Air in Central Heating System - Faulty HW Cylinder?

[MalcolmD]

I have been getting lots of air in my central heating system and having to regularly bleed lots of air from some of the radiators.


I have been advised that the problem may well be a leak in the radiator system below the floor. Possibly a small leak drawing in air. The radiators themselves are all relatively new.


On 2 occasions I have added special leak repair liquid to no effect. I have been advised to replace the supply pipes.


HOWEVER. I have noticed that if I set the system (and 3 way valve) to central heating only (no water heating), then the problem goes away and there is no more air in the radiators


CAN ANYONE PLEASE SUGGEST WHAT MIGHT BE GOING ON? . Malcolm, Taunton, Somerset

 

[erj]

try bleeding the rads with the heating and hot water turned off at the clock

 

[MalcolmD]

Thanks for the reply. I have tried bleeding the system with the heating off but to no avail. I have also had the vent pipe to the header tank extended to avoid suck back of air into the system. ANY OTHER IDEAS PLEASE

 

[Sparkgap]

Location of pump? If the system is under negative pressure you could be drawing in air through rad vents or other places. Whereabouts in Taunton are you?

 

[MalcolmD]

Thanks. That's closer to what I am thinking. I'm in North Taunton. Corkscrew Lane. Its a bungalow. The boiler fairly new and so are the rads but some of the piping might date back to the 50's as far as I know.

Picture Below

 

[Best]

Tighten by hand (clockwise) that wee black screw plug on top of auto vent, just to rule that out first. Also, if you want to check if vent can draw air down, hold a cup of water up to end off vent pipe inside the small feed & expansion tank to see if it sucks.
And just mentioning that leak at bypass valve needs fixed.

 

[MalcolmD]

There are 2 more auto vents in the loft. One on the boiler supply and one on the boiler return. I have turned them all off at the moment while I try and evaluate the situation. I think that leak has been sorted out by one of the plumbers I have had in. Certainly can't feel any moisture there. I will try your cup suggestion and see what happens. Many thanks for your advice

 

[MalcolmD]

As you can see from pictures below the vent pipe has been extended. There is definitely a negative pressure at the vent when the pump comes on but I can't tell if it is enough to draw air all the way down to the bypass valve and cylinder output pipe just below it. What would be the next step in my investigations do you think?

 

[MalcolmD]

Pipe layout loft

 

[MalcolmD]

Just realised that vent pipe is on the supply side. Sorry.

 

[Best]

As you have found it only happens when hot water is being heated, then it must be linked with that. You are correct to turn the auto vents off in attic to test. Frankly usually just manual vents are all that is needed and they won't leak or let air into system.
Raising the vent pipe has been right, but that will not prevent problems with it if the pump and pipework is not correct setup

 

[Best]

I am finding the photos hard to figure out, but I think I am correct in saying it is piped wrong.
It could be piped by doing a close coupled method for the vent and feed pipe, by combining them in a specific way.
Give the heating a try with the auto vents screwed off in attic. Do check the vent isn't pumping up into expansion tank.

 

[MalcolmD]

It is hard to see isn't it. A spaghetti junction of piping. The vent to the supply tank is connected directly to the hot water feed from the boiler. The feed from the tank goes to the return pipe back to the boiler. I can't find any evidence that there is water coming into the tank from the vent but as I say there is negative air pressure at the vent. Have screwed the auto vents in the attic off to make sure they are not sucking air. Is it possible that the pressure differential across the pump is pulling air from the vent pipe down into the system. there are 3 circuits being pumped; the hot water cylinder; the bathroom radiator (normally open) and the remaining radiators

 

[Sparkgap]

Classic arrangement: open vent off the boiler flow, cold feed into the return. From the photo the pump is upside down and pumping downwards, not a good idea! Also reckon from what you say that pipes come off the boiler rise up (into attic? with AAVs?) then drop down again to pump and cylinder meaning quite a bit of it is under negative pressure when the pump is running and drawing in air.
One suggestion would be to resite the cold feed to a few inches from the vent beside the boiler and move the pump onto the flow after that so it pumps up into the attic (making a guess how the system is laid out). That would keep the whole lot under positive pressure and AAVs would work better.
Could drop by on me way back from work and have a look if you can't get it sorted.

 

[SmokeyJ]

Sounds like the cold feed and vent are incorrectly positioned. If I'm following, the vent shouldn't be on the cylinder flow after pump...
Typical cold feed and vent position -

 

[MalcolmD]

In my system the vent to the header tank and the feed from the header tank are on opposite sides of pump. The vent is under negative pressure of the upstream of the pump (possibly sucking in air) and feed is on the downstream side of the pump forcing some warm water up into the tank. Quite different from your diagram. I am most grateful for your help in this.

 

[MalcolmD]

In my system the vent to the header tank and the feed from the header tank are on opposite sides of pump. The vent is under negative pressure of the upstream of the pump (possibly sucking in air) and feed is on the downstream side of the pump forcing some warm water up into the tank. Quite different from your diagram. I am most grateful for your help in this.

 

[MalcolmD]

Interestingly, most diagrams on the net show the feed and vent upstream of the pump but there are some that show the feed on the opposite side like mine. Curious!

 

[Sparkgap]

Interestingly, most diagrams on the net show the feed and vent upstream of the pump but there are some that show the feed on the opposite side like mine. Curious!

Feed and vent positions were historically cold feed into bottom of or return to boiler and open vent off flow or top of boiler. This was was mainly for solid fuel boilers if they lost circulation for whatever reason and started boiling the steam would escape from the vent and cold water from the tank would flow in the bottom of the boiler. When early gravity systems were put in a lot had open vents off the high level pipes to above the level of the tank. When the first circulators (NOT pumps) came out and were used to boost the circulation they were put in the return to the boiler before the cold feed connection and as a result most of the systems were under slight negative pressure but usually there was enough head from the tank to prevent any air getting in.
Later, as pumps came in, pump head increased and pipes became smaller with fully pumped systems putting the pump on the flow became more common. There was a lot of faff in the 70s/80s when we were adding circuits to old gravity systems and in several cases the open vent was plugged off with an AAV and safety valve fitted to discharge over the f&e tank as the whole system was being pressurised by the pump!
Because we are not so worried these days about boilers making steam instead of hot water having the cold feed/vent close together off the flow on open vented systems is more common and with the pump on the flow means the whole system is pressurised and easier to vent.

 

[SmokeyJ]

Vent from cylinder flow and cold feed into return is the way it used to be done with 'gravity' hot water. If the system was upgraded from 'gravity' to 'fully pumped' the cold feed and vent should have been repositioned. It may be you have in the past had a heating upgrade and the cold feed and vent weren't repositioned. I'd get a heating engineer to have a look and alter if necessary.

 

[MalcolmD]

Thanks so much guys for your advice. Looks like I might finely solve this problem after living here several years. I have had 3 different engineers look at this and none of them spotted this problem. I have just turned the pump down to its lowest 'proportional' setting and that has helped enormously and almost fixed the problem. I will try and find a local heating engineer to move the feed upstream of the pump to join the vent.