WWHR - oh yes another set of letters!

[curlewhouse]

Waste Water Heat Recovery. I saw a product like this a short while ago for showers, basically the waste water from your shower runs through a "heat exchanger" (this was in reality simply a straight run pipe within a pipe and it would be easy to make your own more efficient one, but was being sold for some exorbitant price. I briefly considered making one then realised it wasn't worth the bother and I reckoned would cost more in parts than  it would ever recover) and loses some of it's heat to the incoming water, pre-warming it before your boiler or electric shower unit. I also just stumbled across a page about the principle http://www.buildenergy.co.uk/blog/wwhr-as-standard/  . Personally I think it's snake oil. It sounds  a good idea, but given the short time a shower takes (unless you are my daughter!) but I can't see these ever paying back the miniscule amount of heat saved. There is no doubt a lot of heat does go down the drain, you've only got to look at some inspection chamber covers when snow falls to see the temperature difference, but in the case of a bath or washing machine, the refilling has already happened when the hot water flows out, so the incoming water would be most likely stationary at that time. It would be interesting to see a real-world test of one of these. 

Edited August 13, 2016 by curlewhouse

[Crofter]

It's an interesting idea- we discussed it a while back on the forum:

 

 

[jack]

They're only connected to showers.  Depending on the model, you can connect a couple of showers to one.  We have one connected to our two main showers (ensuite and kids' bathroom).

 

In our case, my wife and I tend to shower one after the other, and so do our kids.  That's the best way to use these units, as it takes time for them to start working.

 

As much as the energy saving, which I know is modest, I was interested in maximising the effective amount of hot water we got out of our 250L tank.  I can't say what contribution this unit has made, but we've never run out of water in the 8 months we've lived here, even with multiple showers each a day sometimes (very active family!)

 

 

[ragg987]

I did some calcs of heat recovery vs install cost a while ago. Marginal in our case (no good with baths, there is a "bath" variant but even more expensive), so did not go ahead.

 

As jack says, it may be good where a lot of showers are taken back to back.

[jack]

I also looked at it as something that could easily last 20 years while helping us use a little less energy every day. The focus definitely wasn't payback!

[SteamyTea]

Run the waste water though a W2WHP.  That would cost even more.

[Stones]

Just wondering how prone to blockage these units are?

[curlewhouse]

The straight through version looked very unlikely to block, but also least efficient. I think there might be an application for these in commercial buildings perhaps, Jack's back-to-back showering regime writ large in a leisure centre for example could make for interesting results, where there will be to all intents and purposes a constant flow at certain times. I remember thinking years ago about how good it would be if you could somehow get a massive slinky pipe down into the main sewer network and take some of the heat away!  There must be megawatts of heat going into the sewers every day nationally.

 

A coastal village here in Northumberland used to have a big coal fired power station pumping masses of warm water into the sea for about 40 years - always struck me as insane that all the houses a mere 100 metres away were all having to pay for their own heating systems (though on the plus side some interesting fish used to turn up near the warm water outflow) at the same time as all this hot water was being dumped.

Edited August 14, 2016 by curlewhouse

[jack]

4 hours ago, curlewhouse said:

The straight through version looked very unlikely to block, but also least efficient. I think there might be an application for these in commercial buildings perhaps, Jack's back-to-back showering regime writ large in a leisure centre for example could make for interesting results, where there will be to all intents and purposes a constant flow at certain times. I remember thinking years ago about how good it would be if you could somehow get a massive slinky pipe down into the main sewer network and take some of the heat away!  There must be megawatts of heat going into the sewers every day nationally.

 

There exist products that extract heat from foul water but they all have the problem of dealing with biofilms and blockages more generally. Also, the heat is so low grade on average that you'd need something like a heat pump to get much out of it.

 

All possible, but I think payback time might be an issue given the capital and maintenance costs.

[ProDave]

I think we concluded the best way to recover heat from a bath, was leave the water in the bath until cold, then the heat will be ion the fabric of the building and only cold water going down the drain.
 

[jack]

27 minutes ago, ProDave said:

I think we concluded the best way to recover heat from a bath, was leave the water in the bath until cold, then the heat will be ion the fabric of the building and only cold water going down the drain.
 

Yep, low tech heat recovery!

[SteamyTea]

Yes, let your MVHR unit do the work.  You do still get a 'biofilm' problem, usually called a scum line.  So if you are worried about this, and the environment, then spend the money on a suitable cleaner.