Wanted: Waste water heat recovery for bath

zeupater

Reed_Richards said:

Suppose your hot water supply is at 60 C and you mix it with warmed cold water at 22 C to give you shower water at 41 C which cools slightly in transit so it enters the drain at 40 C.  The hot and cold flows will be equal and lets assume its a powerful shower using 12 litres per minute, that's 6 litres per minute of hot water.

Without the heat recovery your cold water is at 10 C so to get a mixture at 41 C you need 7.44 litres per minute of hot water.  So you are saving yourself 1.44 litres of hot water per minute.  Half an hours' worth of powerful showering would save you 43.2 litres of hot water (and use 180 l of hot water).

I used an online calculator that tells me it takes 2.5 kWh to heat 43.2 litres of water from 10 C to 60 C so the saving would be 2.5 kWh, exactly the figure @waqasahmed estimated.        

Hi

Alternatively, why not simply address the prime issue of 12litres/minute in the first place and save money both on energy & water as well as the investment in heat recovery .... a simple shower flow reducer costing <£5 (sometimes free through water supplier schemes) would have exactly the same effect on energy reduction, half the volume & cost of water being consumed, whilst saving almost £450 & could be fitted later today in next to no time by a competent DIYer with access to one in a local supplier .... ????

Bigger savings with less investment .... aka, moneysaving ...

HTH - Z

Rodders53

I'd worry about Legionella in this lukewarm stored water mixing with heat-treated hot water in a shower.  20-45C are the ideal breeding temperature for the bug. 

Not such a problem if warmed water is feeding the HW cylinder input and being heated to 60C.

zeupater

Rodders53 said:

I'd worry about Legionella in this lukewarm stored water mixing with heat-treated hot water in a shower.  20-45C are the ideal breeding temperature for the bug. 

Not such a problem if warmed water is feeding the HW cylinder input and being heated to 60C.

Hi

That's one of the reasons I thought about when looking at the DIY heat recovery solution I raised earlier ... the recovered heat only pre-heated the sealed flow of mains water into the DHW cylinder, saving energy required to raise the DHW column to the required temperature but not excluding the need for raising the temperature to required safe temperatures ...

HTH - Z

anon_ymous

zeupater said:

Reed_Richards said:

Suppose your hot water supply is at 60 C and you mix it with warmed cold water at 22 C to give you shower water at 41 C which cools slightly in transit so it enters the drain at 40 C.  The hot and cold flows will be equal and lets assume its a powerful shower using 12 litres per minute, that's 6 litres per minute of hot water.

Without the heat recovery your cold water is at 10 C so to get a mixture at 41 C you need 7.44 litres per minute of hot water.  So you are saving yourself 1.44 litres of hot water per minute.  Half an hours' worth of powerful showering would save you 43.2 litres of hot water (and use 180 l of hot water).

I used an online calculator that tells me it takes 2.5 kWh to heat 43.2 litres of water from 10 C to 60 C so the saving would be 2.5 kWh, exactly the figure @waqasahmed estimated.        

Hi

Alternatively, why not simply address the prime issue of 12litres/minute in the first place and save money both on energy & water as well as the investment in heat recovery .... a simple shower flow reducer costing <£5 (sometimes free through water supplier schemes) would have exactly the same effect on energy reduction, half the volume & cost of water being consumed, whilst saving almost £450 & could be fitted later today in next to no time by a competent DIYer with access to one in a local supplier .... ????

Bigger savings with less investment .... aka, moneysaving ...

HTH - Z

It's not necessarily a saving of water bills, if you're getting charged a flat rate 

If you've got a water meter, that's a different story. There is however something to be said for saving water and energy, regardless of ROI

Water security will become an increasing concern. New houses in Turkey have to have to have a rainwater harvesting system

https://smartwatermagazine.com/blogs/graham-mann/new-buildings-turkey-feature-rainwater-harvesting-systems

As time goes on, it wouldn't surprise me if the UK government does the same especially when water security becomes a bigger issue. 

The ROI on that just isn't there but if you can afford it, it's decent imo. It's something I'd do eventually. My main aim is to decarbonise before then recycling water essentially. 

matelodave

You'd save a lot more money by using less water and lowering the temperature, either by having shallower and cooler baths or shorter showers using an eco shower head.

I can manage to shower in two minutes with a 6lpm shower head fed with water at 45 degrees, which means that I'm only using 12 litres of water and not needing to mix it with cold. It would take an awfully long time to amortise the cost of a heat recovery unit and I wouldn't have the disruption of plumbing it in.

We only heat our water to 45 degrees, with a boost to 60 once a week for legionella sterilisation

EricMears

waqasahmed said:

The ROI on that just isn't there but if you can afford it, it's decent imo. It's something I'd do eventually. My main aim is to decarbonise before then recycling water essentially. 

I incorporated rainwater saving into our newbuild 20+ years ago.  Cost was negligible.

System involves a main downpipe from gutter that can be swivelled to supply either storage tanks in basement or a soakaway,  a pump in basement and a smaller storage tank in loft from which WCs are fed.

The only items I actually paid for were the tank in loft and the pipework.   Although most house builders would disagree,  I consider the tank in loft to be essential : if your WCs are mains fed,  spare a thought for what will happen when the water main is disconnected for repairs !

Basement tanks are 4 x 1 tonne 'Transitanks' (aka IBCs) which were donated by a grateful employer spared the cost of disposal but similar ones are usually available at £20 - £50 each.

My pump was also salvaged but could have been bought for less than £100.

Not a lot of people realise but approx one third of all their mains water is used for WC flushing.  Apart from the fact that I consider it a profligate waste of resources to treat that water to potable standard,  using rainwater instead gives an instant 33% 'discount' on metered water charges.  In my particular case reducing the amount of chlorinated water going to a septic tank is also 'A Good Thing'.

EricMears

matelodave said:

You'd save a lot more money by using less water and lowering the temperature, either by having shallower and cooler baths or shorter showers using an eco shower head.

I can manage to shower in two minutes with a 6lpm shower head fed with water at 45 degrees, which means that I'm only using 12 litres of water and not needing to mix it with cold. It would take an awfully long time to amortise the cost of a heat recovery unit and I wouldn't have the disruption of plumbing it in.

We only heat our water to 45 degrees, with a boost to 60 once a week for legionella sterilisation

With a name like matelodave ,  I'm surprised not to see a recommendation for the "Navy Shower" technique !

For those not familiar with the term and CBA to Google,  it involves turning off water once you're wet, lathering then turning water back on to rinse.

zeupater

waqasahmed said:

zeupater said:

Reed_Richards said:

Suppose your hot water supply is at 60 C and you mix it with warmed cold water at 22 C to give you shower water at 41 C which cools slightly in transit so it enters the drain at 40 C.  The hot and cold flows will be equal and lets assume its a powerful shower using 12 litres per minute, that's 6 litres per minute of hot water.

Without the heat recovery your cold water is at 10 C so to get a mixture at 41 C you need 7.44 litres per minute of hot water.  So you are saving yourself 1.44 litres of hot water per minute.  Half an hours' worth of powerful showering would save you 43.2 litres of hot water (and use 180 l of hot water).

I used an online calculator that tells me it takes 2.5 kWh to heat 43.2 litres of water from 10 C to 60 C so the saving would be 2.5 kWh, exactly the figure @waqasahmed estimated.        

Hi

Alternatively, why not simply address the prime issue of 12litres/minute in the first place and save money both on energy & water as well as the investment in heat recovery .... a simple shower flow reducer costing <£5 (sometimes free through water supplier schemes) would have exactly the same effect on energy reduction, half the volume & cost of water being consumed, whilst saving almost £450 & could be fitted later today in next to no time by a competent DIYer with access to one in a local supplier .... ????

Bigger savings with less investment .... aka, moneysaving ...

HTH - Z

It's not necessarily a saving of water bills, if you're getting charged a flat rate 

If you've got a water meter, that's a different story. There is however something to be said for saving water and energy, regardless of ROI

Water security will become an increasing concern. New houses in Turkey have to have to have a rainwater harvesting system

https://smartwatermagazine.com/blogs/graham-mann/new-buildings-turkey-feature-rainwater-harvesting-systems

As time goes on, it wouldn't surprise me if the UK government does the same especially when water security becomes a bigger issue. 

The ROI on that just isn't there but if you can afford it, it's decent imo. It's something I'd do eventually. My main aim is to decarbonise before then recycling water essentially. 

Hi

As mentioned, the cost of water is just one element and even if someone is on water charges based on rateable value the savings applicable to heating a lower volume of water to shower temperatures still applies, especially so at the moment when energy costs are so high ...

Essentially, whether the driver for looking into heat recovery is reduction of cost or moving towards net zero, the argument for considering lower flow rates on a shower still applies, therefore with the solution being so simple, so readily available & so cost effective, any excuse for not even looking into it would seem pretty odd to most considering that the energy savings are pretty much the same as a far more complex and costly heat recovery solution ....after all, the cheapest energy isn't based on any form of recovery, it the energy you don't need to use! ....

Regarding RWH, we essentially use that for around the garden (currently ~1tonne), but, have looked into various solutions to serve the washing machine, WCs etc, both DIY solutions similar to EricMears' and off the shelf pumped packages, probably in the 5 tonne storage range .... it's on the list somewhere but just haven't got around to it yet. The main immediate saving for us would be the diversion of surface water drainage (roof) into a french drain/soakaway and the capping of all outlets from the property to remove the water authority surface water disposal charge ... that'd be a substantial financial saving for us (one of the few remaining!) even without a full RWH system, but then again, you have to be motivated to get around to it and the best installation location for ours would involve destroying and re-establishing both a large paved area & some well established garden planting & that would involve MrsZ's buy-in .... see my motivation loss root cause yet ??  ...

HTH - Z       

silverwhistle

I'm familiar with and use the Navy (yachtie) shower technique sometimes, with my electric shower and on a water meter, unless I am washing my hair too, which takes a little longer.

But I do like a long soak occasionally, like tomorrow after playing football, and I hope the PV diverter does most of the hard lifting. I tend to leave the water in the bath for a bit and use it for soaking muddy kit, and it occurs to me that the OP could do the same but install a MHVR unit in the bathroom to recover some of the heat energy. This would probably be cheaper and deal with any condensation issues.

DullGreyGuy

waqasahmed said:

Magnitio said:

Reed_Richards said:

Where are you trying to recover your heat to?  If it's just the room where the bath is then @2nd_time_buyer 's suggestion is as good as any.  If you are taking a shower leave the plug in.  

https://zypho.uk/zypho-products/zypho-izi-horizontal/

Interesting products. But at the quoted price it would take a very long time to pay for itself.

There is another manufacturer I think. I think Zypho were just the first to market 

Like these guys 

https://recoup.co.uk/?gclid=Cj0KCQjww4-hBhCtARIsAC9gR3aEb95J0b05YjwPWfDgbcLSjxi_ZfUHy22n5zvCdSzJH9bSyK9zxEoaAi1OEALw_wcB

Assuming 50% recovery, and assuming 4 people use 30 minutes worth of shorting a day, surely that saves say 2.5kW? This is on the basis that a half hour shower would use around 5kW

Your maths is way off

The other product claims it can reclaim 1/3 of the heat from the waste water. Whilst you may have put in 5kWh of energy into your hot water you have lost some of that in the piping, you've diluted it with cold water and more heat has been lost to the environment, your skin, warming the bath etc. 

The former product lists a number of ways of potentially using it, without doubt some will be at least partially wasted in any system not run 24/7.

They sound good, from an ecology perspective they are probably worth while getting but the financial return period feels like it will be long.