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You shouldn't let warm water stay in a place with air contact. First it makes air humid and the affected rooms prone to mold, and second evaporation takes energ

You shouldn't let warm water stay in a place with air contact. First it makes air humid and the affected rooms prone to mold, and second evaporation takes energy away, and while condensation might give the energy back you will have wet conditions. It's not very sanitary: warm and wet conditions for some time tend to encourage unwanted life. I armchair thought about a scheme: collect the bath and shower water in a tank in the cellar and use a heat exchanger. However the tank will turn disgusting very quickly. Perhaps a better solution is using a large, long pipe with a flat floor with little inclination, such that warm water does not stay for a long time but flows slowly. Cool the floor of the pipe with a heat exchanger. Perhaps have a contraption to regularly have the pipe brushed from inside and flushed with cold water. Instead of a heat exchanger, use cold water, but this has the problem that fresh cold water needs to flow at the same time as the used warm water. It is worthwhile?

koliber 4y ago
In the winter, the air is very dry in my house and we use humidifiers. By leaving the bathwater in the bathtub, we could kill two birds with one stone. Everything you say about ventilation is right though. I just don't know if calling this unsanitary is universally true. Maybe in climates where humidity is a problem to begin with, adding more humidity leads to less sanitary conditions.
- _nalply 4y ago
  I agree, sometimes I do that, too. However the whole October was warm and humid and I never let the water stay in the bathtub. I carefully check for humidity: it should be between 40% and 60%. Sometimes I hang washed clothes in the room to get more humidity.
mnw21cam 4y ago
I have been considering a grey water solution that would achieve two things. The bath in my house is upstairs, and I have an additional toilet downstairs, almost directly underneath. So, I could let the bathwater drain into a tank above the toilet cistern, from where it can slowly release its heat, and then use the water to fill the toilet cistern for flushing. That way, I save on heating and on water usage, and the water wouldn't be evaporating into the house causing humidity problems. I did the calculations. A 120l bath has water costing £0.53 and heating (by gas, if we assume the incoming water is about 10C and heating it to 40C, about 4.2kWh) costing about £0.44. We probably get out of the bath and drain it when it's about 35C, so if the house is at 22C then there's about 1.8kWh of energy that would get transferred as heat to the house, about £0.19 worth. So, for a very modest cost of installing a tank and a little bit of plumbing (probably less than £200), we'd be saving £0.53 each day (assuming all that water gets used) plus £0.19 each day that we would have the heating on, probably half the year, which is around £230 per year. It'd pay itself off very quickly. The main concern is that the holding tank doesn't become a bacterial breeding ground, so it may need some chemical discouragement every now and again, but that's hardly difficult.
- _nalply 4y ago
  That's a clever solution. In a different vein, I thought about dismantling the mobile air conditioner and putting it in a cold water bath instead sucking air to heat to blow to outside. The lower part is wet anyway from condensate. This summer I did something different and simpler: instead of a hose to outside, I made a wooden frame fitting into the window and strapped the air conditioner to it such that it sucked the air from outside instead from inside. I think it worked better this way because it didn't create negative pressure in the flat anymore and because it could blow the hot waste air directly to the outside. The fans didn't need to fight against the negative pressure and the long hose.

koliber 4y ago

In the winter, the air is very dry in my house and we use humidifiers. By leaving the bathwater in the bathtub, we could kill two birds with one stone. Everything you say about ventilation is right though. I just don't know if calling this unsanitary is universally true. Maybe in climates where humidity is a problem to begin with, adding more humidity leads to less sanitary conditions.

_nalply 4y ago
I agree, sometimes I do that, too. However the whole October was warm and humid and I never let the water stay in the bathtub. I carefully check for humidity: it should be between 40% and 60%. Sometimes I hang washed clothes in the room to get more humidity.

mnw21cam 4y ago

I have been considering a grey water solution that would achieve two things. The bath in my house is upstairs, and I have an additional toilet downstairs, almost directly underneath. So, I could let the bathwater drain into a tank above the toilet cistern, from where it can slowly release its heat, and then use the water to fill the toilet cistern for flushing. That way, I save on heating and on water usage, and the water wouldn't be evaporating into the house causing humidity problems. I did the calculations. A 120l bath has water costing £0.53 and heating (by gas, if we assume the incoming water is about 10C and heating it to 40C, about 4.2kWh) costing about £0.44. We probably get out of the bath and drain it when it's about 35C, so if the house is at 22C then there's about 1.8kWh of energy that would get transferred as heat to the house, about £0.19 worth. So, for a very modest cost of installing a tank and a little bit of plumbing (probably less than £200), we'd be saving £0.53 each day (assuming all that water gets used) plus £0.19 each day that we would have the heating on, probably half the year, which is around £230 per year. It'd pay itself off very quickly. The main concern is that the holding tank doesn't become a bacterial breeding ground, so it may need some chemical discouragement every now and again, but that's hardly difficult.

_nalply 4y ago
That's a clever solution. In a different vein, I thought about dismantling the mobile air conditioner and putting it in a cold water bath instead sucking air to heat to blow to outside. The lower part is wet anyway from condensate. This summer I did something different and simpler: instead of a hose to outside, I made a wooden frame fitting into the window and strapped the air conditioner to it such that it sucked the air from outside instead from inside. I think it worked better this way because it didn't create negative pressure in the flat anymore and because it could blow the hot waste air directly to the outside. The fans didn't need to fight against the negative pressure and the long hose.