Would it be green to leave the bath to get cold?

Astro Pen

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Just got out of the bath and pulling the plug I thought "Suppose I leave the water to get cold before draining it. Would it make a significant contribution to the warmth in the house? Or is it negligible in the scheme of things?
I don't know the calorific capacity of a full bath of hot water but I imagine at least half the copper tank's worth. Multiplied by the average family that is quite a bit of energy.
 

mosaix

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Just got out of the bath and pulling the plug I thought "Suppose I leave the water to get cold before draining it. Would it make a significant contribution to the warmth in the house? Or is it negligible in the scheme of things?
I don't know the calorific capacity of a full bath of hot water but I imagine at least half the copper tank's worth. Multiplied by the average family that is quite a bit of energy.

I seem to remember that there was a building technique that involved an insulated storage tank of liquid beneath a dwelling. Hot water drains from baths, showers, washing etc., passed through the tank and gave up the heat into the liquid. During cold periods vents in the tank opened to allow heat to escape upwards into the dwelling.

I always thought it was a good idea but I’ve never seen of heard of it being implemented.

Regarding the hot bath water - why not? Otherwise it energy (money) literally down the drain.
 

Alex The G and T

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There are far too many undocumented variables in this thought experiment:

Do you have a quick splash and drain very hot water? Or do you have a long soak until the water is tepid and mostly expired, anyway?

Is your bath upstairs, where the drain pipes absorb and retain heat within the walls and subspaces of the house? Or does it drain, immediately, into the earth, outdoors?

Is the Bathtub made of Heat retaining Cast Iron, or thin fiberglass which holds little residual heat?

Do you live in a humid climate where vapor from the residual sumping of warm water contributes to growth of mold or algae, which results in further energy consumption for the cleaning thereof; or possible respiratory infections to the residents breathing stale bathwater vapors?

Of course, the really fidgety, in times of drought, would carry all of that water off in pails, to moisten the potted house plants, flush the toilets; or toss it out on the dryer parts of the gardens in clement weather.

Enquiring minds want to know more details.

(And I add my endorsement to the joys of sharing the bath.)
 

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Which reminds me of the summer of 77 - one of the drought years - when my father rigged a hosepipe from the bath to the garden, and would siphon the used bath water down each evening.
 

Ray Zdybrow

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"Do you live in a humid climate where vapor from the residual sumping of warm water contributes to growth of mold or algae, which results in further energy consumption for the cleaning thereof; or possible respiratory infections to the residents breathing stale bathwater vapors?"

My first thoughts were "condensation" and "black mould".
 

Montero

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Well, in terms of leaving a bath to cool, and condensation, you could always build a cover for the bath that you put on after you get out. Hot tubs have insulated lids. You wouldn't want an insulated one if you are letting the heat out into the house. But if you have any DIY skills, a wooden frame with plastic sheeting on to hold in the evaporation. Could even be made from scrap timber and used plastic sacks. If you built something that is neat, with a hinge in it, you could even have a cover over part of the bath when you are sitting in it, for extra warmth.

An alternative is the neater, but more expensive waste water heat recovery system. Waste Water Heat Recovery Systems - TheGreenAge The costs are probably out of date.
 

Venusian Broon

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Rough maths.

A bathtub holds about 300 litres of water, and assuming you pour a bath at 80 degrees, say, it would release ~75 million joules of heat as it cools down to 20 degrees. (If you went for a very hot bath and just used boiling water, this figure goes up to 100 million joules, but who jumps into a boiling bath of water?)

This is about 0.2 kWh of energy.

I've found a figure that the average energy used to heat a UK home is 12,000 kWh. Or an average of ~32 kWh per day. Thus the cooling bath contributes less than 1 percent of heating by its cooling as above.

However, you probably won't be heating the house in summer a lot, so this contribution in winter is much smaller. Also this 12,000 kWh figure is only for heating. Multiply it by two to get a full amount of energy the house uses for everything.

So in principle it does help a tiny bit....but having a wet room slowly cooling will probably not feel very warm at all.

If you want to save, showering is much better. A 10 minute shower uses half the water that a bath does, and if you want to really save hot water, do a three minute British army shower: First minute shower on, get wetted and start lathering. Second minute turn shower off and really clean your body with the soap suds, then final minute shower back on, wash off everything and then finish. That'd be 1/10th of the hot water used.
 

Christine Wheelwright

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Rough maths.

A bathtub holds about 300 litres of water, and assuming you pour a bath at 80 degrees, say, it would release ~75 million joules of heat as it cools down to 20 degrees. (If you went for a very hot bath and just used boiling water, this figure goes up to 100 million joules, but who jumps into a boiling bath of water?)

This is about 0.2 kWh of energy.

2kWh surely? 2,000 joules per second over 3,600 seconds is 72MJ.

I think it is worth keeping the bath water in the house for a while before draining. But you would want some kind of cover (as suggested earlier) to prevent humidity.
 

Venusian Broon

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2kWh surely? 2,000 joules per second over 3,600 seconds is 72MJ.

I think it is worth keeping the bath water in the house for a while before draining. But you would want some kind of cover (as suggested earlier) to prevent humidity.
Yeah was about to edit this! I put the wrong figure in the kWh calc!
 

Venusian Broon

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In principle it sounds good to urilise the energy. And after @Christine Wheelwright corrected me :giggle: , a full bath is a significant part of your budget.

But in practice it will only usefully heat up your bathroom. And only have minimal effects on the rest of the house.

Cutting consumption of hot water use is probably better. So shallow baths/showers
 

LordOfWizards

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There are far too many undocumented variables in this thought experiment:

Do you have a quick splash and drain very hot water? Or do you have a long soak until the water is tepid and mostly expired, anyway?

Is your bath upstairs, where the drain pipes absorb and retain heat within the walls and subspaces of the house? Or does it drain, immediately, into the earth, outdoors?

Is the Bathtub made of Heat retaining Cast Iron, or thin fiberglass which holds little residual heat?

Do you live in a humid climate where vapor from the residual sumping of warm water contributes to growth of mold or algae, which results in further energy consumption for the cleaning thereof; or possible respiratory infections to the residents breathing stale bathwater vapors?

Of course, the really fidgety, in times of drought, would carry all of that water off in pails, to moisten the potted house plants, flush the toilets; or toss it out on the dryer parts of the gardens in clement weather.

Enquiring minds want to know more details.

(And I add my endorsement to the joys of sharing the bath.)

Dang! What a killjoy! Give me the old days where we diverted the mountain stream into the house. Contractors! Pfff!
 

JunkMonkey

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I have wondered about this (OP) in the past but live in a old, badly -insulated house so the condensation / mold conciderations were more important. Then I realised that because of the absurd ad hoc history of the building the drain from the bathroom runs diagonally under my house so probably looses its heat in my solum thus contributing miniculily to keeping the house from freezing
 

Montero

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Also helps prevent your drain from freezing. One surprise harsh winter when I was at college, the Edwardian hall of residence had a drainage crisis - the cast iron pipes down the four storey building - down the outside - froze and all of a sudden EVERYTHING was backing up.
 

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