The red button underneath our boiler keeps popping out when i have a shower and the water runs cold. It happens a lot for a while then works ok for a bit. I keep getting an engineer out and it works when they see it so they dont believe there is a problem!
the h/water heat exchanger may be becoming blocked or if the problem only occurs with the shower then the mixer valve or shower head isn't allowing enough h/water through try removing shower head and let water run through hose if boiler is ok then head needs cleaning or replacing if not mixer valve needs cleaning or repairing
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix plumbing.