Replacing hot water cylinder, any point in going from open/vented to pressurised?

[Rob]

Currently, I have a "standard" open vented hot water cylinder in and airing cupboard with a cold water tank in the roof.

Imminent boiler replacement may mean the renewal of my existing hot water cylinder (house built in 1985, very hardwater area). Might as well get one with a solar coil as I intended to add a thermal system in the near future.

Is there any point in moving to a mains pressure unvented system? Not bothered about freeing up roof space. Mains pressure hot water would be nice as I currently have a pump for the shower, however, there is no guarantee that mains pressure hot water (by the time it gets to the shower) will be as good as the pumped feed at the moment.

Are there any other reasons to move to a pressurised system, are they more efficient?

Thanks.

[harry]

If the header tank is in good condition absolutely no point.

A low pressure cylinder will be cheaper to buy, probably function better, need no maintenence, less to go wrong and need no relief valve, pressure reducing valve, overheat valve.

Probably best even to replace the header tank if it needs it.

The only good thing about about mains pressure cylinders is there is no tank in the loft to freeze. As long as you have 4 to 6 foot head of water above the shower, no problems there either. You could take the opportunity to raise the header tank if you wanted.

If you get PV electric panels, you can just turn the immersion heater on, no need for a "solar tank" with the extra heat exchanger. Solar tanks are a lot more expensive than the normal ones. And you get an income from the PV electric/FIT too.

[John Rumm]

If you are already happy with the performance of the current system, then you could argue there are only limited advantages to going unvented (no need for a cold tank, wholesome hot water at (close to) mains pressure at all outlets etc). You may also benefit from faster recovery (the primary coils are usually rated to take pretty much the full output of a boiler (mine will take around 22kW)). Insulation on modern unvented cylinders also tends to be very good.

If the current system does not perform well then its worth considering - especially if you were looking to add extra showers etc that would require new pumps.

Before you go the unvented route, you need to check that your mains supply is up to it - both pressure (3.5 bar or better static pressure) and flow rate - the main needs to deliver at least 20 lpm or better.

[John Rumm]

I would say its entirely dependent on the circumstances, and the performance of the whole system as it stands. It really has very little (if anything) to do with the state of the existing header tank.

The header tank could be shot, but the system work well in other respects, in which case replacing the header tank would make more sense. Alternatively you may have perfectly serviceable header tank in a system or property where it will never function well as a complete system, in which case unvented would be a viable option.

Function "better" - not quite sure what you mean? As for the valves required etc, they come with the cylinder.

Adding a solar coil makes the cylinder is somewhat more expensive (probably about 20%) - but not necessarily "a lot". You also have the advantage that you can cobble together a solar thermal system relatively easily from old rads, or a length of MDPE pipe etc rather than needing to spend thousands on a PV system.

[Adam Funk]

Just curious --- does this mean a tank with two coils (one for the boiler, one for later solar use) as well as (probably) an immersion heater fitting?

[The Natural Philosopher]

doesnt need electricity, is one

can be fitted in the loft means the cupboard has more space, is another.

here the pressure is excellent and its the ideal solution.

YMMV on that tho.

DO get a water softener tho.

[John Rumm]

Yup - smaller solar coil at the base, then a normal indirect one above it. So solar can have the first nibble at raising the temp of the coldest bit of the cylinder.

[John Rumm]

or a "whole house" phosphate dosing unit...

[Roger Mills]

FWIW, I've got a vented cylinder and two pumped showers at my main home, and an unvented cylinder and mains-pressure shower in my holiday flat. The mains shower is nowhere near as good as the pumped showers.

[harry]

You should also be aware that additional heat exchangers can be retro- fitted to a hot water cylinder so maybe you don't need to change it at all.

[harry]

The problem with the additional valves is they can go wrong. And you still have to pay for them. Also you might not neccessarily get sufficient volume of water through your mains water pipe for satisfactory operation.

Also they are supposed to be maintained at regular intervals. (More expense)

These things were designed to be cheap to fit to new houses by ill-trained people..

A bit like combi boilers.

Pulling out a working traditional system and fitting one of these is mad.

[Rob]

Thanks for all the replies so far. The header tank is plastic and doesn't appear to have deteriorated in any way. It is insulated and has never frozen.

Having an unvented tank in the roof is being considered. However, despite very good insulation on the tank (210 Litre) it will still lose

2KW per day. Since this is in the roof space then, presumably, this is just lost to the atmosphere. If the tank was in the airing cupboard then this heat loss is being put to some use.

The idea of using PV and the immersion heater is interesting. However, a thermal system is a lot more efficient and will give a lot more bang per buck. There are, admittedly small, payments available from the Renewable Heating Initiative.

[Adam Funk]

Good idea. If you fit a tank with two coils, what do you do with the solar coil until you hook something up to it? Just cover the open ends with insulation?

[Dave Liquorice]

I wouldn't, lots of hot water in a sealed container doesn't appeal, even with safety valves etc...

Have you thought about a thermal store and, due to the hard water, an external plate heat exchanger (so it can be removed and descaled)? Gives you mains pressure HW on demand. Though I see you are already aware that mains pressure might not be good compared to the currently pumped shower.

Also bear in mind you aren't supposed to "suck" on the mains either.

Just replacing the current cylinder (if required) is probably the best bet with the addition of a solar coil. Cylinders are sort of standard sizes, a new one might not be an exact "plugin" replacement but adjusting the pipe work shouldn't be a problem for a competent plumber.

[Colin Stamp]

If I had one, It'd be hooked up to the cooling system of a PC by now...

Cheers,

Colin.

[John Rumm]

Seems like a fairly daft comment since they are not optional extras with an unvented cylinder, but a requirement for its proper operation.

In which case you would not fit one. This is why I said that decisions about this sort of thing should be "entirely dependent on the circumstances" and have nothing to do with old cobblers like what state the loft tank is in.

Check the pressure in the expansion tank, check the function of both safety valves, job done. Hardly rocket science is it?

Nonsense. In fact commercial fitters of these used to require higher levels of training and certification.

(Note that ordinary vented cylinders have now been moved into part G3 of the building regs - so that distinction has gone)

If that were true then there would be no reason to fit a unvented system rather than a combi.

Bold statements made without any qualification, information or knowledge of the circumstance sounds more like madness to me.

[John Rumm]

A bit of pipe lagging over the unused ends perhaps, but you can do nothing if you want - just leave it open. Heat losses from the ends will be limited and might even be desirable in an airing cupboard. (unvented cylinders tend to be better insulated and so can make for a cool airing cupboard otherwise!)

[Adam Funk]

Sealed systems are the default in many other developed countries; besides they're more hygienic.

[Vortex11]

Solar coil is all well and good but you will find that unless it's very large it won't hold all that much energy...also a big issue of taking the tank (and therefoire tap water) to unsafe high temperatures.

I bought and installed an Xcel thermal store from

3 years ago and the best decision I ever made. As it happens it's implemented as a sealed system so no header tanks at all. In effect the store is in the boiler primary circuit, also has a solar coil and a couple of immersion heaters as well.

Because the store can run up to 80C it will hold as much solar heat as can be input from my array (Navitron SFB-30) on a clear sunny day with no problem at all.

[harry]

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Everything is about money. It's cheaper to fit a mains pressure system than a vented one. (Or a combi boiler rather than a traditional system.) There are no other benifits.

As for training, don't make me laugh. There's people here didn't know why gate valves failed through de-zincifying.

I suppose the same ones think an NVQ is adequate training. Britain's "tradesmen" have never been of lower standard.