Short definition
A sealed (pressurised) heating system is a modern closed hydronic loop with no atmospheric vent. The system is filled to about 1 bar (15 psi) cold; expansion is absorbed by an expansion vessel; a pressure relief valve protects against overpressure at around 3 bar (45 psi). Standard residential design today, both new construction and most boiler retrofits.
What it is
The defining feature is what’s not there: no feed-and-expansion cistern in the attic, no open vent pipe, no atmospheric exposure. The whole heating loop sits at controlled pressure, sealed.
The required components:
- Filling loop. A temporary or permanent connection between the cold water supply and the heating return, with a backflow preventer. Used to fill the system initially and to top up after bleeding.
- Expansion vessel. A diaphragm or bladder tank with an air pre-charge that absorbs the volume change as water heats up. Without it, pressure would spike above PRV setting on every heating cycle.
- Pressure relief valve (PRV). Set at around 3 bar (45 psi). Discharges to a drain or pipe routed to a safe location if pressure exceeds the setting.
- Pressure gauge. Reads system fill pressure for diagnostic and homeowner reference.
- Auto-air-vent. Mounted at the high point of the system. Releases small amounts of air that work through to the top during normal operation.
Compared with a legacy open-vented system (UK norm pre-1990s, some old US installs):
- No attic cistern that can leak or freeze.
- Higher operating pressure → can serve taller buildings without head problems.
- No continuous air ingress through the open vent → less corrosion over time.
- Modern boilers ship with most of the components built into the chassis.
Why it matters to a homeowner
If your house has hydronic heat installed or replaced in the last 30 years, it’s almost certainly a sealed system. The gauge on the boiler is the diagnostic readout; the filling loop is the homeowner-accessible top-up; the PRV discharge tells you when the expansion vessel has failed.
The most common homeowner-visible failure: the PRV drips to the drain after the boiler runs. That’s almost always a waterlogged expansion vessel — the air pre-charge has bled out over years, the vessel can’t absorb expansion any more, and pressure spikes to PRV setting on every heat call. The fix is recharging or replacing the expansion vessel, not the PRV. A contractor who replaces the PRV without checking the expansion vessel is treating the symptom.
The other common visit: the boiler locks out with a “low pressure” fault. Top up via the filling loop to 1.0–1.2 bar cold, then watch the gauge over the next week or two. If pressure stays steady, the original drop was just a bleed event. If pressure keeps dropping, hunt the leak — failing valve seals, microleaks at radiator joints, or a heat-exchanger pinhole.
When you’ll encounter this term
- A new boiler install converting a 1950s open-vented WA system to sealed.
- A boiler quote referring to “sealed system pressurisation” as a line item.
- An expansion-vessel replacement scope of work.
- A homeowner-inspection report at sale time noting “sealed hydronic system, pressure within range.”
Common variants and disambiguation
- Sealed/pressurised vs. open-vented. Open-vented has an attic feed-and-expansion cistern and an open vent pipe; less common in modern installs. Most legacy WA open-vented systems get converted to sealed at boiler replacement.
- Sealed (heating) vs. closed (potable). Two different concepts. A closed potable system has a check valve preventing flow back to the main; a sealed heating loop has no atmospheric vent. Both can coexist in the same house.
Common failure modes
- Pressure climbs and PRV drips. Expansion vessel waterlogged. Recharge or replace.
- Pressure drops over weeks despite no obvious leak. Microleak somewhere — failing valve seal, radiator joint, or heat-exchanger pinhole.
- Air-locks at radiators. Auto-air-vent at the high point may be faulty or undersized; bleed individual radiators.
- Cylinder collapse (rare). More an open-vented failure mode; unlikely on properly designed sealed systems.