Drain Water Heat Recovery?

[Mark Siddall]

Hi Doug,
Thanks for clearing that one up. I had a feeling that it was something like that but wasn't entirely certain. By using the passive drain water recovery the heating season will be shortened (marginally) and therefore the contribution during the short heating season will be less.
There is a good review of the failings in the HDD method in the Super Insulated House Book (Dutt, 1985) and also in "Superinsulated Houses and Air-to-Air Heat Exchangers" (Shurcliff, 1985...I think)

Mark

[Doug McEvers]

Mark and Nick,

I appreciate your diligence, in an energy constrained future, all systems must be tweaked for maximum efficiency.

Best,
Doug

[Mark Siddall]

I've been thinking. Is drain water recovery the way to go? Is there a lower cost, lower maintenance option that could reduce the cooling load from the WC?
As luck would have it I think that there is: The insulated water cistern.

This insulated approach would:
1) Increase the time that it takes the mains water (10C) to rise to room temperature (20C) and as a consequence it would reduce the cooling load from the WC (effectively by helping to ride out the peaks and troughs between flushes that would otherwise arise.)
2) Would also raise the outside surface temperature of the cistern so as prevent condensation (that accumulates and drips everywhere and can lead to mould growth.)
3) Be low/no maintenance (or at least on a par with existing cisterns)
4) Require no new products within the home (effectively it's a one for one substitution)
5) Be a technology that could be introduced painlessly at little or no cost compared to standard costs.

Drawbacks:
1) It would probably need a plastic cistern but as it would hopefully recoup the embodied energy/carbon this may be okay.....
2) Obviously this will not save as much energy as DHW recovery... but it may be the least cost first option.

Why is it that we don't have them? Does anyone make them? (A quick google search reveals nothing of interest.)

Mark

[Nick Grant]

Hi Mark

Don't steal our thunder but Alan C, Judith T and I are working on the theory side of this plus data logger in loo cistern! All feels so unlikely that we keep checking the numbers.

Some concealed cisterns have polystyrene jacket to reduce condensation and the new Ifo models (which I have a vested interest in) have a plastic cistern under a ceramic cover which would reduce heat transfer a little (part of the reason for it is condensation).

Whilst generally dismissed as mad geekyness by all the sensible people in the room, someone at EST suggested sending out tea cosies for loo cisterns! Lovely idea but won't make that hard to heat Victorian house any warmer despite the saving.

Only a modest loss but in PH terms is surprisingly significant, first guesstimate around 1kWh/m2.a but will report properly when report finished. Will mention in passing at my talk at Ecobuild.

Nick

[Nick Grant]

Another system:

http://www.ecodrain.ca/en/how-does-it-work

Compact, no fall but is it in proper production and independently tested?

Nick

[Mark Siddall]

Here's a thermogprahic photo for all you water lovers out there that are concerned about the cooling load imposed by flushing the loo. :-)

Mark

[Kate de S]

You've been having fun with that camera Mark! Crying out for insulated cisterns, clearly.

But back on the topic of showers, I often ponder whilst standing in the shower, that the reason I keep the shower running continuously is not to keep myself wet, but to keep myself warm. While shampooing, scrubbing between toes, combing out conditioner etc I don't think I especially welcome water in my ears/on my face etc, but even a warm bathroom is too cold when you are bare and wet. (Very tough teenaged son virtuously turns off shower for these tasks, but he's harder than his mother)

If you could heat the shower cubicle, very locally, and control the shower (ideally with a toe-operated run-back tap!), you might be happy using a lot less water? And at lower temp too, even? Might it require less heat to warm the air/cubicle than to warm the hot water, and might the shower might dry faster afterwards too, with less mould growth etc?

Of course it would need to:
save energy in theory
save energy in practice with real people's showering preferences
not be hideously complicated and expensive to construct, install and run

so probably doomed -- but I'd still be interested in info re the trade-off between hot water use and  bathroom temperature.....

kate

[fostertom]

Would all that be necessary, with effective drain water heat recovery?

[Mark Siddall]

Tom,
The water still comes in at <10C so insulated cisterns are needed.
As for the rest. Some one got there already Kate, to some extent at least: http://www.sunfrost.com/efficient_shower.html

Mark

[Kate de S]

Thanks Mark -- I'm glad someone else has thought about it!

Also would prevent that nasty cold wet shower curtain sticking to ankles issue?

Kate

[fostertom]

Tom,
The water still comes in at <10C so insulated cisterns are needed.

I totally agree about loo cisterns - a significant-sized rad maintained at anything between 5 and 12C 24/7, only reaching 18-20C after being left to do its deed uninterrupted for many hours.

I was referring to leaving the shower running simply to keep warm, and alternative ways to achieve that. I said if the outgoing heat's largely recovered, less need to worry about that.

[john cantor]

Following on from Kate's point about having a warm cubicle. We live in a very big old house, and the showers are typical open-topped glass showers. The draft - cold down, and hot up, is dramatic when the room is only 16°C!
So I have been experimenting with a makeshift cardboard top cover, and a data logger.
(I have tried to attach the XLS logger file)
I put sensors on the shower head, the cubicle air at head-height(dry sensor), one in the plughole, and one mid height in the room.
Results are interesting - water 45°C. hot enough to compensate for cold cubicle. 5 mins in, and I had reduced the water to 43.5°, and I'm having a nice comfortable shower.
I then remove the cardboard lid from the top of the shower, and I become uncomfortable, and feel the need to increase the water temperature, but I resist until point 50 (10 second /reading, so 8.3 mins from start of graph)
What intrigued me was the drain temperaure. It only drops a couple of degrees (average) after the top was removed. This seems to show how much water simply goes in large droplets, or runs straight down your body, onto the shower tray.
(yes I should re-try with an atomising shower head).

When I started this exercise, I thought that a lid on our shower could save energy, but possibly the opposite is true.  Firstly, the shower temperature is not proportional to energy. Electric showers are fixed-heat, the flowrate varies.  With the lid on, I am comfortable, and likely to stay in longer. With it off, I am likely to have a quicker shower.  The best plan would be to have the cubicle nice and warn (as Kate suggested) so that I could stop the water to lather-up. But in our case (old house), I'm sure more heat would be used heating the tiles etc.
So, If I fit permanent lid, we will be more comfortable, the house will be less humid, and we will probably use more energy.
The drain water was on average about 32.5°C over the first 5 mins.  I have thought many times about a heat exchanger, but never actually made one.  It would seem not cost effective unless you shower a lot.  It strikes me that a high thermal mass shower tray might help with short showers. Maybe I'll cast one out of bronze!

[Kate de S]

Fascinating - thanks John! No wonder you have such short showers if its usually so cold in there. Do you really have the water at 45 though? - I would expect that to hurt?

(I know what you mean about the 'bliss' effect - always presumed low flow showers mainly save energy by removing any temptation to linger under the miserable trickle. However, if your 'box' is keeping down condensation, might you be clawing a bit back on the cost of heating ventilation air?)

The electric shower is a special case if constant wattage, fewer opps for energy saving. Have you tried switching off the flow and seeing how quickly you feel cold again?

Our shower I am pretty sure pumps at a steady rate, and alters the mix of HW and CW, so lowering temp would save energy. I will enquire.

Now I need to do some sums based on supposition that I could drop temperature 1.5 degrees with an enclosed cubicle, and also time myself (by counting, I suppose, unless I can borrow a datalogger from him indoors) to see for how long I could turn off the flow during a shower, and add those savings too.

cheers

Kate

[john cantor]

I'll check that water temperaure. Its a new logger and have not checked it properly, but I'm sure one needs a few degrees extra water temp to compensate for the cold air.  I could try turning the water off mid-shower, and fit an audible sensor to one of the logger inputs!

Yes, much of the house cold. The house is too big to do otherwise.  Our next house will be more sensible!  Roll on summer!

[Jon Walker]

I know it fairly expensive (£525) but the Showersave Reco-vert is Gastec/KIWA tested, SAP eligible and WRC approved which no other WWHRS are at present in the UK. A huge part of this cost has gone into research, testing and approval by BRE (they are not cheap). Savings can be calculated either in SAP (about the same benefit as about 2m2 of solar) or by a simple spreadsheet (which can be e-mailed), if you know the shower usage patterns. For ground floor showers there is the Recoh-tray which can be used under a shower or hidden in a floor void. Find out more or contact me via shower-save.com Sure I'm biased because I brought the product into the UK because I really think it plugs a gap (pun intended) in a major area of heat loss in modern houses - hot shower water going down the drain! Jon Walker