanger option, but the OV system is less efficient than a pressurised system, and adding the efficiency loss from the plate heat exchanger could further increase running costs. If your house isn't well insulated, you could find that even with a heat pump-specific tarrif, heating your home could become much more expensive. Plate heat exchangers are also susceptible to blockage and having one side of the exchanger operating open vent just increases the risk of blockages. The risk of blackage can be reduced by adding a system filter and by keeping on top of adding corrosion inhibitors - your service engineer is likely to have a kit to test the concentration of inhibitor in the sytem and can top it up at each service if necessary. Plate heat exchangers can normally be flushed and/or disassembled to clean them, but this take time and effort so increases service costs. The potential for blockage could leave you without heating in the depths of winter.

Shedman

@tacpot12 Thanks for response.

I was led to believe that due to high copper prices and shortage of supplies in the 1980's that a lot of the pipes used then were of lower quality (from Italy or East Europe?) and also why so much microbore was used. Maybe that was incorrect info or urban myth, but I do know of several late 1980's houses in this part of our town that have suffered from water leaks from under screed piping (incl one of my snooker buddies sons who got a water bill for £2000 before it was discovered..god knows where all that water was going) and one of our neighbours (same style and age property in close of 14 houses) had a suspected leak in the underfloor gas pipe running to their lounge gas fire (discovered by leak test during boiler service) so had it capped off. Hence my reservations (maybe unduly pessimistic) about the pipework.

I have considered re-piping but again it would involve a lot of disruption and would likely be cosmetically unpleasing as pipework for the ground floor would probably have to be run down from first floor and boxed in. Plus given our ages we probably will only be in this house for 10-15 years before downsizing so not sure it justifies the extra expense.

I think I'm swaying towards just going with OV gas boiler replacement with some form of PDHW, even given the unknown about what will happen with gas prices over next decade or so vs electric and even though it may not be the most efficient set up, much as I would like to go greener. But even that is proving trickier than I had initially presumed due to boiler position, size of new boilers (Potterton fits neatly in a wall cabinet), change in flue regs, etc….

FreeBear

I was led to believe that due to high copper prices and shortage of
supplies in the 1980's that a lot of the pipes used then were of lower
quality (from Italy or East Europe?) and also why so much microbore was
used. Maybe that was incorrect info or urban myth, but I do know of
several late 1980's houses in this part of our town that have suffered
from water leaks from under screed piping

Not a myth. I've had to deal with a leak or two in a friends house built in the 1990s. The pipes in question were cold water feeds up in the loft…
The problem with copper pipes buried in concrete floors is the cement attack copper. Over time, this gives rise to pinhole leaks that are difficult to pinpoint exactly and also gives rise to major damp in the concrete slab. Replumbing the system and clipping the pipes to the wall is far less disruptive than digging up the concrete. It also gives you opportunity to rationalise radiator location and minimise pipe drops - In a well insulated home with decent double/triple glazing, there really isn't any need to mount radiators under windows.
Replumbed my system a couple of years back with a mix of 22mm & 15mm. Have just two drops from upstairs with radiators mounted on opposite sides of the walls (also have a riser for all the hot & cold pipes close to the boiler). When the time comes for a heat pump, I plan to run 28mm feed & returns behind kitchen base units to connect to a pair of 22mm drops at the back of the house.

NedS

Our house was re-plumbed with 22mm copper (with 15mm tees into radiators) when the heat pump was installed, with the pipework fixed to the walls just above the skirting boards. I appreciate this may not be for everyone, but I don't dislike the 'industrial' look of exposed pipework, and it can be covered with clip on trunking covers if that look is preferred. I did purchase some of the clip on covers, but they remain unused other than some vertical drops I covered on walls.

We could also have boxed it in afterwards had we so chosen at additional expense.

Personally, I value the knowledge that the pipework is fit for purpose (not blocked with sludge inhibiting flow or likely to leak) far more than aesthetics. The exposed pipework also increases our emitter area which helps low temperature heating (i.e, a heat pump).

QrizB

It's years since I studied thermofluids but a plate heat exchanger will cost you a bit of COP since the flow temp from the HP into the head exchanger will be higher than the flow temperature out of the heat exchanger into the house. Eg. you might set a 50C flow temp on the heat pump and only see 45C flow temp in the central heating.

Exactly how much COP you lose will depend on how big (what capacity) a heat exchanger they fit! Bigger is better, subject to costs.