I recently had the hot water cylinder replaced in an indirect gravity fed system. At first it didn't work due to a massive air lock, but when I cracked open the nut on the flow inlet it all started working.
The hot water pipe from the boiler comes up to a T junction.. one way goes up the vent, the other went horizontal into the old cylinder. because the entry point is a few inches higher on the new cylinder, a couple of 90 degree bends have been put in. I have been finding the return pipe getting steadily colder as the days go by, so I crack open the nut again on the inlet pipe - some air spurts out, and then the return pipe goes immediately hot again.
I've been doing this about once a week in the 5 weeks since the new cylinder... is this just residual air in the system from when it was replaced and eventually there'll be none left, or is air getting in? on the old system, due to that T junction, any air building up there would have been able to get up the vent but with the new raised bit it's stuck.
Also, when everything is switched off, if I place my ear to the top of the cylinder i can hear a water trickling noise every few seconds. As no water is moving about when everything is switched off, what could this noise be?