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

[conradh]

Hi Aragorn,

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".

Ah, right - no, I wasn't missing this. A well-designed tank will minimise churn, so that as hot water is drawn out of the top of the tank, and cold water flows into the bottom, the hot and cold layers will stay separate --- at least for a while: I agree they'll even out to lukewarm over the long term. This means that if you make a continuous draw on either a hot conventional gravity-fed immersion cylinder or an unvented cylinder, you'll get very hot water until you've drawn off almost all of the tank.

In contrast, with a heat store you're not drawing off a hot layer, you're running the incoming cold mains in a spiral through most of the tank, so after you've pulled half a tank's worth of water through the heat store, the whole store will be half way to cold, and it won't be able to raise the temperature of the mains water you're pulling through it by anywhere near the amount it could at the start of the exercise. The heat store can't benefit from the hot/cold layering in the unvented cylinder because it needs to run the incoming mains through a lot of its stored hot water in order to transfer enough heat to the mains water.

This is illustrated in this company's "why-we're-better" page:

http://www.systemdesigner.co.uk/documentation/Xcel vs Competitors.htm

... obviously you can't trust someone's own blurb about their own product, but the behaviour you should note is that the output temperature from the "normal" heat stores drops pretty quickly immediately you start using them. Their supposedly-superior product has an external pumped plate heat exchanger which improves usability a lot (and by some people's terminology makes it a heat bank), but adds complexity to the system.

I dare say that with a much larger heat store this might not be so much of an issue, but if we're talking about a small flat, the heat exchange spiral through the store runs through most of the tank from what I understand.

Edit: I should point out that I'm only linking to that graph to show the heat store's temperature behaviour. The graph is fundamentally misrepresentative for its intended purpose, because it's showing constant water flow, not constant heat flow: in the first minute, the 180l heat store will have released about 1.3kWh worth of hot water (18l at 75C), where their heat bank will have released less than 0.9 (18l at 55C). As a result you'd use significantly fewer litres per minute from the heat store than from the heat bank in order to get your 38C shower!

Conrad

 

[myrica]

Electric shower as previously said.
Rich

Nope, electric showers are rarely very good, especially for 70 quid I suspect. Currently I get 3.5l/min from a shower off the bath-taps, and as a girl with long hair I end up taking longer showers than I would otherwise. With a decent, thermostatic shower I can be done in 3 minutes; my rubbish shower takes me 11 mins.

With a £70 electric shower running at 4.5l/min, that's 10 minutes of essentially boiling a kettle on expensive electricity.

Having done some rough sums on running a decent electric, 9.5kw shower, I came to the conclusion it was better to have a shower that utilises the cheap night-time electricity. It'll use less water, less energy and less money. Initially more expensive, but longer term returns.

Next choice is to keep gravity-fed system but install a shower pump, or replace with unvented. Pump again is initially cheaper to install but adds to the electricity usage anyways. The current 178l tank is also far bigger than I ever need (unless I have 2 people staying which is rare). So, the initial expense of having a smaller, unvented system installed, instead of a pump wiith the ridiculously big cylinder, should also mean longer term savings.

Plus I work outdoors so there is nothing worse than getting back from a muddy day in the hills to a crap shower

 

[ianmcd]

Hello again Ann

You seem to be getting loads of mixed replies, why not give these guys a call and tell them your requirements they are the ones I use and as they make both systems they will give you impartial advice, I have always found them really helpfull

Cheers Ian

http://mcdonald-engineers.com/blog/index.php/tag/hot-water-cylinder/

 

[myrica]

Funnily enough, having looked again my vented cylinder is a McDonald cylinder. Wonder if they'll do a part-exchange...

Thanks though. I've got a couple of companies to contact to try to get some more technical data, so I'll add McDonald to the list.

 

[conradh]

mcdonald-engineers.com

"ianmcd" recommending mcdonald-engineers.com? I don't think there is a relationship, but you gotta at least say something

Nice to see a Scottish engineering company producing something in this arena. Their unvented cylinder is called Powerflow. Unfortunately they tell you almost nothing useful on their site: no information about its standing loss, insulation thickness, etc.

I'm mainly looking at 120l cylinders: have a 120l vented one and think I've run out of hot water once in five years (live alone, have guests on and off). I've had a look around some brands, and of Ariston Aquabravo, Gledhill Stainless LIte, Santon PremierPlus, Heatrae Sadia Megaflo, OSO Super S, Albion Ultrasteel and Dimplex EC-Eau, the ones with lowest advertised standing loss appear to be Dimplex, Gledhill and Albion (all 1.1 kWh/day or less; the others seem to be around 1.3 or 1.4).

- Dimplex use the thickest insulation by default (60mm vs 50mm for everyone else), so it's not surprising they make the best claims. They can't be bothered to reply to questions from a prospective customer though, which gives me a really good feeling about their post-sales support. Big brand, small customer.

- I've heard complaints about poor customer support from Gledhill as well, but an intelligent (as opposed to sales monkey) member of their staff 'phoned me the day after I emailed them an enquiry, which was a good sign.

- A relative who uses the Santon ones recommends them pretty highly, but Santon mainly sell into industry so small installers don't use them as much.

- The Heatrae cylinder is interesting for its use of a built-in air bubble instead of an external expansion tank; much neater but requires periodic (seemingly trivial, and obvious when it's necessary) maintenance (user-provided though: no plumber necessary).

In the end I suspect that to a large extent choice of manufacturer is much of a muchness, dependent on who your installer is comfortable with, what fits in your space (e.g. does the hot water outflow come from the side or out the top), and how much you like their guarantee. Differences of 0.3kWh/day in heat loss are worth only a few quid a year, and if your cylinder is inside your flat, that heat loss goes to making your airing cupboard and flat warm anyway.

Eh, massive digression. I share your pain re: pathetic showers though Annaonion.

Conrad

 

[myrica]

Yeah I spotted the surname thing but no one in the company seems to have that surname anymore so I safely assumed it wasn't an inside job

Some of those you listed are glass-lined (Aquabravo), some are copper (PowerFlow). I'm not keen on glass, I've been warned off by boyfriends dad who is a former heating engineer. If the lining goes, bad things can happen, like a thermos flask I guess (although I don't plan to drop my water cylinder so regularly).

Also been steered towards steel rather than copper due to corrosion etc. Ariston Primo is steel, McDonald SteelFlow too. It may not be such an issue these days, but then most seem to be steel now & surely there's a reason for that.

I read good things about Oso too somewhere (I think the term used was "wee beauty") but initially discounted them as their website looked like the tanks didn't go down to 120l but it seems I misread the site.

[Edit: I lie. It was an indirect Vaillant described as that. Same poster was recommending Oso over MegaFlo & Santon. V.old forum post though.]

Thanks for the info re.other brands & technical bits. I reckon the expansion vessel vs. air-bubble debate is also much of a muchness (depending on the expected life & cost of the vessel).

Further digression: I have had lots of crap showers when in holiday cottages for work. Worst week I had at work though was when staying in a cottage with no shower, being filthy & cold at the end of the day, and bits of insect & wasp coming out of the taps. I refused to wash til I got home! Can't wait to have an awesome shower to come back to

Anna

 

[bolshy]

Only briefly scanned this topic, but it looked like there was some confusion amongst some of the posts....An unvented cylinder is basically a hot water cylinder which operates at mains pressure and stores a limited amount of useable hot water, approximately 2/3 of it's capacity - you draw the hot water from the cylinder in a similar way to a vented one. It requires annual maintenance.
A thermal store is a hot water reservoir with a header tank, the stored hot water stays in the cylinder, and the cold mains is passed through it in a coil picking up heat and coming out as hot water. No hot water is drawn off the cylinder, but it cools as cold mains is passed through it. In my experience, for large quantities of hot water, the thermal store is better with good recovery time too. It's only limited by the decreasing temperature of the store, an unvented cylinder will always be limited by it's volume.

 

[hansthebear]

Steel is cheaper these days, copper is inert.

 

[ianmcd]

mcdonald-engineers.com

"ianmcd" recommending mcdonald-engineers.com? I don't think there is a relationship, but you gotta at least say something



ha Ha good one never even thought of any connection, no nothing to do with me Im afraid, the two lads that own it are actually called Stuart if I remember correctly

 

[SimonH2]

If I can add my 2d worth ...

A thermal store should also show considerable stratification during use. As cold water enters at the bottom, it will absorb considerable heat due to the high temperature difference between cold mains and store. If the whole store is hot, then by the time the water is part way up the coil, it will be hot and will take little or no further heat out. So the bottom of the store will cool of the fastest by a good margin.

Obviously, as you draw off heat, the store will cool, but it will tend to do it bottom-up, and you'll start losing temperature at the top of the store as well. But as long as any level of the store is above the temperature of the mains, it will still be cooling down as water is drawn off.

This effect will be more pronounced if the draw off rate is low - the faster the flow, the less time the water remains in the coil to absorb heat, so the more heat removed from the upper part of the store. Which leads on to ...


Like a combi, a thermal store has a finite heat transfer rate. Thus it's important to size the store correctly for the intended use. If you draw off water too fast, then the water will be leaving the top of the coil before it's had chance to fully heat. I have limited experience, but the 170l store I put in the flat can supply "nice hot" water at a rate limited only by the mains and internal plumbing - we get up to 6 bar or more here (reduced to 3 bar with a reducing valve) and "quite generous" flow rates that go with it.

To some extent, you can reduce the downsides of a thermal store by keeping it hotter - and in fact many manufacturers quote "quite high" store temperatures as it makes the figures look good. With all electric it won't make any difference to the efficiency of heating, only a slight increase in standing losses.


Now, if you are in an area prone to scaling, you need to consider where the heat exchange happens. In an unvented cylinder this is at the immersion heater - so the heater will fur up. In a thermal store it's in the coil so the coil will fur up. Both are treatable - it's just a case of what the effects are.
A furred up immersion heater will reduce heat-up times (and I guess impact on heater lifetime) but not affect draw off rate. A furred up thermal store coil will limit your draw off rate but not affect the immersion heater.

As to safety, unvented cylinders are safe - because we have regs which mandate the controls which make them safe (ie the risk is controlled). I wouldn't be worried about one from the safety POV. Most thermal stores tend to be open vented which removes most (bit not all) of the risk from cylinder failure. I believe there are unvented thermal stores as well, but I'd have thought their applicability was limited since they would appear to have the disadvantages of both types.


As the OP might have realised while searching on forums, there seems to be a "dislike" of thermal stores amongst plumbers. Like most industries, it seems the plumbing world has it's fair share of people who won't consider anything out of their normal range of experience. Even where the criticism might have some element of truth, these don't apply to the OP.

The OP already has space for the store or cylinder and F&E tank. While I happen to like stores, either option would appear to be suitable. The unvented has the advantage as there's no secondary heat exchange - just the one from heater to water. Both will give much the same flow as pressure as this is determined by the water supply (subject the note above about the limits of the heat transfer coil ina store).
So it will most likely come down to practicality (can the safety drain be managed for the unvented), price, and what the local plumbers can cope with.