Sealed CH System - apparent airlocks in boiler

[DavyS]

I have 7 year-old Potterton Kingfisher CFL 100 boiler operating a sealed room heating and hot water system.
For years, air in the system used to accumulate in the upstairs radiator nearest to the boiler. Simple to vent.
About 6 months ago, air stopped accumulating in any radiator and the pump could not start the circulation going unless switched to its fastest speed.
When the pump is first switched to its fastest speed then I hear a gurgling in the boiler, followed by a swishing noise (air bubbles?) in the pump and the circulation starts and things are fine for the rest of the day. Ditto noises if I open the hot water primary feed zone valve.
But the problem is back the next morning.
There is no sign of any water leaks
There are no air bleed valves except an automatic one at the highest point of the hot water primary feed.
The system pressure is 1.5Bar and constant.

I am completely baffled; any ideas?

 

[Armo74]

If the pump is struggling there is more than likely a build up of magnatite restricting the flow and what you are venting could be hydrogen which is a bi-product of magnatite which is usually more associated with open vented systems.When did you last clean and flush your system and add inhibitor?

 

[45yearsagasman]

With a system pressure of 1.5bar it is not possible for air to get in your system from outside. Have you checked for hydrogen build up?
When venting a rad, collect the gas in a inverted tin and apply a match.

 

[DavyS]

If the pump is struggling there is more than likely a build up of magnatite restricting the flow and what you are venting could be hydrogen which is a bi-product of magnatite which is usually more associated with open vented systems.

But surely I could not have sufficient hydrogen build up to 'air lock' the boiler every 24 hours???
And once the circulation starts where is the gas going? - it must get trapped in a radiator but I dont have gas in the radiators.

 

[petertheplumber12]

I have 7 year-old Potterton Kingfisher CFL 100 boiler operating a sealed room heating and hot water system.
For years, air in the system used to accumulate in the upstairs radiator nearest to the boiler. Simple to vent.
About 6 months ago, air stopped accumulating in any radiator and the pump could not start the circulation going unless switched to its fastest speed.
When the pump is first switched to its fastest speed then I hear a gurgling in the boiler, followed by a swishing noise (air bubbles?) in the pump and the circulation starts and things are fine for the rest of the day. Ditto noises if I open the hot water primary feed zone valve.
But the problem is back the next morning.
There is no sign of any water leaks
There are no air bleed valves except an automatic one at the highest point of the hot water primary feed.
The system pressure is 1.5Bar and constant.

I am completely baffled; any ideas?

You are getting a build up of air because you have an automatic air vent for the hot water but you need one for the heating circuit since that is not using the hot water AAV.That is why the air builds up.

 

[DavyS]

Thanks everyone for your suggestions.
I have now come to the conclusion the problem is probably not an air lock (cos when the circulation starts it would just move the air to a radiator which it doesnt) but the opposite - a blockage at the bottom of the boiler caused by sludge.
So I intend to completely shut the system down (especially the zone valves), attach a hose to the drain tap at the bottom of the boiler and then turn the filling loop on and thus flush out the boiler with mains pressure.
Anyone see any problems with this?

 

[petertheplumber12]

Just remember to think about the way the water flows through the boiler, and then make sure that the water that is pushing the sludge is going in the opposite direction to it natural flow so the sludge is sent back into the system and can the be drained out.

 

[Onetap]

Anyone see any problems with this?

What could possibly go wrong?

Well, it's mains pressure, so if you manage shut the outlet route, the system will be subjected to mains (high) pressure. You may have rusted, weakened radiators, that are full of black sludge and that won't take a high pressure. Please tell us that you have new white/oatmeal coloured carpets, to complete the set.

Second, the filling loop usually has quite a narrow restricted bore, since it doesn't have to provide much flow rate. You really need a high flow rate and you need to close radiator valves so that the maximum flow goes through one radiator at a time (se above re shutting all outlet valves).

If I were doing it, I'd use a hose connected to a full-bore ball valve, via a pressure reducing valve.

Besides which, I'm not convinced that the problem is due to sludge, but that's your opinion and I'm in no position to question it.