Green Building Forum - Grey water heat recovery...

Grey water heat recovery...

James Norton — Mon, 17 Nov 2008 21:13:22 +0000

Ok although I am in principle largely in favour of the 'leave-the-plug-in' method but I have recently been thinking about getting that mains incoming cold water up a few degrees particularly in the winter with the solar DHW panels already stretched to say the least.

The idea so far is split the incoming mains near entry to house as you do with one line going to 'boiler' and one to the taps. Passivhaus pipe lagging (50mm all round).

The boiler one doesn't actually go to the boiler it goes to the unvented cylinder through a flat plate heat exchanger in the thermal store powered by solar, wood stove boiler and immersion or gas boiler in that order.

If I were to get the baths and showers to direct their waste water to a tank before leaving the house, I then create an enormous coil holding 50 or so litres of cold incoming water this way the water can sit for a good long while but still keep my mains pressure hot water.

Thought I could construct said coil from 30m or so 22 HDPE pipe, ie as it comes of the roll. Alternately 4x 10mm copper coils 22-10mm manifold at each end. Sling the thing out in a 100l tank, perhaps larger to fit the coil and then stick something in the middle of the coil to reduce the volume.

How to I empty the waste tank bottom first? (to keep the hottest water at the top in the longest) Is the 4x 10mm coil a dumb ass idea?, Does HDPE not conduct enough?

As you can tell, thinking aloud really... all input welcome...

J

Grey water heat recovery...

Gotanewlife — Mon, 17 Nov 2008 22:27:23 +0000

Pretty soon you would have a really manky tank, wouldn't you? All that hair and scum, all mixed up with suds and crud kept nicely warm, OK I'm not sugesting you are dirty as such but I just had the pleasure of cleaning out 2 'U' traps - yuk! Sorry, I hate to raise a problem but then not give a solution but I don't have one, however, I'm sure I saw on here mention of a shower that does exactly does this heat recovery thing.

Grey water heat recovery...

JackyR — Mon, 17 Nov 2008 22:47:02 +0000

http://www.shower-save.com/pdfs/Recoh-vert%20grey%20water%20heat%20recovery.pdf
This is the shower heat-recovery thing mentioned, and includes an additional skin between contaminated and mains water "which is obligatory in most countries".

Grey water heat recovery...

GreenDIY — Tue, 18 Nov 2008 13:28:05 +0000

In France, it is recommended to have a "bac Ã graisse" for grey water to remove fat before going to the septic tank so you could install one of those and plop the pipe in.

But (house layout permitting) you could could maybe remove the bath panel and coil the incoming cold mains pipe around the bath?

Grey water heat recovery...

Mark Siddall — Tue, 18 Nov 2008 13:38:25 +0000

Thin film heat recovery seems to be quite popular in the USA/Canada:
http://www.gfxtechnology.com#
http://www.renewability.com/dhrt.htm
http://www.watercycles.ca/
http://www.shower-save.com/pdfs/Recoh-vert%20grey%20water%20heat%20recovery.pdf

No sure how this UK system opperates. The description suggests that it may be prone to maintenance issues though (I get the impression that it is a sump type design)
http://www.aki.co.uk/page/warmit

Mark

Grey water heat recovery...

James Norton — Tue, 18 Nov 2008 17:50:19 +0000

Thanks Mark, will have a look.

William, Jackie

My concern with a coil around this is that unless its actually sat there for while you won;t get enough transfer of heat...? Although Jackie's gadget does claim the contrary.

J

Grey water heat recovery...

Mark Siddall — Tue, 18 Nov 2008 18:16:01 +0000

James,
Thin film technologies, such as GFX and Recoh, only work well for continuous flow devices like showers as they pass the incoming cold water through the heat exchanger to preheat it before it enters the domestic hot water heater (SHW tank or whatever).....They are not suited to much else. That said much of the DHW is used from bathing rather than taps so if you avoid baths in favour of showers you can reduce your DHW consumption quite substantially (and avoiding showers in favour of a wash hand basin for every other day helps even more).

These system are, apparently, relatively low maintenance (I can certainly believe it compared to sumps and tanks.) The GFX information really help to explain the technology and there are a number of independent reports on this particular system. I've not heard of one being used in the UK and would be interested to hear about anyones experiences if they have.

I think that sumps and tanks have many potential problems with bacterial and mould growth since it won't be hot enough to kill the nasties. Legionaires could also be a serious risk.

Grey water heat recovery...

JackyR — Tue, 18 Nov 2008 20:01:46 +0000

Strangely, legionella in the sump wouldn't pose much hazard (AFAIK). It might grow there, but unless you plan to inhale the water you'll be fine. The danger is in storing "clean" water (which can naturally contain legionella) at nice, cosy, breeding temperatures and then spraying it around - eg shower, spa bath - to create an inhale-able aerosol.

So legionnaire's is a potential problem with all pre-heat systems, as by their nature they break the "keep cold water cold, keep hot water hot" rule. Wookey would say, Go ahead and use anyway, not many of you will die! But if you're installing for others it's a definite no-no.

According to their website, legionnaire's is theoretically possible with the Shower-Saver, but only a minor concern due to the small quantity of stored "clean" water. http://www.shower-save.com/pdfs/Recoh-vert%20installation%20guide.pdf. (NB Haven't used this kit, just read up on it after saw on GBF.)

Grey water heat recovery...

James Norton — Tue, 18 Nov 2008 20:04:05 +0000

Mmmh perhaps easier said than done...

The gunk and nasty thing perhaps mitigates toward the single length of HDPE so that the outgoing grey water can never get at the the fresh incoming cold. re the nasties and, 'spose I would have to arrange a weekly purge or something, with gunk trapped and dealt with once month.

Thing is it just seems to make no sense to chuck out 50% of the heat in the water you've just done all you can to get up to temp and then bring in a few hundred litres of icy cold water...

There has to be a way!!

J

Grey water heat recovery...

jamesingram — Tue, 18 Nov 2008 20:19:02 +0000

slightly smaller store area , but how about a indirect cylinder turned up side down
run the cold main through the coil and the grey water into the tanks cold water feed
the bottom would be coned shape and the waste water could leave through what would normally be the hot water out (using 40-50 pipe)
connect a u- access trap ( to remove sludge etc) and then bring the waste pipe back up to create , something like a big
toilet u trap

any good or nonsense ?

cheers Jim

Grey water heat recovery...

chrisinbrighton — Wed, 19 Nov 2008 22:32:50 +0000

JackyR,
I had never heard about legionnaires being formed in warm `clean` mains water,surely this can also happen where cold pipes run alongside hot or even if a cold pipe runs under a hot roof?
It must scupper alot of hot water ideas.
How real is this danger and are there specific regulations.

Thanks,

Chris

Grey water heat recovery...

JackyR — Thu, 20 Nov 2008 00:17:48 +0000

Posted By: chrisinbrightonsurely this can also happen where cold pipes run alongside hot or even if a cold pipe runs under a hot roof?

Yup.

There are two excellent and easy to understand papers about legionella, with specific ref to solar hot water systems, in: http://energytech.at/pdf/task26_4_industry.pdf

Basically, legionella occurs naturally in ground and surface water and breeds well between 25 and 45 degrees. Only some legionella is the type which causes legionnaire's, and only some infected people become seriously ill (so an unknown number of people get it but aren't diagnosed).

I seem to remember about 12 people die each year in the UK, another 50-120 being quite ill (??). Many of these cases are from DHW; some follow "trip away from home" but not necess abroad, leading to speculation that legionella grew to dangerous levels in the unused home plumbing. About 10 percent of DHW systems show legionella present in some position.

Google for UK regs - there's lots of stuff about maintenance routines eg flushing toilets in hotels/student residences once a week during empty periods, to prevent legionella population building up. Common preventative method for showers: pasteurize water at temps from 55 degrees to 70 degrees, depending on whether you want to kill it in hours or seconds.

This is all off the top of my head, from reading up a few months ago, as I can't use multiple windows to Google right now (new release of Safari keeps crashing ). So for heavens sake check it all, don't take my word. Should have written thread while still remembered it all...

Grey water heat recovery...

JackyR — Thu, 20 Nov 2008 00:18:33 +0000

Here's that paper again, but clickable: http://energytech.at/pdf/task26_4_industry.pdf

Grey water heat recovery...

Ed Davies — Thu, 20 Nov 2008 00:20:20 +0000

Chris, AIUI, the problem is when there's water "lurking". In a straight pipe run it's not a problem even if the water is a bit warm for a while as it all gets flushed through each time the tap is run. Unused branches off the main pipe run are best avoided though. More significantly, it's in the muck at the bottom of a tank or where ever the water doesn't get changed regularly that the real problems can arise.

Grey water heat recovery...

abomb1969 — Thu, 20 Nov 2008 22:05:56 +0000

That heat recovery system for showers is nearly 500 quid.

Grey water heat recovery...

Gotanewlife — Thu, 20 Nov 2008 22:45:02 +0000

Good grief that's an insane amount of money.

Grey water heat recovery...

James Norton — Fri, 21 Nov 2008 18:10:43 +0000

My idea was 'store' the water in a 50m long 22mm pipe but by store I mean say a few hours, the grey water is also 'stored' but for a similar amount but in a tank shortly before discharging in to the soil outlet therefore never the twain meet.

The preheated water then passes on to the plate heat exchange in the thermal store to get lifted up to operating temp. As the grey water will be at most 25 degrees or so the preheat won't make in preheated water that hot, but if its coming in at 2 degrees in the winter even lifting it to 10 would make a decent impact.

Jim,

good idea about the tank, would be keen to use relatively off the shelf bit of gear, not sure how much water would be in the coil but the transfer would be better.

J

Grey water heat recovery...

JackyR — Fri, 21 Nov 2008 18:30:25 +0000

Sounds the sort of thing that's fine for your own use (modulo any regs about double skins between grey and clean water), but you'd get in trouble for installing for others. Commercial installs have to be user-proof, surely.

And yes, time is a major factor in legionella growth - that paper has tables on all this...

Grey water heat recovery...

JackyR — Fri, 21 Nov 2008 18:53:37 +0000

Someone pls check my sums.

Assume 22 mm pipe to have internal diameter 20 mm, therefore radius 10 mm = 1 cm.
Capacity of 1 cm length of 22 mm pipe is therefore Ï€ x 1 x 1 x 1 = 3 cm^3 (approx).
Capacity of 50 m of 22 mm pipe = 50 x 100 x 3 cm^3 = 15 000 cm^3 = 15 litres.

This feels wrong - and James talks of only 30 m of pipe holding 50 litres...

Grey water heat recovery...

James Norton — Fri, 21 Nov 2008 19:45:57 +0000

Oops, just checked the back of my beer mat, I got it the wrong way round it was 30l in 50m not 50l in 30m and I'd used 28mm pipe and not accounted for pipe thickness...

On that basis = r=14 gives 30.1l, r=13 (which is the right no.) gives 26.5l still a decent amount though

J

Grey water heat recovery...

Gotanewlife — Fri, 21 Nov 2008 19:54:29 +0000

So if you do this put the details on here because i'd probably be able to DIY it as well - mmmmmmm - give it a try anyway. And i'll be deeply up to elbows renovating house next summer so could build it in......if it work

Cheers.

Grey water heat recovery...

djh — Fri, 21 Nov 2008 20:55:39 +0000

On http://www.gfxtechnology.com" > it says: "Fig. 1: Whole House Model S3-58-LC GFX is equivalent to a pair of 30â€ single coil â€œGâ€ Models having their drainpipes connected in series and coils in parallel ..."

Have I lost the plot or is this exactly the wrong way around? Wouldn't you want the drains in parallel and the heat-gaining coils in series?

Grey water heat recovery...

jamesingram — Sat, 22 Nov 2008 12:09:37 +0000

james
i've seen a similiar system used to preheat the water main for a combi-boiler with the tank acting as a thermal store heated by a solar thermal panel. the tank the right way up though.
The water in the coil would have heat transferred to it as it passed through the grey water even after the 'stored' volume was used up. the problem would be as you suggested earlier removing the sludge without getting covered in it. some kind of diverter to soil pipe , on the access trap could be used to drain down and clean
Diy only, but going back to your first idea , how about
a 50 gallon+ cold water storage tank used to store the grey water outside , in a insulated box near a gully or soil pipe
fit a 28+mm gate valve to drain down into the gully and off to the sewerage farm, also fit 110mm access plug in the bottom to clean out
at the top of the tank fit a 50mm waste over flow into the 110mm soil pipe(with a u trap) or staight to the gully-trap
stick your water main 50m coil in the tank and fix it near the top ,below the overflow .
lid on and insulate the watermain pipe into house etc..
Second hand kit would be best as I 'd imagine the energy recovered would take a while to payback the embodied energy of new
materials.
If you're prone to the occassional use of bleach cleaner, this would enter the tank and maybe help reduce the growth of festering funk inside.

cheers Jim

Grey water heat recovery...

Mark Siddall — Mon, 24 Nov 2008 12:52:55 +0000

abomb1969
In terms of costs: DWHT is a great concept that "just" needs to be manufactured on a larger scale to push the cost down. I estimate that at the current price of the thin film DWHR could take 17yrs to paypack (10 l/min shower), however, if harmonised with changing from a 10 l/min shower to a 6 l/min shower, it could take 8 yrs to payback.
(Assumes gas heating to DHW and metered water. 3 people, 4min/shower ....and a shower every day!)

Mark

Grey water heat recovery...

James Norton — Mon, 24 Nov 2008 13:44:55 +0000

mmmh..... some consideration of the exchange rate and the following graph, cause our you hero to reconsider the GFX...

Grey water heat recovery...

Mark Siddall — Tue, 25 Nov 2008 13:14:15 +0000

James,
I think that pie chart that you offer over estimates the water consumption from shower use (in the UK.) If you google "BNWAT28: Water consumption in new and existing homes" you'll find the average is:

Shower: 23%
Bath 15%
Total: 38%

......Anyway percentages are fickle things so don't get caught up on them they are only relative to a given benchmark(you need to know how the benchmark was determined).

Mark

Grey water heat recovery...

TomN — Tue, 25 Nov 2008 14:52:39 +0000

I'm not sure that I follow the concern over Legionella...

From what I can see of the Recoh units, the waste water from the shower travels through a heat exchanger with a double barrier meaning that there won't be any contamination of the incoming mains water. This water is then drained away as usual.

Where could the contamination occur?

Grey water heat recovery...

James Norton — Tue, 25 Nov 2008 16:27:10 +0000

Mark,

Point noted, its probably a lifestyle thing as well

J

Grey water heat recovery...

JackyR — Tue, 25 Nov 2008 20:31:30 +0000

With a temp diff of at most 23 degrees (your estimated <25 degree sump, 2 degree winter mains - but measure mains as figures vary wildly) and 28 mm HDPE, surely it'll take hours to have a noticeable impact on the incoming cold water? And the temp diff may often be much less.

So you'll get pre-heated water in slugs of 26.5 l max, hours apart. A bath is usually 50 to 150 litres, a low-flow shower 4 to 10 l/min.

It's not that you wouldn't get a few joules out of this system in the winter, it's that I just can't imagine it being worth the embodied energy, including installation energy, and cleaning cost (chemicals, water, your time).

The proprietary shower kits are narrow-piped, copper and operate at higher temps, so their heat-recovery figures should be much better. But the financial figures... whew!

Grey water heat recovery...

James Norton — Wed, 26 Nov 2008 13:23:14 +0000

I think your right Jacky, and even though expensive - I probably have better things to do with my time than mess about with huge tanks of mucky water, that could easily add up to the cost of the GFX...

J