Heat Recovery from Grey-water

[Richard A Downing]

I'm taking a year off from renovations during 2009, but next spring I'm planning to renovate the main bathroom on my bungalow here in sunny Cumbria. As you probably know it gets quite cold here and I continually plot ideas to save heating oil - I used 1000l since the beginning of November.

The boiler is a new Mistral Condensing 50-90 unit, and I have an OSO pressurised 210l water tank (thanks Dr.D, I know about thermal stores, and in 20 years that'll be the way I'll go). I'll lay in lots of wall insulation, of course, and if I can, the heating will go underfloor as I did in the Sitting Room, with insulation under (I used 100mm in the Sitting Room). The loft is >250mm insulated.

So it seems to me that the biggest waste of heat left in the bathroom will be the grey-water from SWMBO's daily baths - she does not do showers. I'm planning on a smaller bath (sssh!) But is there a simple way of using the waste heat in the grey-water for heating (aside from letting it go cold in the bath tub before pulling the plug)?

R.

[Tim Downie]

Call me a killjoy but I think that amount of heat economically recoverable this way in a domestic setting is minimal. You might come across the odd enthusiast but I think you'll find most folk here will say the same.

Tim

[Dave Liquorice]

Only 1000? We've got through nearly 2000 since the end of November. Will be due to order another 2000 in about 10 days time...

Even if you can effectively recover it what are you going to do with such "low grade" heat? Any medium you recover the heat into is only going to be around the upper 30's C at best. Too cool even for under floor heating.

How much actual energy is there in a bath full of water? Is it worth recovering given that it is such low grade?

[Richard A Downing]

Two active young-for-our-age oldies. No kids. Few visitors. Lots of insulation. 20 degree thermo setting. South-facing big K-glass/Argon windows.

OK, I'm convinced. Scrap that idea....

Thanks.

R.

[Andrew Gabriel]

You could just leave it to cool down before pulling the plug. That way, the heat has passed into the house. The moisture it generates by evaporation might not be welcome though. Could cover the top to prevent evaporation, but then it will take much longer to cool.

If you're into DIY refrigeration, another option would be to pump the heat from the bath back into the hot water cylinder. You could affix evaporater pipework to the back of a (metal) bath, and have a condenser coil in the bottom of your hot water cylinder. Then you cool or even chill the bath water, dumping the heat back into the hot water cylinder. Almost certainly not worth it, but might be good fun. ;-)

I've thought about recovering the heat from a shower. That's easier because you can just construct a reverse flow heat exchanger to transfer the waste heat to the hot water cylinder inlet flow, as they'd both be flowing at the same time.

[Mike Tomlinson]

In article , Richard A Downing writes

I've often wondered if this could be used to warm up the cold feed to a mixer shower (or even an elcetric one). It'd need to be a thermostatic mixer, however.

[TheScullster]

"Richard A Downing" wrote

Others seem to dismiss the heat recovery option. It may be more energy/cost efficient to look at filtering and re-using the grey water for toilet flushing/garden watering. Leave you to do the maths though :)

Phil

[meow2222]

By doing that you would recover the heat, as the warm water would sit around long enough to go cold. That would be useful 9 months of the year.

Evaporation from leaving bathwater in situ might be countered with a few drops of oil. What effect doing this would have on bath cleaning I dont know.

NT

[Tim S]

Mike Tomlinson coughed up some electrons that declared:

[RobertL]

So have I, and i think it could. A counterflow heat exchanger might recover 80% of the heat. Yo ushoudl then be able to have a high flow rate shower running on a small electric shower heater.

Robert

[Andrew Gabriel]

That's what you call a hot flush...

[Andy Dingley]

Does "counterflow" have any real meaning when we're talking about such a small flow, that's so intermittent?

[meow2222]

You wont get good recovery if its not counterflow.

NT

[Onetap]

What a super idea.

Just let me think; why do lubricated air compressors (as against oil- free compressors) often come complete with oleophilic and activated charcoal filters for connection to the receiver's condensate drain?

Ah yes; that would be because it is very environmentally unfriendly and very illegal to put oil into the drainage system.

Is there any area of technology in which you don't hold your expertise in high regard and don't feel a compulsive need to propagate your half- baked ideas across the internet?

[Rod]

Who is going to be the first to tell Johnson & Johnson?

[Mike Tomlinson]

In article , Tim S writes

[Tim S]

Mike Tomlinson coughed up some electrons that declared:

Sadly, one of my showers-to-be drops into an underfloor drain (as in under concrete) so I can't use one there. The outflow from the bath (with a shower) does have loads of room to install random widgetry like this - not sure yet if it would be worth it (will people have a shower in the bath or go round the other end of the house and use the shower?). As neither have yet been built, this is the question :)

But it seems very obvious to me that the above will recover useful amounts of heat, costs not much to make and therefore should have a bit of payback.

[meow2222]

Indeed, or the many skin cleaner and hair conditioner manufacturers that put oil in their products. Perhaps the effect of the detergent in the bathwater went unnoticed.

To mr onetap, I dont recall claiming to be offering an expert opinion there. This is a discussion thread, for the purpose of discussing possibilities.

NT

[Jules]

Well, dumping used sump oil into the bath *is* a possibility, I suppose :-)

[Onetap]

Baby oil? Mineral oil? On a baby? Why? Do they rust? Just hand out a COSHH data sheet with every bottle. Is it just me that associates the phrase 'mineral oil' with the words cancer and testicular.