Shower waste water heat recovery

[Tinbum]

On talking to the person that is doing the SAP calculations for my new build he mentioned devices for collecting the heat from waste water from the showers which is used to warm the cold water to the shower. Has anyone any experience?

[Joeboy]

No, although the thought has crossed my mind many times.

[Tinbum]

I have seen them before but I've previously worked on the principle that the heat is generally lost to the fabric of the house before it exits the house itself so isn't wasted as such.

[Stinsy]

This is one of those things that makes sense when you first think about it but isn't practical.

• Showers only run for a few minutes a day.

• Lots of the heat is lost between the shower-head and plughole.

• You need to avoid freezing the outgoing water.

[Joeboy]

I have seen them before but I've previously worked on the principle that the heat is generally lost to the fabric of the house before it exits the house itself so isn't wasted as such.

Ventaxia do the single unit MVHR, that would grab a reasonable amount?

[AE-NMidlands]

If I was designing one it would have to be easily accessible and come apart very easily for cleaning as the heat exchange surfaces will quickly get coated with slimy stuff and soap scum which will cut its effectiveness. Hence the need for a very simple (and therefore inefficient) heat exchanger, maybe just a couple of microbore copper pipes for the clean water inside a wider waste-water tube? Counter-current flow for the best heat-exchange.

Keeping it clean just by regular dosing with chemicals of some kind has both financial and embedded energy costs as well. It's horrible when shower trays don't drain properly...
A

[Oldgreybeard]

A friend fitted one of these heat recovery units a few years ago, a sort of concentric copper pipe arrangement that forms the vertical part of the waste pipe from the shower. It recovers around 30% of the heat that the shower uses, by pre-heating the cold water feed to the hot tank. Not caused any issues at all AFAIK, as the grey water from the shower just runs down the centre copper pipe, which is around 50mm in diameter.

I looked at making one, as the ready made ones cost and arm an a leg, but there's a problem with the water regs, in that they demand a double pipe arrangement - you apparently cannot run potable water with just a single skin separating it from grey water. My idea was to buy a length of 50mm copper pipe and then wrap 4 parallel runs of 8mm microbore pipe tightly around it, with the 4 runs of microbore being connected in parallel. I planned to solder the pipes together to get good heat transfer.

As it turned out I'd not got the space to install such a thing, as ideally it needs to be vertical to work well. The secret is to have an offset feed from the shower waste going in to the top, so it makes the waste water spin around as it descends, making it stick to the walls of the pipe and maximise heat transfer.

A 30% saving in hot water energy use seemed very worthwhile to me, although most of our hot water comes from solar, so there wouldn't have been much of a cost saving.

[Bugtownboy]

If you have a water saving shower head, turn the temp down and shower for 2 mins, how much energy are you really going to save ?

[Tinbum]

Just looking quickly their are some claiming about 60% efficiency but the price is

https://www.ukhps.co.uk/heatrae-sadia-s ... 33370.html

[Stinsy]

Just looking quickly their are some claiming about 60% efficiency but the price is

https://www.ukhps.co.uk/heatrae-sadia-s ... 33370.html

[Expletive deleted}

For the same money you could fit an extra kW or two of PV, or fit an air-to-air heat pump, etc. they'd make a much bigger difference than this thing of dubious efficiency!