eandg

Waste water heat recovery

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I've had a look at this technology and it sounds a bit too good to be true based on the sales pitch - 70% heat recovery, 4-6 points SAP boost and £80-100/year savings for a capital cost of £350-700 (based on pricing from Powerpipe). Has anyone installed? Do they work with ASHPs/DHW cylinders as easily as they do with a combi boiler? Do you need one for every shower unit or just one at or around your water tank? 

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I believe it works by preheating the cold supply to the shower mixer. So you use less hot water.

 

Should work as long as your hot supply is hot enough to need cold water mixing in. If you don't use any cold water (because your hot water isn't very hot) then it won't.

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14 minutes ago, Temp said:

I believe it works by preheating the cold supply to the shower mixer. So you use less hot water.

 

Should work as long as your hot supply is hot enough to need cold water mixing in. If you don't use any cold water (because your hot water isn't very hot) then it won't.

Thanks - which means it wouldn't work particularly well with an ASHP then, presumably?

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Probably not if the DHW is close to the required shower temperature so little if any cold is used. 

 

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If the system is something that will benefit you I’d include it 

I would worry about the final Sap as it’s meaningless With online companies that will score you very high for a fee of less than £100

I don’t think many people take any notice of SAP ratings  

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7 hours ago, Temp said:

Probably not if the DHW is close to the required shower temperature so little if any cold is used. 

 

It can still be useful but you'd need to use the WWHR to preheat the cold supply going into the cylinder instead of (or as well as) the cold feed to the shower. 

I'll also go on a limb and suggest even without this, your DHW doesn't need to be that much above showering temperature for it to work well so long as the shower thermostatic mixing valve is working properly (necessary for any WWHR install). While the TMV will start off only drawing a small amount from the cold feed, as that cold water warms up it will have to mix in a higher and higher proportion of cold to achieve desired temperature, thus reducing the DHW water usage as intended. Overall the system would be working right at the boundary though, so just a small decrease in DHW/cylinder temperature and it suddenly wouldn't work at all as you say. 

 

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8 hours ago, eandg said:

Thanks - which means it wouldn't work particularly well with an ASHP then, presumably?

 

I've had one for the last 4 years, installed with an ASHP. This isn't accurate.

 

First, most people use their ASHP to heat water in the tank to at least 50 deg C (@ProDave takes his to 48 deg C, and that's the lowest I'm aware of anyone on here using). Most people like their shower water to be no hotter than around 43 deg C. So you need to add cold water to bring the temperature down. But guess what - if you add some of the heat from the waste water to the cold water before the mixer, then you can use a higher proportion of that (preheated) cold water to achieve a given temperature at a given flow rate. A higher proportion of cold water means a lower proportion of hot water, hence energy is saved.

 

Even better, to get the nearly 70% efficiency you mention, the system needs to be connected so that the preheated cold is supplied to both the cold side of the shower mixed and the cold feed to the DHW cylinder. Higher temp water into the cylinder means less energy to heat it to the final temperature.

 

In short, the numbers work even if you're using a lower temperature heat source like an ASHP. In fact, I'd argue that waste water heat recovery is particularly suited to ASHP-based systems. There's a big difference in the amount of hot water that can be supplied by a tank at 50 deg C (ASHP) and a tank at, say, 75 deg C (gas boiler). In most households, showers consume a large proportion of all generated hot water. I'd estimate in our case that showers account for at least two-thirds of our usage. Waste water heat recovery increases the effective amount of hot water you can get out of your DHW cylinder. That lets you either use a smaller cylinder, or go for a standard cylinder and get more time in the shower.

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45 minutes ago, jack said:

 

I've had one for the last 4 years, installed with an ASHP. This isn't accurate.

 

First, most people use their ASHP to heat water in the tank to at least 50 deg C (@ProDave takes his to 48 deg C, and that's the lowest I'm aware of anyone on here using). Most people like their shower water to be no hotter than around 43 deg C. So you need to add cold water to bring the temperature down. But guess what - if you add some of the heat from the waste water to the cold water before the mixer, then you can use a higher proportion of that (preheated) cold water to achieve a given temperature at a given flow rate. A higher proportion of cold water means a lower proportion of hot water, hence energy is saved.

 

Even better, to get the nearly 70% efficiency you mention, the system needs to be connected so that the preheated cold is supplied to both the cold side of the shower mixed and the cold feed to the DHW cylinder. Higher temp water into the cylinder means less energy to heat it to the final temperature.

 

In short, the numbers work even if you're using a lower temperature heat source like an ASHP. In fact, I'd argue that waste water heat recovery is particularly suited to ASHP-based systems. There's a big difference in the amount of hot water that can be supplied by a tank at 50 deg C (ASHP) and a tank at, say, 75 deg C (gas boiler). In most households, showers consume a large proportion of all generated hot water. I'd estimate in our case that showers account for at least two-thirds of our usage. Waste water heat recovery increases the effective amount of hot water you can get out of your DHW cylinder. That lets you either use a smaller cylinder, or go for a standard cylinder and get more time in the shower.

Great, thanks. And yes - showers (particularly as kids get older) will make up for most of our hot water usage. 

 

I presume the systems are all much of a muchness? And is one for the property sufficient or one for each shower (the numbers start to change if you need three installed)?

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1 minute ago, eandg said:

Great, thanks. And yes - showers (particularly as kids get older) will make up for most of our hot water usage. 

 

I presume the systems are all much of a muchness? And is one for the property sufficient or one for each shower (the numbers start to change if you need three installed)?

 

I don't know about the difference between units, other than that the longer vertical ones are quite a bit more efficient than the horizontal under-tray type units.

 

From memory, the one I bought recommends connecting up to two showers per unit. We have three upstairs showers, and just connected the two that are used most often (I think the guest shower's been used two or three times in four years!) I personally think you'd get away with connecting even three showers into the system.

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Just now, jack said:

 

I don't know about the difference between units, other than that the longer vertical ones are quite a bit more efficient than the horizontal under-tray type units.

 

From memory, the one I bought recommends connecting up to two showers per unit. We have three upstairs showers, and just connected the two that are used most often (I think the guest shower's been used two or three times in four years!) I personally think you'd get away with connecting even three showers into the system.

Great, thanks. We have two upstairs showers and a downstairs one that'll be used for guests/mucky kids in the same way so I'll look into connecting a single unit upstairs then. 

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The other thing to bear in mind if sharing units is you need to be able to route the drains of the showers into the same vertical stack. unfortunately our two upstairs showers are in opposite corners of the house so no way to get them sharing a heat recovery unit. 

The downstairs shower is directly beneath the upstairs one, but again can't share a vertical WWHR as there's minimal vertical drop from the downstairs shower into the horizontal drain below the slab. We could have them share a horizontal WWHR unit beneath the downstairs drain but they're less efficient anyway and not really designed for sharing. So on balance, it's probably only the ensuite that will get one installed.

Likewise routing the cylinder inlet via the ensuite WWHR is going to be a bit of a PITA but I think it can be done (in reality, just one more large and lagged pipe run coming back from shower to plant room). I have a slight concern in the pressure drop this will introduce (in addition to a water softener) but I guess worse case, it's easier to retrospectively bypass it than add it in!

 

 

1 hour ago, jack said:

I've had one for the last 4 years, installed with an ASHP. This isn't accurate.

 

This is reassuring to know. After writing the thing above I took a shower myself and was contemplating the margins must be enough to have it work reliably. I had been imagining 45 vs 48ºC which seemed tight, but in reality it's more like 40 vs 50ºC which should be plenty of operating margin.

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40 minutes ago, joth said:

The other thing to bear in mind if sharing units is you need to be able to route the drains of the showers into the same vertical stack. unfortunately our two upstairs showers are in opposite corners of the house so no way to get them sharing a heat recovery unit. 

The downstairs shower is directly beneath the upstairs one, but again can't share a vertical WWHR as there's minimal vertical drop from the downstairs shower into the horizontal drain below the slab. We could have them share a horizontal WWHR unit beneath the downstairs drain but they're less efficient anyway and not really designed for sharing. So on balance, it's probably only the ensuite that will get one installed.

Likewise routing the cylinder inlet via the ensuite WWHR is going to be a bit of a PITA but I think it can be done (in reality, just one more large and lagged pipe run coming back from shower to plant room). I have a slight concern in the pressure drop this will introduce (in addition to a water softener) but I guess worse case, it's easier to retrospectively bypass it than add it in!

 

 

This is reassuring to know. After writing the thing above I took a shower myself and was contemplating the margins must be enough to have it work reliably. I had been imagining 45 vs 48ºC which seemed tight, but in reality it's more like 40 vs 50ºC which should be plenty of operating margin.

Thanks. Our two upstairs showers will be c.3m apart but across the doglegged stairs - hopefully no issues with using the same WWHR. I'll need to speak with a plumber to confirm. 

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1 hour ago, joth said:

This is reassuring to know. After writing the thing above I took a shower myself and was contemplating the margins must be enough to have it work reliably. I had been imagining 45 vs 48ºC which seemed tight, but in reality it's more like 40 vs 50ºC which should be plenty of operating margin.

 

Even if there's zero cold water being used at the mixer, as long as the system is plumbed to preheat the cold feed to the tank, you'll still recover perhaps ~50% of the outgoing heat. 

 

1 hour ago, joth said:

The other thing to bear in mind if sharing units is you need to be able to route the drains of the showers into the same vertical stack. unfortunately our two upstairs showers are in opposite corners of the house so no way to get them sharing a heat recovery unit. 

 

Ah, yes. We were lucky that we were able to have the two most-used showers back-to-back, and close to the main vertical stack.

 

1 hour ago, joth said:

Likewise routing the cylinder inlet via the ensuite WWHR is going to be a bit of a PITA but I think it can be done (in reality, just one more large and lagged pipe run coming back from shower to plant room). I have a slight concern in the pressure drop this will introduce (in addition to a water softener) but I guess worse case, it's easier to retrospectively bypass it than add it in!

 

The pressure drop can be planned for, but all the extra pipework will mean that the system will take more time to come up to full efficiency. Not the end of the world, and the longer the shower, the greater the overall efficiency. It's probably at its worst for people taking very short showers, especially if you aren't having people jumping in one after the other.

 

Re: softeners, to minimise pressure drop, make sure that the connecting flex connectors are actually full bore. Our "full bore" 22mm connection kit came with flex connectors that had 11mm restrictions at both ends.

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8 hours ago, jack said:

 

First, most people use their ASHP to heat water in the tank to at least 50 deg C (@ProDave takes his to 48 deg C, and that's the lowest I'm aware of anyone on here using).

Our DHW is stored at 45C heated by the EASHP.

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15 hours ago, PeterStarck said:

Our DHW is stored at 45C heated by the EASHP.

 

I stand corrected!

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I spent a bit of time measuring shower temperature, right at the output from the thermostatic mixer.  Anything over 40°C feels pretty hot to me, and we run ours at about 38°C, which is hot enough for my wife and just about OK for me (I'd prefer it to be slightly cooler, but I can't be bothered to keep adjusting the temperature every day).  Interestingly, the hotel we stayed in a week or two ago had a detent on the shower temperature control, that set it to 38°C.  A button needed to be pushed to change to any other temperature.  Made me wonder whether 38°C is some sort of agreed standard for shower temperature.

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10 minutes ago, JSHarris said:

Made me wonder whether 38°C is some sort of agreed standard for shower temperature.

I seem to remember, from years ago, it is/was.

 

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13 minutes ago, SteamyTea said:
25 minutes ago, JSHarris said:

Made me wonder whether 38°C is some sort of agreed standard for shower temperature.

I seem to remember, from years ago, it is/was.

 

100°F

~Body temperature.

I guess, it's an obvious number to go for...

 

 

26 minutes ago, JSHarris said:

I'd prefer it to be slightly cooler, but I can't be bothered to keep adjusting the temperature every day

LOL hadn't thought of this -- another justification for the double-TMV shower in our ensuite 🙂

 

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