Choosing the correct Central Heating pump from DIYnot shop?

[charlesr]

Hi
Hope you can help
My Central Heating pump is packing up. An engineer serviced the 10 year old IMI Powermax 135 about a year ago and said that the pump was going to go soon (bearing rumbling) and recently it's started to get worse, so you can hear the rumbling properly. He said it was an easy job to change it.
However, I'm having trouble choosing which pump to get. I think the choices are:
this
or this
The info says "Use Gold 5m pump for systems up to 60K BTU. Use Gold Star 6m for systems 60K-120K BTU."
How can I figure out if my system is above or below 60k BTU?
My current pump looks like this if it's any help:

Many thanks
Charles

 

[breezer]

if you look carefully the site you will be getting it from is Screwfix, i would get the bigger of the two, since a big pump can be turned down , but a smaller one can not be turned up

if you are going to change it make a note on the wall / floor which way pump sits

 

[charlesr]

Sane advice. Thanks
Having never fitted one before, how would I know if it was set too high (or low)?

 

[oilman]

If it's on a combi, they usually get turned up full, then turned down if they make too much noise. (very scientific...not). If it's to set up the heating correctly, the flow and return temperatures are monitored, and the pump adjusted to make the temperature difference 11 deg C (or near as poss) when the system has warmed up to its working temp.