Hello BigMac. Thanks for the suggestions, and apologies for the delay in responding. I decided I ought to try out your 3 steps while the system was misbehaving, and as per sod's law, it's been running perfectly since you posted your reply (well, it was still singing, albeit a little less vociforously, but all the rads were heating properly). Until today, that is. It's back to its tepid attempt to heat the house. I've checked the pump - no problems there. I was not able to stop the rotor. I've been into the loft, and the expansion tank had about 4 inches of water in it. My son bled one of the upstairs rads (no air, just water), and shortly after he started, the filler to the expansion tank started first to drip & then flow at about the same rate as it was coming out of the rad. After several minutes, the flow from the rad showed no sign of abating, and the filler pipe was still trickling into the expansion tank. He stopped bleeding the rad, and after a little while, the trickle from the filler pipe slowed & stopped. Any more ideas?

Hi Bigmac .. thanks for coming back so quickly. It's a Potterton Profile 80e, with a vent straight out of the wall behind it. The system is open vented, with a Grundfos Selectric pump. It looks to be on setting 2, but i've not tried turning it both ways to confirm that. I'd not bled any of the rads, because when they were hot, they were hot to the top. I've just run round now, bleeding them all. Got a tiny bit of air (and i mean tiny) from a couple, but the rest were straight water. Upstairs, the water ran clear, downstairs most but not all were a nasty black. The hot water was not getting enough heat to keep it hot, so had to put on the immersion heater to give it a boost. The boiler ran this evening from 8:41 to 8:49 when it stopped (the house is still cold). The singing appears to be coming from the boiler itself. Within a minute of the burn starting, the singing appeared, but in the house, it was only apparent in the bathroom (the boiler is in the garage). The singing stopped before the burn finished (during the last 3 minutes of the 8 mins of the burn, the singing was intermittent, mostly just the normal boiler sounds with just occasional flurries of song). The gas has been off for over 25 mins now, but the fan is still blowing air out of the vent (the exhaust gas is barely warm) - is this normal ? Update: it came on again at 9:47, stayed on for 2 minutes & then shut off (not because of the timer, or thermostat). The fan was on constantly from 8:40 to 10:00 (when i called it a night & went in to warm up in front of the gas fire).

Hi Bigmac, gashead, et al. While you're being helpful, perhaps you could have a think about my boiler (it's a 10yr old Potterton). Last summer, at the annual service, the BG engineer held a magnet to one of the copper pipes in the airing-cupboard to show me that I'd got iron sludge building up. He recommended a power flush, but we baulked at the cost (£600), and we didn't have any cool patches at the bottom of any of our radiators. Around about Christmas, our heating system started to sing to us. I couldn't find out where exactly it was singing ... seemed to be all over. Now it's decided to go on strike. The temperature in the house is quite cool (the thermostat clicks when I turn it down to about 13C, whereas it's currently set at 18C), the radiators are tepid, yet the pump is going, the boiler control is set to max, the boiler vent is venting, but only the pilot light is on (so the vent exhaust is cool). Is the BG man going to tell me that he told me so? Is what we're now experiencing a symptom of a bunged-up system, or is it more likely to be a control failure somewhere? Any thoughts?