Boiler hissing constantly - why?

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My combi boiler has been making a constant hissing noise for the past couple of days - similar to how it is when we are using hot water. Any ideas why? Is it a job for a plumber?

thank you

is it firing?

have you got a water meter? Is the bubble constantly turning?

go outside and look at the wall behind it. Is water coming out of the small, bent, copper pipe?

when was it last serviced, and by whom?

Gas engineers are not plumbers.

Hi - thanks for replying so quickly!

Yes it is firing, we don't have a water meter.

There is nothing coming out of the outlet bit outside, steam, water, nothing.

It was last serviced about 18 months ago I think - we are currently renting, so it wasn't arranged by us which is why my memory is a bit sketchy.

The timer/thermostat is an old Switchmaster 900, but the boiler is only about five years old or so.

Could it be the pressure - I guess it could do with another service.

thanks again

if it is firing when you are not drawing hot water from the tap, then probably either it is topping-up a hot-water store (some boilers have a small internal cylinder) or you have a leak. If you have not noticed it, it might be under the floor.

Observe the gas meter and see how much it goes up per day. In summer, about half a meter a day will be enough for showers and washing up.

p.s.

feel your radiators. Are any of them hot?

No hot radiators.

Sorry I didn't understand the bit about firing - it only fires up when the CH/water is on, not randomly between times. I thought you were trying to establish whether it was working as it should be.

I'll take a look at the meter to see how much gas we are using.

If it's a leak, it is likely to be 'just' costing us money, it wouldn't be dangerous? We do have a carbon monoxide detector that isn't going off or anything like that.

thank you

no, I think it would be a water leak. Gas leak is not impossible, but less likely.

carbon monoxide would only be if the flue was leaking, which would not cause a hiss.

look at your filling loop and verify that it is turned off at both ends and that the boiler pressure gauge is at the normal range.

will do - thank you

i have contacted our letting agents and it is due a service anyway it seems so i think it will be looked at in the next few days, which is reassuring.

thanks again for all your advice

Could be lime scale in the system particularly the element (ie the bit where the water gets boiled iyswim).

A kettling noise (like a singing kettle) due to limescale in the heat exchanger is not unususal, but is only heard when the boiler is firing, which does not seem to be the case here.

well just to update, it was a cold water leak, it was under the kitchen floor and the plumber has been here all day fixing it. pigletjohn - thank you