If your boiler pressure is “fine at night” but low in the morning, it’s rarely random. Overnight drops usually point to what happened during the last heating cycle: expansion, discharge, contraction, then a low reading when cold. This guide breaks down the exact patterns that separate a small leak from an expansion vessel or PRV problem.
Part of the “hot water but no heating” cluster: Hot water but no heating (hub). If low pressure is stopping the heating, pair this with: boiler pressure too low (what it means).
Before you chase leaks under floors or start topping up every day, capture two readings: cold pressure and hot pressure. Pressure should rise a bit when the system heats (that’s normal), then settle back when it cools.
The overnight clue is simple: if the system dumped water while hot (often via the PRV), it will look “okay” until it cools down — then the gauge reads low in the morning.
So don’t just tell me “it drops overnight”. Tell me: what does it do when the heating runs?
If pressure goes up near 3 bar when hot, that often points to the expansion vessel or PRV.
Water expands when heated and contracts when cooled. In a sealed system, that expansion has to go somewhere — that’s the job of the expansion vessel: a diaphragm tank with an air cushion behind it.
If the vessel is flat (lost its air charge) the system has nowhere to “store” expansion volume. Pressure rises quickly as the heating runs, and the PRV (pressure relief valve) can lift to protect the boiler. When it lifts, it dumps water out of the system through the discharge pipe.
Then later — usually overnight — everything cools, the water contracts, and the gauge reads low. So the morning drop is often the cold-side effect of a hot-side event.
If your gauge is showing boiler pressure too low during the day, that’s the “symptom layer” — this page helps you identify whether the loss is coming from hot-cycle discharge (vessel/PRV) or a true leak.
If you want the foundation explanation first, read: boiler pressure too low — what it actually means. This page is the “pattern decoding” layer on top.
“Overnight” doesn’t automatically mean “leak”. It means you’re observing the system at its coldest, lowest-pressure state. The job is to determine whether the system lost water (leak/discharge) or simply revealed a loss that happened earlier. These are the most common pathways I see around Orpington/Bromley homes.
This is the classic: you top it up to 1.2–1.5 bar, heating runs, pressure climbs rapidly, PRV opens briefly, then when cold it’s low again. People chase “leaks” for months when the system is actually dumping water.
Home clue: check the outside discharge copper pipe for dampness after heating has been on.
Once a PRV has lifted, it may not reseal perfectly. That creates a constant slow drip outside — easy to miss if it’s near a drain or evaporates.
Home clue: dry paper towel under the discharge outlet (outside) can reveal a “barely there” drip.
Small leaks at radiator valves, towel rail joints, or push-fit connections can dry before you see puddles. Under suspended floors, you may never see water — just pressure loss.
Clue: no big hot spike; pressure just trends down regardless of heating cycles.
Constantly topping up introduces fresh oxygenated water. Oxygen accelerates internal corrosion, producing sludge (magnetite) and sometimes hydrogen gas. That gas collects at high points and causes circulation oddities.
Clue: black water from bleed points, frequent air, noisy pump, cold patches on radiators.
Sometimes the pressure isn’t the root issue — it’s a symptom of repeated bleeding and instability. If you’re chasing cold upstairs radiators, the system might need balancing/circulation attention too.
Pair these if relevant: signs of an airlock and radiator balancing explained.
On some boilers the leak is internal: pump seals, joints, heat exchanger pathways. This needs proper inspection — not DIY case removal.
Clue: damp marks under boiler, corrosion staining, pressure loss even when heating hasn’t been used.
Many boilers won’t run the heating circuit below a minimum pressure because it risks pump damage and unsafe operation. So you wake up to a cold house, top it up, it works for a day… then repeats.
That loop is exactly why the hub exists: Hot water but no heating. Overnight pressure loss is one of the cleanest mechanical causes of that symptom.
If your system is also noisy (banging, kettling, gurgling), that’s a separate diagnostic layer: it can point to scale/sludge or air/cavitation. A proper service helps prevent repeat faults: boiler service checklist (what’s included).
If pressure rises aggressively when heating is on, you’re often looking at expansion control, not a mystery leak.
The gauge reads lowest when cold. That’s why morning readings reveal problems that happened during heating cycles.
Daily top-ups are a symptom management loop that can accelerate corrosion and sludge formation over time.
I’m Stuart — I answer the phone, turn up, and do the work. I’m based around Orpington and regularly work across Bromley and BR7. This page is written from real “pressure drop” call-outs where the fix is diagnosis-first — not parts guessing.
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Overnight is when the system is cold, so pressure is at its lowest. The key is whether water left the system earlier (leak or PRV discharge) and the low reading is just revealing it.
Repeated topping up introduces fresh oxygenated water which can accelerate corrosion and sludge formation. It’s okay as a short-term measure to restore heat, but it’s not a long-term solution.
A common signature is pressure rising aggressively when heating runs, then later dropping low when cold. Pair this page with: boiler pressure too low.
Look for a copper pipe that exits the wall (often near the boiler location). Any drips, wet patch, or staining is a major clue — especially after the heating has been on.
Pressure issues can overlap with circulation/water quality problems. These help: radiator balancing and signs of an airlock.
If you’re in Orpington, Bromley or BR7, send your postcode and your 3 readings (cold / hot / morning). That pattern usually tells me whether we’re dealing with a leak, PRV discharge, or expansion vessel behaviour — and what the right fix is.
Related guides: pressure too low • hot water but no heating • service checklist
