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[I_want_an_Omega]

Ok, conventional CH & DHW system.

There is a problem with one of the motorised valves being a bit sticky, so will bite the bullet and change both - and the pump as well as they are all 15 years old and access isn't great.

So, my question - my current Grundfos pump is simple on/off, but new ones seem to be "self modulating" and determine the flow rate themselves. They (appear) not to have any other control inputs, so how to they determine what rate to pump at? Is it done by water temp?

Any ideas from those in the know?

Thanks - Rob

[Kevin Wood]

I'm guessing it works by pressure difference, i.e. as the radiator valves shut down there is more resistance to flow, more pressure difference across the pump and the pump detects this and slows down. It can't work by temperature as in some systems the pump is in the "flow" pipework from the boiler where the temperature will be almost constant.

I've no experience with them, but I think I'd probably go for one of their UPS models that has a switchable 3 speed manual control rather than the more smart pumps. This way you can manually set the speed to suit the system, it's less complex with less to go wrong, and I'd worry that the smart pumps fire up at full speed first thing every morning, making a load of noise, and there's nothing you can do to adjust them. That said, as I say, I haven't experienced one myself.

[Marks DTM Calib]

I have run a Wilo smart pump for some years( 6 ish at a guess) and no issues at all, it just works (and pretty well to boot).

They detect the back EMF of the motor to determine the load and adjust accordingly.

Be aware though that they dont work well on S plan setups unless you either have a towel radiator always running with no thermostatic valve fitted.