If your boiler pressure keeps dropping, topping it up isn’t a fix — it’s a temporary patch. The pressure is only “low” because water is leaving the sealed system (or the system can’t handle expansion properly). This guide explains the real mechanics in plain English, without dumbing it down.
This page supports the main cluster hub: Hot water but no heating. If your heating won’t come on because pressure is low, start here.
“Low pressure” usually means the boiler’s pressure gauge is below roughly 1.0 bar when the system is cold. Most sealed central heating systems like to sit around 1.0–1.5 bar cold (it rises when hot). Different boilers vary slightly, but that range is a solid rule of thumb.
If the pressure is a bit low, you can often top it up via the filling loop — but only do it if you’re confident you know which valves you’re turning. If you’re not sure, stop and call someone who does this daily.
Important: if you’ve got any smell of gas, scorch marks, or the boiler is making loud banging noises, do not try to troubleshoot it — switch off and get it checked.
If pressure rises a lot when heating is on (e.g. 1.2 → 2.8+), that often points to expansion vessel problems.
A sealed heating system is a closed loop of water. The pump circulates it, the boiler heats it, and radiators release the heat. The pressure gauge isn’t there to look pretty — it tells you whether there’s enough water in the loop for a continuous water column, and whether the system can operate safely.
Pressure changes for two reasons: (1) temperature (water expands when hot, contracts when cold) and (2) volume loss (water leaving the system through a leak or a safety discharge).
If the pressure keeps dropping, one of these is true: water is escaping or the system is dumping water when hot then showing low pressure when cold.
The most important clue is the pattern. That’s why this page links directly to: pressure loss happening overnight. Overnight drops often relate to what happened during the last heating cycle.
In Orpington/Bromley homes, the “low pressure” call-out is usually one of these. The trick is not listing them — it’s understanding the mechanical signature each one leaves. That signature is what separates diagnosis from guesswork.
When the vessel fails, the system can’t absorb expansion. Pressure rises fast when heating is on, the PRV opens, and water exits the system through the discharge pipe. When everything cools down, you’re left with low pressure.
Signature: pressure looks “fine” cold, then spikes hot (often 2.7–3.0+), then drops later.
PRVs are designed to open under high pressure. But once they’ve lifted, they sometimes don’t reseal perfectly. That leaves a slow ongoing leak — not always visible, especially if it drips outside.
Signature: wet patch at the outside pipe even when heating hasn’t been on much.
Tiny leaks can evaporate before you notice. Look at radiator valves, towel rail joints, pipework under floors, and around the boiler itself. A slow leak drops pressure steadily over days.
Signature: no big hot-pressure spike; just a gradual decline.
If you constantly bleed radiators, the system is likely generating gas internally — often hydrogen from corrosion. Each bleed releases gas but also lets the system lose small amounts of water, and introduces fresh oxygen (which feeds corrosion).
Signature: frequent air, black water, radiators cold in patches, recurring pressure issues.
Hard water can accelerate scale in the boiler’s heat exchanger. That causes hotter localised zones, stronger expansion behaviour, and can contribute to repeated PRV lifting in marginal systems.
Signature: kettling noise + pressure behaviour + repeated faults in winter.
Occasionally there’s an internal leak (pump seals, heat exchanger issues, internal joints). This needs proper inspection — don’t strip cases off yourself.
Signature: visible damp inside case, pressure drops even when system is off.
People get confused because they can still run a hot tap, but the radiators won’t come on. On many combi boilers, domestic hot water can still fire briefly, but the heating circuit will lock out if system pressure is too low.
That’s why these two pages are built together: Hot water but no heating (hub) and this pressure guide. Low pressure is one of the cleanest “mechanical causes” of that exact symptom.
If you’re getting cold radiators as well as pressure issues, this one pairs next: radiators not heating but hot water works.
If it spikes hot and drops cold, think vessel/PRV. If it steadily drops, think leak. Pattern matters more than “the number”.
Repeated bleeding can accelerate corrosion by introducing oxygen. It solves the symptom, not the cause.
Proper diagnosis tests vessel charge, checks discharge, and confirms the system is stable under heat — not just topped up.
I’m Stuart — the person who answers the phone, turns up, and does the work. I’m based around Orpington and I’m usually working across BR5/BR6, Bromley and BR7. This guide is written from real pressure-related call-outs — not rewritten from generic plumbing sites.
Checkatrade score from 67 reviews.
Read them here:
SKR Plumbing & Heating on Checkatrade.
Most sealed systems sit around 1.0–1.5 bar cold. It will rise when hot. If you’re regularly below 1.0 bar, it’s worth investigating why.
Water expands as it heats. The expansion vessel absorbs that change. If the vessel isn’t working, pressure rises too high and the PRV may open.
It can be. Repeated topping up introduces fresh oxygenated water, which can accelerate corrosion inside radiators and pipework and contribute to sludge. It’s fine as a temporary measure, but it’s not a long-term solution.
A common pattern is: pressure looks okay cold, rises a lot when hot, and then later drops low. Also check the outside discharge pipe for dripping. Proper confirmation requires testing the vessel charge correctly.
Go here: pressure loss happening overnight. Overnight drops often reveal what happened during the last heating cycle.
If you’re in Orpington, Bromley or BR7 and your boiler pressure keeps dropping, send your postcode and the pressure pattern (cold vs hot). I’ll tell you the most likely cause and the right next step.
Related guides: hot water but no heating • pressure drops overnight • proper boiler service checklist
