Boiler overflow leaking but no loss of pressure

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 due to the hot summer. It drips about once every 20 seconds, and you get a very small puddle on the floor about 10cms diameter on the patio. Hardly anything noticeable.

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 wouldn't a faulty expansion tank 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?

The boiler is set to 1.5 bar and when the system's on it creeps up to about 1.7 bar (in the winter months perhaps as high as 1.8/1.9 bar when the heating is on full whack) and returns back down to 1.5 bar when cold. Never drops or rises when not on, and it doesn't overflow when the boiler isn't on either.

 

That's right.

I don't know about the overflow pipe. It comes out directly behind the boiler cupboard and the water tank is two floors up in the loft, quite a way away, so shouldn't think it's connected but not sure.

Noob question - what's a cylinder blow off?

 

Wait, no, sorry, it's vented. I have a storage tank upstairs next to the hot water cylinder, which is next to the expansion tank.

Sorry, misread that earlier.

(And we had a storage tank overflow problem about a year ago, which we solved with a ball valve, and that was a different pipe (plastic I think it was). This is just the copper pipe that comes out of the back of the boiler cupboard.

 

That's a different pipe (see bottom of above post) so it can't be that.

Everything would seem to suggest that it's either the expansion tank or the valve, but I can't work out why I'm not losing any pressure at all.

 

If there was air in the system, could that push a bit of water by? Or does it not work like that?

The pipes around the boiler are quite noisy when it's on and the cupboard door is open. Whooshing noises as though it's sucking water around (I know it probably is, but never heard this noise before on another boiler) and you can hear bubbling (faintly) from within the pipes.

 

That's correct mate.

Boiler in cupboard downstairs with pump and one of those filter things above it (It was powerflushed last year and had it fitted then)

Copper cylinder in the loft alongside the storage tank for the water

A smallish red expansion tank (I'd say perhaps three times the size of a kettle, maybe not quite that big) sits beside the copper cylinder

Next to the boiler is the tap/filling for the boiler pressure. This is set to 1.5 bar. This setting has, since we moved in two years ago, often gone up when the system has been on a long time to about 1.9 bar, and for just the hot water can creep up to 1.7 bar at times. It doesn't drop below 1.5 bar though and has been at 1.5 bar for at least the last three months, which is the last time we had the system drained down to have three radiators replaced with newer models.

I have a pipe coming out of the fascia that is the overflow from the storage tank (and it's not that that's leaking)

The pipe that is leaking is a copper pipe coming out of the back of the boiler cupboard, which I'm told is the overflow pipe for the boiler/heating/hot water. This is the pipe that is dripping. As I say though, only about one drip every 30-40 seconds after the boiler has been heating the hot water for about 20 minutes.

Another thing I thought was, could it be the valve is just starting to go, and when it gets hot (i.e. the pipes get hot with the hot water) it is allowing a little bit of water by? Or the same I suppose could be said for the expansion tank.

I just don't get why I'm not losing pressure.

 

That's exactly it. Copper vessel (with green insulation), cold water storage tank (quite large) and a small kettle that's the expansion tank/pressure tank.

I can't get pictures at the moment as there's a pile of boxes between the loft hatch and them as we've stuck stuff up there while we decorate a room. I don't fancy moving all that until I really have to.

 

That's the bit that gets me - why no loss of pressure.

Filling valve appears to be turned off. I've quickly turned it on and off again, and there's no noise of it filling up at all (even a very small turn of it causes quite a noise and a sudden rush of water through) and I think that would leak all the time would it not (and also cause the pressure to rise while not in use).

If it was losing pressure I wouldn't even be trying to second guess.

I've just turned the boiler temperature down this morning - it was still up near maximum after the winter (no need for it to be that high now) and although I was home later, the patio seemed dry...I'll check again in the morning but wonder if it's linked?

 

I turned the boiler down more last night and checked this morning...still doing it.

So it sounds more like the expansion vessel then (which I've been calling expansion tank by mistake). A friends dad, a plumber (who unfortunately is rammed and can't make it round to fix it) thinks that it might just need recharging with pressure, but I've no idea how to do that. Wondering if just paying the £120 to get the two new parts fitted is just going to be the best bet. I suppose they will at least be two new parts that should last a lot longer than what we currently have.