Pressure keeps dropping

1 Common causes

Vaillant's website lists the main reasons: bleeding radiators, a leak from the appliance or the system, usually at radiator valves or components, and pipes and radiators that corrode or loosen over time². Its page on leaks adds failed seals and a faulty heat exchanger inside the boiler, both for an engineer⁵.

If your pressure dropped right after you bled radiators, that is expected: top up and recheck. A drop with no such reason is the case to take seriously. Vaillant says a sudden fall may mean a major leak, while a gradual one may respond to a top-up³.

2 Why repeated top-ups are a problem

Worcester's 4000 manual explains the cost. Constant repressurising is an indicator of potential leaks, and it dilutes the inhibitor in the system, so corrosion can follow⁴. That reasoning links this fault to sludge and inhibitor. Worcester's oil guide adds that excessive venting or water leaks mean a service engineer should inspect the installation⁶.

Other makers state it shortly. Baxi and Main both read a regular need for water as a fault or leak, and send you to the installer¹⁷. Ideal's Logic+ guide says that if pressure continues to drop, a Gas Safe registered engineer should be consulted⁸. How low is too low differs by model, so see boiler pressure.

3 What you can check

None of this involves opening anything.

• Radiators and valves: look for damp patches, rust and flaking paint. Baxi's Main guide and Vaillant's site both point here⁷².
• Pipes: wipe a pipe dry and put a plate underneath to see if it really drips, since pipes also sweat from condensation².
• Skirting boards, floors and ceilings: look for water stains².
• Underneath the boiler: a visible drip suggests a boiler leak, and needs a Gas Safe registered engineer².
• Outside: the pressure relief pipe ends on an outside wall or towards the ground, and water there is a clue; see pressure relief valve⁹.

Note what you found, and how often you topped up. It speeds up the visit.

4 What to do about a leak

Vaillant advises that when a leak is spotted, you turn off the water supply and the heating, contain the water and call a Gas Safe registered engineer⁵. It warns that water reaching electrics can cause faults⁵. It also says not to repair loose or broken joints unless trained².

If pressure falls again immediately after a top-up, stop trying to refill it³. The how-to for a single top-up is under repressurising a boiler.

5 Pressure that drops, then rises

A gauge that swings can have a different cause from a steady loss, such as a tired expansion vessel. Worcester says a recurring significant decrease or increase should go to the installer or engineer⁴.