Hi Carole there are several reasons that can cause this.
1. Natural water loss i.e. needs topping up once every 3 months or so.
2. Expansion vessel broken and activating the pressure relief valve, check for water coming out of overflow pipe on the wall outside of boiler
3. You have a leak on the system, usually a radiator valve weeping slightly
Goodluck
Stag Heating TMSGAS, December 2010
