Unvented cylinder vs thermal store (1 floor flat, electric)

[conradh]

Hi,
I'm in a one-floor flat, currently with a cold water tank about 4 foot above floor level and an electric immersion tank underneath it at floor level (no gas, Economy 10 electricity). This obviously is a disaster for anything except a bath: there's a shower hose attached to the bath taps, but you can't stand up and get hot water. I'm looking at re-engineering the hot water system. The two options appear to be an unvented hot water cylinder and a thermal store (both electric: again, there's no gas here). The cold water pressure is good I think: don't know how to measure it, but I get 40 litres a minute out of the cold tap. The local water (Edinburgh) is probably the softest you can get. The building is listed, converted to "modern" habitation in the 70s, and as far as I can tell has plastic stacks, which is a bit of an issue re: unvented system --- my understanding is that I need metal pipes to ground level for that. It's not impossible: while replacing the interior drain might be unfeasible, there's a plastic drainpipe from the gutters just outside the bathroom which I might be able to replace with cast iron.
The pros I've spoken to so far know enough to say that unvented's gonna be hard, but don't seem to have a lot of experience of the two, so I'd like opinions from people who *know* the difference...
So my question is, what are the considerations between an unvented system and a thermal store? Looking at the specs for thermal stores, I typically see "limit 20 litres per minute", much lower than my cold tap; also it seems to me that where with a hot water cylinder that you draw from directly, you'd expect that until the cylinder runs out of hot water you'll get just hot water. However with a thermal store you'll be reducing its temperature all the time you use it, so its output water will get steadily cooler with use --- a bit like the difference between certain kinds of rechargeable battery: sudden death vs gradual decay. I think I prefer the idea of a shower which works well until it doesn't, to the idea of one which gets steadily worse from the moment you start using it. That said, the annual maintenance cost associated with unvented seems a fairly big negative. Anybody in a position to compare and contrast from personal experience?
Conrad

 

[ianmcd]

I think an unvented is your best bet, as for the annual maintenance cost it shouldnt be more than say £65 I do a lot in edinburgh admittedly in the same group of blocks of flats at £40 each but I can get 10 a day if booked well in advance, when you say "metal pipes to ground level" I suspect you are referring to the pressure releif pipework, it used to state that this had to be metal throughout its entirety but this has recently changed to a material capable of withstanding the maximum pressure and temperature it may have to withstand (cant remember the exact figures off the top of my head) but yes it is possible to use non metallic pipes now

 

[conradh]

I think an unvented is your best bet

Thanks for the reply Ian! Can you tell me why you'd prefer unvented? It does look as if the pressure relief pipework is going to be hard/expensive to do if it's even at all possible, so I'm trying to find out why unvented would be better than a thermal store... As I said, my gut feeling is that a thermal store may not offer the same pressure or consistent temperature as an unvented cylinder, but I haven't found anyone clearly stating this.

Conrad

 

[ianmcd]

Hi Conrad yes a thermal store would also suit your needs and doesnt need a PRV I only said an unvented out of personal preference, the PRV outlet from the unvented does not need to terminate at ground level as long as it is returned to the wall where it exits then that is fine, if there is only one bath/shower room in the flat I doubt if you would really notice any hot water depletion when using from either system

 

[minimouse]

I'm in a similar postion and quite confused as which will be better in the long term...

 

[conradh]

I'm in a similar postion and quite confused as which will be better in the long term...

Yeah, I don't know. Advocates of thermal stores argue that they're simpler and safer because you don't need three layers of safety devices or a special certification to install them - or to notify building control officers about the installation, or to pay for annual maintenance, where unvented cylinders have all of these undesirable characteristics. Further, thermal stores apparently provide excellent "mains pressure" hot water which is safe to drink and use in cooking because it's literally live mains cold that's been heated up this instant. This all sounds amazing.

But there's a guy on another plumbing site who keeps recommending thermal stores in preference to unvented, and everyone there treats him like he's a complete lunatic (admittedly he seems to overdo it on the "they could EXPLODE!!!1!" argument), implying that unvented is much better but never saying why. So yeah I really haven't been able to find any clear argument on the actual merits that differentiates the two - or more particularly, why anybody would ever even consider installing an unvented cylinder since they're so much hassle and thermal stores seem to tick all the boxes you could want. Confusing and frustrating.

Conrad

 

[minimouse]

It's extremely frustrating. I found MEP associates helpful - 01892 6699 78 they give consumers advice but without surveying they can't really comment.

Different plumbers have varying experiences and will tend to advise on what they are familiar with.

I am going towards the Thermal but one plumber advised me it's outdated and thermals fur up but the immersion element is not as costly to replace as the parts of an unvented. I am looking into the Santon as there is no air bubble build up. My pressure is not great either.

I have a problem with limited space - can only go 800 high... I've looked at mcdonalds who will make a bespoke one for £1090 + vat.

I am worried about water running out like you...

It's a tough call. Let me know what you decide as I may follow suit...

 

[myrica]

I felt I should sign-up and add to this. I'm also in Edinburgh, also in a listed, 80s converted building, also looking at replacing a vented, gravity system with unvented. And having read conrad's post I'm now also concerned over what types of pipes are required etc etc...

So I called Building Standards at the council to ask. He said it was not the material of the soil stack that is an issue, but the material of the pipework from the cylinder to the stack. Which will be installed by the plumber and they ought to use "HPO" type plastic. Not ABS or polyvinyl-something-or-other. Building Officer seemed to be dead certain, correct plastic pipes emptying into the soil stack, job's a good'un.

I am *hoping* that this really is the whole story (he ought to know surely). But I would recommend calling them to ask the same question (and perhaps asking it more clearly than I managed to babble out).

Also, Scottish Water are coming out to measure water pressure entering my property and under the kitchen sink. At no cost. Bonza.

And I have also come across the guy shouting about how unvented cylinders will explode. Everywhere else in the world seems to use unvented quite happily, and nowhere bothers with our vented systems, so I suspect that's balls

Anna

 

[conradh]

Yeah, that's the message I'm beginning to get: I think the law was relaxed a bit recently, so it does seem to be safe and legal. The special plastic is just a heat-resistant one as far as I understand it. You'll end up with a "tundish" in your airing cupboard, which will have hot water visibly pouring through it if the cylinder is overflowing or whatever.

Explosion guy does seem irrational. Touch wood

Also seems that some manufacturers are now certain enough about their product that you may only need to get the thing checked every two years, maybe even less frequently.

Well done on getting SW to measure your pressure: they just told me "it'll be over 1 bar". Ended up buying a 15 quid pressure gauge which you screw on where your washing machine connects: worked a treat. The pressure does vary a lot during the day though: the lowest pressure I measured was about 20% less than the highest.

Another interesting thing to note is that if you're on Economy 10 you can typically get away with a tank that's one size smaller than for Economy 7, since the time between reheats is shorter -- you only need enough hot water for 4 or 5 hours at a time, rather than the whole day. So basically size the tank so everyone can take a shower and you should be set. The smaller the tank is, the lower the standing energy loss since you've got less hot water just sitting there getting cold while you're not using it. A loss of 0.5kWh per day amounts to about 13 quid a year on my tariff if I remember right, so choosing a tank with better insulation and just-right size saves a little bit over the long run.

Conrad

 

[dcawkwell]

Electric shower £70 quid from argos?

 

[Aragorn84]

you do realise that with unvented the water will still gradually get cooler?

As you draw off hot water, your feeding cold in to replace it,thus the temperature of the cylinder will drop. Your not going to get a cylinder full of hot water then it snaps to cold like you suggested.

The reduced maintenance of the thermal store seems to me to make it the better option?

 

[conradh]

Aragorn84: either I'm misunderstanding you or you're misunderstanding me. Everything hot cools down. Even if you're not drawing off hot water, any cylinder will lose heat over the course of the day, and it'll lose more if it's not well insulated. It's called standing loss. As soon as your immersion comes back on, it'll start to heat the cylinder back up to your desired temperature, and that energy is essentially waste because of the fact that the cylinder isn't perfectly insulated. The figure is measured in the energy lost per day, which will typically between 1 and 5 kWh/day, depending on cylinder size, insulation, etc. Better insulation or a smaller cylinder will lose less heat and so there's less waste. That's all I was saying.

I don't understand what you mean by "Your not going to get a cylinder full of hot water then it snaps to cold like you suggested."

I do think that reduced maintenance is a strong argument in favour of thermal stores. However it appears that a thermal store will only offer half of the water flow of an unvented cylinder on the same mains supply, and it also appears that the temperature of the water from a thermal store (using an internal coil, not an external pump and heat exchanger) will drop linearly from pretty much the moment you start using it, where with unvented the temperature will remain high until you've used almost the whole tank, at which point the temperature will quickly drop to cold. Both of these are points against thermal stores and for unvented I think. It's a difficult decision to make, and I think anyone making it needs to think carefully about what they want. The process isn't helped by the fact that it seems to be impossible to find an unbiased comparison between the two.

Conrad

 

[rpevans]

Electric shower as previously said.
Rich

 

[Aragorn84]

conrad: you said that a disadvantage of the store was that the tank slowly cools as you use the water throughout the day, with the suggestion that unvented will give you fully hot water until the water runs out:

also it seems to me that where with a hot water cylinder that you draw from directly, you'd expect that until the cylinder runs out of hot water you'll get just hot water. However with a thermal store you'll be reducing its temperature all the time you use it, so its output water will get steadily cooler with use --- a bit like the difference between certain kinds of rechargeable battery: sudden death vs gradual decay.

The bit your missing is that as soon as you draw water from a hot water cylinder, be it vented or unvented, its immediately replaced with cold water. It is after all the incoming cold water that pushes the hot water out of the cylinder and down the pipes to your tap! Thus both systems will infact exhibit the same "gradual decay".

 

[conradh]

Re: electric shower...

Even adding in the cost of standing loss, the fact that night rate electricity is half the price makes a cylinder cheaper. By my reckoning they're about on a par if all you do is have one 5 minute 4.5 litre-per-minute shower per day, but as soon as you shower longer (or have two shower users, or have a higher shower flow rate, or take a bath) the cylinder wins - by 30% at a 15 minute shower.

Add in the fact that a 70 quid electric shower (Creda 850DL, I looked it up) can only produce 4.5 litres of hot water per minute, and the electric shower loses big time. They're just not as nice, and you have to shower longer because of the slower flow. My current system is 4l/min, and it's hateful.

If you keep the cylinder at a lower temperature and have an under-sink unit for 60C washing-up water, then the difference will be even more pronounced.

Conrad