boiler keeps getting air locks, why?

[dave144]

I've just moved into a house with a Baxi Boston floor standing open flue boiler. Ive only been in the house a month and have lost all hot water twice due to air locks in the cylinder. Any advice as to how and why this keeps happening? A family relative who is a plumber has suggested pressurising the cylinder to do away with the cold water feed tank.

I've always had combi's so this thing is alien to me. Maybe time to invest in a combi system?

[V8covin]

I'm certainly no plumber and don't know anything about your type of boiler but in my system my boiler kept shutting down due to air in the system that was caused by a faulty 3-way motorised valve.

[dirkgently]

If I understand correctly you are running out of hot water. The common problem is a blocked float valve on the cold water cistern. The problem probably occurs when you run two baths in quick succession, the cold cistern cannot fill fast enough to keep pace with demand and air is drawn into the cylinder cold feed,poorly installed pipework traps the air.

[Ferg]

As Dirk says, or are you talking about water being there, just not hot?

A pic of the pipework near the cylinder would be useful.

[dave144]

The water gets cooler each time it is demanded not always from running baths. The central heating gets red hot even though there is no hot water We get by with as we have an electric shower but is still less than ideal.

I have tried everything and I mean everything!!! Bled all the rads with the system shut down starting from bottom to top. Then putting the cold water feed from the washing machine onto the hot to push the air out..still nothing. Have tried twice to crack each union around the cylinder resulting in me getting soaked each time. I will try to get a few pics of the shoddy home built system tomorrow.

About mid way up the cylinder is the overflow with a bleed point, tried that too but yep you guessed it still nothing.
As an Engineer it pains me not to know how to fix something so simple. Got the wife's uncle whos a plumber coming round, so fingers crossed. Do you think pressurising the cylinder is a good idea?

[Ferg]

So the water is coming out of the hot tap fine, but not heating?
If it's not circulating it must be isolated somehow. Is this a fully pumped system, with motorised valves? Or a gravity hot water system? It may be a heavily sludged return.

[dave144]

It's a gravity fed hot water system. The hot water outlets pipes from the boiler get scalding hot but the airlock is causing the stat to trip out I think? Either way the hot water never reaches the tank.

[Deva Link]

From your descriptions it's hard to isolate whether you've got an issue with the gravity circulation from the boiler to the cylinder, or you've got a problem with the hot water supply to the taps.

The system should look like this:

[Ferg]

If you're getting water when you break the top coil connection then it isn't an airlock. Sludge, more like.

[dave144]

How the dickens do you remove the sludge? Thanks for the diagram. Sounds like when I was robbed by the last plumber all he did was get the sludge moving and now its settled again.

[Ferg]

You could put a sludge remover in which will mobilise it and then you can drain it out. Or powerflush... or cut pipework out.

[Deva Link]

dave144 said:

Then putting the cold water feed from the washing machine onto the hot to push the air out..still nothing.

I still don't get this bit - do you sometimes turn on the hot tap and no water at all (hot or cold) comes out?

[dave144]

I still get cold water from the hot tap when the darn thing plays up.

The idea of the hot and cold washing machine feeds is that the cold water is a higher pressure than the hot and "should" push the air lock up and out into the cylinder cold water tank. Lots of people swear it works, not convinced myself.

[Ferg]

That is to cure a different issue.

[Paul Drawmer]

What happens if you turn off the CH (by turning down the 'stat) and have the HW on? Test to see if the return pipe from the cylinder to the boiler warms up. If the boiler is firing but shutting down without the hot going to the cylinder, it's sludged up.

What size are the feed/return runs to the cylinder?

[christof.h]

put an automatic vent on the flow and return at your highest point

[Ferg]

Deva Link said:

I just noticed that the pipes come out of the bottom of that boiler!! Not a great idea on a gravity system!!!

[Deva Link]

Ferg said:

Deva Link said:

I just noticed that the pipes come out of the bottom of that boiler!! Not a great idea on a gravity system!!!

Next you'll be complaining that the feed and vent pipes are too far apart!

[Ferg]

Deva Link said:

Ferg said:

Deva Link said:

I just noticed that the pipes come out of the bottom of that boiler!! Not a great idea on a gravity system!!!

Next you'll be complaining that the feed and vent pipes are too far apart!

Nope. Not an issue unless it's fully pumped.

[dave144]

Well had the plumber out who's the wifes uncle and he put it down to a number of factors.
First was the fact that the cold water feed to the tank was not high enough above the cylinder. Second was the shocking DIY installation by who ever fitted the system way back, with some of the pipe fittings only hand tight. The pipes feeding the coil in the cylinder had limsescale deposits at the joint with the cylinder indicating a leak at some point.

He got us up and running again by "boiling off" the cylinder? didn't ask too many questions as I'm going to replace it with a combi now. The cost to pressurise and replace the cylinder isn't far off what the cost of a new boiler will cost, bearing in mind we'll get it at a "family price"