waste water heat recovery (WWHR) systems

Never tried them but we'll be installing them in our new builds where possible. The claimed benefits seem well worth the relatively small cost.

Very simple!

Would add one but no where to fit it....

Yeah, that's the main issue, getting sufficient space to guarantee flow, and access to clean.
I've got a couple of mates with them, and they do seem to need a thorough clean fairly often.

Good idea on a new build where you can make sure you have space and rearrange stuff to make sure.

One of them has it boxed into the corner of the kitchen, against the wall of the utility room.

You can get horizontal units that fit under a bathtub I think, might even get one to go under a shower tray?

it seems the horizontal ones recover much less heat than the vertical ones.

In our world, the shower is going right above a brand new garage extension, so we have space for a vertical one there

Seems a great idea, how much do they cost? Just need something like this for the flue coming out of the log burner.

it seems the horizontal ones recover much less heat than the vertical ones.

I could fit one under the bath though....

Not a hope of fitting a vertical one.

Interesting. One of those "why didn't I think of that" things. Would be interested to see how much they cost, can't actually find a price anywhere. Given that my partner swears a shower using <100l isn't physically possible, payback time might not be too long, and retrofitting might not be out of the question.

I looked into it when I redid the bathroom. It would be easy to fit as the drain runs vertically next to the feed. The cost didn't seem worthwhile. There's some DIY solutions too but we just take shorter showers. Less waste water, less waste heat

Would be interested to see how much they cost, can’t actually find a price anywhere

About £700

So me leaving the bathwater in after I get out can now be rebranded installing a temporary heat store, and not just me forgetting to pull the plug out.

As the possessor of a shower and having had three long haired teenagers grow up mostly in them, I'm not rushing to put anything between the plughole and the outside world, the regular trap was grim enough, can't imagine a huge copper coil.

Horizontal one is only 10%....

ok Confusing numbers:

1. Up to 10% heat transfer to incoming cold feed 
2. Up to 31% heat recovery efficiency

https://www.baxi.co.uk/new-build/products/flues-and-accessories/baxi-assure-horizontal-shru

Also £700...

They go after the trap, so the only thing you get in there is soap residue and related greasy gunk.
Shouldn't be any hair holding it all together like you get in the trap.

The cheapest i've seen was about 250 quid, but it certainly looked like a bottom of the range device, simple and untidily made, and i don't think i'd want it installed anywhere in a heating/hot water circuit...

Anything over 5 mins in a shower is excessive and boring - you can’t even relax and read a book! 🤣

There’s only so many times you can wash the same bits! 🚿🍆🧼

Got me thinking how I could make a simple contra-flow heat exchanger. 40mm dia copper tube is likely expensive. Aluminium on the other hand readily available on eBay, 40mm inner waste surrounded by a 50mm outer carrying the cold feed water. Just some end pieces to machine up. Pity I never put together a metal working machine shop I always wanted. (Was into model engineering years ago!)

A decent 10 litre/minute shower is 21kW down the plug hole if the water temp has been raised 30°C.

I just had a genius idea... Underfloor bathroom heating that's basically just a reservoir for your shower water. You don't really need a heat exchanger, you just want to store the water in the house til it's cooled down, rather than pipe it out so a simple tank anywhere with a delay valve is all you need, but the underfloor is elegant.

Til it leaks, that's inelegant. But I only come up with genius ideas, it's some other poor sod's job to deal with them

so trying to work out the ROI on one of these - could someone check my maths?

Example system is a recoup hex

RECOUP.CO.UK "https://recoup.co.uk/documents/Recoup_WWHRS_Brochure.pdf"

which states (in the way we'd probably use it) a 50% heat recovery - lets assume that's accurate.

The unit is £600 inc vat, and lets say its going to cost me £150 to get it installed as part of our extension build, so £750 all in.

I measured the incoming water temp (11.8C), our shower temp (as close as I could - 41.5C) and our drain temp - 37C.

if its 50% efficient, it will recover ~12.6C out of the water coming down the drain, adding it to the water going in.

In our house we have 2 adults taking showers, lets say a total of 10 showers a week (shower at work the rest of the time), each, I dunno, 10 mins long (that's a guess, I'm going to start timing it to work out). For the first 30 seconds I recon the recovery kit won't add anything (as the water won't have reached the bottom/the heat will go into warming up the unit itself) and for the last 30 seconds it won't add anything (as the water going up just stays in the pipes and cools back down) - so we have 180 minutes of showers that might be affected per week, with the water being increased in temp by ~29C.

Our showers feel powerful (mains pressure hot water tank) so lets say 15l/min (edit: measured with a pan at around 16l/min) - 2700l of water per week. raising that by ~29C requires about 93kwh of energy per week, whereas raising it with the unit in place would use 53kwh of energy - a saving of 40kwh per week. lets say our boiler is 85% efficient, we'll be saving 47kwh per week.

We are away maybe 7 weeks of the year if you add up all the weekends, so 45 weeks * 47kwh is a saving of 2115kwh per year or £158 per year - payback is 5 years on an initial price of £750

that seems like a no-brainer, but am I missing something? if we drop shower length to 5.5 minutes (as I said, we haven't measured it) the payback length doubles, but its still an easy win

each, I dunno, 10 mins long

That seems at the upper end for length of time for a shower.

Semi-daft suggestion

Shower with the plug in, let it drain after it's cooled, the heat energy has gone into the air/room

Need quite a deep shower tray for that and not ideal for the second person showering.

The first time that it needs cleaned out 90% of people will just call a plumber and have it removed. In terms of fitting them to new builds anyway.

Would seem sensible on the dish washer and washing machine too?

Second insulated tank for all waste water with heat exchanger to hot water tank?

You're probably being over optimistic on the savings (everyone I work with usually is), but for the investment cost you only need to include the "additional" costs over a standard shower, so its not the full £750.

So if a "normal" shower would be £500 fitted the investment cost is only £250.

You’re probably being over optimistic on the savings (everyone I work with usually is), but for the investment cost you only need to include the “additional” costs over a standard shower, so its not the full £750.

So if a “normal” shower would be £500 fitted the investment cost is only £250.

the exchanger thing is £600 on its own (!) so the additional cost is I recon £150 for the plumber to hang it and whack a few pipes in where needed.

A few of them look unblockable (ie on the waste side they look just like a standard pipe), but maybe I'm being optimistic? My wife does tend to get quite a lot of hair down there..

Shower with the plug in, let it drain after it’s cooled, the heat energy has gone into the air/room

only useful for the ~6 months of the year that we're heating the house, and gets a tonne of condensation into the air.

Code requirement in some US states.

They're typically smooth walled copper pipe with a copper supply wrapped around the outside pre heating the water that goes back to the water heater.

I'm unsure where there's a resulting increase in blocked drains though as the waste water doesn't run through or over any coils.

I don't think leaving the hot water in the tub or DIY routing it into an underfloor heating system in the bathroom would be effective.  You want to reduce boiler usage rather than just absorb the waste heat (and humidity) into the house, especially during the summer when you wouldn't want the heat anyway. And in winter there are not many that would be happy to have the house colder in the morning when showering only to warm up over the next hour or two.

The unit is £600 inc vat

Which, given it's a PVC pipe with a copper pipe (35mm x 3m is about £65) inside and a couple of fancy fittings is insane. It's a basic counter flow heat exchanger and one that likely won't be operating at anything like decent efficiency since it's going to quickly start fouling on the hot side. Even the cold side will eventually get fouled up reducing efficiency even further. Not sure about flow velocities or the fact they seem to be sloshing the hot side round the pipe in a vortex, possibly another efficiency loss if it's got an air gap in the middle of the flow.

If I was going to do it I'd want it in a shell and tube exchanger that can be easily serviced, especially on the shell side as you would want the hot (manky) stuff there. You'd probably want a really good trap as well to prevent build up of hair.

It's a nice idea but I think in reality it's going to get really complicated really fast if you want it to work well which adds cost, inconvenience and ultimately longer payback. A catch tank with a heat pump would probably be a far better solution overall.

The price is high, but other similar systems seem to be a similar price. Could you explain the fouling? It seems like the hot water is just chucked down a normal diameter pipe (albeit copper) with another (also copper, thin diameter) pipe wrapped around the outside carrying the cold water up. If the hot waste was in thin pipes the fouling issue would be obvious but I (as a layman) can't figure it out with the layouts I've seen