Bath heat reclamation

More of an idle bit of curiousity than something I’m pursuing with utmost seriousness, but having seen heat reclaimers for showers I’m curious to know if anyone knows of something similar for baths.

I’ve thought about it and I can imagine it could be achieved with something like a radiator under the bath that slows the rate at which the water drains while radiating heat. Maybe even something with a small heat pump (as you’d find in a fridge or tumble dryer). I’ve not encountered any products even remotely like this and I’m left wondering why that would be. Does anyone have any ideas?

The cheapest and most efficient form of heat recovery for a bath is to leave the water in it until it reaches room temperature.

Unless you filter the water leaving the bath your radiator method would clog up in no time.

I’m familiar with that approach. It feels like there must be a happy medium where we don’t have to flush all that lovely warm water away if someone else wants to use the bath but not the bath water…!

I’d thought about the clogging thing too, but it seems odd to me since shower heat recovery units don’t seem to suffer from that problem - something else I’ve been confused by.

A shower heat recovery unit is basically a copper drain pipe with the cold supply to the shower wrapped round it. It works over the period of the shower, preheating the shower water.

A drain based bath heat exchanger as you describe would provide ambient warmth for the house.
If your bath drains in 3 minutes you somehow either get warmth for 3 minutes or have to store the water as it cools. Slowing the flow will cause precipitation of suspended matter (skin, soap, dirt, hair). This needs to be dealt with. Unfortunately I don’t see how it can be economical to do, but on a purely carbon saving basis it might be possible to make a “profit” if all the materials used are rescued scrap.

Incidentally an anywhere near efficient shower heat recovery unit probably never pays for itself either, whether accounting in money or carbon.

I leave bath water in the bath, heat recovery ventilation operates while humid.
So room is warm for getting dressed
Can also use bath water for flushing the toilet - water has a carbon footprint!

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