If you manage rented property in London, you’ll get the call: “the boiler’s losing pressure again.” Here’s the decision tree we give our letting agents before they call us.
Step 1: Is the system still running?
Yes — pressure between 1 and 1.5 bar, boiler firing normally, hot water working? → Re-pressurise and monitor (see Step 2)
No — pressure below 0.5 bar, boiler won’t fire, no hot water? → Emergency callout (Step 4)
Step 2: Re-pressurise (only if pressure is low but system is running)
Modern combi boilers have a filling loop under the boiler — a silver braided hose connecting to a small tap. Turn the tap until the pressure gauge reads 1.0-1.5 bar, then turn it back off. Takes 2 minutes. Do NOT repressurise if there’s a visible leak — call a Gas Safe engineer first.
Step 3: Why is it losing pressure?
Three common causes:
- Small leak in a radiator — usually a valve or joint. Look for water staining on carpets below radiators, or water dripping from the valve. Tighten the valve gently with a spanner. If still leaking, callout.
- Pressure relief valve dripping — a small copper pipe exits the boiler and goes outside (often to the outside wall). A drip here after re-pressurising is normal. Constant dripping is not — callout.
- Expansion vessel failed — the pressure drops every 2-3 days after re-pressurising. This is a service engineer job, not a tenant-fixable item.
Step 4: When to call us
Call if:
- Re-pressurising didn’t hold (pressure drops within 24 hours)
- There’s any visible water leak
- The boiler is making new noises (kettling, banging)
- The pilot light keeps going out
- Any smell of gas — call 0800 111 999 (National Grid) FIRST
What we do on a boiler pressure callout
The engineer arrives, identifies the leak source or component failure, gives a fixed quote for the repair. Common jobs: replace pressure relief valve (£80-£120), re-pressurise and check expansion vessel (£60 call-out), replace expansion vessel (£150-£220 parts + labour).
Annual service
If the boiler is losing pressure more than once a year, it’s due a service. Gas Safe annual service is £70-£90 and catches 90% of these issues before they become emergencies. We offer portfolio clients a fixed-price annual service across all their properties.
Don’t do this
Tenants sometimes pour inhibitor into the system to “stop leaks”. Inhibitor doesn’t fix leaks — it just slows corrosion. If you have a leak, the right answer is repair, not chemicals.