Met a chap today who is selling heat recovery units for shower waste water. He claims the heat exchanger is 60% efficient and the system has Certification to back this up. At a unit cost of around £450/ unit it would be interesting to see if the numbers stack up. Anyone looked into these?

Lots of them about - is this about the thing in principle, or about a particular make? what make?

There's the common vertical drop-pipe type, many versions from US/Canada, also a German one that's a flat coil that fits under the shower tray - in fact I have an idea it's an option on the Bette lift-out-floor shower tray.

8 years later, and they still seem to be about the same price... There's also a UK manufacturer about now (Recoup), with a part-plastic, part-copper design.

I made an all-copper one myself for < £100 in parts, but it's not WRAS compliant since no double-walled exchanger (EN 1717).

Yet another manufacturer gets around the double-walled requirement by mandating that it's installed upstream of the shower U bend (sold in the UK as Heatrae SHRU). Overall I suppose it's not as good, since if it springs a leak, it'll be chucking fresh water down the drain I suppose, but I suppose that's pretty unlikely (most likely to leak from the solder joins I suppose in which case all the leakage paths would be fresh water into the room). Nice to know you can do that design with the £100 DIY option, and still comply with the regs tho'.

I'm quite happy with mine as it allows me to run two showers simultaneously from a minimum size combi boiler (for me the boiler replacement savings outweigh the the running cost savings).

Also worth noting for these that if you're running from a combi boiler that the boiler will be running quite a lot more efficiently (maybe 90% instead of 70%) in addition to the obvious savings of cutting a basic 50% off the hot water usage.

Must get around to putting some temperature sensors on my DIY one, to do some real-life efficiency monitoring

My initial esimates gave a pay back period of around 15 years I think (including labour).

Cheers,

Tim.

Put a double check value on the water feed before it goes into your shower heat recovery, so if you get a leak, it is unlikely to get back to your drinking water tap.

I think you are looking at it from the wrong end. If the shower is 45C, then that means half the energy is coming from the heat exchanger rather than the heater so your showers are approx twice as efficient as they were (modulo start/end effects). That sounds worth having. Certainly for £100, less so for £450, although if your shower tray costs £1200, then it's neither here nor there!

Could you not make one that used a heat pump and a coil to preheat a header tank of water.
Would require a bit of certified refrigerant gas working and an vented shower system.

A lot of the energy from a bath or a shower goes into the air, a good heat recovery unit will help in the winter.
My water temperature rises and falls by 15°C between winter and summer (mean is 13°C). In the winter it can drop to 5°C and up to 20°C in the summer. May be easier and cheaper to fit a trace heater/small immersion heater to the incoming mains and run it from some PV.
Some sums would be needed.

My bath/shower probably uses around £100 of E7 electricity a year. Which is really way too low to spend more than £50 quid on.

My water costs more than the heating

I agree, though there are ways to make a very small heat pump act like a larger one for a short period of time.

The main problem of over cooling waste water would be gunking up of the waste pipe.

Well done Tim, I like people who make things, your apparatus looks great! I'm also a fan of Combi Boilers, heating cylinders of water is inefficient and they waste a lot of space in a house. The problem with Combi's, as you've mentioned, can be insufficient water pressure and the capacity of the boiler to heat 2 showers simultaneously. Your heat exchanger idea should substantially boost the ability of a Combi to power 2 showers. I saw some shower heat recovery units http://www.wagner-solar.com/waerme/produkte/waermerueckgewinnung.html at this years Passive House Conference that were Passive House Certified to recover up to 65% of the heat used when showering.

If we build Passive Houses with Tri-Solar Roofs, FreshR HRV and Shower Heat recovery we should be able to run the houses with a small amount of electricity.
FreshR and Solar Heating cuts the heating demand from 15kWh/m2.annum to 1kWh/m2.annum.
Solar Heating and Shower Heat Recovery can cut the hot water demand from 20kWh/m2.annum to 1kWh/m2.annum.
3-4kW of Solar PV can cut the electrical demand by 60%, the rest can be powered by straight electricity, eliminating heat pumps, but it shouldn't cost any more than £100/annum to run the house.
My best performing 180m2 house uses €140/annum for heating, hot water and electricity, it has no PV.

I've been doing a Google search and found 2 reviews of the Zypho's:

https://www.kbbreview.com/15941/news/zypho-shower-heat-recovery-system-are-now-available-in-uk/

https://specificationonline.co.uk/articles/2018-06-07/zypho/after-successful-trials-zypho-shower-heat-recovery-device-now-available-to-improve-uk-energy-efficiency

Makes me wish we'd been able to install them, but that's not how it went and hopefully somebody else can benefit from them.