Draining the System

2010

I have an open vented heating system with header tank in the loft.
I need to change a 3 port motorised valve body where the spindle is leaking.
Do I need to drain all the system or just partial?

Mr.Generous

Take the opportunity to drain down completely and re fill with inhibitor. Sludge in the system causes all sorts of issues. Flush it out while its empty.

nickcc

Depends where the valve is located. As previous post draining and flushing is the best option but as long as you drain below the height of the valve then the valve can be removed without water leaking out. You obviously need to either isolate the supply to the header tank or tie the ball valve up.

a&akay

You can turn off the cold feed to the system, if you have such a valve, and block the overflow over the top of the feed tank in the attic with a bung. When you undo the valve connections, vacuum will hold back the water apart from a very slight dribble. Has to be less than 32 feet (6m) from the level of the water in the feed tank to the valve. I installed an isolation valve in both the pipes mentioned and have used this method on several occasions to replace leaking valves with no problem.

Alex1983

You can’t fit isolation valves to the open vent pipework, it’s a safety pipe, if it’s turned off by mistake you will have a dangerous situation.

I bung most of the time but be weary, it doesn’t always work, if there are automatic air vents they need shutting and if you have a old primatic hot water tank bunging is not possible.

nofoollikeold

If you are going to drain down, wholly or partially, remove the water from the small loft tank (Feed and Expansion (F&E) tank) and clean the tank out before you drain. Otherwise you risk washing any muck / sediment present in the tank into the rest of your system.

Drain down wholly or partially, replace valve, refill and check there are no leaks. When happy, drain down to the bottom of the F&E, then drain out 2 or 3 more litres. Pour the inhibitor into the F&E tank, then refill to normal level (usually about 4 inches / 100mm depth of water). That way, the inhibitor flows into the system and doesn't just stay in the F&E.