Replacing copper HW tank.

We've just had a traditional sixty-year-old DHW system replaced by an unvented, mains-pressure system in my late mother's bungalow, which we intend to move into soon. Several comments:

It's based on a Joule tank,

The DHW cylinder itself is much bigger that I expected; this may just be what the plumber chose to install, rather than being necessary. My comment to the plumber was that it looked like a section from a Saturn

A potential disadvantage is that in very cold weather, such as we've just had and where the mains supply may be off for several days, is that you have no water at all. At least in our present place when the water was cut off for 36 hours, we could draw down the water from the tank in the loft, boiled where appropriate, but that wouldn't be an option with an unvented system unless some way was found of retaining the tank, in which case why bother with an unvented system at all.

Well, it's not like we have any problems with what there is now. We occasionally need to switch on an immersion heater (our little boy has several baths a day - he likes to play with his toy ships), and the upstairs hot water pressure is a bit low (but I put a pump in, so it's okay). It's just that sometimes, thinking of selling the house in the future, everything might be a bit 'old-fashioned'. The thought of mains-pressure HW does seem a little scary. And I don't really like over-complicated things (which is why we still have the 30 year old boiler).

Does it do what you want?

AFAIK the main reason to have a tank/heat store is if your demand for hot water is greater than easily supplied by the mains flow. Often a compromise with a combi boiler.

How old is your boiler? People normally consider changes like this when changing the boiler. If the rest of the system is working well then just fitting a new (possibly slightly larger?) cylinder may be the easiest option.

Other reasons are to free off space used by the cold water storage and remove worries about freezing pipes in the winter.

I still can't decide if I did the right thing in switching to a combi.

A previous house with a big hot water tank and a shower pump served two showers simultaneously with lashings of hot water.

The current house has a combi sized to supply two showers simultaneously but doesn't in any way match the flow rate.

So worth checking the maximum flow rate of the cold mains before you move away from a cold water storage tank. Noting also that if your mains water is cut off then a storage tank in the loft can be an unexpected bonus.

Hard or soft water area?

If you are in a hard water area with unsoftened water then your tank may have more crust on the inside than the outside.

Cheers

Dave R

It does. I just wonder if I should be 'keeping up with the times', that's all.

The boiler is a 30 year old Thorn Apollo. As long as I replace the thermocouple and top socket every five or so years, it just keeps going. I don't want to replace it with something less reliable that I can't easily fix myself.

Soft.

We've had a pressurised hot water system for a few years now. As has been said, it's relatively complex. The tank is well insulated and doesn't take much for the boiler to keep the temperature up when the water's not being used. Generally, we're very pleased with it - in our case it's all in the cellar (the 'plant room' as the installer called it because it sounds better than 'coal hole') and doesn't take up any otherwise useful room.

The downside is that you can only get out the flow that you can put in. In our case, that means that you can't have two showers at once, or have a shower while the bath is running. If that's an issue for you, don't do it.

Cheers

If you are keeping the current boiler, then your options basically come down to either stored hot water, or some kind of thermal store / heat bank arrangement.

The setup you have now is simple enough and can work well - however it has pros and cons and does not suit all properties or usage patterns.

An unvented cylinder will adds a bit of complexity close to the cylinder

- but does away with the need for a cistern in the loft. It has the advantage of delivering mains pressure (or close to it water to all the hot taps - so good for showers without the need for pumps etc). You can still have an immersion backup. To work well though you would need to make sure you have a decent flow rate from your mains cold supply.

Have a look at the lists of pros/cons etc here:

The thermal store options are similar - basically a big tank of hot water - however its used indirectly to heat the incoming cold water on demand. A slightly different set of pro's an cons, but handy for example if you want to incorporate several heat sources like solid fuel and solar as well as a boiler / immersion.

You can buy them ready to go or cobble one together from parts:

Here is the basic layout you can expect:

Historically it was the case that only unvented cylinders were covered by building regs (Part 3 document G). These days *all* cylinders are covered by the building regs. See approved document G for more info.

They also need to be installed by someone who knows what they are doing! (although that applies to any cylinder - unvented ones may be less well understood by some plumbers).

Modern installs should also include things like mixing valves to limit the maximum temperature at the point of use, but again that applies to all stored water systems.

Yup unvented cylinders are often available in larger capacities than traditional ones. (although you can still do the same trick doubling up smaller cylinders like you can with a vented system to get more capacity)

Personally I fitted a 210L one which is probably about 5' tall. The outer shell and integral insulation can make them look a bit more "solid" than a normal tank. If I were doing it again, I might go for the next size up.

Sometimes upgrading the connection to the mains is way of solving a number of related problems (lack of flow rate, freezing etc). A nice

As a matter of interest, do you think that is likely to be worth doing if the water board side connection is 2m of 25mm?

The tank suppliers require the tank pressure to be below some maximum figure, 3 bar in our case, controlled with a pressure reducer, so it doesn't necessarily run at mains pressure.

But we also have a reducer on the incoming mains. I don't know what it's set to, it has no gauge (but that is due to be remedied!).

There is also an expansion vessel on the DHW, as well as one on the CH.

You mean using a larger pipe than theirs? It depends on the circumstances... If you have say 40m of 1/2" pipe after that, Then any increase will make a significant difference. The cost of the pipe is usually only a small part of the overall cost of swapping it, so you may as well over rather than under spec it.

(Flow resistance is a function of both diameter and length so a short run of something narrower is not necessarily a show stopper)

I have no gass so am all electric and so my airing cupboard has just got the traditional cylinder and immersion heater in it. It would be nice to be able to dial up a temperature and fill a bath, and not have to wait for the shower to stabilise its temperature, but is it worth the expense of duplication for such wonders of science? Brian

Unless you have cause to think the crusty pipe connections may fail I'd suggest you don't fix what isn't busted. Do note that installing an unvented cylinder should be done by a competent and certified installer. Incorrectly installed it is a potential bomb. I think there's a requirement for building regs approval for swapping a vented one now as it comes under conservation of energy but iirc prelagged complies as long as the temperature controls are up to date and immersion stat is compliant.

TBH, if when making the decision to stick with a new conventional vented system or go unvented, I knew then what I know now, I wouldn't have changed. By all means replace the tank if you think it needs it, but leave all else alone. IIABDFI.

I replaced my hot water tank with one that came covered with 2" of foam which I then painted. Much better than the previous jacket. I like having an immersion heater as a backup in case my boiler is not working.

I did get something for the old copper tank, based on weight a lot of which was crud.

I just put in about 8m of 25mm, but you're making me wonder if I should have used 32mm!

The new tank is likely to come with a integral foam insulation jacket so if replacing like with like capacity wise you may need more space for the new tank.

When they changed our cylinder the weight of crud combined with corrosion was enough for the bottom to fall out...

In theory the new one will give you hot water continuously. The coil will absorb all the heat the boiler can give.

Andy.

While often stated, I am not convinced this is anything other than a bit of FUD that serves the makers of combis etc well. Yes you can turn one into a bomb in theory, but you would have to try *really* hard...

Firstly you are not going to super heat any cylinder with a boiler as the primary heat source. So you would need to use an immersion as the primary heat source.

Secondly, they come pre equipped with at least two safety interlocks (over temperature and over pressure relief valves). You would have to defeat both.

Lastly the immersion heaters also come with a thermostatic control and separate overheat protection. You would need to defeat both of those also.

(probably worth keeping in mind that an vented cylinder can kill you as well)

It also comes under G3 - where the proper venting requirements, sanitation protection, and thermostatic mixing requirements are specified.

In comparison, I made the change and am very pleased with the (massive) improvement in performance...

It really all depends on the circumstances...