Vented/unvented mains pressure hot water systems

I went to look at a new property and forgot to ask what type of heating system it has. What I saw in the airing cupboard was a tall cylinder with what looked like two ‘ball’ shaped pressure vessels above it. There is a gas boiler plus a small PV array on the garage. Could this mean its a vented system? (Hope so)

If it’s got pressure vessels then it’s probably unvented.

We’ve got unvented cylinder with an expansion doo dah above it.

It’s a normal gas boiler + solar tubes on the roof. The cylinder is just hot water storage when the boiler is on a timer. Luckily in the summer we get free hot water when it’s not too cloudy

One will be for the solar thermal array which will be on its own circuit filled with glycol heats water indirectly via a coil. The other vessel is for the hot water system. The tank probably had 2 coils with the second being feed from the boiler. If not then the backup from the solar thermal will just be a immersion heater.

Unvented is higher pressure. Vented is the low pressure

@TheBrick does this make it an unvented system? Is there any reason to disregard a property with an unvented system? Eg annual service costs, risk of explosion etc

My unvented has one expansion vessel for the hot water and one for the heating.

I’d say if you have a large family then even the 6ft cylinders can empty quickly with teenagers in the shower for ages.

For smaller families then it’ll be right, you’ll just need to learn to have the boiler set to cope with the hot water demands. It should have an immersion heater by-pass anyway which is fine for an emergency

Only two of us so should be fine.

Unvented systems are good. No danger or annual costs. Ok technically they should be checked, but you can check the safety pressure / temperature relief valves your self in a minute.
Good hot water flow (better than a combi), able to harvest solar, either direct thermal solar or PV solar. Free hot water for ~8 months a year. The bigger the tank the better, I have a 300 litre tank, yesterday 13Kwh from PV > water, today 3.3KWh, but still not needed a boiler top up, as of heat carried over from yesterday is plenty.

Just had my 30 year old gravity system changed to unvented. Night and day. Cylinder and boiler in the garage, landing now twice the size and loft ripe for conversion in a couple of years as nothing in it.

Super high pressure showers x2 .

Cylinder is 270L and we havnt run out yet with two teenage girls but we are quite resources conscious so never had long showers.

One will be for the solar thermal array which will be on its own circuit filled with glycol heats water indirectly via a coil.

Well seeing as the OP said there’s PV on the roof then maybe not!

Certainly sounds like an unvented system though.
All unvented cylinders have an expansion vessel, sometimes they’re visible (like the one you’ve seen) but some are integral (i.e.megaflo) so you can’t see them.

We’ve been at this house for 18 years and put in an unvented system when we moved in. It’s a 250L megaflo and it’s been ample for a family of 5 including 3 girls who are now 18+.

Only two of us so should be fine.

We have a tiny ‘pencil’ HW tank, but I run it at 75C so there’s enough HW after mixing with cold to run showers etc. In summer we have more than enough and I turn it down, in winter the mains CW temp is noticably lower and we get a max of 15 mins shower time to share between the two of us.

I had to fit mixer units to the sink HW tap as it comes out pretty scolding raw from the tank.

75c is proper hot – I can’t get near that (61c) with the immersion thermostat on max!

That sort of temperature water goes a long way!

75°C is bonkers !!!

Well seeing as the OP said there’s PV on the roof then maybe not!

Ha good point, I think I read what I wanted and had solar thermal in my head.

I’d say if you have a large family then even the 6ft cylinders can empty quickly with teenagers in the shower for ages.

For smaller families then it’ll be right, you’ll just need to learn to have the boiler set to cope with the hot water demands. It should have an immersion heater by-pass anyway which is fine for an emergency

We find that if the boiler is running, it can reheat the water at almost the same rate we can get through it (family of three people + one that thinks she is a mermaid). Occasionally we end up switching on the immersion too, but that’s the exception.

75c is proper hot – I can’t get near that (61c) with the immersion thermostat on max!

Our in indirectly heated by the gas boiler, so in theory can go as hot as the max boiler water temp, which is probably 80-85C.

I went from vented to unvented 5 years ago and it is much much better, but vented is perfectly functional for sure. Needed a pump for the shower but other than that all good.

My unvented has two pressure vessels – one for the hot water pressurisation and one for radiator circuit expansion. Some might only have one pressure vessel.

Our in indirectly heated by the gas boiler, so in theory can go as hot as the max boiler water temp, which is probably 80-85C.

I’d be VERY surprised if it didn’t have a thermostat on the cylinder for the motorised valve on the hot water circuit from the boiler.

I can always tell when our hot water is the production of the PV – it’s noticeably hotter because the immersion thermostat goes higher than the thermostat for the boiler circuit.

We find that if the boiler is running, it can reheat the water at almost the same rate we can get through it (family of three people + one that thinks she is a mermaid). Occasionally we end up switching on the immersion too, but that’s the exception.

same for us. Given the boiler is more powerful than a combi, I suppose it should be no surprise

question for the masses. As I understand it, as a tank empties hot water, new cold water is pushed into the single vessel. If this is the case, why does the water suddenly go cold instead of gradually getting less and less warm over time? if you use 10% of the water in the tank (heater off), surely the newly topped up water should be an even temperature that’s 10% closer to ambient temp?

I’d be VERY surprised if it didn’t have a thermostat on the cylinder for the motorised valve on the hot water circuit from the boiler.

Some time ago when I had studied the regs I think this is a requirement to avoid over heating. NC valve on input of coil. Like all safety stuff fail to safe.