why do you need to change it ? you could have had work done on the system causing it to drop, bleeding the rads, extra rad installed. etc (if not then you have a leak in the system) no reading means the system needs refilling this raises the pressure in the system causing the gauge to rise, limit to about 1/0 - 1.5 bars my system has 2 gauges one a dial gauge the other is digital readout
i will say this the boiler won't light without the system being pressurized hence the 1/0-1.5 bar figure.
wirecutterman, June 2009
