Pressurised Heating Systems – Plumbing Tips

Posted by in Plumber Talk, on August 31, 2017

A brief video about Pressurised Heating Systems. How to drain them and fill them up. How to vent air from them. How to pressurise them. For more information visit our website at


In this video I’m going to tell you how to drain down a pressurised heating system. This particular system doesn’t have a feed and expansion tank in the loft or a ball valve for you to hold up or anything like that. It’s completely sealed and airtight
so you have an expansion vessel, cold water fill and a pressure valve, and a pressure release valve as well. I’ve managed to locate the pressurization filling loop and I’ll just show you that now. Here’s the pump, that’s the flow from the boiler. This pipe coming down here is the feed pipe Here is our expansion tank. That will take any expansion for when the water heats up. So, there is a rubber diaphragm in there This side is air and this side here is water. THen we have our pressure gauge which currently reads about 1 bar when it’s cold.
The heating system should be at about 1 and a half bar when it’s warm.
There’s our first valve from the mains comes down here and there’s our second valve there. turn both of those off. Remember when draining any system
to open up any 3-port or 2-port valves.
put our hose on and drain the system down.
You have your boiler with it’s flow to the radiator and it’s return back.
You’ll have a cold feed coming in to a
valve, then a hose that you saw upstairs when I showed you
That will go through a manifold that connects to the expansion vessel. This has a rubber diaphragm inside. On one side of that you have water
and on the other side you have air. There is a small schrader valve so that you can pump the air
bag up to a certain pressure.
Usually you’ll pump it up to 1 bar
Also you have a high pressure relief valve. Now, for safety reasons there should never be any valves between the
heating system or the components of the expansion vessel and the high
pressure relief valve. That’s very important. When testing the air pressure in the air side of the expansion
vessel, only ever do that when the system is drained down and there is no water pressure in there otherwise
you’ll get a false reading. There is
also a check valve that in a situation where the heating system pressure got above that
of the cold water mains coming in, you couldn’t push heating system water back
into the main supply come downstairs and find the
lowest point of the system. Here’s our drain off just here on this lowest radiator.
So, pop a hose on here. Open that fully up and then go back upstairs.
So we come up here, open this air vent up
make sure that these two valves are open so the thermostatic valve
is fully open and the lockshield at the other end is open
and open up the valve. Do that to every
other radiator upstairs and after about 10 minutes, and do it to all of the rads downstairs.
If you’re working on the radiators just upstairs
then you won’t need to drain the one downstairs, will you? Once all the
water has stopped coming out of the end of your hose and you’ve vented all the radiators that you want to work on upstairs and downstairs you can now work on everything
You can now fill the system up again.
Right, remove your hose and then shut off your drain off.
go upstairs and go around all of the radiators upstairs and downstairs and shut all their air vents.
go back to where the pressurization unit is and the fill loop is and open those valves and start to fill the system
Just open that. Now we are reintroducing
water into the system. You should see dial start to
rise as pressure starts to build.
Pressurize it up to about one and a half bar, then go back downstairs to the
radiators and vent them out.
As soon as there’s no more air coming
out and it all stops go back upstairs and top the system up again
Keep going around doing that until you have water coming out of each radiator and you’ve turned it off
and then as soon as that’s done go back upstairs, pressurize the system to one bar and
you should be done. what you do is pressurize it up to a bar, you
bleed the rads. When no more air comes out shut that radiator. Go back upstairs
repressurize to a bar, open that radiator, Leave the system at one bar
and then when it comes and gets hot and you turn it all on it should only get up to
one and a half bar


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