Many sealed residential boiler systems sit around 1–1.5 bar when cold and rise somewhat as they heat, but the correct range depends on the manufacturer. The right reference is the appliance manual and gauge rather than applying one universal number to every installation.
Use the range specified for your model
The manual usually states the normal cold pressure and when a reading needs attention. Open-vented systems or specialized configurations are not interpreted the same way as a modern sealed boiler.
Some pressure rise during heating is normal
As water heats, it expands and system pressure usually rises. A modest change is expected; a large increase toward the warning range can point to an expansion-vessel or other system problem.
Repeated low pressure usually has an underlying cause
If you need to top up frequently, look for leakage around radiators, valves, and visible pipework. Repeatedly adding water without finding the cause can mask a fault and introduce fresh water into the system.
High pressure or safety-valve discharge needs attention
Do not block or alter a safety discharge. If pressure repeatedly enters the high range, water comes from the relief outlet, or the appliance shuts down on pressure, internal diagnosis should be left to a qualified technician.
Sources
- U.S. Department of Energy — Energy Saver — Practical information on home heating, cooling, thermostats, and energy efficiency.