On 27/04/2017 16:16, Tricky Dicky wrote:
Used to get a similar problem at our old house after draining down the DHW due to partial air locks when refilling The simplest solution was to connect the washing machine hose cold feed to hot feed and give it a quick blast at mains pressure until water came out of the main tank overflow, problem solved.

Richard

When we had the cottage in Langthwaite we had that problem every time
the system was re filled after the winter drain down.

Simple cure was to block the outlet of the kitchen mixer tap with a
thumb whilst both taps were turned on thus forcing cold back up the hot
pipe. Wait for gurgly noise, turn off.

Mike