So I turn on the hot water tap and cold water comes out, as I'd expect. I wait a while. Just before the hot water makes its way through I get a few volent spurts of air. This happens from all taps.
Any ideas? - Standard open vented system for hot water only.
You ran the bath and basin taps simultaneously, and loft tank couldn't refill quite quickly enough and some air got into the pipes?
Owain
There are air bubbles collecting in the top of the HW cylinder. The air comes out of solution when the water get fairly hot. It is quite likely that you have an older system which can't control the HW temperature. With the cold snap in the weather the heating is now on for extended periods and the HW cylinder is getting quite hot.
If the radiators are working OK you could try turning the boiler down a bit. Note that a common failure mode of the thermostats on older boilers is to become gradually less sensitive causing the boiler temperature to get hotter over the years.
Alternatively there is a restriction in the cold feed into the HW cylinder and as you draw HW off air is drawn down the vent pipe into the HW pipes. This is less likely and does exactly fit your symptoms.
oops missing 'not' ... exactly fit your symptoms...
The draw off connection at the top of the HW cylinder does not rise vertically into the base of vent pipe, with the feed T'd into that vertical section. When you heat water the disolved air comes out and collects in the pipe work/top of the cylinder, when you turn on a tap the air is forced along the pipe work to the tap.
| Bodgit wrote: | > So I turn on the hot water tap and cold water comes out, as I'd expect. | > I wait a while. Just before the hot water makes its way through I get a | > few volent spurts of air. This happens from all taps. | > Any ideas? - Standard open vented system for hot water only. | | You ran the bath and basin taps simultaneously, and loft tank couldn't | refill quite quickly enough and some air got into the pipes?
Tap water contains dissolved air, which comes out under some circumstances. Those who have a combi will note that the hot water comes out milky at times. This milkiness is air and clears very quickly. Not all components of air dissolve equally easily in water, IIRC carbon dioxide dissolves most.
What about a faulty cyclinder thermostat?
MBQ
This sounds the most likely. There are high ceilings in this part of the house, and for some bizarre reason a false ceiling has been built in the hallway about 4'6" lower than the original ceiling. The hot water cylinder is mounted on its side in the void, and all the pipework has been squeezed into this area. I haven't looked recently but I don't remember any pipes rising directly from the cylinder, and there are certainly some that run horizontally above it. Sounds like this is the cause.
Thanks everyone for your suggestions.
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