dnb

Working out a scheme for DHW

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8 hours ago, dnb said:

What I really need is a near infinite capacity thermal store that has next to no loss. Then I can average the energy across the seasons... So, a 10m^3 concrete cube buried deeply  ;)in the garden ought to do it. ;)

 

One or two have tried this, using things like a very large central water tank as a seasonal store.  On another forum, years ago, a few of us discussed ways of using a large tank filled with a phase change material, as used in the Sunamp (sodium acetate) or something simpler like paraffin wax.  Somewhere I've read an article (might have been in an old copy of Passive House magazine) about a house that had a very thick column up the centre, with a helical staircase curved around the outside.  This column was a massive, well-insulated, water tank that acted as a seasonal store.  Even using water, that can store around 4 times as much heat as concrete for a given mass, it's challenging to make a tank big enough to work as a seasonal store.

 

 

8 hours ago, dnb said:

I got the first draft of the SAP calculations this afternoon. It will change a bit, I am sure, because some design details have been entered as guesses. It indicates the house will be annoyingly close to carbon negative, but won't quite achieve it. Still, it's a good A rating so nothing too bad for a first go at house building. 

 

You should be able to go carbon negative with a bit of PV.  Our house came out with an EPC of A107 and a CO2 of -0.9 tonnes/year and we're not PH certified.  The thing that made the  difference was the 6.25 kWp PV array.

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7 hours ago, dnb said:

I got the first draft of the SAP calculations this afternoon. It will change a bit, I am sure, because some design details have been entered as guesses. It indicates the house will be annoyingly close to carbon negative, but won't quite achieve it. Still, it's a good A rating so nothing too bad for a first go at house building. 

 

Have a look at shower waste water heat recovery units. Good for a few SAP points, only a few hundred quid, and will make your hot water last longer.

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45 minutes ago, jack said:

Have a look at shower waste water heat recovery units. Good for a few SAP points, only a few hundred quid, and will make your hot water last longer.

Something I may look at, sometime.

Can't remember, do you have one fitted @jack

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Just now, SteamyTea said:

Can't remember, do you have one fitted @jack

 

Yes, we have one that's connected to our ensuite and main bathroom showers.

 

Easy enough to install if you're building from scratch or completely gutting a house. Retrofitting is harder, but there are options for fitting, eg, underneath shower trays.

 

They aren't a quick payback, but you can go for a slightly smaller hot water tank, plus I don't see anything wrong with a product that lets us use less energy for a couple of decades, with little to no maintenance.

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1 minute ago, jack said:

 

Yes, we have one that's connected to our ensuite and main bathroom showers.

 

Easy enough to install if you're building from scratch or completely gutting a house. Retrofitting is harder, but there are options for fitting, eg, underneath shower trays.

 

They aren't a quick payback, but you can go for a slightly smaller hot water tank, plus I don't see anything wrong with a product that lets us use less energy for a couple of decades, with little to no maintenance.

 

Easier I think with shower-baths, but I do not see many new self-builders fitting those.

 

Ferdinand

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Is shower water really warm enough to be useful by the time it's fallen to the shower tray and flowed across it? I'm sceptical. I have some waterproofed DS18B20 (1-wire temperature sensors) in the post so should try some measurements when they're here.

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6 minutes ago, Ed Davies said:

I have some waterproofed DS18B20 (1-wire temperature sensors) in the post so should try some measurements when they're here.

I got loads of them kicking about, could set some up and see what is happening.  Not taken the bath panel off in over 10 years since I fitted the shower pump.  Probably about time I had a look in there (soft water is great).

A mate of mine did his second year university project on waste water heat recovery.  All I seem to remember is that he thought the extractor fan took out more energy from the bathroom than the waste.

 

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11 minutes ago, Ed Davies said:

Is shower water really warm enough to be useful by the time it's fallen to the shower tray and flowed across it? I'm sceptical. I have some waterproofed DS18B20 (1-wire temperature sensors) in the post so should try some measurements when they're here.

 

 

If used to preheat the cold water before it is heated to DHW temperatures, then yes, I'd say any recovered heat is useful.

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17 minutes ago, jack said:

They aren't a quick payback, but you can go for a slightly smaller hot water tank

Do you think I could use one to pre-heat the water in the header tank, which is almost above the bath?

 

1 minute ago, JSHarris said:

If used to preheat the cold water before it is heated to DHW temperatures

Snap

Edited by SteamyTea

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24 minutes ago, SteamyTea said:

Do you think I could use one to pre-heat the water in the header tank, which is almost above the bath?

 

 

No, pre heating water in an open header tank to "warm" is a very bad idea.

 

Use it to pre heat in real time, the cold  water entering the hot water tank.

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47 minutes ago, jack said:

 

Yes, we have one that's connected to our ensuite and main bathroom showers.

 

Easy enough to install if you're building from scratch or completely gutting a house. Retrofitting is harder, but there are options for fitting, eg, underneath shower trays.

 

They aren't a quick payback, but you can go for a slightly smaller hot water tank, plus I don't see anything wrong with a product that lets us use less energy for a couple of decades, with little to no maintenance.

 

Remember what product you used @jack? The cheapest ones seem to be the non-branded vertical ones, but these are less flexible in terms of fitting them in the void I assume?  Megaflow have a few horizontal ones, and then at the other end of the scale are these: https://joulia.com/en/products/

 

Maybe WWHRS deserves it's on topic, seems it's not something that's really been discussed too much before if I go by search results.

 

Edited by Dan Feist

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IIRC we had some lengthy discussions on waste water heat recovery a fair time ago, may have been on this forum's now-defunct predecessor though.  Back then one key point discussed was compliance with the water regs, as they demand that there be an indicator space between the potable water side of the heat exchanger and the waste water side.  This complicates the design and potentially reduces efficiency, but at that time it seemed that there were systems available that got around the water regs challenges. 

 

There's a bit of discussion in this thread from last year:

 

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1 hour ago, Ed Davies said:

Is shower water really warm enough to be useful by the time it's fallen to the shower tray and flowed across it? I'm sceptical. I have some waterproofed DS18B20 (1-wire temperature sensors) in the post so should try some measurements when they're here.

 

The outgoing drainwater (typically mid 30s deg C, I guess, for a typical shower) is used to preheat the incoming cold water during the shower.

 

You can also connect it up so that it also preheats water supplied into the hot water tank/combi. This arrangement has the highest efficiency - they claim up to mid 60s% efficiency IIRC. 

 

Edited to add link to explanation of different connection possibilities: https://recoupwwhrs.co.uk/installation/installation-methods/

 

36 minutes ago, Dan Feist said:

Remember what product you used @jack? The cheapest ones seem to be the non-branded vertical ones, but these are less flexible in terms of fitting them in the void I assume?  Megaflow have a few horizontal ones, and then at the other end of the scale are these: https://joulia.com/en/products/

 

I think it was the largest Recoup model (Pipe+HE, from memory, but it's been a while!)

 

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1 hour ago, SteamyTea said:

I got loads of them kicking about, could set some up and see what is happening.  Not taken the bath panel off in over 10 years since I fitted the shower pump.  Probably about time I had a look in there (soft water is great).

A mate of mine did his second year university project on waste water heat recovery.  All I seem to remember is that he thought the extractor fan took out more energy from the bathroom than the waste.

 

 

When my bath panel was removed last week it turned out that the waste had not been connected for perhaps 5 years +.

 

😫

 

When I looked into these he devices about 3 years ago, prices were prohibitive from a payback POV, as Jack has highlighted ... could not find anything much under £300. In the end these should be £50.

 

But for me as an LL these would also have the dual purpose of managing regulatory risk attaching to EPC values.

 

Ferdinand

 

 

Edited by Ferdinand

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10 hours ago, JSHarris said:

You should be able to go carbon negative with a bit of PV.  Our house came out with an EPC of A107 and a CO2 of -0.9 tonnes/year and we're not PH certified.  The thing that made the  difference was the 6.25 kWp PV array.

 

It has already included my solar PV plans - it took me to an EPC of 106. Probably 3 more panels would swing it.

 

9 hours ago, jack said:

Have a look at shower waste water heat recovery units. Good for a few SAP points, only a few hundred quid, and will make your hot water last longer.

 

Good call. Thanks! I'm only looking for a small boost to the numbers, and given the wife and daughter's love of showers it will pay back well over the years.

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7 hours ago, jack said:

The outgoing drainwater (typically mid 30s deg C, I guess, for a typical shower)

 

Yeah, but that's what I sceptical about. Water coming out of the shower head will be in the high 30s or early 40s °C and I suspect the temperature will drop considerably in the time it takes to fall the height of the shower. Then it runs across the shower tray getting cooled by that and by evaporation as it goes. Of course there are huge number of variables, whether you just have a warm shower or one that flays your skin off, the height of the shower head, the room temperature and initial humidity, the length of the shower, the starting temperature of the shower tray, the material of the shower tray (that's probably a major one, fibreglass will behave a lot differently to steel or stone) and so on.

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Getting back to the original topic (the diversions have been very interesting and insightful, but I have to appear vaguely sane when I tell my long suffering architect what I've decided to do next week)

 

The latest version of my modelling tool has led me to the following conclusions:

 

* The difference between PV and ETs is actually quite small when just considering DHW.

* If there is a limit on roof space (like my current house - and that's why I'm building another) then ETs are probably the better solution for DHW - ETs are more efficient per m^2, until the store is full.

* It's not just the cost of panels and ETs (obvious, I know, but let's follow the logic). If PV (or ETs) are going to be there anyway, then increasing the amount of either is a much smaller delta cost.  So there's a big saving when one or other technology achieves zero presence. (and a nice saving in complexity)

* I can't remove all the PV - it makes no sense at all from any point of view. So it looks like ETs are not going to be part of my build after all.

 

A chart to justify my position.

image.png.1addaf6fa2ce65caf30336666c6fe1cf.png

The X axis represents the number of 3.9m^2 ET assemblies on the roof. The remainder of the roof is used for PV. The roof area is held constant, and the calculations account for quantization of panels. (The numbers differ from previous iterations because I included another kW or so of PV on the SE facing roof having looked at tree heights - and my chainsaw ;) - today)

 

So as can be seen, adding ETs reduces the import costs at every level, even when a lot of the energy collected in summer is waste heat that has no practical use until I build the swimming pool. The best payback however comes from the simple PV system primarily because of the reduction in infrastructure and each addition of a block of ETs always extends the payback time, with the extensions getting gradually larger as the waste heat becomes ever more significant.

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Yes, that's the general conclusion I've come to: ETs aren't terrible but, if you've got the roof space, don't really make sense just for DHW. I have lots but will use them primarily for space heating.

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Someone, over at the other place, designed a simple system that was, on the face of it, under tubed (may have been flat panels).

His thinking was that by eliminating all risk of boiling, the control system was simple and reliable.

I seem to remember that worked well at scavenging to raise the base temperature up.

I can't remember what he did for the final temperature boost though.  Probably electric immersion.

4 hours ago, dnb said:

A chart to justify my position.

They are always good.

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1 hour ago, SteamyTea said:

Someone, over at the other place, designed a simple system that was, on the face of it, under tubed (may have been flat panels).

His thinking was that by eliminating all risk of boiling, the control system was simple and reliable.

I seem to remember that worked well at scavenging to raise the base temperature up.

I can't remember what he did for the final temperature boost though.  Probably electric immersion.

 

 

 

My limited experience of fixing a neighbours ET system (twice now) has been that the only problems seem to relate to over heating and the control system that is supposed to prevent that.  It seems that if the risk of overheating could be removed, with no need for active intervention by the control system, then reliability would be enhanced a fair bit.  Anecdotal, I know, but pretty much every problem I've ever heard of with solar thermal systems has been associated with the control system or overheating prevention system.

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There's a lot to be said for the Solartwin- style system. Freeze-tolerant but can be setup to drain back, boil-on-stagnation tolerant too.

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none of these technologies have to be roof mounted if that is the reason for not having balanced mix .

size of tank and where you put the solar heat in and where ufh coils are will make a tremendous difference as to how much of the year they can contribute.

 over heating is purely a function of tank size to panel size  and stopping them boiling is not hard .I only had problem once and then it ran for years with no problems 

you could of curse do a very simple belt and braces approach and have a heat dump linked in --like an old house raidiator on a divert valve and max temp sensor .

please do not under estimate how much lower temp water they will give even in the winter  if enough panel size  --to run UFH - and then if you had been doing that with pv --before 

the Pv ouput could then be sold back or used for something else.

Istill believe a mix of both with,suitable tank size and DESIGN  and  control will  be good economical system 

 tank must be as tall as poss to get thermal stratification and dhw taken from top only _mixed with cold when it leaves tank

 UFH coil in middle

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1 minute ago, dpmiller said:

There's a lot to be said for the Solartwin- style system. Freeze-tolerant but can be setup to drain back, boil-on-stagnation tolerant too.

I can,t speak for all evac tubes ,,but  one model  thermomax   did have a bimettalic strip inside so when tube got too  hot it disconnected from manifold

me being greedy did not go for that tube,as i thought my tank size and usage for UFh would not be a problem --wrong

TANK was TOO SMALL 

certainly considering a custom tank  maybe 8ft -tall  and min 5-600 litres and made like the SOLVIS tank which directs solar heat to the level it is closet to in the tank without stirring up the contents

 

 

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24 minutes ago, JSHarris said:

 

 

My limited experience of fixing a neighbours ET system (twice now) has been that the only problems seem to relate to over heating and the control system that is supposed to prevent that.  It seems that if the risk of overheating could be removed, with no need for active intervention by the control system, then reliability would be enhanced a fair bit.  Anecdotal, I know, but pretty much every problem I've ever heard of with solar thermal systems has been associated with the control system or overheating prevention system.

An even more simple over heat protect would be heat dump linked by a mechanical themostat that diverts when temp gets too hot - 

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1 hour ago, JSHarris said:

 

 

My limited experience of fixing a neighbours ET system (twice now) has been that the only problems seem to relate to over heating and the control system that is supposed to prevent that.  It seems that if the risk of overheating could be removed, with no need for active intervention by the control system, then reliability would be enhanced a fair bit.  Anecdotal, I know, but pretty much every problem I've ever heard of with solar thermal systems has been associated with the control system or overheating prevention system.

This is precisely why a drain back system is more robust IMO. I've fitted a dozen or so solar thermal systems and not had any issues with the drain back ones (3 in total). 

 

The solar twin solution with silicone hoses is a neat solution but prone to blocking up in hard water areas due to it being a direct system. Can be overcome by going indirect but means a twin coil tank is needed. 

 

Edited by Miek
More info

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