Central Heating expansion tank overflowing

Rdwill

Hi All

I have just fired up my central heating for an hour to give it a mid summer test / run out.

I notice that when the central heating is running the overflow pipe that sticks out of the underside of the roof is dripping / almost running. 

I have stopped the central heating and the leak from the pipe has also now stopped.

Can someone diagnose the problem for me please?

For info, I usually use the immersion for heating the water as I have solar panels and a immersun gizmo, so this is the first time I've run the boiler since about April.

Thanks in advance.

NSG666

There will be a small tank in the loft (feed and expansion tank) and the ball valve is failing and has let too much water into the tank. When you turn the heating on the water in the system expands and the water level in the tank rises. If the water in the tank is too high to start with then it will reach the overflow which is what you are seeing.

Either call a plumber or if you want to fix it yourself you need to check whether it's a part 1 or part 2 float valve then buy a new one from e.g. Screwfix and replace it. Don't be tempted to try and change the washer, it rarely works and a new valve and float is less than a tenner.

Rdwill

That's really helpful, thank you. I'm sure I've changed a ball valve before.

I was worried it was something to do with the cylinder or radiator system.

I will get up there and have a look.

Jeepers_Creepers

You go up there with a torch/inspection lamp, and someone else turns on the heating.

Note the water level in BOTH tanks before you start. There is an overflow pipe exiting a few inches down from the top edge, and the water level should ideally be a few inches lower than this to allow room for expansion as NSG says.

(a) see what the water levels are like before you start, and (b) monitor them both with the boiler turned on.

EssexExile

Ours overflowed when the pipe feeding the water from the tank down to the system got blocked, another possibility.

Section62

Pumping over could cause enough splashing in the F+E tank to be another reason for an overflow drip.

On a wrongly configured system a sticky valve or something after a period of non-use could be a trigger for pumping over when it hasn't previously been a problem.

An inspection of the F+E tank when the system is running (and the overflow dripping) will be a must to figure out the actual problem.

Rdwill

I bled all the radiators, there was quite a bit of air in one and ran for an hour. 

The problem did not repeat.

So fingers crossed I think its cured itself.

I'll give it another hour later just to confirm.

Thanks all.

Jeepers_Creepers

Blimey - didn't see that coming.

Pleased it's sorted. Worth checking your rads again in a few weeks to see if air has reappeared. If it has, it may also be worth checking whether it is air, or hydrogen. The latter would signify internal corrosion.

On which point, also worth adding a dose of inhibitor to the F&E tank - it should work its way into the system in use.

NSG666

Rdwill said:

I bled all the radiators, there was quite a bit of air in one and ran for an hour. 

The problem did not repeat.

So fingers crossed I think its cured itself.

I'll give it another hour later just to confirm.

Thanks all.

Sorry to be a killjoy but I don't think that you will have cured the problem causing the original symptoms but bleeding the radiators certainly needed doing.

In the feed and expansion (f&e) tank, when the water in the system is cold it will be at a low level and when it is hot it will be at a higher level. Under normal circumstances at the higher level the water never reaches the overflow.

The float valve is used to add water to maintain the minimum level when cold but when it starts to fail it allows the cold water level to rise. You do not notice anything at this point but when the system gets hot and the water expands, because it's starting point was higher it reaches the overflow and you hopefully spot it. Were it cooler weather and you were using the heating twice per day then you would see water coming out of the overflow twice per day as the cold water in the system heats up.

By bleeding the radiators you have lowered the level of the cold water in the f&e tank so it probably won't overflow for a day or two (possibly longer) but the faulty float valve will eventually allow too much water in and it will reach the overflow again.

grumbler

NSG666 said:

Rdwill said:

I bled all the radiators, there was quite a bit of air in one and ran for an hour. 

The problem did not repeat.

So fingers crossed I think its cured itself.

I'll give it another hour later just to confirm.

Thanks all.

Sorry to be a killjoy but I don't think that you will have cured the problem causing the original symptoms but bleeding the radiators certainly needed doing.

...

By bleeding the radiators you have lowered the level of the cold water in the f&e tank so it probably won't overflow for a day or two (possibly longer) but the faulty float valve will eventually allow too much water in and it will reach the overflow again.

Well, when heated, air expands much more than water, so this could well be the problem. When cold, it contracts and ball valve tops the tank up. Then, when hot, it expands and the tank overflows. As simple as that.

Jeepers_Creepers

grumbler said:

NSG666 said:

Rdwill said:

I bled all the radiators, there was quite a bit of air in one and ran for an hour. 

The problem did not repeat.

So fingers crossed I think its cured itself.

I'll give it another hour later just to confirm.

Thanks all.

Sorry to be a killjoy but I don't think that you will have cured the problem causing the original symptoms but bleeding the radiators certainly needed doing.

...

By bleeding the radiators you have lowered the level of the cold water in the f&e tank so it probably won't overflow for a day or two (possibly longer) but the faulty float valve will eventually allow too much water in and it will reach the overflow again.

Well, when heated, air expands much more than water, so this could well be the problem. When cold, it contracts and ball valve tops the tank up. Then, when hot, it expands and the tank overflows. As simple as that.

Ooh - good point :-) Neffer thought of that.