Green Building Forum - DHW heat recovery

DHW heat recovery

MarkBennett — Wed, 26 Oct 2011 14:22:33 +0100

Posted By: qeiplMy 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.

I have to say that I've though about something similar although I was thinking about a motorised mixing valve that progressively made the water temperature colder the longer it ran rather than suddenly went from hot to cold. Not sure I'd ever be brave enough to implement such a thing though.

DHW heat recovery

joe90 — Wed, 26 Oct 2011 14:54:39 +0100

Looking at this from another angle:- I plan a new build with no insulation under the slab and using this thermal mass to store summer heat from windows etc, what if the soil pipe was looped within this area before it left for the sewer and heat from ALL waste would help top up the slab temprature? just a thought!

DHW heat recovery

Paul in Montreal — Wed, 26 Oct 2011 15:11:22 +0100

I looked at installing a shower drain water heat recovery unit - cost would have been around Can$500 (big copper to fix around a 4" cast iron drain pipe). I calculated the payback time to be around 40 years or so as my hot water heating cost was already low due to pre-heat from the GSHP. The path from the bathroom to the drain is relatively long in my house so a decent amount of the heat is emitted back into the heated enveloped anyway. Not so useful in summer, but I don't have hot showers in the summer anyway.

For places like gyms where the showers are running all the time, it makes perfect sense. Modern (commercial) laundries usually re-use the heat from drying and washing anyway.

Paul in Montreal.

DHW heat recovery

qeipl — Wed, 26 Oct 2011 15:28:05 +0100

Posted By: joe90Looking at this from another angle:- I plan a new build with no insulation under the slab and using this thermal mass to store summer heat from windows etc, what if the soil pipe was looped within this area before it left for the sewer and heat from ALL waste would help top up the slab temprature? just a thought!

For this to work you'd have to separate the hot waste from the cold, otherwise the cold waste would be removing energy from your seasonal store and depositing it into the sewer (or septic tank).

DHW heat recovery

joe90 — Wed, 26 Oct 2011 15:41:16 +0100

Good point qeipl, easy enough with a new build, hot stack and cold stack, teed together above to vent through roof and joined together in manhole outside. Cold soil pipe insulated within slab and shortest route, hot soil pipe not insulated and longest route within slab.

DHW heat recovery

RobinB — Wed, 26 Oct 2011 16:08:01 +0100

Couldn't you just have an e-n-o-r-m-o-u-s U bend on the shower drain that would hold a showers-worth of water and contirbute that heat to space heating? Would be very cheap! A sort of midway between "Tony's leave it in the bath til it's cold" (too steamy) and these expensive heat recovery contraptions. Could also be applied to sink or bath.
RobinB

DHW heat recovery

SteamyTea — Wed, 26 Oct 2011 16:11:58 +0100

I am working on a different idea at the moment, if I find my temp loggers I should get better data as I am working with 20 kg of water this time and a pumped heat exchanger. More to follow.

DHW heat recovery

crusoe — Wed, 26 Oct 2011 16:20:50 +0100

Condensation? No JSH, that's for old, cold houses, surely - and has never been a problem if ventilation is working properly - in most cases a window ajar, or open fire going. Modern, warm houses with full ventilation/heat recovery - not a problem! We're not talking swimming pool context here, simply a small bath.

I suspect PIM's 40-year figure would extend beyond a century if 3-minute showers and new recovery kit were factored in.

As for showering, just get a deeper tray, or shower over the bath. The plug ain't removable 'til the temperature drops. That'll sort qeipl's neices too. Overflow? It will only happen once. I can feel a few patents coming on. While we're at it, sorting the bleeding obvious, that is, please join my campaign at www.letsnotfixtheunbroken.com

DHW heat recovery

SteamyTea — Wed, 26 Oct 2011 17:06:55 +0100

www.letsnotfixtheunbroken.com is broken

DHW heat recovery

Paul in Montreal — Wed, 26 Oct 2011 17:11:12 +0100

Posted By: crusoeI suspect PIM's 40-year figure would extend beyond a century if 3-minute showers and new recovery kit were factored in.

My 40 year figure is because energy is cheap where I am and my gas consumption (for cooking and hot water) is quite small already. Even if I reduce by half when it's only a few 10s of dollars a year maximum. My drain stack travels at least 30feet through the heated envelope anyway. By the time the water disappears below ground level in the basement, the soil pipe is barely warm to the touch after even a 10 minute (horror!!!) shower.

When I have a bath, I do just leave the water in until cold before pulling the plug (but more, in winter, to get extra humidity into the house!).

Paul in Montreal.

DHW heat recovery

JSHarris — Wed, 26 Oct 2011 17:16:07 +0100

Posted By: crusoeCondensation? No JSH, that's for old, cold houses, surely - and has never been a problem if ventilation is working properly - in most cases a window ajar, or open fire going. Modern, warm houses with full ventilation/heat recovery - not a problem! We're not talking swimming pool context here, simply a small bath.

Not me. Damon made the point about condensation. I agree with you, it's only a problem with poorly insulated houses (or should be).

DHW heat recovery

JSHarris — Wed, 26 Oct 2011 17:17:45 +0100

Posted By: RobinBCouldn't you just have an e-n-o-r-m-o-u-s U bend on the shower drain that would hold a showers-worth of water and contirbute that heat to space heating? Would be very cheap! A sort of midway between "Tony's leave it in the bath til it's cold" (too steamy) and these expensive heat recovery contraptions. Could also be applied to sink or bath.
RobinB

One option is just that, the Recoh-Tray system: http://shower-save.com/recoh-tray.html

DHW heat recovery

JSHarris — Wed, 26 Oct 2011 17:21:57 +0100

Posted By: qeiplMy 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.

Just fit a submarine shower valve. These are often just push button operated and give you a 30 seconds of water to wet yourself, then the water goes off while you lather up, then you push the button again and get another 30 seconds of water to rinse off. Bit of a bugger if you haven't fully rinsed off when the water goes off, because you only get two presses of the button every three days..............

DHW heat recovery

SteamyTea — Thu, 27 Oct 2011 09:07:03 +0100

Test done, here is the graph. After 18000 seconds (5 hours) it is hardly worth carrying on as the pump is taking almost as much as the energy transfer. Was simply a plastic pipe stuffed into the hot and cold holding tanks with a pump in a reservoir. Performance could be easily improved with a copper pipe and better insulation on the parts open the ambient temp. This may improve the response time of 1200 second (20 minute). Good fun though.

DHW heat recovery

JSHarris — Thu, 27 Oct 2011 10:16:25 +0100

There's nothing like a good experiment to show how something will work!

A system that retained hot waste water for a few hours, allowed the heat to be exchanged to incoming cold water (or air, if room heating was the objective) and then emptied the waste water away, would seem to have some merit. The snag is making something like this relatively simple. There needs to be a way of flushing the holding tank and draining it, as all the stuff I've read suggests that grey water goes very manky, very quickly, if stored in a tank for more than a few hours.

DHW heat recovery

SteamyTea — Thu, 27 Oct 2011 10:25:18 +0100

Posted By: JSHarrisThere needs to be a way of flushing the holding tank and draining it

Thermal valve (either mechanical or electronic, maybe timer as well) and gravity. I think of the holding tank as just a wide pipe.
I have an idea (from my spa bath days) about water to air heat exchangers that I may try later, I think I have the kit laying around there somewhere.

A simple 'proof of concept' experiment is generally cheaper, quicker, easier and more definitive than modelling every time, and I did mathematical modelling for my BSc dissertation

DHW heat recovery

crusoe — Sat, 29 Oct 2011 19:01:18 +0100

Sorry JSHarris/Damon confused.... though you did agree with Damon, JSH.

Touche PiM!

Steamy - link was a joke.... And I think your laudable commonsense experiement shows this to be, as a few of us who have been around the block a few times suspected, a very questionable use of funds. Well done.

We already have temperature-sensitive submarine valves, guys. Grumpy old guys like me falling asleep in the bath, waking up when it's cold, realising this - and letting the water out.

DHW heat recovery

chrisinbrighton — Sat, 29 Oct 2011 22:51:08 +0100

I have often thought about shower recovery systems and was wondering if a metal shower tray was used could the heat be taken directly from this -as opposed to the pipe.
It might be cold for your feet unless on an wooden? island above...

DHW heat recovery

JSHarris — Sun, 30 Oct 2011 09:44:28 +0000

Posted By: chrisinbrightonI have often thought about shower recovery systems and was wondering if a metal shower tray was used could the heat be taken directly from this -as opposed to the pipe.
It might be cold for your feet unless on an wooden? island above...

The Recoh-Tray is similar to this. The best image I could find of it is in this PDF file: http://shower-save.com/pdfs/Recoh-tray%20installation.pdf

DHW heat recovery

Citrus — Fri, 30 Mar 2012 09:16:17 +0100

There'll be 7 adults in the house I'm moving to soon, and no immediate funds for solar hot water, so the DHW sounds very appealing to me. If I lived on my own it wouldn't be worth it, but with 7 people it should give back something useful. Does anyone fancy drawing a diagram of a DIY version? I get the idea, but am quite bad at extrapolating what something looks like from written descriptions, and I am in no way a plumber. I do like to have a go at things though :)

DHW heat recovery

GreenPaddy — Sat, 31 Mar 2012 15:06:28 +0100

Found this at Eco-build;

www.zypho.eu

no idea of price or efficacy.

DHW heat recovery

DamonHD — Sat, 31 Mar 2012 18:15:53 +0100

And the fact that the entire front page is Flash or something equally vile...

DHW heat recovery

SteamyTea — Sat, 31 Mar 2012 21:41:42 +0100

You didn't miss much
Got to this and stopped reading.
OK, I read a bit more, the PDF of the technical stuff is here:
http://www.zypho.eu/images/Ficha_Tecnica_EN.pdf

DHW heat recovery

MarkBennett — Sun, 01 Apr 2012 18:04:29 +0100

Posted By: GreenPaddyFound this at Eco-build;

www.zypho.eu

no idea of price or efficacy.

My take on this device:

http://hardhouseblog.wordpress.com/2012/03/26/ecobuild-zypho-shower-heat-recovery/

DHW heat recovery

RobL — Mon, 07 May 2012 13:05:00 +0100

Re One or two copper pipes between mains cold and waste water -

In our case the DHW tank is fed from a tank in the loft, so we have isolation from mains water for sure. Could I therefore make a simple&efficient copper heat exchanger that only has one barrier from cold to waste? As mentioned here, the waste would form a film on a vertical copper pipe, cold water running in a sleeve contraflow to this.
An important considereation is the pressure drop of commercial units, and I note that the type with microbore soldered around a copper waste pipe aren't suitable for gravity fed DHW.

DHW heat recovery

Citrus — Mon, 07 May 2012 22:51:59 +0100

I am still keen on the idea of these systems, but realised the other day that the waste pipe from our bath goes straight out the wall and down the outside. Major palaver to replumb it to the inside?

DHW heat recovery

RobL — Wed, 09 May 2012 21:28:39 +0100

I measured the temp of our shower, to get a bead on the numbers - there's a piccy of a graph I made below.
I measured the cold and hot feeds into the thermostatic power shower, and the U bend waste water temp. The flow rate under test was a rather embarrasing 10l/minute. I know this should be a lot less, but that kind of "improvement" would not go down well.

In brief, I calculated:
One typical shower used 1.8KWh of hot water
1KWh could be recovered if the hot feed only was preheated with 100% efficiency.
1.3KWh could be recovered if hot and cold feed were preheated with 100% efficiency.
The U bend water temp is 3degrees lower than the shower water outlet temperature - ie almost all of the heat really is flushed straight away.

Our daily use is 2.5 showers, and 7KWh of gas-> hot water+cooking, which corresponds well to the above.

RobL

DHW heat recovery

SteamyTea — Wed, 09 May 2012 23:22:00 +0100

Halve your shower time is another way

DHW heat recovery

JSHarris — Thu, 10 May 2012 07:21:56 +0100

Posted By: SteamyTeaHalve your shower time is another way

Just fit submarine shower valves. They have a timer and on-off button. The first press of the button gives you 30 seconds of water, then the water turns off while you lather up. The second press gives another 30 seconds of water to rinse off. Any subsequent press of the button has no effect, so you're limited to 1 minute of shower water in total.

DHW heat recovery

RobL — Thu, 10 May 2012 10:57:11 +0100

Installing a timed shower would not get internal planning approval.
Installations without planning approval invoke removal of privileges.