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Feed and Vent Combined on Open Vented System?

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arran197

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Went to a job today and the feed and vent pipe were the same thing, 22mm up to the header in the loft.

Tenant was complaining that the rads need venting every time they use the heating. The water level was a bit low, so raised it up, but can't think of anything else it can be, as the pump can only really be sucking in water from the vent because it's wet! Maybe a leak somewhere?
 
Was the water in the tank clean, or was there a lot of crap inside?

The 'air' could possibly be hydrogen caused by corosion inside the radiators.

Of the pump could be pulling air in from a small leak on the system.

Happy to hear other suggestions.
 
Intaking air maybe through an AAV if fitted.
 
Quite possible vent combined with feed in an attempt to prevent the problem occuring. My old boss did that once, don't know if it worked - stopped working for him shortly afterwards.
 
Dannypipe said:
Was the water in the tank clean, or was there a lot of crap inside?

The 'air' could possibly be hydrogen caused by corosion inside the radiators.

Of the pump could be pulling air in from a small leak on the system.

Happy to hear other suggestions.
Click to expand...

Tenant didn't mention any leaks and I would have said that the amount of times the rads need venting would eliminate the corrosion. But it has to be coming from somewhere. Has been a week since I went and not been called back, so will see.
 
Combined 22mm vent and feed or 'wet vent' is something i have been installing for years with on problems
 
combined feed and vent might be common but is it correct to pipe up a system like that? i was taught expansion feed sperate within 6 inchs of each other behind the pump. most systems i go to that have problems are either the combined f+e or thrown in anywhere
 
A combined feed and vent is a recognised way of piping especially for low head installations. It cannot draw air (or pump over for that matter) if piped that way.
Many older badly piped systems have been changed to combined F&E to stop them drawing air. Usually after some numpty has changed a pump and set the speed too high because they don't know any better.
A low water content boiler fitted to an existing system that worked perfect with the feed teed off the return will cause problems unless the position of the feed is altered. This is because the old cast boiler acted like a neutraliser and slowed the speed of the water which the new one won't do.
Here is a drawing from a baxi HE
untitled2.jpg
Note how they recommend the use of an air seperator (basically a section of bigger pipe) to slow the water as it passes.

Arran197
Continual venting of rads encourages more corrosion as oxygen (through the fresh water) is constantly being introduced to the system. Without getting too technical, this oxygen reacts with the steel parts of the system (rusts) which produces magnetite (sludge) and as the oxygen is taken from the water, in the process, leaves hydrogen which is more than likely what is being vented.
Smell it or put a match to it (at your own risk, just make sure the curtains are well away). It burns with a near invisible flame.
 
Hi. The combined cools feed and vent was first introduced by the gas board.
Probably to reduce the amount of call back on the large number of gas installation in the late 60is and early 70is which were carried out by contractors. Reducing the need for good design.
It was frowned upon at the time as solid fuel installation were common. But it was often used on solid fuel systems as a result of acceptance on gas. Which often created dangerous situations during power cuts etc.
 
Done the old trick of holding a jam jar upside down over the bleed valve and then lighting the hydrogen collected. The resulting POP has em running fer cover screaming " ok how quick can you sort it out?" One powerflush coming up thankyou.
 
I took combined feed and vent out this week as I was struggling to remove air from system drain down. My theory is venting will be harder as feed water is pushing down the same pipe. Fit vent then feed preferably where pipes leave boiler allowing boiler to vent a little easier, although the worcester ri has a neat little venting system on heat exchanger, bonus!
 
to be honest i prefer to install combined feed and vent, and when i go to a system were the 15 feed is blocked i always cut out and turn the vent into a 22mm combined feed and vent, quickest way and stops blockages again in future at the "negative point" or "neutral point" or what ever they call it at college :)
 
WaterTight said:
Quite possible vent combined with feed in an attempt to prevent the problem occuring. My old boss did that once, don't know if it worked - stopped working for him shortly afterwards.
Click to expand...

this was common, usually due to poor installations, check were feed vent and pump are in relation to each other, i suspect the vent is under negative pressure when it kicks in
 
tamz said:
A combined feed and vent is a recognised way of piping especially for low head installations. It cannot draw air (or pump over for that matter) if piped that way.
Many older badly piped systems have been changed to combined F&E to stop them drawing air. Usually after some numpty has changed a pump and set the speed too high because they don't know any better.
A low water content boiler fitted to an existing system that worked perfect with the feed teed off the return will cause problems unless the position of the feed is altered. This is because the old cast boiler acted like a neutraliser and slowed the speed of the water which the new one won't do.
Here is a drawing from a baxi HE
View attachment 3644
Note how they recommend the use of an air seperator (basically a section of bigger pipe) to slow the water as it passes.

Arran197
Continual venting of rads encourages more corrosion as oxygen (through the fresh water) is constantly being introduced to the system. Without getting too technical, this oxygen reacts with the steel parts of the system (rusts) which produces magnetite (sludge) and as the oxygen is taken from the water, in the process, leaves hydrogen which is more than likely what is being vented.
Smell it or put a match to it (at your own risk, just make sure the curtains are well away). It burns with a near invisible flame.
Click to expand...

good post tamz, on a slightly different note, i dont like the air seperators, they were v popular a decade ago but fell out of favour thankfully, i found they made the pump noisy, and didnt work as they should, ever seen them fitter upside down! depending on type. Some boilers allow ombined feed and vent, should only really be done if man instructions say
 
This system had no air separator fitted. Could this be the problem? Surely if you are venting the system of the 'air' or hydrogen, you are actually replacing it with water and not air from atmosphere.
 
arran197 said:
This system had no air separator fitted. Could this be the problem? Surely if you are venting the system of the 'air' or hydrogen, you are actually replacing it with water and not air from atmosphere.
Click to expand...

no the problem will be the position of the feed and vent in relation to the pump
 
Any open vent system has to have a feed and or vent regardless of where it is connected ,
I am posting due to recent (past 18 months) having installed a good few wood burners combined with mains pressure hot water cylinders / solar / oil / heat pump.

There is no hard and fast way other than manufacturers instructions which some times are not suitable for the instalation.

ps. back when they were intsalled there were no standards
 
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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!
 
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
 
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.
 
Can you explain further about the gas board introducing the cold and feed pipe into one.
 
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?
 
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