Mains pressure HW cylinder

It's either a pressurised HW system or a Heat Store.

And indeed is a far better way to do t8ings IMHO. lots of high pressure hot water.

The 'continent' has been doing it this way for years....

This sounds like an unvented mains HW cylinder, if it had a number of pressure releif valves on it. OTOH if it did not then it was a themal store which also produces water at mains pressure.

If you looked around you'd likely find a valve which cuts the supply of HW to it saving you having to cut of the all the water.

Thanks for that - I'm just surprised that I've never come across something like this in my travels! And I've been in many airing cupboards over the years!

I saw no evidence of pressure relief valves so I guess it must have been a thermal store.

There was indeed a valve on what appeared to be the incoming feed. Wound it right down tight, didn't seem to make any difference (so at a guess it couldn't have been the feed).

PoP

Sending email to my published email address isn't guaranteed to reach me.

Is it something like one of these?

Yes I know it has to be certificated... it will be ;-)

details.

PoP

My published email address probably won't work. If you need to contact me please submit your comments via the web form at

In fact it could have been the feed. IIUC Most of these mains pressure direct water storage systems are designed so that you can extract water from them faster than the mains can replenish it - hence turning off the supply would still leave you able to draw off hot water until the tank is empty.

Not so, they have a rubber diaphragm inside to maintain a level of pressure even if the mains water is turned off. Mine will still run for a while even after I turn off the feed.

Mike

That's right! AIUI, they have a built-in expansion vessel (or equivalent thereof) which stops the pressure getting too high when the water is heated. Presumably they also have a non-return valve on the mains inlet to stop hot water flowing backwards into the mains.

When you close the mains feed and open a hot tap, the energy stored in the expansion vessel will drive a certain amount of water out through the tap.

I don't feel so virginal in having asked this original question after all these splendid answers! Thanks to all for contributing!

PoP

My published email address probably won't work. If you need to contact me please submit your comments via the web form at

Sounds very interesting.

What sort of life span do pressurised tanks have? Does the diaphram not die at an early age?

Would it be better in the long run for separate storage tank and expansion tank?

John

The diaphragm will certainly lose pressure over time, I gerenerally had to re-charge the system 3 times over the 5 years I had it for. Not too much of a pain really, more than compensated by the brilliant level of pressure with the hot water. A shower that scraped your skin off!

Mike

Unvented cylinders, being fed directly from the incoming mains, stop as soon as you take away the incoming mains pressure. Is yer traditional low-pressure tank-in-the-roof jobbies wot can supply water faster than the mains can supply it.

How easy is it?

Car tyre foot pump?

John

Agreed espeically if the cylinder is a Megaflow which use a trapped air pocket.

Most makes have an external expansion vessel which does indeed contain a diaphragm. These however will likely not need either recharging or any maintenance in 5 years. Their life is however perhaps 10-20 years at best unlike the vessel to which they are attached which has essentially an indefinite life.

Megaflow and perhaps one other brand use a trapped pocket of air. This pocket of air however dissolves into the high pressure water over time. This leads to excessive pressure fluctuations during heating with the result that a) The air pocket has to be renewed. (essentially a partial draining of the cylinder). b) One of the relief valves (usully the 6 bar over-pressure unit), operates then becomes get limescaled and fails.

Typically the air pocket needs to be renewed annually.

Except where they use combi boilers. I think we are on a slow path to 65% combi boilers 35% unvented cylinders.

So is this air pocket in direct contact with the water, with no intervening rubber diaphragm? If so, isn't the control of its volume somewhat hit or miss?

Is replacing a relief valve a DIY job?

John