Re-using the heat in bathwater in Britain • Hacker News Board

Re: Re-using the heat in bathwater in Britain

#91

Post by bombcar » Sun, Oct 30, 2022, 8:46 PM UTC

z3t4 wrote: post #33

wcoenen wrote: post #7

I tried this once on a cold winter day: I let the bath cool down to room temperature (which was <20C) before letting it drain, with the bathroom door open. The result was a lot of condensation on the windows in the surrounding rooms. I figured the high humidity was going to cause more damage than it saved money, so I scrapped the idea.

It could also be that your windows are not insulated enough. For 20C and 40RH the windows need to be colder then 6C to get condensation.

Moisture has to go somewhere even if you don't see it visibly condensing on windows.

Either it's being absorbed or you have something removing moisture from the system (venting, etc).

Re: Re-using the heat in bathwater in Britain

#92

Post by nsteel » Sun, Oct 30, 2022, 8:46 PM UTC

cameronh90 wrote: post #56

newaccount74 wrote: post #44

Earlier quoted context omitted.

Ventilation with heat recovery sounds like it could work in those conditions, but just barely. 80% @ 15C would turn into 55% @ 21C.

Unfortunately, very few homes in the UK have any air ducting. I'm looking at installing a MHRV system at the moment, but planning the ducts is a nightmare. Brick walls, lintels and structural joists everywhere with no headroom between floor and ceiling. We absolutely should be doing it in new builds though, and we're not.

New build flats have some air ducting. At least, mine did in two rooms (it wasn't for AC). But maybe you meant new build houses.

Re: Re-using the heat in bathwater in Britain

#93

Post by postultimate » Sun, Oct 30, 2022, 8:48 PM UTC

I've been doing this, except I have a bath. It finally dawned on me that heating water and leaving it to cool was exactly the way the house is normally heated, so it was costing me nothing to bath as long as I liked. I live there now.

Re: Re-using the heat in bathwater in Britain

#94

Post by bombcar » Sun, Oct 30, 2022, 8:49 PM UTC

sixstringtheory wrote: post #82

In addition to things like heat exchangers or reducing the amount of time/water in a shower, is there any work being done around optimizing the amount of time a hot water heater keeps the water at high temp? I’ve thought about this for extended absences but this could also bring potential benefits if it just doesn’t run for certain portions of the day or middle of the night.

People have done that (some recent post on HN was about using a RaspberryPi to do just that, including a "get hot enough to remove legionaries' disease").

Likely for most people the tankless heaters would be a better option, or using in-floor water-based heating instead. Something like this: https://www.radiantcompany.com/system/opensystem/

Re: Re-using the heat in bathwater in Britain

#95

Post by ericpauley » Sun, Oct 30, 2022, 8:49 PM UTC

ArmandGrillet wrote: post #53

Tangentially related: has someone found something less energy intensive than taking a bath while bringing the same degree of warmth? I've been looking at heated blankets but they're all made of polyester which is not comfortable due to the moisture being trapped under the blanket. Heated seats in some cars are the closest sense of comfort I've found but that technology doesn't seem to exist for regular seats.

A heated mattress cover has been a game changer for me. You can use your existing comfy blankets and it only takes a very small amount of heat to really make a difference. It definitely pays for itself in being able to reduce temperature set point at night.

Re: Re-using the heat in bathwater in Britain

#96

Post by twic » Sun, Oct 30, 2022, 8:51 PM UTC

maherbeg wrote: post #11

I was actually looking at heat recuperation systems for bathrooms recently and stumbled upon https://ecodrain.com/en/ - if we hadn’t already remodeled our bathrooms I’d have loved to have installed this. For now we make do with a heat pump water heater which is very efficient.

There are numerous manufacturers of these things, eg:

https://powerpipehr.co.uk/

https://showersave.com/vertical-wwhrs/

https://recoupwwhrs.co.uk/

Ultimately they're just cleverly arranged copper pipes, so the barrier to entry for new manufacturers is low!

I am keen to get one - my shower sits directly above a cupboard on the floor below, where it would be simplicity itself to fit.

This article suggests the energy savings are not particularly big:

https://www.thegreenage.co.uk/tech/waste-water-heat-recovery...

... but there are several comments reporting the opposite!

Re: Re-using the heat in bathwater in Britain

#97

Post by TrueSlacker0 » Sun, Oct 30, 2022, 8:53 PM UTC

SnaggyJoker wrote: post #70

georgeg23 wrote: post #28

Earlier quoted context omitted.

You could place a plastic mat on top to limit evaporation.

You could also switch to a water conserving mist shower and reduce the amount of heated water used for the shower. You could also reclaim the water from the shower for use in the toilet by turning the supply to the toilet off and manually transferring it to the toilet tank. There are lots of things you could do to save resources but most things are not done and are not going to be done because it is inconvenient and…

Combo that with toilet sinks and a lot of water would/could be saved per year/person.

Re: Re-using the heat in bathwater in Britain

#98

Post by bombcar » Sun, Oct 30, 2022, 8:53 PM UTC

yjftsjthsd-h wrote: post #10

sweettea wrote: post #6

Not discussed: hot water in houses increases humidity. In modern well sealed houses, that can result in unfortunate mold growth.

I've always experienced extremely low humidity in winter months, so I would think the net effect might be a reasonable positive if this saves me needing to run a humidifier. Am I missing some detail? Or is that just unusual?

Moisture is the #1 destroyer of buildings in its various forms.

https://www.buildingscience.com/bookstore/books/moisture-con...

Moisture control and mitigation is a very important part of building to last, and building to minimize maintenance and energy usage (simplistic "insulate everything and seal all air leaks" can result in a doomed structure).

Re: Re-using the heat in bathwater in Britain

#99

Post by bombcar » Sun, Oct 30, 2022, 8:56 PM UTC

ArmandGrillet wrote: post #53

Tangentially related: has someone found something less energy intensive than taking a bath while bringing the same degree of warmth? I've been looking at heated blankets but they're all made of polyester which is not comfortable due to the moisture being trapped under the blanket. Heated seats in some cars are the closest sense of comfort I've found but that technology doesn't seem to exist for regular seats.

Heh get a dry suit (like a wetsuit but sealed) and then fill it with hot water? Should only use a few gallons.

Re: Re-using the heat in bathwater in Britain

#100

Post by twic » Sun, Oct 30, 2022, 8:58 PM UTC

afandian wrote: post #40

maherbeg wrote: post #11

I was actually looking at heat recuperation systems for bathrooms recently and stumbled upon https://ecodrain.com/en/ - if we hadn’t already remodeled our bathrooms I’d have loved to have installed this. For now we make do with a heat pump water heater which is very efficient.

I wonder how different it would be to use an off the shelf heat exchanger, e.g. this one? https://www.ebay.co.uk/itm/263272702860 You'd probably have to filter the shower water before sending it through, but beyond that I wonder how different the principle is. Maybe being easy to clean is a significant factor in drain showers.

I think doing adequate filtration of the full waste water flow without losing most of the heat would be a challenge. And remember you're also sending soap suds down the pipe, which will pass the filter and leave deposits inside the exchanger.

Also, if you're just using gravity to push the waste water through, what flow rate would you get on one of those units, and how many would you need to handle the full flow?

The most persuasive waste water heat exchangers i've seen are based around the a large-diameter pipe, with conical sections connecting it to the input and output, where the waste water flows down the inner wall of the pipe in a thin layer, and the cold water flows up the outside. It's not as space-efficient as a conventional heat exchanger, but it avoids the problem of blockage.

That would also be fairly easy to improvise, i think, given that you can buy copper pipe, or even buy copper foil and wrap it into a large, thin-walled cylinder.