The "Solar thermal or PV? ASHP or burnt fuel of some kind?" thread has drifted off to a discussion on waste heat recovery from DHW, so I thought I'd summarise all I've been able to dig up on this topic so far in one thread. If nothing else it may help someone who later uses the "search" function.

I've found (thanks to tips from others) two shower waste heat recovery devices, the Recoh-Vert (at a reputed Ă‚ÂŁ575 + VAT) and the itho Shower Heat Recovery Unit. Both are very similar, essentially just two concentric copper pipes with the warm waste water running down the centre and the cold water feed running up the outside. Both look as if they need to be mounted vertically to perform at their best.

The other solution I've found is from Earth Save Products and looks like it will recover waste heat from any warm water source in the house. It's called the Heat Squirrel and consists of a 120 lt tank, buried beneath floor level, with a heat exchanger coil inside, through which the cold water feed to the DHW system runs.

The performance claims for the shower waste heat recovery systems indicate that they may reclaim 50 to 60% of the waste heat from shower water. I've not seen any data for the Heat Squirrel, but suspect that it might be a bit better, as, despite the lower overall waste water temperature, it has what looks to be a more efficient heat exchanger. As incoming cold water is almost always a fair bit colder than the inside of the building envelope, this system seems to have the potential to preheat incoming water much of the time, even if only by a modest amount.

The last option is a DIY shower waste heat recovery device, made from ordinary large bore copper pip and some fittings. This is almost certainly the lowest cost option, as even buying pipe and fittings from your local plumber's merchant should allow a unit to be built for less than Ă‚ÂŁ100.

Overall I'm leaning towards a combination pre-heating system for cold water feeding the DHW system. The Heat Squirrel looks to be a good option for recovering low-grade heat from mixed waste grey water, but the shower heat recovery system looks more effective at recovering the higher grade heat from a shower, perhaps. I'm seriously thinking about using a combination of a home made shower waste heat recovery device along with a Heat Squirrel buried in a well-insulated cavity in the slab, with a suitable access hatch. Cold water would come into the house and have a direct feed to the cold taps, with all the rest going to the Heat Squirrel, and then on to the shower heat recovery device before feeding the DHW system.

Any thoughts?

<blockquote><cite>Posted By: tony</cite>The problem with showers is that by the time the water gets to a waste pipe it has already lost nearly all of its heat.</blockquote>

According to the blurb from the manufacturers of these things, they reckon that water leaving the shower head at 42 deg C will have cooled to around 37 deg C by the time it gets to the drain. The 5 deg heat loss will already have been recovered to the house (assuming effective MVHR on the shower room exhaust), so if the unit can recover another 15 or 20 degrees from the remaining water, without costing the earth or creating problems, it has to be worthwhile.

<blockquote><cite>Posted By: tony</cite>CAT used a under floor tank but not sure how much heat was recoveravble.</blockquote>

Having been reading more on tank heat recovery systems I'm less optimistic than I was when I saw the Heat Squirrel mentioned above. It seems that grey water very quickly turns manky when stored, even for short periods, so such tanks need drain systems and also need to be regularly flushed out.

<blockquote><cite>Posted By: tony</cite>I leave water in sink/bath (though these are rare) in winter until cold and recover the heat into the house rather than water, costs nothing and you get it all</blockquote>

Good strategy if you use baths or sinkfuls of hot water.


Having looked more closely at the regulations, it seems my simple concentric copper pipe system wouldn't be legal. Apparently there has to be a double wall between the waste pipe and the cold water pipe.

The easy way around this is to change the design so that the cold water feed is made up from several paralleled coils of microbore pipe, wound around a large diameter copper waste pipe. This is just as easy and cheap to make as the concentric pipe system and would be legal to boot. I think that it's performance could be enhanced by soldering the wound pipes to the waste pipe, as this would enhance thermal conductivity a fair bit. In practice it might not matter, though. An experiment is called for, I think!

BTW, here's a related posting on my blog following ECOBuild this year:

http://hardhouseblog.wordpress.com/2011/03/06/ecobuild-shower-heat-recovery/

Providing you have some sort of HRV to take out the moisture I'd second Tony's

'I leave water in sink/bath (though these are rare) in winter until cold and recover the heat into the house rather than water, costs nothing and you get it all'

Mark,

Thanks for the blog link, and I agree with your comment above. The point about getting the water to stick to the side is interesting, because I'm not convinced (from looking at the Recoh-Vert pictures) that they do it as well as they could with their "rotator". There should be merit in spinning the water, by introducing it to the downpipe tangentially, which would not only get it to stick to the wall effectively (for good heat transfer) but would also slow the descent rate somewhat, by forcing the water to adopt a spiral path down to the outlet at the bottom. If the water could be spun at a faster rate then I think the system should work slightly better.

I'd have thought that a better way of doing it would be to use a larger bore central waste pipe, with a tangentially fitted standard bore waste pipe going in at the top, at right angles horizontally. This should induce a fair degree of spin (enhanced by Coriolis effect if you get the entry angle correct for your hemisphere!). The principle would be a bit like the "wall of death" motorcycle trick, where going around fast enough sticks the bikes to the wall.

Experiment done, data collected and I predicted (from above idea) that the water temperature should be half the start temperature difference plus the cold start temperature.

So I started with cold at 17C (0.3kg), hot at 46C (0.3kg), so (46-17)/2+17=31.5 C
Ended up with with 29 C. The difference being that it was open to ambient temperature (20C) at the (see picture), so some was lost to the room (argument for having it indoors).

Posted By: JSHarris The point about getting the water to stick to the side is interesting, because I'm not convinced (from looking at the Recoh-Vert pictures) that they do it as well as they could with their "rotator". There should be merit in spinning the water, by introducing it to the downpipe tangentially, which would not only get it to stick to the wall effectively (for good heat transfer) but would also slow the descent rate somewhat, by forcing the water to adopt a spiral path down to the outlet at the bottom. If the water could be spun at a faster rate then I think the system should work slightly better.

This is effectively what the Recoh-Vert rotator is doing, or am I missing the point? The only thing you can do to increase the time that the waste water is available for heat to be removed is to minimise the fall speed of the water when it enters the exchanger, after that it's gravity and friction controlling the descent. Of course, you could also have a longer heat exchanger, but that is unlikely to be very convenient for most houses.

The point to bear in mind is that the Itho versions don't have a rotator, and they show very similar heat recovery efficiencies to the Recoh-Vert. I did discuss this with Itho at ECOBuild and they said they had looked at a rotator, but decided it didn't add much to the efficiency and they didn't think it was worth doing.

Posted By: JSHarrisI'd have thought that a better way of doing it would be to use a larger bore central waste pipe, with a tangentially fitted standard bore waste pipe going in at the top, at right angles horizontally. This should induce a fair degree of spin (enhanced by Coriolis effect if you get the entry angle correct for your hemisphere!). The principle would be a bit like the "wall of death" motorcycle trick, where going around fast enough sticks the bikes to the wall.

Yes, this would remove much of the vertical velocity from the waste water but would complicate the system a little and I'm not convinced that it would really add much efficiency.

Apparently Earth Save Products are still offering the Heat Squirrel and reckon it works OK, so that would meet the "needs to sit for a while" approach quite well. Having read some of the horror stories about letting grey water sit in tanks I'd need to do more research to convince myself that such an approach was viable in the long term.

The surface area in the Heat Squirrel looks to be quite high, as it's essentially a standard heat exchange coil, AFAICS. I'm guessing that it has a moderately high volume, but no doubt it could be enhanced further. What I don't understand is how it complies with water regs, which required a double wall to separate waste from clean water.

It seems that the fundamental problem with waste water heat recovery is the one of holding waste water in some form of tank, although if you have a sewage holding tank (for a pumped system or treatment plant) then there should be a way of utilising that I'd have thought.

All told the simpler shower only heat exchangers look like a pragmatic choice, as they are still going to recover a lot of the heat and have virtually nothing to go wrong.

Too little return for effort IMO. Diminishing returns and all that. Concentrate efforts where results are more tangible- the 80% heating bill. Too many ofthese fringe technologies take up too much time and effort to really ever pay back. If you have the time and lots of waste materials I suppose it may be fun, but I would have to apportion my time to the things which were of most use.

I was asked to capture heat from a customer's engine after he had parked in the garage, and I had the same answer. And while the customer is using gas or oil to heat the water in the first place, I would say they are barking up the wrong tree.

Space heating, what is that

<blockquote><cite>Posted By: qeipl</cite>There is also a fundamental problem with shower waste heat recovery. By the time the bulk of the waste water has accumulated in the heat recovery unit the shower has been turned off, so the supply to the cylinder stops, so there is no heat recovery to the cylinder. Trying to solve that problem for a maximum gain of 0.3kWh per shower seems a waste of effort.</blockquote>

That's where the devices like the itho SHRU and Recoh-Vert come in. They extract around a (claimed) 60% of the waste heat in the shower outflow whilst the shower is running and return that energy directly back to the cold feed at the same time. They don't have a tank, being basically just a vertical heat exchanger. This is presumably on top of the heat recovered back from the moist air in the shower via an Ecocent or similar heat recovery system. Overall, it may be that 70% or so of the energy used to heat the shower water gets recovered, which seems quite reasonable to me.

JSHarris: Tip: Leave your bath or sink water til it goes cold, then drain it, and this discussion becomes pointless. As for showers, qeiple has it about right IMO. And anyway, if you extract all the heat, the rat rescue society locally will be busier than ever this winter helping rodents with hypothermia....think I'm joking?

<blockquote><cite>Posted By: crusoe</cite>JSHarris: Tip: Leave your bath or sink water til it goes cold, then drain it, and this discussion becomes pointless. As for showers, qeiple has it about right IMO. And anyway, if you extract all the heat, the rat rescue society locally will be busier than ever this winter helping rodents with hypothermia....think I'm joking?</blockquote>

OK if you have baths, or use sinkfuls of hot water, but again, it's "horses for courses", isn't it? If you're main use of hot water is showers, then it makes sense to recover heat from them if it's economically viable. Even getting 50% of the energy back is a big benefit, if your major energy input goes on DHW.

Posted By: qeiplI'm a 3-minute shower man which means I use about 1kWh of hot water from my ASHP DHW cylinder.

I would agree that for a single 3-minute shower then it is fairly pointless. By the time you get the heat exchanger up to temperature the shower would be over. For several 3-minute showers in series as the family members all get showered then it might make more sense.

For my sins, I'm not currently a 3-minute showerer and my kids certainly aren't. Perhaps I should be more strict with myself and family but this is unlikely to be enforceable in the short term and instead I am trying to ensure a reasonably energy efficient way of waking up in the morning. For me and my family I am clear that shower water heat recovery should provide benefits.

Whether it does or not is another matter and it'll be interesting to see. (I do plan to monitor temperatures across the heat exchanger and DHW flow to see how effective it really is.)

Posted By: Mike (Up North)Hi,
The straight through design came about for shower based operations rather than the domestic market as such. For example - sports changing rooms, hair salons, industrial wash downs etc etc. So it came from seeing all that hot water going down the drain fairly constantly for most of the day.

Cheers

I can see the value of heat recovery in commercial situations where there is a substantial volume of hot water going down the drain over an extended period. For domestic volumes I'm convinced it's a waste of time.