Heat recovery from shower water

[solar_cambridge]

When you're trying to recover heat from grey water its going to be the law of diminishing returns. Much better to conserve the energy on the input side of the equation - gas, solar and reduce the amount of water through the nozzle. The other factor is consumption and if you've got people taking 20 minute showers thats a bigger question to be addressed rather than trying to save a few % of heat from the waste water. 

Of course they may tell you where to go or just ignore you and take 20 minute showers anyway. Perhaps come up with a system design that only gives batches of 30 litres of hot water every 10 minutes rather than infinite amounts. That would certainly focus the mind of shower users.

[mistadave]

I find that a deep shower tray with a slightly blocked waste works very well, allows the tray to fill up so more of the heat is lost to the room than the drain, and also prevents people from showering too long as the tray would overflow!

[Philip R]

Just have a small HW tank, when the HW expires, the shower comes to a rapid conclusion. Excellent way of dealing with wasteful teenagers!

When trying to heat incoming water with wastewater, be aware of the water regulations and the requirement to prevent contamination of potable water. You do not want to contaminate the supply by backflow of cat 2/3/4 or higher water into the cat1 supply!

PhilipR

[trubble]

the waste pipe heat exchanger made me think...

I could put a piece of 15mm copper pipe inside a 42mm waste pipe across the bottom so that the incoming cold could be pre heated by the waste cold going out. but over 1 or 2m in length how much heat trasfer would you really get?

Incoming mains - 6-15°C depending on time of year. Outgoing shower waster - 30-35°C max, so a delt T of 10-29°C. Actually in summer when incoming mains is around 15°C the shower waste is more likley 25°C.

anyone have any ideas on how to work out sensible temperture increase for 1m or 2m lenght of heat exchanger? 1°C, 5°C??

I've kinda done this. Shower waste in 40mm, compression onto just over 2 metres of 42mm copper, mounted vertically - shower is first floor level. Sleeved around this with 54mm copper, with reducing tees either end - 54/42/22 tees. Soldered up, so mains water is between the 54 and 42, feeding the cold side of the shower tmv, feeding into sleeve from the bottom, magically appearing warmer at the top. Also fitted a mains bypass to link out the whole 54/42 'under mains pressure' part - in case of leaks! No idea of efficiency, but it does make quite a difference touch-wise - temps before and after definitely different. Without ebay it'd be a non-starter due to materials costs