Central heating pump

[Killo123]

Hi new on here.
Am slightly confused with a water pump on central heating so need help.
If my central heating is off and I call for hot water only the pump starts up.
Someone has told me that is wrong as the pump should only come on when asking for heating to radiators.
Could someone please advise.
Cheers

 

[Mottie]

Unless you have gravity hot water, it will come on to pump hot water through the coil in the hot water cylinder.

 

[Hugh Jaleak]

Fully pumped system has pumped CH and HW. Flow from pump is controlled by Motorised valves. The old Gravity HW systems had large 1" primaries from boiler to cylinder, HW cylinder used gravity circulation, when CH was required the pump started up.

 

[Killo123]

Lottie and Hugh
Thanks for info and speed of your reply.
Helps to understand how it works.

 

[Hugh Jaleak]

Basically the early central heating systems were large bore pipework, and worked on gravity circulation. As things became more advanced, it was found smaller bore pipework could be used for heating, if a pump was fitted. HW still remained on larger bore pipework relying on gravity for circulation, worked well but little control over HW temp. To overcome this some fitted a motorised valve in the return pipe from the cylinder, coupled to a cylinder stat, to close the valve and stop circulation when the cylinder was up to temp.

It was then realised if the pump was used to circulate water though both HW and CH circuits, with the use of stats and motorised valves, much more control could be given. Modern cylinders with better insulation would keep water hotter for longer and newer, lighter, smaller, low water content boilers came about, making systems more efficient.

Still no substitute for good insulation though, the less heat lost, the less needed to put back to maintain room temperatures.

 

[CBW]

Also pumps are used in combi boilers, which again kicks in for either hot water or heating.