thermostat in hot water tank stopping pump

periodically when the hot water is timed to go off we get a loud bang. We got our maintainers and they noted that when the thermostat on the hot water tank determines that water has reached desired temperature it switches the pump off. If the central heating is on the pump switches itself off and then comes straight back on again. They informed me that the wiring must have been wrong from when it was first installed as the pump should remain on even if hot water tank is warm enough. They will not fix it under maintenance and advise me to contact the housebuilders (house 5 year old, Is the behavior i am experiencing with the thermostat/pump normal and could it be causing the loud noise.

Assuming you have a Y-plan system then this sounds normal behaviour apart from the bang. So where does the big bang come from?

What's your **** boiler pretty please !

If the CH is off and the HW is satisfied the pump should go off, possibly after an overrun period.

SOme maintenance contract! Who is it??

It is an ideal classic 230. It is maintained by British Gas, but since turning thremostat down in hot water tank shuts off the pump (allbeit briefly if heating on) they say it has been wired up wrong and will do nothing. I'm going to look damn silly if the housebuilders send someone and it is wired up corectly. Guess I will have have to trust BG`s view as I know nothing better. Though if noise still appears or pump should work like this I will be straight back onto them.

If your system is a Y-plan (has a mid position 3-port motorised valve) then, as ArtfulBodger mentioned, it is normal for the boiler and pump to shut off when the cylinder stat stops calling for heat.

While there is a requirement for hot water, the boiler/pump are powered directly from the cylinder stat. When the cylinder stat reaches its set temperature, it switches the power from the boiler/pump to the motor of the 3-port valve. This causes the valve to move to the central-heating-only position. As the valve reaches this position it operates a micro-switch that connects the output from the room thermostat to the boiler/pump. Hence, while the valve is moving the boiler/pump are briefly not powered.

If the bang is from the boiler, and occurs as the main burner ignites (explosive ignition), then the usual cause is the pilot light assembly - it is partially blocked (resulting in a pilot flame that is too small) or it is badly positioned (relative to the main burner).