1st post. I have a gas C/H with a gravity feed to the hot water secondary circuit. Several times a day I hear bubbling/gurguling sounds (that may coincide with the room thermostat demanding heat). Checking the C/H header tank in the loft, I can feel that the tank is hot. There is a pressure relief pipe from the feed to the secondary H/W circuit going to the loft "up and over" the C/H tank that hangs over the top of tank. The pipe is open ended and several inches above the water level. I assume the header tank water is hot due to water surging up the relief pipe. Is this anything to worry about? As a secondary question, if you don't mind, several years ago I had a tall radiator fitted in the bathroom replacing a short rad that was connected to the secondary H/W heater circuit. As resulted with the feed to the top of the rad connected to the secondary circuit in the loft space (but is below the bottom of the header). This connection point is the same pipe as the "up and open" pressure relief pipe. The bathroom rad never gets hot throughout, with just the top section getting hot. This seems to indicate the rad flow is intermittent and top-down. On a gravity fed H/W system should the heat supply enter at the top or bottom of the secondary circuit. Thanks.