What is a pressursed hot water cylinder and an immersion heater?

rottcodd

Probably a stupid question, but I'm a FTB and about to move into somewhere that has according to the survey "a modern pressurised hot water cylinder. There is an electrical immersion heater fitted to the hot water cylinder" 

Could someone explain as if you are talking to a stupid person what exactly the cylinder is, what the immersion heater is, what do they both do, and how do they interact with each other, and anything I can do to reduce energy costs as much as possible? 

MultiFuelBurner

No stupid question an immersion is a way of heating your hot water in the hot water cylinder by electric.

It may be you have gas in the property your are purchasing is that mentioned in the particulars. Such as gas central heating etc

Netexporter

And most hot water systems are now (mains) pressurised, rather than gravity fed from a tank in the loft (sometimes with added dead pigeon).

Rodders53

The correct term is "unvented" hot water cylinder. They need regular annual maintenance by a G3 qualified plumber.
They supply HW at the same pressure as the cold so usually give a good shower flow without pumps, far better than electric instant showers or those from an open/vented tank.

If electric only they should ideally have two immersion elements and the lower one (for a full cylinder of HW) should be on a cheap rate overnight electric rate (E7 or similar).

If the property is already bought and you are about to move in this 'due diligence' question about the hot water system is being asked far too late... 

BellaBlondykeTheThird

Rodders53 said:

The correct term is "unvented" hot water cylinder. They need regular annual maintenance by a G3 qualified plumber.
They supply HW at the same pressure as the cold so usually give a good shower flow without pumps, far better than electric instant showers or those from an open/vented tank.

If electric only they should ideally have two immersion elements and the lower one (for a full cylinder of HW) should be on a cheap rate overnight electric rate (E7 or similar).

If the property is already bought and you are about to move in this 'due diligence' question about the hot water system is being asked far too late... 

I wonder how many people bother to get their unvented pressurised hot water system serviced every year?

Reed_Richards

If you are in an old and unmodernised house and try to take a non-electric shower you may find it very difficult to adjust the temperature, a tiny tweak of the tap will take it from too hot to too cold.  The reason for this is that the cold water is at the high pressure of the feed from the water main whilst the hot water is at a much lower pressure fed from a small header tank in the loft.  Sometimes both hot and cold water are fed from a header tank in which case water from a shower on the floor immediately below the loft will come out at a near trickle.

A pressurised hot water cylinder gets around this problem.  It's also more hygienic as various dead insects (and occasionally birds) could end up in the header tank.  The downside is that to adhere to regulations you should get a pressurised (="unvented") hot water cylinder checked every year so that is an additional cost.     

Gerry1

Presumably a downside of an unvented system is that your property becomes uninhabitable the moment that the supply fails, e.g. a burst water main?

Admittedly, that's a pretty rare event but not impossible.  At least a big tank in the loft buys you some time even if you have to boil the water for drinking.

QrizB

BellaBlondykeTheThird said:

Rodders53 said:

The correct term is "unvented" hot water cylinder. They need regular annual maintenance by a G3 qualified plumber.

I wonder how many people bother to get their unvented pressurised hot water system serviced every year?

Hopefully all of the ones who have watched this Mythbusters, at least!

https://www.youtube.com/watch?v=28ixJLtQBCc

markin

They have a pressure relief valve, so should never ever actually blow up, The is usually a gauge around the white expansion tank.

Some tanks have an integrated expansion tank on top of the hot water tank.

https://www.youtube.com/watch?v=KIEBTFHV_MU

QrizB

markin said:

They have a pressure relief valve, so should never ever actually blow up

Making sure the PRV hasn't stuck is (or should be) part of the annual service. If you don't have the service, you risk it not opening on demand.

peter3hg

QrizB said:

markin said:

They have a pressure relief valve, so should never ever actually blow up

Making sure the PRV hasn't stuck is (or should be) part of the annual service. If you don't have the service, you risk it not opening on demand.

For it to explode like in the Mythbuster clip the electric immersion heater would need to be permanently on, the thermostat in the immersion heater would need to fail, then the thermal cut-off in the thermostat, then the PRV, then the separate expansion relief valve and finally the expansion vessel would need to hold on far beyond it's rated max pressure.

Pretending that the Mythbusters clip is a possible outcome of a properly installed system, even one that is not actively maintained, is fanciful in the extreme.
I'm not saying people shouldn't have them serviced but we can dispense with the fearmongering of catastrophic outcomes that in the real world aren't going to happen.

Electricity is a far more likely cause of danger in the home but very few people have regular EICRs outside of landlords.