Feed and Vent Combined on Open Vented System?

[arran197]

Had to combine a feed and vent pipe this week. I went to the property to re-connect the gas supply after it becoming tenanted after being void for a while. The system was old, but had a 4 year old Worcester Ri installed. The boiler overheated. So I looked further, found loads of air in the system that wouldn't clear. Header tank was full, but the fill pipe must have a blockage somewhere, so rather than trusting the old under the floor pipework on a sealed system, my supervisor and I agreed that combining would be the best bet.

Worked a bloody treat!

 

[TerryH]

I have been asked to replace several radiators on a combined feed and vent system. I've never worked on a system like this before but I do know that filling a system and purging all the air can be an arse even with an open vent. 🙁
What's best to do?
I have three ideas;
Fit an air bleed valve where the vent would have been;
Run a conventional 22mm vent in and then simply cap it off to keep the system air tight on re-fill;
Bottom fill the system (a mains fed hose filling from the drain off points)?
Any suggestions would be appreciated,
TerryH

 

[tamz]

Just drain it and do the job. It should fill again no problem. If not fill it with a hose through a drain valve.
Combined F&E are no different to any other open system for filling.

 

[Inverness]

Can you explain further about the gas board introducing the cold and feed pipe into one.

 

[Deleted member 25704]

Some good points already made - legend Tamz on the nail as always!

I would add that boilers and heating systems connected to combined cold feed and vent systems should have an over heat device in addition to the boiler thermostat. Boilers in the past, such as potteron profiles & neataheats and some of the old glow-worm low- -water-content boilers had combined cold feed and vents. These always worked great provided you followed the instructions, positioning the cold feed/expansion in the Neutral point before the circulator, which gives a positive pressure system.

As for solid fuel systems there is always the risk of water boiling so it would be risky to have a combined cold feed and vent installation. When water boils there must be an open vent of adequate size terminated safely.

However, times move on - even unvented hot water systems are installed, linked to solid fuel boilers dunsley baker neutralizer link-up system . This said, there are greay areas around this type of installation - do unvented cylinders manufacturers allow their products to be fitted to solid fuel systems?