Waste water heat recovery

[Crofter]

In a rare idle moment, I was contemplating the torrents of hot water that disappear down the plughole whilst having a shower, and thinking what a huge waste that is. A quick google reveals that heat recovery systems are possible, but I didn't stumble across any off-the-shelf systems.

 

One of these days I fancy building a system myself. Plenty of space under the suspended floor. My initial thoughts are to swap out a length of the 40mm plastic waste pipe for a compatible metal pipe, and then wrap microbore copper around the outside in a close spiral. Encase in insulation, and then feed the incoming cold water in at the low end of the microbore, and feed this in to the existing thermostatic shower.

 

Potential complications:

- how will my shower cope with the increased temperature of the cold water feed?

- how long will the heat exchanger need to be to extract a useful amount of energy?

- will it be practical to make a leak-free joint between plastic waste pipe and the metal section?

 

I think it could work much better if I was able to do something more elaborate e.g. take the waste water and feed it into e.g. 28mm copper, overwrapped with microbore, which should make a much better heat exchanger. Perhaps two such HE in parallel, to ensure adequate flow rate. But I would see this getting blocked with hair etc very easily, and no doubt it would be a nightmare to find a way of connecting it to a 40mm waste...

 

Curious to see if anybody else has given this some thought.

 

[Alexphd1]

http://shower-save.com

https://www.heatraesadia.com/products/renewables/waste-water-heat-recovery.htm

[Jeremy Harris]

There are commercial units.  The people that sell the Ecocent were selling a thing called the Heat Squirrel a while ago, and may still sell it.  It sounded dodgy to me in terms of compliance with the water regs, but it was a chamber in the main drain into which the waste water was fed.  It had a heat exchanger coil in it and that was connected to the incoming cold water feed to the hot water system, as a pre-heat system.

There's also at least one waste water heat recovery unit on the market that's designed to be fitted vertically.  The Recoh-Vert was probably the first : http://shower-save.com/products/recoh-vert.html  but there are a few others around now.

http://www.ithodaalderop.com/products/heatinghot-water/shower-heat-recovery/

http://www.recoupenergysolutions.co.uk/products/

 

I looked at making one, but the required air gap between the twin walls needed to comply with the separation of fresh and waste water in the regs was a potential problem.  I was going to just use a length of 50mm copper waste pipe wrapped with four parallel coils of copper microbore pipe, with a for way manifold at each end to join the microbore back into 15 or 22mm.  50mm copper waste pipe will fit standard 50mm waste pipe compression fittings, but you need to fit a swirl unit at the top to get the warm waste to stick to the inside face of the waste pipe.  A tangential pipe brazed at right angle to the top of the vertical waste should create enough of a swirl to work.

[jack]

We have one of these installed, connected to the drains of our two most frequently used showers.  I was slightly concerned that our thermostatic valves (Mike Pro by Crosswater) might have trouble coping with warm water being fed to the cold side, especially as the feed temperature changes over the first couple of minutes, but it's been perfectly fine.  

There are two main types: one with three or four parallel copper pipes spiralling down the outside of a central copper pipe, and another with two concentric pipes such that the incoming cold water passes through the space between them.

You also need to look into connection types.  You can send the pre-heated water to just the shower(s), or tee off after the unit and send the pre-heated water to both the shower(s) and the cold feed to your DHW system.  There are apparently some efficiency gains to be had with the latter system.

These devices are in SAP and I understand some developers throw them in where they need easy SAP points (well, that's what the suppliers suggest!) 

There are question marks over payback, but with four very active people in our house, we seem to take a lot of showers, often in quick succession (the best scenario for heat recovery).  We only have a 250L tank, so my thinking is that we probably get a bit more effective hot water than we would without the recovery unit.  We probably don't save that much money, each year, but I see no reason why the unit shouldn't have a long life, so it'll keep contributing a little bit for many years to come.

[Crofter]

Thanks for the links, gents. These units all appear to assume a vertical waste pipe. In my house it would need to be horizontal- would this be a problem? I presume it would actually help with heat exchange as the waste water would be in contact with the pipe for longer, albeit only along the bottom.

[jack]

Look at the retrofit+ and various "drain" options here: http://www.recoupenergysolutions.co.uk/products

[PeterW]

2 hours ago, jack said:

 

2 hours ago, Crofter said:

Thanks for the links, gents. These units all appear to assume a vertical waste pipe. In my house it would need to be horizontal- would this be a problem? I presume it would actually help with heat exchange as the waste water would be in contact with the pipe for longer, albeit only along the bottom.

So I would look at 42mm copper pipe - you can buy it by the foot on ebay - and then braze or solder 2 pairs of 10mm pipes to the bottom of it. That will give you the flow and soldering will enable the heat transfer much better. I would go as far as to make it up  with lengths of solder between the pipes and then heat the lot as it will solder the joints much better. Wrap the whole lot in insulation and you're done.

It will affect the flow rate a little, but its probably quicker and much cheaper than those commercial systems that rely on immersion of a 15mm pipe in warm water.

[Jeremy Harris]

10 minutes ago, PeterW said:

So I would look at 42mm copper pipe - you can buy it by the foot on ebay - and then braze or solder 2 pairs of 10mm pipes to the bottom of it. That will give you the flow and soldering will enable the heat transfer much better. I would go as far as to make it up  with lengths of solder between the pipes and then heat the lot as it will solder the joints much better. Wrap the whole lot in insulation and you're done.

It will affect the flow rate a little, but its probably quicker and much cheaper than those commercial systems that rely on immersion of a 15mm pipe in warm water.

There has to be an air gap.  The regs prevent you from having what amounts to a single wall between a pipe containing potable water and a waste water pipe, so brazing or soldering the cold water pipe to the waste water pipe would be a breach of the regs.  It's why the efficiency of the commercial units is rarely better than 50% or so; they have to comply with the regs and leave a tiny air gap and also provide a visible indication that either pipe has failed, by means of drips coming out through that air gap drain space.

[PeterW]

7 minutes ago, JSHarris said:

There has to be an air gap.  The regs prevent you from having what amounts to a single wall between a pipe containing potable water and a waste water pipe, so brazing or soldering the cold water pipe to the waste water pipe would be a breach of the regs.  It's why the efficiency of the commercial units is rarely better than 50% or so; they have to comply with the regs and leave a tiny air gap and also provide a visible indication that either pipe has failed, by means of drips coming out through that air gap drain space.

Thanks - I am struggling to see how the Recoup+ units work to achieve that as they definitely seem to use a "composite" pipe that has no apparent air gap, but also has no drain point to show of any leaks..?

 

 

That would indicate that a mechanical linkage (ie contact) is fine, but a soldered joint is not. In reality, neither is likely to fail, and the high pressure to low pressure route is more likely to occur.

[Jeremy Harris]

The Recoup units don't have smooth walls where they are pressed together, there are longitudinal grooves so that any leakage can flow down,escape from the lower vent and be seen, which slightly reduces the contact area, but allows compliance with the regs.  There used to be a cross-section graphic showing this, but I've had a look and cannot find it, it may have been taken down because others were copying it.

I agree that a failure is both unlikely and even less likely to result in contamination, but then the regs were not designed to be flexible enough to cover waste water heat recovery, and haven't been amended.

 

[PeterW]

So in essence if you were to use a solid coil of copper at 4mm to wrap a 42mm pipe, put that inside a 50mm pipe and then put a water jacket on that, you could meet the regs...

Just seems very odd..! 

[Jeremy Harris]

It is odd, but I looked into it, to the extent of making up a prototype short version, using just an annular water jacket, before I discovered the quirks in the regs.  IIRC I may have partially documented it on the GBF, as it was several years ago.  I bought short lengths of pipe from ebay and brazed the thing up.  The bit I had to play around with was getting the tangential inlet to work.  It's essential that there is a lot of swirl in the waste water, so that it both contacts the side of the waste pipe and to slow it down, vertically.  There was a bit of playing around to get the water to swirl enough that it would stay stuck to the inside face of the waste pipe.

Just dumping waste water down the thing meant it barely touched the sides, and heat transfer efficiency was pretty low.

[PeterW]

Horizontal would work better as the bulk of the water sits on the bottom of the pipe but given I could make this up for about £30 in bits it would need a lot of savings for it to pay for itself !

[PeterW]

Always wondered how this meets the regs as it recycles water in the shower - I assume it basically says that until it leaves the unit it is not "waste" water

https://www.hamwells.com/e-shower

[Jeremy Harris]

I think the general definition (although I've not seen it written down) is that waste water only becomes waste after the trap.  If this is the case, then there is scope for making a single-walled version.  It may well be how the Heat Squirrel ( http://esavep.com/index.php/air-source-products/waste-water-heat-recovery.html ) got away with it, as that definitely did not have a twin wall pipe system when I looked at the cutaway drawing that they had on a stand at a show years ago.  It was just a tank to hold waste water with a conventional coiled pipe heat exchanger in it that the incoming cold feed went through.  Maybe it was also a big trap, and worked by not having a trap on the feed to it. 

I discussed it with the ESP people a few years ago, as I was very interested in it (and in their Ecocent and the other Chinese ASHPs they were selling), and they assured me that it was legal, even though I couldn't see how on earth it could be.  Then again, very nice and helpful people as they are, I'm not at all convinced that ESP are really that ethical; one look at their use of images and diagrams taken directly from the Genvex manual, and the use of a photo of a Genvex in their advert for their Chinese-made copy of a similar unit, has left me wondering a bit about whether they are overly concerned with the fine detail of laws and regulations.

 

[ProDave]

Interesting thread.

If I were DIYing a heat recovery system, I would not get bothered about the regs. All you have in essence is a cold water supply pipe running next to a waste pipe, nothing against that in the regs. the fact you [cough] solder the cold water pipe [/cough] to the waste pipe makes it no more likely that waste water will contaminate the cold flow.

My thoughts,

It would surely work much better in a long horizontal run at a shallow fall, feed the cold water in at the outlet end and take the warmed water out of the end nearest the shower trap.  On the basis the water will flow along the bottom of the pipe, just a couple of heat collector pipes in a straight line following say the bottom 1/3 of the pipe rather than wrapped around it.

Now I can see this working for a shower, to pre heat the "cold" feed a little bit. and having an individual heat recovery unit for each shower, feeding the cold feed to that shower.

But what about a bath?  A bath uses a lot more water. It lets it all go qickly when you pull the plug, and even if you had a heat recovery system where is the warmed water going to feed?  So it seems only a viable idea for a shower then?  for a bath you need somewhere to store the heat and a heat pump to extract the heat from it.  I know, why not just leave the water in the bath after you have finished until it has gone cold (wared the room) before pulling the plug?

[Crofter]

What about a check valve? That would eliminate the possibility of contaminating the whole mains system in the house, which I agree is a completely theoretical risk anyway.

 

I'm not going to lose any sleep over how this would apply to a bath- I would estimate that less than 5% of our usage is baths, the rest is showers (each have maximum of one bath a month, compared to daily shower).

[PeterW]

49 minutes ago, Crofter said:

What about a check valve? That would eliminate the possibility of contaminating the whole mains system in the house, which I agree is a completely theoretical risk anyway.

 

I'm not going to lose any sleep over how this would apply to a bath- I would estimate that less than 5% of our usage is baths, the rest is showers (each have maximum of one bath a month, compared to daily shower).

Between 1 and 5 you need an air gap - its a pain ! It's the reason why rainwater fill systems need a tundish arrangement as they cannot bridge between mains and rainwater even when it has been filtered and purified.

 

[jack]

2 hours ago, ProDave said:

So it seems only a viable idea for a shower then?  for a bath you need somewhere to store the heat and a heat pump to extract the heat from it.  I know, why not just leave the water in the bath after you have finished until it has gone cold (wared the room) before pulling the plug?

They're expressly designed for use with showers, where you have a constant drain at the same time you're using cold water. 

We hardly ever use baths anyway!  And yes, leaving baths to cool down is a useful way of gaining back most of the heat in winter (possibly quite a good fit with MVHR!)

[Onoff]

One of my mad musings is to build some form of bio-gas generator, basically feeding it with garden and kitchen waste along with manure etc. Was thinking a heavily insulated mass concrete "box" on the lines of the Mother Earth News one. To speed up the reaction I thought about a home brew solar thermal panel running the pipes through the "compost".  Then I thought as I'd want such a unit out of way round the back of the house (it'd be down slope too) why not run the waste water pipes through it too and make use of that heat.

(It'll likely never happen of course ).

[SteamyTea]

20 hours ago, Crofter said:

Thanks for the links, gents. These units all appear to assume a vertical waste pipe. In my house it would need to be horizontal- would this be a problem? I presume it would actually help with heat exchange as the waste water would be in contact with the pipe for longer, albeit only along the bottom.

I think it would kill the heat transfer.

When water 'falls' down a vertical pipe, it actually swirls around the inside of the pipe wall.

Put that same pipe horizontally and you create a D shape (twiddled over 90°), this has a much greater volume to area ratio.  That will ruin the heat transfer.

 

When we discussed this a few years back, I took a different route from Jeremy and looked at transferring heat in a batch method.  Basically the waste goes into a holding tank, then via a heat exchanger into the cold water storage.  It worked, but was slow, needs space and was not cost effective at all.

This idea could be modified with a W2WHP, but then getting even more costly.

 

Main thing is to do the sums first.

A normal shower should not need more that about 50 kg of water at 38°C max.

Incoming mains in winter is probably very rarely below 8°C.

So:

50 [kg] x (38 - 8)[°C] x 4.2 [kJ.kg^-1.K^-1] = 6300 kJ or about 1.7 kWh

That is about 32p at my expensive day rate, 14p at my night rate.

Now a 250lt tank to service 4 people with hot showers is right at the limit of its capability, so probably storing at 65°C (or maybe a bit more), but the mean temperature will probably be nearer 55°C, so that is 49350 kJ or 14 kWh.

This will probably give you standing losses of around 4 kWh/day so you have 10 kWh of usable energy in the hot water (losses between 32p - 72p.day^-1 or £117 to £265.year^-1).  So look at insulating the store more and reducing the temperature a bit.  A couple of sheets of Celeotex type material will cost 50 quid.  Turning down the temperature costs nothing.

Another method is to get an aerating shower head, had one when I lived in the USA (it also pulsed and did wonderful things on other settings).  it worked well, but the flow was still very high.  I would like to try one out on my lower (11 kg.min^-1) flow shower here.

Another alternative is to bite the bullet and get instantaneous water heating (maybe in series with a Sunamp/PV).  If that costs £7000 to install, and you can save close to £300 a year, then 'only' 23 years till you hit break even.

Easier to use less.

 

[Guest Alphonsox]

The simplest (and cheapest) form of waste water energy recovery for a house fitted with MVHR must be an uninsulated holding tank. Basically just keep the warm waste water within the thermal envelope and use the MVHR to distribute the energy. As mentioned above this works fine for a bath so why not extend this method for showers as well ?

[Jeremy Harris]

It would work to an extent, but the MVHR will be very, very slow to re-distribute heat (because of the low heat capacity of air and the low flow rates) and the tank will lose heat more and more slowly as it cools, so could take a long time to get down to room temperature.  It's also only a benefit during the heating season, which is only two or three months for a well insulated house, if that.

The combined slow heat distribution and cooling of the tank then bears on the capacity of the tank.  To be effective it may well need to have enough capacity to store waste water from several showers/baths, so could need be around 200 to 500 litres to cope with showers or baths taken within a few hours of each other.  That's pretty big, especially for something that would need to be bypassed with a valve for around 8 or 9 months of the year when room heating is either not needed or would be a nuisance.

Hot water demand is pretty much the same all year around, so if there is a way to reduce that from waste heat then that is a significantly greater benefit.  At a guess, you might be able to halve your hot water energy needs with a very good system.

[Ferdinand]

So the "Ockham" answer to this is:

1 - Use a shower over a bath.

2 - Shower with the plug in, and leave it there for 30 minutes.

3 - Or, more sensibly, have a thing like a grid or a trivet to raise you above the level of the shower floor.

Ferdinand

 

 

[Jeremy Harris]

6 minutes ago, Ferdinand said:

So the "Ockham" answer to this is:

1 - Use a shower over a bath.

2 - Shower with the plug in, and leave it there for 30 minutes.

3 - Or, more sensibly, have a thing like a grid or a trivet to raise you above the level of the shower floor.

Ferdinand

 

 

Yes, as you can choose whether to keep the heat or chuck it away depending on whether or not you need it.

I still like the idea of pre-heating the incoming cold water that's feeding the hot water system with waste heat, and wish the regs were changed to make this more practical.  Recovering 50% or so of the waste heat from hot water usage seems a worthwhile saving in a house where hot water energy use dominates the overall energy consumption.