Britons! Warm your homes by leaving bathwater in the tub instead of heating the sewers with it.

Originally published at: Britons! Warm your homes by leaving bathwater in the tub instead of heating the sewers with it. | Boing Boing


I see that the article was updated to include the first issue that came to my mind:

I doubt that many Britons have humidifiers in their homes so I don’t think that’s much of a mitigating factor.

There definitely are ways to try to capture the heat energy from warm bath water (and the article does mention the existence of drain water heat recovery systems) but I think that leaving water exposed to the air in a tub isn’t a great way to do it. Maybe if you put a cover over the tub so that the heat transfers without as much evaporation or adding humidity to the home it would be something to consider.


I’m surprised he beat the Tories to bestowing this advice upon the citizens of the nation their policies have made so prosperous.


IANAP (I am not a physicist) so I don’t know how to do the math on this, but it seems like evaporative losses would represent only a small proportion of the heat lost by a cooling bathtub. I doubt anything close to an entire liter is evaporated when a bathtub of water is left to cool.


I agree that nowhere near a liter of water would be likely to evaporate in a short period of time so that wasn’t a great example for the author to give. But a steamy hot tub would inevitably add humidity to the room, and once the tub was down to room temperature any further evaporation would continue to pull heat from the air, much as a swamp cooler does.


Farts [warm ones], It’s farting indoors for all of the wonderful UK this Winter.


My father encouraged us to do this way back in the 70s. He also hacked the thermostat so that he could turn it down lower than the minimum factory setting. One of the positives was that I once kept a glass of milk on my bedroom windowsill for over a week, stayed fresh the whole time.

I always attributed my parents’ “waste not” attitude to the fact that they grew up in England during WW II.


I’ve found if I take a short, hot shower in my en-suite bathroom, the temperature in the adjacent main bedroom goes up by 0.5 of a degree.I assume that’s due to the hot water vapour from the shower because it’s a walk-in type and doesn’t have a plug (though I could get one.)


Humidity is an enemy! You gonna get problems in your walls ya’ll.


It will pull the energy from the hot water, until the water is at room temperature. During this time, you will definitely still be hearing the air.

Also, if that water then condenses on the walls or mirror, it’s returning that heat to the building.

That said, humidity is a concern, and also all this goes right out the window if you keep your bathroom fan on.


My bathroom is downstairs in an extension so I don’t know that it contributes that much to the temperature of the wider house. Besides, it’s beyond the (also an extension) kitchen with its glass roof which must be a much worse thing for the temperature…


Not sure this would make cool yet humid British homes feel nicer to be in, even if the thermodynamics of it all work out.

IIRC, actually having your house a little humid in the winter DOES make it warmer than a dry house. It is why some people have a dehumidifier in their central air and turn it off in the winter.


I’d bet that is pretty region, temperature, and heating system dependent! Forced air heating in the winter where I am, we’re fighting to increase humidity in the house wherever possible. Without using humidifiers, 20-30% relative humidity is pretty common, which is very uncomfortable (unless you really enjoy static shocks with every movement.) I already start opening the bathroom door and turning the vent fan off during showers then, some water sitting in a tub and evaporating for a little bit would be a net benefit.

Other scenarios, absolutely something to keep an eye on.


Hey UK, you mean like similar to every bath tub in Asia? Lessons learned or not yet learned by you though common knowledge for centuries.


But that hot water keeps the grease in the drains moving, helping prevent the formation of fatbergs!


The sentient mould in my bathroom would like a word.


For many, in badly built and maintained housing, this is a major problem. Even drying clothes becomes a nightmare.

And it is a tad over-optimistic to think people in need of making such marginal savings can actually afford to heat a bathtub of water in the first place. If they can spend money on heating water it is likely to be very short showers or ‘flannel bathing’.

As an exercise in domestic application of physics principles, fine. As a public information tip, not very fine at all, really.


I’m wondering how many nanoseconds of heat is actually contributed, considering the fact that bath water starts cooling immediately and only takes minutes to be less than optimal for a bath, let alone heating an entire home.


A better system is drain water heat recovery which transfers heat from the warm drain water to the new cold water coming into the water heater.


Yes, and the linked article does mention that system. Works for showers too, not just baths.

One nice thing about it is that even if the water is fairly cold by the time it’s going down the drain it’s still probably significantly warmer than the fresh water coming in to your home through the municipal pipe, so you’ll still be getting some energy back.