Combi start up problem

[wiggs]

When mains cold is used, the combi fires up for about 10 secs.
I could understand this if the hot water demand was controlled with a
pressure sensor but I think they use a flow sensor.
What do you reckon is happening please?

 

[Paul Barker]

common as muck. Don't fire, just start fan. Check valve on cold feed might fix it, but will make water delivery noisy. One of the many inconveniences of a combi.