Why does my boiler pressure go up when the heating is on?

A sealed heating system is a fixed volume of water. Heat it and it expands, and something has to absorb that expansion. That job belongs to the expansion vessel, so how the gauge behaves is really a report on whether the vessel is working.

Safety

What you'll need

• Access to the boiler's pressure gauge
• A note of the cold and hot readings over a few days

Step by step

1. Read the gauge cold.With the heating off for several hours, the needle should sit around 1 to 1.5 bar. Many boilers have the acceptable band marked on the dial.
2. Read it again fully hot.After an hour of running, expect roughly 0.3 to 0.7 bar higher. That is normal thermal expansion and it should come back down as it cools.
3. Record both over several days.One reading tells you very little. The pattern — where it starts, where it peaks, whether it returns — is the diagnosis.
4. Check the filling loop is closed.A filling loop left even slightly open lets mains pressure into the system continuously, so the pressure climbs and never comes back down. This is the most common cause and the easiest to fix.
5. Disconnect a flexible filling loop when not in use.Good practice, and it removes the possibility entirely. Many systems have a keyed or lever-operated loop instead.
6. Suspect the expansion vessel if it climbs steeply.A vessel that has lost its air charge cannot absorb expansion, so a small temperature rise produces a large pressure rise. The classic sign is a big swing between cold and hot with a low cold reading.
7. Look for discharge outside.The pressure relief valve dumps water through a small pipe to outside when pressure gets too high. Water dripping from that pipe is a definite fault signal.
8. Do not simply keep draining it down.Bleeding water off to lower the pressure works for a day and removes inhibitor from the system. It also masks a fault that will get worse.
9. Note whether it also loses pressure.A system that climbs when hot and then sits below normal when cold is losing water somewhere as well as failing to absorb expansion.
10. Call an engineer for the vessel.Recharging or replacing an expansion vessel involves draining the system and working on pressurised components inside or beside the boiler. It is engineer work.

Tips

• Write the cold and hot readings on a note by the boiler for a week. It turns a vague worry into a clear pattern.
• Disconnect the flexible filling loop after topping up. A loop left connected is the most common cause of creeping pressure.
• A rise of half a bar between cold and hot is normal and not worth worrying about.

Common mistakes

• Bleeding radiators to reduce high pressure — It lowers the pressure temporarily but removes treated water and inhibitor from the system, and it does nothing about the cause. The pressure climbs again the next time it heats up.
• Leaving the filling loop connected and slightly open — Mains water leaks continuously into the sealed system, so the pressure keeps rising until the relief valve discharges. It also risks contaminating the mains supply.

If it doesn't work

Questions people ask