Central Heating Pump Comes On When Switched off

[WSB]

I have switched off the central heating at the controller (set it to OFF) and drained the central heating system down.

NB : My water heating as always is set to continuous.

After some time, I heard the Grundfos central heating pump come on and make a whirring sound (presumably as it is running dry).
I immediately switched off the hot water at the controller too and the pump stopped too.

I'm pretty confused here as I thought the central heating pump could only come on when the central heating was switched on at the controller and would have no reason to come on for hot water heating at all?

Ideas anyone?

 

[Justin Passing]

no reason to come on for hot water heating at all?

Most heating systems installed in the last 30+ years are "fully pumped".
If you can tell us what motorised vales you have or put a picture up that would help.

Ideas?? Turn the mains supply to the whole boiler/heating system off while it has no water in it!

 

[WSB]

Thanks Justin,

Am I right in saying that when the boiler switches on for hot water heating, it uses the central heating pump to pump the water from the boiler, through the coiler in the cylinder and back to the boiler again?

My understanding was that the pump is for pumping water to the rads only.

If it is used to pump it from the boiler to the cylinder coil, then that makes complete sense.

It would mean though that it wouldn't be possible to have my central heating system drained and still be able to heat water, as the pump would be required for the latter. No want I want, with a drained system, as that would mean the pump running dry.

Am I right?

 

[Justin Passing]

Yes, you're right.
Google "Y plan" and "S plan" for descriptions of what you may have.

See if you have an electric immersion heater in your HW cylinder. Usually at the top with a thick cable coming away to a switch, if you're lucky.
Then you can heat your hot water (fairly slowly).

 

[WSB]

Thanks Justin!
You were a great help. The penny has dropped now and I've learned something new.
All good stuff except when my missus gets in, I'll get an ear bashing.
Got the immersion on now, to try and appease her
Thanks again.