How to choose the right pump speed

[akist]

The pump is the UPS2 25-80 - it has six speed settings in two sets of three. I am not sure how to set the pump speed for best economy of gas.

Flow is set to 45C for now. I know that we want a low temp return to allow for most condensing.

I have some ideas but they are conflicting. Are there any rules of thumb to follow?

 

[DP]

With that pump I assume you have a big house And a hot water cylinder.

With hot water zone not calling, I would be running the boiler at say 70 degrees and each radiator drops the temperature by 20 degrees.

With any property smart radiator valves are the way forward. I cannot see beyond the Honeywell offering

Pump speed in essence is the one that removes the generated heat from the boiler So boiler is not short cycling.

 

[akist]

I would be running the boiler at say 70 degrees and each radiator drops the temperature by 20 degrees.

Would I be right in assuming that the slower the speed, the cooler the return will be, as hot water spends longer inside the radiators?

 

[muggles]

Slower is better, generally. What boiler do you have? Pumps are sized based on the resistance in the system and the load they need to shift. It's not a rule of thumb thing, it can be calculated (or you can use Grundfos' graphs) https://product-selection.grundfos....48?pumpsystemid=2250188845&tab=variant-curves

 

[Johntheo5]

Would I be right in assuming that the slower the speed, the cooler the return will be, as hot water spends longer inside the radiators?

Basicall, yes, but one has to look at the whole picture.
If your rads are sized to give full output at a dT 20C,the flow temperature must be 80C, so you then have flow/return/dT/output
80C/60C/20C/100%, a 1500 watt rad will output 1500 watts. The flow rate must be 0.72LPM/kw, 25kw "worth" of rads require a total flowrate of 18.0LPM.
If you now reduce the flow temperature, as you have, to 45C then (assuming flowarete remains unchange) you will get, flow/return/dT/output,
45C/38C/7C/33.6%, a 1500 watt rad will now output 500 watts. Now, a return temp (which defines the condensing effect) of 38C will give excellent condensing/boiler efficiency. If you want to achieve a still further reduction in return temperature but don't want a reduction of rad output of 500 watts then if you increase the flow temperature to 55C and reduce the flowrate to 0.18LPM/kw, you will get....
flow/return/dT/output, 55C/28C/27C/33.6%, the 1500 watt rad will still output 500 watts but now with only a return temperature 28C and still better condensing.
This is why weather compensation can work so well, relatively simple to reduce the boiler flow temperature without any need to reduce the flow rate.

Your 8M UPS 2 pump is very powerful, the 3 constant speed CC settings give 6M, 7M & 8M, even speed 1 at 6M is not required in most cases so I would (if not allready done so) suggest PP (proportional pressure) mode setting 2 and see how you get on.
If the boiler displays the flow/reurn temperatures then note them or if you have a IR gun measure them at the boiler flow/return.