Hiss on central heating - part 2

[lvegas2009]

Have got a hiss coming from my central heating/hot water circuit, and trying to isolate the cause. I feel pretty sure it is to do with the hot water, but could be introduced by the boiler.

If I turn the mains water off, the hiss goes.
I have a tap on the cold water circuit just before the boiler (to isolate if neccessary). If the water is switched off here, the hiss disappears. I can stil run cold water rest of house, but obviously cannot get hot using this method.

If the stopcocks are open, and I run hot water from any tap, the hiss will disappear for up to a few minutes, and then starts again. Is this the hot water demand relieving the pressure off a leak for a few minutes?

My suspicon is
1) something in boiler is bleeding air (worcester 30cdi)
2) there is a leak on the hot water circuit. If it is, it does not appear to be upstairs as no damp marks on celing.

Unfortuntately, I think mains water and hot water are under the kitchen in concrete, under cupboards etc. Before I start pulling things about, is there a way I can help isolate whether cause is the boiler or a leak?

Thanks in advance

 

[stevetheplumber]

sounds like you have a leak on the hot water pipework or possibly in the boiler can you get a valve cut in to the hot water pipes after the boiler to eliminate the boiler being the problem my bet would be on pipework below floor as its not showing from upstairs