Unvented system confusion.

[Chris B]

OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared things up.

If I have an unvented system and I need to bleed a radiator the pressure in the system will drop as I bleed it. Having completed the bleed I now need to top up the system using the filling loop until a set pressure on the gauge. My assumption here is that incoming water is compressing the air in the expansion vessel as I top it up. Having reached the required cold pressure I isolate the system at the filling loop. Now as the radiators heat up the water in the system expands out into the expansion vessel and the pressure rises to the working pressure. All the water I have been talking about so far is the heat transfer fluid (ie that with inhibitor).

Now the real crux of the problem - I simply want to change a hot tap washer. The tap does not have one of those little quarter turn isolator valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised. Do I simply turn off the rising main at the cold water inlet point and let the pressure out of the hot tap? If so how to I "repressurize" the system when the job is done?

Any help appreciated.

[Tim+]

You?re only pressurising and depressurising the heating circuit when you top up with the fillip loop or bleed a radiator.

The hot water system is just a ?through the boiler/heat exchanger? system so just turn off cold inlet to boiler, change washer, job done.

Tim

[Chris B]

No, its the 185 lt of hot water in the pressurised cylinder I'm worried about......

[John Angus]

Surely there is a valve on the output of the pressurised cylinder that stop hot water coming out of that that can be used to change a hot water tap washer ?

[tabbypurr]

You don't need to do anyting, the incoming mains water will repressurise it. The 2 pressured circuits are different in nature.

NT

[Michael Chare]

If you turn off the mains, does any water come out of the hot tap?

It maybe that to get water out of the tank you have to let air in by loosening a compression joint at the highest point in the pipe work between the cold water feed to the house and the hot water tank. Turning on a cold tap might also let the hot water out.

[Andy Burns]

If you turn off the mains pressure cold going into the cylinder, there will be no hot water coming out of the cylinder ...

[John Rumm]

So far, that is all spot on.

ok, first thing to realise is that the term "unvented" can be used for two separate and unrelated systems... it can be used to describe a sealed primary CH system - which it what you alluded to above. However that is more commonly simply called a "sealed system".

It can also be used to described a mains pressure hot water *storage* system. Which has the cold main feed directly into a hot water cylinder. The cylinder needs to be rated to take that pressure and have a number of other safety systems included with it. Including its own expansion vessel. (the cylinder could be heated by a vented or unvented CH system, or an immersion heater).

However many people will have mains pressure hot water provided via a combination boiler - that heats water on demand - there is not stored hot water.

In the case of a combi, shutting off the cold main supply to the boiler (or indeed the whole house), will also stop the supply of hot water.

With an unvented cylinder, shutting off its cold main supply will also shut off the hot water - but it will still continue to supply a few litres of hot which is expelled by the pressure from the expansion vessel that belongs to the cylinder (not the one in the boiler).

[John Rumm]

With an unvented cylinder there is another pressure vessel that copes with the expansion of the DHW as it heats. Shown as item 10 here:

Once the cold feed to the cylinder is shut off, you will still get several litres of hot water out. However let it run and that will reduce to nothing as the pressure vessel de-pressurises.

There should be no need to drain the cylinder itself. Once there is no mains pressure to push the hot water out of the top of it, no hot water will flow.

[John Rumm]

+1, yup what he said - I left that off my earlier explanation.

[harry]

The pressurised heating system and the water in your hot water tap are different unlinked systems. Turn off the incoming cold water supply (stoptap) and then you can work on your taps.(Hot and cold)

[The Natural Philosopher]

Until someone flushes a loo...

then the loo will allow air into the cold side of the system and the hot water will all come out.

[The Natural Philosopher]

That depends on where you turn it off.

Critically, on whether there is a possibility of unsealing it downstream of where you have switched it off.

[Chris B]

Thanks to all for your help, just one last thing: when "repressurising" the HW cylinder there is no gauge I need to look at? I just turn the rising main back on and the cylinder design just takes care of the pressure?

[RaadhikaBhatia]

replying to Chris B, RaadhikaBhatia wrote: Good Discussion. Need more answers to get the informative notes. The number of tips and techniques will help us. RaadhikaBhatia |

[Andy Burns]

Such as the tap with the leaky washer ...

[GB]

Not in my experience, but in theory the head of water within the pipes could push a small amount of hot water out of the cylinder. There's nothing wrong with TNP's theory, so I guess it's stymied by anti-syphon valves in the toilet cistern.

Just to be on the safe side and to avoid an argument with TNP, leave the hot tap open and flush all the toilets in the house before changing the washer.

My 2p-worth: turn the boiler off today and change the washer tomorrow!

[GB]

Yes, that will do.

You might want to download the manual for your particular DHW cylinder and follow the procedure for recharging the inbuilt pressure vessel. That's supposed to be done annually but I know there are systems that have been installed for years without it ever being done - mine for instance! You are also supposed to test the pressure relief valve, etc.

Yours, living dangerously, GB

[Chris B]

Right. One brave pill just consumed.......

[The Natural Philosopher]