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    •  
      CommentAuthorJSHarris
    • CommentTimeOct 24th 2011
     
    The "Solar thermal or PV? ASHP or burnt fuel of some kind?" thread has drifted off to a discussion on waste heat recovery from DHW, so I thought I'd summarise all I've been able to dig up on this topic so far in one thread. If nothing else it may help someone who later uses the "search" function.

    I've found (thanks to tips from others) two shower waste heat recovery devices, the Recoh-Vert (at a reputed ÂŁ575 + VAT) and the itho Shower Heat Recovery Unit. Both are very similar, essentially just two concentric copper pipes with the warm waste water running down the centre and the cold water feed running up the outside. Both look as if they need to be mounted vertically to perform at their best.

    The other solution I've found is from Earth Save Products and looks like it will recover waste heat from any warm water source in the house. It's called the Heat Squirrel and consists of a 120 lt tank, buried beneath floor level, with a heat exchanger coil inside, through which the cold water feed to the DHW system runs.

    The performance claims for the shower waste heat recovery systems indicate that they may reclaim 50 to 60% of the waste heat from shower water. I've not seen any data for the Heat Squirrel, but suspect that it might be a bit better, as, despite the lower overall waste water temperature, it has what looks to be a more efficient heat exchanger. As incoming cold water is almost always a fair bit colder than the inside of the building envelope, this system seems to have the potential to preheat incoming water much of the time, even if only by a modest amount.

    The last option is a DIY shower waste heat recovery device, made from ordinary large bore copper pip and some fittings. This is almost certainly the lowest cost option, as even buying pipe and fittings from your local plumber's merchant should allow a unit to be built for less than ÂŁ100.

    Overall I'm leaning towards a combination pre-heating system for cold water feeding the DHW system. The Heat Squirrel looks to be a good option for recovering low-grade heat from mixed waste grey water, but the shower heat recovery system looks more effective at recovering the higher grade heat from a shower, perhaps. I'm seriously thinking about using a combination of a home made shower waste heat recovery device along with a Heat Squirrel buried in a well-insulated cavity in the slab, with a suitable access hatch. Cold water would come into the house and have a direct feed to the cold taps, with all the rest going to the Heat Squirrel, and then on to the shower heat recovery device before feeding the DHW system.

    Any thoughts?
    • CommentAuthortony
    • CommentTimeOct 24th 2011
     
    The problem with showers is that by the time the water gets to a waste pipe it has already lost nearly all of its heat.

    CAT used a under floor tank but not sure how much heat was recoveravble.

    I leave water in sink/bath (though these are rare) in winter until cold and recover the heat into the house rather than water, costs nothing and you get it all :smile:
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 24th 2011 edited
     
    <blockquote><cite>Posted By: tony</cite>The problem with showers is that by the time the water gets to a waste pipe it has already lost nearly all of its heat.</blockquote>

    According to the blurb from the manufacturers of these things, they reckon that water leaving the shower head at 42 deg C will have cooled to around 37 deg C by the time it gets to the drain. The 5 deg heat loss will already have been recovered to the house (assuming effective MVHR on the shower room exhaust), so if the unit can recover another 15 or 20 degrees from the remaining water, without costing the earth or creating problems, it has to be worthwhile.

    <blockquote><cite>Posted By: tony</cite>CAT used a under floor tank but not sure how much heat was recoveravble.</blockquote>

    Having been reading more on tank heat recovery systems I'm less optimistic than I was when I saw the Heat Squirrel mentioned above. It seems that grey water very quickly turns manky when stored, even for short periods, so such tanks need drain systems and also need to be regularly flushed out.

    <blockquote><cite>Posted By: tony</cite>I leave water in sink/bath (though these are rare) in winter until cold and recover the heat into the house rather than water, costs nothing and you get it all</blockquote>

    Good strategy if you use baths or sinkfuls of hot water.


    Having looked more closely at the regulations, it seems my simple concentric copper pipe system wouldn't be legal. Apparently there has to be a double wall between the waste pipe and the cold water pipe.

    The easy way around this is to change the design so that the cold water feed is made up from several paralleled coils of microbore pipe, wound around a large diameter copper waste pipe. This is just as easy and cheap to make as the concentric pipe system and would be legal to boot. I think that it's performance could be enhanced by soldering the wound pipes to the waste pipe, as this would enhance thermal conductivity a fair bit. In practice it might not matter, though. An experiment is called for, I think!
  1.  
    You beat me to it on the comments regarding the storage tank idea.

    Similarly on the WRAS approval for a DIY system. There are systems in the US that do use the wrapped microbore pipe approach (see http://www.gfxcompany.com/) and I have even seen suggestions that they have been imported and used in the UK, but they are not WRAS approved and I suspect anyone doing this is on dodgy legal grounds. Apparently in Canada you can pick them up off the shelf at your local DIY store.

    We currently plan on using the copper version of the Itho device, mainly because it is more obviously supported in the UK (i.e. they actually bother replying to emails about the technical installation) as well as being significantly smaller than the Shower-save. The prices are effectively the same (or at least were).

    In my mind it is clear that DWHR is a good thing, and well worth doing, at least for a new build, where it is reasonably easy to minimise the pipe lengths. We will have three showers feeding it with a short run (<4m) and another with a slightly longer run (~6m).

    Of course, it only really provides benefit for showers, but this is the bulk of our DHW consumption, especially now the kids are starting to prefer showers over baths, so well worthwhile.
    • CommentAuthorMarkBennett
    • CommentTimeOct 24th 2011 edited
     
    BTW, here's a related posting on my blog following ECOBuild this year:

    http://hardhouseblog.wordpress.com/2011/03/06/ecobuild-shower-heat-recovery/
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 24th 2011
     
    Having looked (albeit briefly) at WRAS approval and the water regs themselves, I'm reasonably sure that a home made system that uses off-the-shelf approved pipework and fittings should be OK. 8mm copper pipe can be bought as an approved product, as can 54mm copper waste pipe. As long as I used approved fittings to connect everything up, then in principle it should still be legal (I think) as it would just be a slightly more complex cold water feed pipe, that happens to run in very close proximity to a waste pipe (which seems to be legal, as far as I can see).

    The only issue I can see is over the possibility of getting the "cold" water feed pipe up to temperatures where Legionella might start to multiply in the water stuck in the pipe. I think the way around this is to ensure that the DHW system is able to pasteurise the water periodically, as, I believe, the Ecocent system can (it seems to have what's referred to as an "antisepsis" cycle that can be programmed in).

    I'm guessing that the crazy price of the commercial units may well be down to the cost of gaining approval and the need for the double wall pipe to comply with the regs. The thin air gap between the cold water pipe and the waste pipe walls in the Recoh-Vert must impact on its efficiency, which maybe why it is larger. It's not clear from the itho information how they get around the double wall requirement.
    • CommentAuthorjms452
    • CommentTimeOct 24th 2011
     
    Providing you have some sort of HRV to take out the moisture I'd second Tony's

    'I leave water in sink/bath (though these are rare) in winter until cold and recover the heat into the house rather than water, costs nothing and you get it all'
    • CommentAuthorMarkBennett
    • CommentTimeOct 24th 2011 edited
     
    Posted By: jms452Providing you have some sort of HRV to take out the moisture I'd second Tony's

    'I leave water in sink/bath (though these are rare) in winter until cold and recover the heat into the house rather than water, costs nothing and you get it all'


    This is clearly the right strategy for a bath or a basin - there's basically no other sensible approach that works or doesn't require extensive cleaning on a regular basis.

    However, for showers I don't think it is the right strategy. Firstly you end up wading around in a small amount of dirty water (more concentrated than a bath full). Secondly, in a well insulated house, you don't need to capture the shower water heat into the house envelope for most of the year. There's probably more than enough around anyway.

    In my opinion, for showers it is much better to use a DWHR type system since it reduces the energy consumption at source, rather than using all the energy and hoping that the waste energy has useful value. If most of your heat is coming from solar thermal then it makes little difference carbon wise, but it can be a useful trade-off between the heat recovery vs solar thermal area, especially when trying to get away with solar only DHW in the spring and autumn.
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 24th 2011 edited
     
    Mark,

    Thanks for the blog link, and I agree with your comment above. The point about getting the water to stick to the side is interesting, because I'm not convinced (from looking at the Recoh-Vert pictures) that they do it as well as they could with their "rotator". There should be merit in spinning the water, by introducing it to the downpipe tangentially, which would not only get it to stick to the wall effectively (for good heat transfer) but would also slow the descent rate somewhat, by forcing the water to adopt a spiral path down to the outlet at the bottom. If the water could be spun at a faster rate then I think the system should work slightly better.

    I'd have thought that a better way of doing it would be to use a larger bore central waste pipe, with a tangentially fitted standard bore waste pipe going in at the top, at right angles horizontally. This should induce a fair degree of spin (enhanced by Coriolis effect if you get the entry angle correct for your hemisphere!). The principle would be a bit like the "wall of death" motorcycle trick, where going around fast enough sticks the bikes to the wall.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 25th 2011 edited
     
    The way I see this as a problem to be solved is as a 'batch' process. The reasons for this is that when it comes to energy transfer, time plays an important part (see Newtons Law of Cooling). Also there is not a continuous flow of hot waste water in the domestic setting. If you could dump the hot waste into an insulated holding tank that holds a heat exchanger (or a copper coil), energy can be transferred to the incoming cold supply. This could then be delivered to an insulated header tank before it enters the cylinder/boiler/heater. The heat exchanger (or copper coil) fluid volume would have to match the holding tank volume to get maximum efficiency of 50%. That bit could be tricky as the waste hot volume is variable (time would need to be infinite to reach maximum efficiency as well), but a vertical coil of the same volume as the surrounding waste water could deal with that.
    I measured a 'bath load' at 76 kg at 43C, so assuming 2 bath loads (or a long 15 minute shower) and an 'exit' temperature of 35C, that is 150 kg, you should be able to rise the incoming (assuming 3C mains temp worst case) by 16C (19C to header tank). When the holding tank had dropped to 19C (or a bit above) then a thermostatic valve can dump that batch of cooled water to the drain.
    Now I know that there is not infinite time, but cooling is an exponential decay with the majority of the work being done at the beginning rather than the end. So the first couple of hours do more than the last (over a 24 hour period). But a bit of behavioural change could make a difference here (bath/shower/wash up/washing machine all in quick succession of run concurrently).

    I feel an experiment coming on.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 25th 2011 edited
     
    Experiment done, data collected and I predicted (from above idea) that the water temperature should be half the start temperature difference plus the cold start temperature.

    So I started with cold at 17C (0.3kg), hot at 46C (0.3kg), so (46-17)/2+17=31.5 C
    Ended up with with 29 C. The difference being that it was open to ambient temperature (20C) at the (see picture), so some was lost to the room (argument for having it indoors).
      Heat Exchanger 2.jpg
      Heat Exchange Chart.jpg
  2.  
    So each batch would only need to be held for 10-15mins...?

    J
  3.  
    Posted By: JSHarris The point about getting the water to stick to the side is interesting, because I'm not convinced (from looking at the Recoh-Vert pictures) that they do it as well as they could with their "rotator". There should be merit in spinning the water, by introducing it to the downpipe tangentially, which would not only get it to stick to the wall effectively (for good heat transfer) but would also slow the descent rate somewhat, by forcing the water to adopt a spiral path down to the outlet at the bottom. If the water could be spun at a faster rate then I think the system should work slightly better.


    This is effectively what the Recoh-Vert rotator is doing, or am I missing the point? The only thing you can do to increase the time that the waste water is available for heat to be removed is to minimise the fall speed of the water when it enters the exchanger, after that it's gravity and friction controlling the descent. Of course, you could also have a longer heat exchanger, but that is unlikely to be very convenient for most houses.

    The point to bear in mind is that the Itho versions don't have a rotator, and they show very similar heat recovery efficiencies to the Recoh-Vert. I did discuss this with Itho at ECOBuild and they said they had looked at a rotator, but decided it didn't add much to the efficiency and they didn't think it was worth doing.

    Posted By: JSHarrisI'd have thought that a better way of doing it would be to use a larger bore central waste pipe, with a tangentially fitted standard bore waste pipe going in at the top, at right angles horizontally. This should induce a fair degree of spin (enhanced by Coriolis effect if you get the entry angle correct for your hemisphere!). The principle would be a bit like the "wall of death" motorcycle trick, where going around fast enough sticks the bikes to the wall.


    Yes, this would remove much of the vertical velocity from the waste water but would complicate the system a little and I'm not convinced that it would really add much efficiency.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 25th 2011
     
    If each batch was a third of a litre and everything else was equal.
    The system properties (the k in the cooling law) is multiplied by time. System properties are things like material, areas, volumes, shape, turbulence, stratification.

    The trick will be to maximise surface area (greatest practical area to volume ratio) to reduce time of transfer.
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 25th 2011
     
    Apparently Earth Save Products are still offering the Heat Squirrel and reckon it works OK, so that would meet the "needs to sit for a while" approach quite well. Having read some of the horror stories about letting grey water sit in tanks I'd need to do more research to convince myself that such an approach was viable in the long term.

    The surface area in the Heat Squirrel looks to be quite high, as it's essentially a standard heat exchange coil, AFAICS. I'm guessing that it has a moderately high volume, but no doubt it could be enhanced further. What I don't understand is how it complies with water regs, which required a double wall to separate waste from clean water.

    It seems that the fundamental problem with waste water heat recovery is the one of holding waste water in some form of tank, although if you have a sewage holding tank (for a pumped system or treatment plant) then there should be a way of utilising that I'd have thought.

    All told the simpler shower only heat exchangers look like a pragmatic choice, as they are still going to recover a lot of the heat and have virtually nothing to go wrong.
  4.  
    Hi,

    I had a look at many of these devices a long time back. I think the key is the separation between the flows so I can’t immediately see how the squirrel complies. I think the other devices immerse both pipe runs in a water filled jacket (i.e. a small thermal heat store) so the transfer become indirect and would require 2x leaks to contaminate the water supply. It may be that they use a continuous finned coil (e.g. solar coil) with no fittings within the volume so all external connections, which after all is what is on your solar DHW cylinder and as that contains solar fluids and is deemed ok, there must be a precedent (likewise boiler primary water).

    Indeed there are some horror stories of the vessel types ending up looking like the wash basin u tube trap. The key to the vertical column is a spreader to push the flow to the pipe walls, there is a tendency with different flow rates/volumes for the flow to pull away from the sides and form a twisting spout right down the middle thereby missing the sides altogether. Unfortunately with various flow rates from different baths/showered/basins etc it’s actually very difficult to get a consistent result. Hence the use of the retained volume approaches.

    A home made solution might for example use a large butt or drum with a roll of 1 or 2 inch mdpe pipe coiled on a frame and immersed in it. Whilst the MDPE doesn’t offer the best heat transfer its cheap enough to put in a size that offers a big enough internal volume (say 100m of 32mm at internet price about £100 contains about 60 lit of water). A large self insulated water tank with access for cleaning and piped in/out is all that’s needed. Best fitted in the basement and best used near a drain in case of leaks. No major temperatures so should be ok with plastics. You could elect not to insulate at all and just take that temp to the room, or simply use some form of duvet jacket rather than rigid insulation.

    You need the room and the will to do it but if you use quite a bit of water then it makes sense, after all these are only really forms of what used to be called an “economiser” – names on the tin isn’t it.

    Cheers

    Mike up North
    • CommentAuthorcrusoe
    • CommentTimeOct 25th 2011
     
    Too little return for effort IMO. Diminishing returns and all that. Concentrate efforts where results are more tangible- the 80% heating bill. Too many ofthese fringe technologies take up too much time and effort to really ever pay back. If you have the time and lots of waste materials I suppose it may be fun, but I would have to apportion my time to the things which were of most use.

    I was asked to capture heat from a customer's engine after he had parked in the garage, and I had the same answer. And while the customer is using gas or oil to heat the water in the first place, I would say they are barking up the wrong tree.
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 25th 2011 edited
     
    <blockquote><cite>Posted By: crusoe</cite>Concentrate efforts where results are more tangible- the 80% heating bill. Too many ofthese fringe technologies take up too much time and effort to really ever pay back. If you have the time and lots of waste materials I suppose it may be fun, but I would have to apportion my time to the things which were of most use.</blockquote>

    Exactly. With a well-insulated, well-sealed, house hot water IS a big element in the energy bill! That's the whole point of doing this, trying to address what will be the biggest energy demand in the house.

    Of course, if you're talking about a poorly insulated house, one where winter space heating dominates over year-round DHW in terms of overall energy consumption then I'd agree; best address the heat loss from the building before looking at DHW energy savings, if investment funding is limited.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 25th 2011
     
    Space heating, what is that :wink:
    • CommentAuthorqeipl
    • CommentTimeOct 26th 2011
     
    I thought about shower waste heat recovery before I built my house and concluded that the gains weren't worth the hassle.

    I'm a 3-minute shower man which means I use about 1kWh of hot water from my ASHP DHW cylinder.

    Some of the 1kWh is returned to the cylinder via the heat pump by way of the shower room vent, let's call it 20%, so my net loss from the cylinder is 0.8kWh.

    Let's be pessimistic and say the average annual COP of the heat pump is 2, so I'm using 0.4kWh of electricity per shower, which costs less than 6p.

    Even if your heat recovery unit managed to recover 50% of the waste energy your saving will only be 0.3kWh of electricity - less than 3p.

    There is also a fundamental problem with shower waste heat recovery. By the time the bulk of the waste water has accumulated in the heat recovery unit the shower has been turned off, so the supply to the cylinder stops, so there is no heat recovery to the cylinder. Trying to solve that problem for a maximum gain of 0.3kWh per shower seems a waste of effort.
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 26th 2011
     
    <blockquote><cite>Posted By: qeipl</cite>There is also a fundamental problem with shower waste heat recovery. By the time the bulk of the waste water has accumulated in the heat recovery unit the shower has been turned off, so the supply to the cylinder stops, so there is no heat recovery to the cylinder. Trying to solve that problem for a maximum gain of 0.3kWh per shower seems a waste of effort.</blockquote>

    That's where the devices like the itho SHRU and Recoh-Vert come in. They extract around a (claimed) 60% of the waste heat in the shower outflow whilst the shower is running and return that energy directly back to the cold feed at the same time. They don't have a tank, being basically just a vertical heat exchanger. This is presumably on top of the heat recovered back from the moist air in the shower via an Ecocent or similar heat recovery system. Overall, it may be that 70% or so of the energy used to heat the shower water gets recovered, which seems quite reasonable to me.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 26th 2011
     
    The more I ponder the details the harder it seems to get. A holding tank, while relatively cheap and effective takes up room and has health risk potential and can only really feed a header tank with energy (so combi boilers out generally).
    Ideally, cooling the flow of waste while it is happening is the better way to go, but this is fraught. To do this as passively as possible will take a very large surface area as there are only a few seconds (as long as it takes to drain a bath) to recover the energy.
    I still think there are ways around it though and will ponder it more.
    • CommentAuthorcrusoe
    • CommentTimeOct 26th 2011 edited
     
    JSHarris: Tip: Leave your bath or sink water til it goes cold, then drain it, and this discussion becomes pointless. As for showers, qeiple has it about right IMO. And anyway, if you extract all the heat, the rat rescue society locally will be busier than ever this winter helping rodents with hypothermia....think I'm joking? :shocked:
    •  
      CommentAuthorDamonHD
    • CommentTimeOct 26th 2011
     
    Retaining the hot (bath) water until it has cooled, in the absence of good MHRV, will create a huge condensation problem.

    Rgds

    Damon
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 26th 2011
     
    <blockquote><cite>Posted By: crusoe</cite>JSHarris: Tip: Leave your bath or sink water til it goes cold, then drain it, and this discussion becomes pointless. As for showers, qeiple has it about right IMO. And anyway, if you extract all the heat, the rat rescue society locally will be busier than ever this winter helping rodents with hypothermia....think I'm joking?</blockquote>

    OK if you have baths, or use sinkfuls of hot water, but again, it's "horses for courses", isn't it? If you're main use of hot water is showers, then it makes sense to recover heat from them if it's economically viable. Even getting 50% of the energy back is a big benefit, if your major energy input goes on DHW.
    •  
      CommentAuthorJSHarris
    • CommentTimeOct 26th 2011
     
    <blockquote><cite>Posted By: DamonHD</cite>Retaining the hot (bath) water until it has cooled, in the absence of good MHRV, will create a huge condensation problem.

    Rgds

    Damon</blockquote>

    True. In my case (and in qeipls) the plan is for the Ecocent to take care of that problem. Because it can extract more latent heat from the water vapour (I suspect) than a straight forward MVHR unit it makes that strategy work reasonably well (at least for those that take baths).
  5.  
    Posted By: qeiplI'm a 3-minute shower man which means I use about 1kWh of hot water from my ASHP DHW cylinder.


    I would agree that for a single 3-minute shower then it is fairly pointless. By the time you get the heat exchanger up to temperature the shower would be over. For several 3-minute showers in series as the family members all get showered then it might make more sense.

    For my sins, I'm not currently a 3-minute showerer and my kids certainly aren't. Perhaps I should be more strict with myself and family but this is unlikely to be enforceable in the short term and instead I am trying to ensure a reasonably energy efficient way of waking up in the morning. For me and my family I am clear that shower water heat recovery should provide benefits.

    Whether it does or not is another matter and it'll be interesting to see. (I do plan to monitor temperatures across the heat exchanger and DHW flow to see how effective it really is.)
  6.  
    Hi,
    The straight through design came about for shower based operations rather than the domestic market as such. For example - sports changing rooms, hair salons, industrial wash downs etc etc. So it came from seeing all that hot water going down the drain fairly constantly for most of the day.

    Cheers
    • CommentAuthorqeipl
    • CommentTimeOct 26th 2011
     
    Posted By: Mike (Up North)Hi,
    The straight through design came about for shower based operations rather than the domestic market as such. For example - sports changing rooms, hair salons, industrial wash downs etc etc. So it came from seeing all that hot water going down the drain fairly constantly for most of the day.

    Cheers


    I can see the value of heat recovery in commercial situations where there is a substantial volume of hot water going down the drain over an extended period. For domestic volumes I'm convinced it's a waste of time.
    • CommentAuthorqeipl
    • CommentTimeOct 26th 2011 edited
     
    My teenage nieces came to visit this summer and would have emptied the 300 litre HW cylinder if left to their own devices in the shower.

    It made me think that some sort of timer valve on the hot feed to the shower mixer would be a good thing.
    Every time the shower is turned on it sets the hot feed timer to run for a certain number of minutes, after which the hot feed is terminated for the same number of minutes.

    It could incorporate a digital display that counted down how many seconds you had left before getting the cold water treatment.
   
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