Shower waste water heat recovery

[Gareth J]

Would like to see real world delta Ts. From experience trying to cool milk from about the same temperature at about 30l/min, a setup of this size is needed. The copper is 28mm and water flowrate is more than double the milk flowrate. Cools to within a couple degriof water temperature.


Clearly a shower heat recovery isn't really comparable; lower flowrates, 1:1 fresh:waste ratio etc. But my point is, I find it hard to imagine such a relatytiny surface area can exchange enough heat to make it worthwhile.

[Stinsy]

Would like to see real world delta Ts. From experience trying to cool milk from about the same temperature at about 30l/min, a setup of this size is needed. The copper is 28mm and water flowrate is more than double the milk flowrate. Cools to within a couple degriof water temperature.


Clearly a shower heat recovery isn't really comparable; lower flowrates, 1:1 fresh:waste ratio etc. But my point is, I find it hard to imagine such a relatytiny surface area can exchange enough heat to make it worthwhile.

It really doesn’t take much calculation or experience to know that these shower heat recovery devices are a waste of time/money.

[Gareth J]

It really doesn’t take much calculation or experience to know that these shower heat recovery devices are a waste of time/money.

I recall relatively simple heat exchanger performance equations, though im just going by gut feeling, I think you're right - it should be pretty straightforward to work out an idea of performance easily.

[Stinsy]

It really doesn’t take much calculation or experience to know that these shower heat recovery devices are a waste of time/money.

I recall relatively simple heat exchanger performance equations, though im just going by gut feeling, I think you're right - it should be pretty straightforward to work out an idea of performance easily.

The thing that completely and utterly licks this idea into touch is that there has to be two barriers between the potable and waste water...

[Bugtownboy]

Given the starting temp, the amount lost to the atmosphere from the shower droplets, shower enclosure, shower occupant(s) and sitting in the trap.

Add the inefficiencies of the heat recovery device, is it worth it.

Be better spending the money on a shower cut off at 3-5 minutes

[Paul_F]

Clearly a shower heat recovery isn't really comparable; lower flowrates, 1:1 fresh:waste ratio etc. But my point is, I find it hard to imagine such a relatytiny surface area can exchange enough heat to make it worthwhile.

Rough cut they'll extract about half of the heat available in the water, i.e. the outlet temperature will be about half-way between cold inlet and waste inlet temperatures.

It really doesn’t take much calculation or experience to know that these shower heat recovery devices are a waste of time/money.

That all depends what you're trying to do. In my case I'm looking to build a Passivhaus - which means essentially the heating system will end up being sized by the required recovery time on the cylinder. With showers being the main hot water demand, a 50% efficient heat exchanger will nearly halve the recovery time on the cylinder, allowing me to fit a smaller (more efficient and better matched) heat pump. So at a system level, the £500 cost of fitting one is very attractive since it permits big savings elsewhere - as well as a 30% reduction in effective hot water demand over the life of the building. For retrofit, not so attractive.

Given the starting temp, the amount lost to the atmosphere from the shower droplets, shower enclosure, shower occupant(s) and sitting in the trap.

Add the inefficiencies of the heat recovery device, is it worth it.

Be better spending the money on a shower cut off at 3-5 minutes

Horses for courses. I know I wouldn't get my family to do that.

[Gareth J]

At very least, request data on the expected performance wrt differing flow rates and inlet temperature differentials. If they can't, or won't supply these, they've either not tested it or, they aren't proud of its performance. With the charts, you can compare it to the sort of shower you find acceptable. Or, maybe it'll inform your choice about the type of shower you fit.

A step further, if still skeptical, would be to ask if they'll guarantee that their data will be reflected in real world performance and, if they will, before you fit, rig up a test bed to check.