Changing a shower mixer valve.

[Auto Addict]

Set up is hot and cold water tanks + pump.

Do I drain down the hot and cold water tanks and isolate the pump?

[Sir Tigger KC]

Do you have isolator valves in the pipes just before the mixer valves?

[Doctor Gollum]

Do you not have a shut off valve on both tanks 

Too slow... Must be on Spanish time

[Auto Addict]

Do you not have a shut off valve on both tanks 

Too slow... Must be on Spanish time

I think so, but what about the pump?

[Sir Tigger KC]

Yes I'd have thought that there must be isolator valves somewhere, so he doesn't have to drain the system. 

Ideally, to the shower so he can carry on using the water elsewhere. 

[Sir Tigger KC]

Do you not have a shut off valve on both tanks 

Too slow... Must be on Spanish time

I think so, but what about the pump?

It probably has a fuse somewhere that you can pull. 

[Doctor Gollum]

Do you not have a shut off valve on both tanks 

Too slow... Must be on Spanish time

I think so, but what about the pump?

It probably has a fuse somewhere that you can pull. 

It certainly should have some form of disconnect/breaker attached to the circuit.

[Sir Tigger KC]

Is it a shower unit or shower over the bath. 

If it's over the bath, remove the panel and you might find isolator valves under there along with the pump.

[Auto Addict]

Is it a shower unit or shower over the bath. 

If it's over the bath, remove the panel and you might find isolator valves under there along with the pump.

No it's a separate shower.

There is a valve to stop the cold water tank draining and a valve to shut off the hot water tank to the taps, I think.

[Auto Addict]

Might have to get a plumber in, getting too old for this.

[Auto Addict]

'Kin hell, rung around a few plumbers - they all want an £80 call out charge before they start!

[Doctor Gollum]

That's the trade off. Although the plumber at the end of the road can service our boiler for that...

[Viral_Jim]

Might have to get a plumber in, getting too old for this.

Most shower pumps, if installed correctly should be connected using flexi hoses (to reduce vibration and noise) like the ones I have linked below. The ones I've had experience of have integrated shut off valves that you can operate with a flat blade screwdriver (again, pictured on the one I linked). If you turn off these valves and power down the pump you should be able to attack the job without touching the tank(s). 

https://www.screwfix.com/p/salamander-pumps-15mm-x-3-4-straight-anti-vibration-coupler/4677p

Fair warning though, if its been installed a while, the twisting of the valve to the off position had been known to cause them to drip, meaning you would have to drain down the tank. But one step at a time hey! 

[YZ250]

You SHOULD have an isolator on both hot and cold feed's BEFORE the pump and a source of electrical isolation for the pump, like a switched fuse. This would then allow you to work on/replace the pump/shower without restricting the water to bath/basin/ toilet etc.
There should be no need to drain down if it's installed as above. 

[Allenm]

If you are only changing the valve, you shouldn't need to be worried, just remove electricity from the pump, isolate the water and change it.
20 minute job if the entry points are the same on the new valve.

even if the valves don't quite close properly, removing the power from the pump will stop it turning on, and you will get a dribble of water at worst, but as it is in a shower, who cares!