# At last: a practical Energy Harvesting proposal?

Gold Member
Summary:
Use the waste warm water from your plumbing system.
I was enjoying my (very short) hot shower this morning and realised that the lovely warm water was flowing over my body (sorry - too much information) only once and then disappearing down the drain.

This water - and what comes out of the washing machine, dishwasher etc. could be at a seriously 'useful' temperature for a heat pump. There must be many kWh lost over a day. You can find systems for shower heat recovery, advertised but they seem to be passive and are low hanging fruit, afaics. A whole house system would be better and would involve a hot water storage tank (which I have).

Obviously the heat exchanger plumbing would need to be maintained (cleaned out) on a regular basis and I would imagine that separate drainage would be needed for foul water. Also, the discharge water temperature would need to be well above freezing but the usable heat would be a lot more, per electrical kWh, than what can be obtained from an air source heat pump in winter. An installation could be a lot cheaper than heat pumps that are being considered today.

Many years ago I heard of a scheme to recycle heat from a house via a ventilation system where outgoing air is passed through a heat exchanger to warm up incoming air. That's a similar principle.

I ask myself why there aren't dozens of these alternative systems to be found working in houses all ov er the country.

Drakkith
Staff Emeritus
I ask myself why there aren't dozens of these alternative systems to be found working in houses all ov er the country.
Ah yes, something else that needs regular maintenance and can break or leak all over my house...
Honestly they probably just don't save very much money for their added cost and complexity.

bob012345, PhDeezNutz and russ_watters
Baluncore
2021 Award
Before the water becomes waste, heat is lost to the air, and to your body.
Hot water evaporates to steam, then condenses on the ceiling and the mirror.
Hot air, and saturated air rise, so the heat moves to the top of the room, not to the drain.

Many years ago I heard of a scheme to recycle heat from a house via a ventilation system where outgoing air is passed through a heat exchanger to warm up incoming air.
By the way, that works both for recovering heat and: cold too

I ask myself why there aren't dozens of these alternative systems to be found working in houses all over the country.
Unless you have a really beefy heat pump to do the job 'on the fly', like that Zypho thing does, you need to store the wastewater for a time. Not impossible, but also not really convenient since it may mean a m3 class insulated tank somewhere (below shower level, likely).

The more thorough the design, the more problems and costs pops up. So far just wasting that heat was not really painful.

Gold Member
So far just wasting that heat was not really painful.
So far. I agree but things are changing and Energy will be increasingly relevant - that's both for heating and for building things.
As for the quantity of hot water that would need to be stored, in a house with a hot tank (and that will soon have to be the norm because Combi Boilers will die out) we'd need' only something like that volume. 'On tap' heat will not be a sustainable method and heat storage will be necessary in any case.

I like the cooler idea . btw.

Summary:: Use the waste warm water from your plumbing system.

I was enjoying my (very short) hot shower this morning and realised that the lovely warm water was flowing over my body (sorry - too much information) only once and then disappearing down the drain.

This water - and what comes out of the washing machine, dishwasher etc. could be at a seriously 'useful' temperature for a heat pump. There must be many kWh lost over a day. You can find systems for shower heat recovery, advertised but they seem to be passive and are low hanging fruit, afaics. A whole house system would be better and would involve a hot water storage tank (which I have).

Obviously the heat exchanger plumbing would need to be maintained (cleaned out) on a regular basis and I would imagine that separate drainage would be needed for foul water. Also, the discharge water temperature would need to be well above freezing but the usable heat would be a lot more, per electrical kWh, than what can be obtained from an air source heat pump in winter. An installation could be a lot cheaper than heat pumps that are being considered today.

Many years ago I heard of a scheme to recycle heat from a house via a ventilation system where outgoing air is passed through a heat exchanger to warm up incoming air. That's a similar principle.

I ask myself why there aren't dozens of these alternative systems to be found working in houses all ov er the country.

There is already many systems for this on the market.

"Greywater heat recovery system" is the water heat recovery you are talking about. And there is both systems that is just a box you put under a shower cabinet, and systems that you put in the drainpipes that demands more installation work. The nice thing about the box under the shower cabinet solution is that it is really cheap and does not demand any installation work (just connect the drain pipe to it, and reroute the cold water intake to the shower), and it is the biggest water heat waste source in the house. And it is easy to clean out because the drain water usually just run in a open gutter with the clean water going in a copper pipe in that gutter.

And balanced ventilation systems with heat recovery via a Enthalpy Wheel or other forms of heat exchangers has been a rule that every new building has to have in Norway for many years now, and that is probably the same in other countries as well.

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Laroxe and russ_watters
gmax137
just connect the drain pipe to it, and reroute the cold water intake to the shower
Does it increase the time the water runs before you "get" hot water at the shower head? Where I live (desert) the cost of the water far exceeds the cost of heating it up.

EDIT -- oops, just realized you said the cold water line runs thru the drain. OK, never mind...

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sophiecentaur
Gold Member
Ah yes, something else that needs regular maintenance and can break or leak all over my house...
If the use of energy becomes as big an issue as 'they' predict then a bit of complexity may be the only way to maintain the comfortable life style. Our lives will change and sooner than we might have hoped, I think.
that is just a box you put under a shower cabinet,
The simple version uses no heat pump so I'd assume it would only work to heat incoming cold water to a electric shower, perhaps. Or it could go in the cold feed to a thermostat which would then just 'top up' from the hot feed. The advertising blurb seems to imply that people use 50% of their hot water for showers. I would argue that the way to cut down on their bills would be to take
And balanced ventilation systems with heat recovery via a Enthalpy Wheel has been a rule that every new building has to have in Norway for many years now, and that is probably the same on other countries as well.
Ah yes; that's the term for it. It's not a thing that the UK have introduced in any quantity. Even a neighbour with a 'built from scratch' green home didn't have one of them.
fewer showers.
Before the water becomes waste, heat is lost to the air, and to your body.
Hot water evaporates to steam, then condenses on the ceiling and the mirror.
I'm not in the habit of laying in my bath until the water is room temperature. By the time I rise up from it (too much info again) the water is still comfortably warm and the bubbles will reduce the evaporation significantly so I don't think that's a valid objection / comment. Perhaps a lid could be put on top of the bath until it has actually cooled down to near room temperature.

Energy can be saved by multiple use of the same bath water. Not impossible. When I was a small boy, staying with my grandparents, Grannie got first go in the galvanised bathtub, followed by me and then Grandad. The water was hand drawn and heated up in a 'copper' - then emptied out into a ditch at the rear of the cottage. (Once a week.) Ahh, the simple life.

Where I live (desert) the cost of the water far exceeds the cost of heating it up.
Yet more factors at work. There are parts of the world where the water supply is being over-stretched and the populations will need to address that situation much more than they appear to be doing. It's just the same as with use of Energy. Life will need to change.

gmax137
Baluncore
2021 Award
I was enjoying my (very short) hot shower this morning and realised that the lovely warm water was flowing over my body (sorry - too much information) only once and then disappearing down the drain.
I'm not in the habit of laying in my bath until the water is room temperature.
I am sorry, I thought you were having a shower.

anorlunda
Staff Emeritus
I suggest that it would take less money, save more energy, and be more fun to do a DIY rooftop solar hot water project. Even at latitude 65, it can be very effective.

Gold Member
I am sorry, I thought you were having a shower.
I started off in the shower but you were suggesting that all the heat was lost into the room and not down the drain. The only possible way that could happen would be if a bath were allowed to cool down so I assumed we were now in the bath.
There are many paths for heat to be lost but the drain is the main one when having a shower. (And also for anything but a stone cold bath.)

Gold Member
I suggest that it would take less money, save more energy, and be more fun to do a DIY rooftop solar hot water project. Even at latitude 65, it can be very effective.

View attachment 293785
Definitely but hot water from any source has heat in it that's worth 'harvesting'. And a harvesting system would work at night.

Gold Member
OK then. Many of you have told me about existing possible systems. BUT how many PFers actually have any heat recovery working in their homes? How many such systems are operating in places you know of?
Why hasn't our government taken notice of any possibilities, other than ground or air sourced heat pumps? (Daft question , I know - but that would take us into politics.)

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As for the quantity of hot water that would need to be stored
Regarding water storage: I don't know how this works in other countries, but we still has these electric boiler things and (here) 2-300l storage for hot water is enough for an usual household (parents & kids).

The m3 class tank I mentioned would be needed to store the wastewater till the heat pump can do its work.
... Energy will be increasingly relevant - that's both for heating and for building things.
... heat storage will be necessary in any case.
I think it'll be a bit more complex than that on long run. My bet is, that while energy as we know it right now will be on shorter supply, there will be significant amount of intermittent energy available - if you have adequate amount of heat storage, yes.

sophiecentaur
Gold Member
I think it'll be a bit more complex than that on long run.
That's right. Thing is, a lot of waste water is 'cold' in any case and wouldn't be the best to get heat from for hot applications. If we are really interested in kWh per buck then hot waste would be best value. It's not unthinkable to use selective storage of waste, on the basis of its temperature and the circumstances. A m3 tank may be bigger than needed but, compared with all the other systems, it's not a lot. We may have to deal with a lot of changes.

In a heat pump world, domestic radiators will need to be very large as the water will not be at 60C. Any system will need to be smart and flexible.

Sorry @Drakkith but you may well have a lot more pipes in your home if you want the comforts of a 2020s home.

Rive
Drakkith
Staff Emeritus
Sorry @Drakkith but you may well have a lot more pipes in your home if you want the comforts of a 2020s home.
My house was built in the 1950's and I pay about $25 a month for gas for the water heater, and much of that is probably just a flat connection fee. Would any alternative system save me significant amounts of money? sophiecentaur and russ_watters Science Advisor Gold Member My house was built in the 1950's and I pay about$25 a month for gas for the water heater, and much of that is probably just a flat connection fee. Would any alternative system save me significant amounts of money?
Do you just have a thick overcoat or a big dog to keep you warm? I'm impressed.

But can that be the whole story? Whatever energy source you use, things are bound to hit you (ore someone else) at more than £25 pm. Even if you are off grid, there must be some possible harvesting of wasted heat.

Drakkith
Staff Emeritus
Do you just have a thick overcoat or a big dog to keep you warm? I'm impressed.
I live in Louisiana. It's currently 67 F in my house without turning the heat on.
Besides, we're just talking about the hot water heater, not the heating system.

But can that be the whole story? Whatever energy source you use, things are bound to hit you (ore someone else) at more than £25 pm.
But realistically how much of an impact could recovering warm wastewater make? I haven't seen any numbers in this thread yet in that regard.

sophiecentaur
OK then. Many of you have told me about existing possible systems. BUT how many PFers actually have any heat recovery working in their homes? How many such systems are operating in places you know of?
Why hasn't our government taken notice of any possibilities, other than ground or air sourced heat pumps? (Daft question , I know - but that would take us into politics.)
Yes i do. I live in Norway in a apartment from 2018 so it's got a balanced ventilation system with heat recovery of the exhaust air just like all newer houses and apartments is required to have in Norway. But i'm not impressed by the efficiency of the system because i use more electricity now than i did in my previous appartement of approximately the same size that was from the 50's that had bad isolation. But the passive air exchange in the old apartment was probably pretty bad to. I have the fans on my current system set to medium speed because there is a lot of radon where i live so i want a good air exchange to not accumulate to much of it inside.

And after Norway has sold all of its hydro power to the EU because they decided to shut down their gas, coal and nuclear power plants and drained our water magazines so we suddenly got really high electricity prices i am considering to add a graywater recovery system to the shower to. Sadly i'm not allowed to install a air to air heat pump by the housing association, but that would have made the biggest impact on the electric bill. Because it is not normal to use gas for heating in Norway, and newer buildings almost never have chimneys so i can not install a wood burning stove either. And the EU even want to ban wood burning stoves...

anorlunda
Staff Emeritus
And the EU even want to ban wood burning stoves...
Norwexit.

Tom.G
Gold Member
I live in Louisiana. It's currently 67 F in my house without turning the heat on.
Besides, we're just talking about the hot water heater, not the heating system.

But realistically how much of an impact could recovering warm wastewater make? I haven't seen any numbers in this thread yet in that regard.
Fair comment about the figures. I based my idea on the advertised figure of 50% for hot water use in the link I posted. That would be a lot of money in our house.
The UK government doesn't help here and I hesitate to go for any significant scheme because there are no incentives, despite the statements that are made about the climate situation etc.. But that's no excuse, I guess.

Baluncore
2021 Award
Because it is not normal to use gas for heating in Norway, and newer buildings almost never have chimneys so i can not install a wood burning stove either. And the EU even want to ban wood burning stoves...
The UK government doesn't help here and I hesitate to go for any significant scheme because there are no incentives, despite the statements that are made about the climate situation etc.. But that's no excuse, I guess.
If a nation or an economy does not manage energy wisely, it's currency will be devalued by inflation. All economies are in competition, to preserve their buying power in the international marketplace. The united states of Europe's € is an interesting case to watch, as the component nations have different resources and philosophies.

Nugatory
Mentor
Why hasn't our government taken notice of any possibilities, other than ground or air sourced heat pumps? (Daft question , I know - but that would take us into politics.)
It’s not politics, it’s economics. The low-hanging fruit is harvested first; the residual heat from single-family residential water usage is higher in the tree.

In a heat pump world
It may worth to consider that every solution mentioned here has its own footprint and cost. And complicated systems tends to have both at the 'terrible' range.

It's like the well known paradox around insulation and heating. Once you invest into insulation, you cut the energy need: the more you invest, the lower. So the calculated return of an investment on a complicated high efficiency heating system will become further and further away in the future.

From financial standpoint, to install anything else but the most basic climatization or low efficiency (cheap) gas convector in an almost-passive house is just madness. It has no return point at all.
This stands for the footprint too.

Hot water is a bit different, of course, but hot water alone is again, a tricky matter.

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sophiecentaur and Drakkith