Boiler overflow leaking but no loss of pressure

[kentmonkey]

Got an odd one here.

My boiler overflow pipe started leaking we estimate sometime around six weeks ago (we noticed it dripping three weeks ago with a bit of green limescale on it, now it's got more limescale).

It only leaks when the hot water comes on for 40 mins in the morning and in the evening, about 20-30 mins after coming on. Obviously no heating at the moment. It drips about once every 20 seconds, and you get a very small puddle on the floor about 10cms diameter on the patio.

However...no loss of pressure. It doesn't leak at any other time and we have no loss of pressure. Our plumber is going to change the valve and the expansion tank just to be sure, but I can't work out why we're not experiencing loss of pressure or, indeed, why the pressure gauge doesn't go shooting up when the boiler is on (as I would have thought a faulty expansion tank would do that).

The system was drained down earlier in the year for three new radiators, would the expansion tank have had to have been repressurised then, as nothing was done to it then?

It's going to cost about £120-130 to have the works done to replace the valve and expansion tank, which isn't terrible but not brilliant either (been expensive year) so just want to rule out anything else that might not cost anything before we have it done.

 

[kentmonkey]

Another thing...the pipes that supply the boiler sound like they have quite a lot of air in. When it's on you can hear very faint noise, almost like fizzing/bubbling, coming from the pipes.

Sure that's nothing to do with it but thought it might still be worth posting.

 

[petit_pablo]

What boiler is it?
What type of hot water cylinder do you have in the airing cupboard? Vented or unvented?

Its a bit presumptious of your plumber to be changing expensive parts just incase. Get a proper diagnosis.

 

[kentmonkey]

I'm assuming unvented. It's in the loft, and the expansion tank is next to it. How can I tell the difference?

He is a Gas Safe registered plumber who we have used before (and he's done a good job before), and I guess this is just his experience telling him it'll be one of those two items, but I can't understand why we're not getting any changes in pressure if it's either the expansion tank or the valve that's faulty.

And it really is just dripping ever so slightly.

EDIT: Ah, must be vented as we have a cold water tank upstairs, and our hot water isn't mains pressure.

 

[petit_pablo]

What is the actual boiler? Make and model.

If the PRV is dripping then it needs seeing to although I doubt the expansion needs changing, just recharging.
It's also possible your pressure guage is blocked, giving you a false reading.

 

[kentmonkey]

Pressure gauge goes up and down so it shouldn't be that. Filled to 1.5 bar, goes up to 1.7 bar when the hot water has been on for about half an hour.

I'll check the boiler make and model.

 

[kentmonkey]

petit_pablo, it's a Potterton 70l from what I can tell.

About 10-12 years old I think.

 

[Agile]

Seems your well thought of plumber is just a parts changer!

Better ones diagnose the problem and then do what is necessary.

Recharging the expansion vessel would be a good start for a non-partschanger!

Tony