Heat loss from thermal stores (and UVCs)

[dnb]

Reading a few threads on here suggests that thermal stores don't have a good track record for having good insulation and low heat loss. I would like to understand how bad this is because as far as I can see, it is going to be the single biggest cause of my new house overheating in the summer. (In the winter I could be persuaded to install an extract vent to the MVHR in the thermal store cupboard to heat the whole house! )

 

The lack of insulation is not necessarily a problem if I know about it now and allow space to build a cupboard with additional insulation in the design. But the key thing here is why can I seemingly not believe the manufacturer data for heat loss? Is it some obscure test where the thermal store is tested when it is only half-warm? If I could understand the test then I might be able to use this to design the additional insulation. Or, even better, does anyone have any real data about how their thermal store lost heat?

 

Note that I live in a part of the country where the water is hard, bordering on agressive, so this makes me think a thermal store might cause fewer problems because the bulk of the limescale is contained in only one heat exchanger rather than spread throughout a cylinder.

[Nickfromwales]

Hi @dnb

Why would you not broaden your options and fit a water softener? The aggressive hardness will not just affect the hot water device, but everything else there too!!

[dnb]

I have a water softener in the current house. It is annoying to keep it topped up and it makes the water taste bad so I had to re-arrange the plumbing to put it on the feed to the boiler only. And it doesn't eliminate the problem - it merely extends the intervals between having to put the boiler heat exchanger in the untrasonic tank! This is a side issue - I would primarily like to understand heat loss measurements from thermal stores and UVCs to decide what I need to design in to make the DHW efficient.

[joe90]

Heat loss from a cylinder is relative to not only the insulation level but also the temp of the water within it. My EPC recently told me my UVC has a loss of 1.95kWh measured, when I questioned this (as my DHW is set at 48’) they said it’s standard to measure the loss at 60’ and it could not be altered. Our airing cupboard is barely warmer than the house so I question the losses.

[SteamyTea]

1 hour ago, dnb said:

I would like to understand how bad this is because as far as I can see, it is going to be the single biggest cause of my new house overheating in the summer

You can treat it like any other heat loss problem as long as you know the surface area of the cylinder, the thickness and conductivity of the insulation, the temperature of the stored water and the temperature of the room.

Just a bit of arithmetic.

12 minutes ago, dnb said:

it makes the water taste bad so I had to re-arrange the plumbing to put it on the feed to the boiler only

Who initially plumbed it in to the main cold tap.  Sounds like a bad installation.

My Mother lives in a hard water area, she, at 90, can top up the softer with salt.

[dnb]

1 minute ago, SteamyTea said:

Who initially plumbed it in to the main cold tap

Not the main cold tap. It fed all the cold taps in the house except the kitchen. I got mightily fed up with having to descend 2 floors just to get a drink of water at night. The current house had so many plumbing problems that you wouldn't believe possible (It was built in the 80s recession) and the silly water softener was the least of the problems. Things like the pipes from the cylinder sitting 20mm above the floor joists!

 

Essentially, I hate the water softener. I can fill it up with salt easily enough when I remember, but there must be a way to design out needing one since it only does half a job and causes other annoyances in the house. Descaling one side of a single heat exchanger once in a while seems a much simpler idea.  

 

6 minutes ago, SteamyTea said:

You can treat it like any other heat loss problem

I understand that. It's not the question I am asking. I want to know that if a manufacturer says a product loses 2kWh per day, what were the conditions of the test? Are UVH and thermal stores tested with the same standard test? Can I believe the manufacturers claims or is it designed to allow them to claim good performance but not actually deliver it? (You know, like car MPG figures used to be measuered) And above all, does the thermal store (or UVC) meet the manufacturer claims in the real world?

[dnb]

28 minutes ago, joe90 said:

...UVC has a loss of 1.95kWh...

Thanks. Just the sort of thing I am after,

 

That's just less than a 100W light bulb in there on the whole time, assuming 1.95kWh/day. It sounds like it is an over estimate to me, although it depends how well insulated the cupboard is.

[SteamyTea]

12 minutes ago, dnb said:

I want to know that if a manufacturer says a product loses 2kWh per day

I think they test in a slightly peculiar way.

They start with a cold cylinder, heat it up, run some water off, which refills the tank with cold water.  While measuring the heat loss.  So basically they are testing the lower thermal value of the heat loss, not the highest thermal value.  So in effect they are measuring a temperature difference of about 10 to 15 °C rather than 35 °C.  All a bit a cheat as it does not take into account issues like Economy 7 heating where the cylinder may be unused for 18 hours.

@JSHarrisknows a bit more about the testing standard.

I measured my DHW losses a while back (I measure everything I can) and regardless of which method I used, I always had very high losses, in the order of 3.5 kWh/day.  My DHW usage was around 1.5 kWh/day.  I added a lot of extra insulation and got it down to a more reasonable 1 to 1.5 kWh/day, but that is really still too high.

[dnb]

Thanks. Those losses do seem really high compared to your usage. I estimated the family usage quite a bit more than that (but perhaps it's more people and more teenage daughters...) 

 

It would seem the losses are trying to model heating with a system boiler and then using the water shortly afterwards - making the test temperature much lower as you say. This isn't the way a store would be used in any sensible low energy house.

 

So the plan has to be to insulate the tank as much as possible. I should ask the SIPs people to bring the larger offcuts with them - there's bound to be some scrap pieces that would be big enough to make a thermal store cupboard. If I can use the same stuff as the roof I might be able to get losses down to 0.5kWh/day.

[Nickfromwales]

8 minutes ago, dnb said:

So the plan has to be to insulate the tank as much as possible.

You would do yourself a favour if you read @JSHarris blog as he did exactly all of this, and then binned his TS. At the very least fit an UVC, other than that a SA ( as Jeremy fitted ) will have the lowest losses in the marketplace. About 2x the cost of the equivalent UVC though.

Edit : SA = Sunamp

[joe90]

This from my report, 

 

“Measured cylinder loss: 1.95 kWh/day

Permitted by DBSCG: 2.24 kWh/day”

[SteamyTea]

28 minutes ago, dnb said:

Those losses do seem really high compared to your usage

Perversely, if I used more DHW, my losses would be lower.  It is a quirk of Economy 7.

One thing I did several years ago was to limit my E7 window to the last 3 or 4 hours of the period.  This stopped the cylinder sitting there at maximum temperature while I slept.

21 minutes ago, Nickfromwales said:

and then binned his TS

He had a combined DHW cylinder and header tank, so rather hard to reduce losses.

I just have a bog standard vented cylinder, so the losses should be about the same as an unvented one (assuming similar shape, size, insulation levels and temperature range).

Edited June 23, 2019 by SteamyTea

[dnb]

So why does a UVC come out so much better than a thermal store? Is it simply the temperature doesn't need to be as high to deliver the same water? Or are they built to a better insulation standard? If I'm feeding the UVC with the same inputs and using the water at the same rate, why are the losses wildly different? (Assuming equal volume and surface area of course)

 

Either way, UVC or thermal store, it doesn't matter how much insulation is put in if the heat can't be removed eventually. Hence the idea of extraction.

 

Sunamp is a strong no at the present time. In my opinion the technology needs time to mature for it to become properly refined - there are far too many subtlly different options to pick from and they are probably all the same inside, but for a (no doubt very expensive) bit of software on a microcontroller somewhere inside. And they are an expensive way round the problem right now. Once they get cheaper I would certainly reconsider.

[SteamyTea]

4 minutes ago, dnb said:

So why does a UVC come out so much better than a thermal store?

I have never had a satisfactory answer to that.  Excluding the combined cylinder/header tank ones, where you tend to heat the cold store with waste heat from the main cylinder, the physics is the same.

One reason given is that there are losses though the pipework, but I find that a bit dodgy as pipework should be sufficiently insulated.

Edited June 23, 2019 by SteamyTea

[joe90]

As I said my losses (judging by the lack of temp in the airing cupboard (not very tech eh!). ) appear low, and I believe @ProDave has found the same with his identical  DHW tank and similar lowish water temp.  Now that my build is finished (nearly) it is my intention to fill the rest of the space around the tank with leftover insulation so it will get even better (but the wife might complain that the airing cupboard is not warm enough ?

 

isnt a thermal store run at hotter temps than a DHW tank? (Or did I make that up?)

Edited June 23, 2019 by joe90

[dnb]

2 minutes ago, SteamyTea said:

Perversely, if I used more DHW, my losses would be lower.

 

I a similar one in the solar thermal thread. If I used twice as much water, solar thermal became cost effective in my DHW model...   But let's not open that one up again.

 

Nice plan with the E7 use. I was looking at writing a control system that looked at what the likely morning water use would be, took a guess at how much sun there would be and then heated the tank accordingly. If it was wrong then it would correct itself by using high cost electricity to support the evening water use.

[dnb]

2 minutes ago, joe90 said:

Now that my build is finished (nearly) it is my intention to fill the rest of the space around the tank with leftover insulation so it will get even better

 

Do this around the outside of the bath too. The wife won't complain about it.

 

I wonder if you could arrange for a 2 compartment airing cupboard? Although it might be too difficult at a late stage in the build... Something I could think about though.

Edited June 23, 2019 by dnb

[joe90]

1 minute ago, dnb said:

Do this around the outside of the bath too.

 

A tip I got from another forum from a passive house builder (who was nearly as tight as me ?) was to let the bath water sit there after a bath till it was at room temp, it  helped heat the house, better than some fancy heat recovery system in the drains,!!!

[dnb]

4 minutes ago, joe90 said:

isnt a thermal store run at hotter temps than a DHW tank? (Or did I make that up?

 

I think so. Or at least the option is there to do that if you put enough energy in to it.

[Nickfromwales]

11 minutes ago, SteamyTea said:

He had a combined DHW cylinder and header tank, so rather hard to reduce losses

Nope, it was a TS. Open pipe one with a header tank. Referred to, IIRC, as a combination tank.

Good point about the inherent additional losses with that though, apples with apples  

[PeterW]

6 minutes ago, dnb said:

So why does a UVC come out so much better than a thermal store? Is it simply the temperature doesn't need to be as high to deliver the same water? Or are they built to a better insulation standard?

 

Thermal Store tends to be a higher storage temperature and also needs to have a higher storage volume for the same DHW volume. As  the store temperature is higher, heat losses are higher.

 

You can get them with custom amounts of insulation - Newark Copper let you specify and you can also get it to 100mm or more if requested.

 

If you want to not have a whole house softener, consider something like a combiphos unit, or even fit a double inline filter unit and go 5 micron then siliphos in a pair of 10" filter housings. I'm filtering at the main sink and a drinking water tap using a 10" filter with activated carbon inserts. They are about £15 for the unit, filters are much cheaper than a Brita.

 

 

[dnb]

1 minute ago, Nickfromwales said:

...apples with apples

 

That seems to be the really hard part about this whole house building thing!

 

2 minutes ago, joe90 said:

let the bath water sit there after a bath till it was at room temp

 

The wife pretty much does that. An insulated bath can be enjoyed for hours apparently...

[SteamyTea]

1 minute ago, dnb said:

That seems to be the really hard part about this whole house building thing

Just basic physics really.  Treat it as such and the rest falls into place.

[Nickfromwales]

And don't buy a TS ?

[dnb]

1 minute ago, SteamyTea said:

Just basic physics really.  Treat it as such and the rest falls into place.

 

Physics is not a problem. I have a couple of qualifications there... It's having to back calculate all of the manufacturer supplied data to find their definition of an apple and seeing what simplifying assumptions the tests allow/force them to make that's annoying.  For instance SAP ratings aren't really helping at the end of the scale I (and others here) want to work to.