Purge a solar loop with a filling station, never with the system pump: circulate the glycol mixture for 20–30 minutes at 3–4 bar so the velocity carries bubbles to the air separator, then vent the high point and close the automatic air vent's isolating valve. Air that comes back weeks later is a leak or steam, not trapped air.
Why does air keep collecting in a solar circuit?
Because a solar loop is the worst case for air in hydronics: its highest point is on the roof, its fluid regularly passes 100 °C, and the gas solubility of water-glycol falls as temperature rises. Heat the fluid and dissolved air comes out of solution exactly where it is hardest to remove.
Four sources dominate:
- Commissioning air that was never fully flushed out — by far the most common.
- Stagnation steam. When the collector boils, vapour is driven into the pipework and condenses unevenly, leaving pockets behind.
- Open automatic air vents. Left open, they release vapour and glycol during stagnation, and can draw air back in as the loop cools and pressure drops.
- Sub-atmospheric pressure at the top of a system filled to too low a pressure — air is pulled in through fittings that never leak fluid.
A loop full of air announces itself: gurgling in the pump station, a pump that whines but moves nothing, collector temperature soaring while the store stays cold, and a ΔT that swings wildly.
How do you purge a solar loop properly?
With a filling and flushing station — a pump, a reservoir of premixed fluid and two hoses. The system pump has neither the head nor the flow to shift air to the roof.
- Cool the collectors. Fill early morning, in the evening, or with the collectors covered. Pumping cold glycol into a 150 °C absorber flashes it to steam instantly.
- Connect the station's flow hose to the fill valve and the return hose to the drain valve, and close the isolating valve between them so the fluid is forced around the whole circuit.
- Circulate for 20–30 minutes at 3–4 bar. The point is velocity: keep the flow well above the roughly 0.6 m/s at which bubbles are entrained and carried rather than left behind. Fluid returning to the reservoir should be free of bubbles for several minutes before you stop.
- Crack the manual vent at the collector high point briefly during circulation, then close it.
- Reverse the flow for a few minutes if the station allows, to shift air out of the reverse-return legs.
- Set the fill pressure, close the drain, and stop the station.
- Close the automatic air vent's isolating valve on the roof. It has done its job during filling; leaving it open during operation risks losing fluid as vapour during stagnation.
- Run the pump at full speed for 15 minutes, then re-check pressure and repeat the purge the following day. A second short purge after 24 hours removes the air that has come out of solution overnight.
Which symptom points at which fault?
| Symptom | Likely cause | First check |
|---|---|---|
| Gurgling in the pump station, pump noisy | Air pocket in the circuit | Purge with a filling station |
| Collector hot, store cold, ΔT unstable | Air lock in the collector array | Purge; check high-point vent |
| Air returns every few weeks, pressure falling | Leak drawing air in, or vent releasing fluid | Pressure test; inspect roof vent |
| Air returns after every hot spell | Stagnation steam | Check store limit and expansion vessel sizing |
| Pump runs, no flow at all, no gurgle | Seized pump, stuck check valve, or closed isolating valve | Manual pump vent screw; valve positions |
Where should the air devices actually be?
An air separator belongs in the solar pump station, on the hot side, where the fluid is hottest and air least soluble; Solimpeks lists both pump stations and air purgers among its solar thermal accessories. A manual vent belongs at the absolute high point of the array. An automatic float vent may be fitted at the high point too — but only with an isolating valve beneath it, closed during normal operation.
Pipework geometry does the rest. Every run should rise continuously toward the collector or fall continuously back to the station; a pipe that goes up, along, down and up again builds a permanent air trap that no purge will clear.
When is the problem not air at all?
If a full purge does not fix it, stop chasing air. A pump that will not move fluid may be seized after months of standing — many circulators have a manual release screw. A check valve stuck closed produces identical symptoms. In a drain-back system the loop is meant to contain air, and the fault is usually insufficient pipe fall (it must be a continuous 2–4%) or a pump too weak to lift fluid to the roof on every start. And in a system with several collector rows, uneven flow between rows mimics air locks exactly — the fix there is balancing valves or a reverse-return layout, not more venting.
Frequently asked questions
Can I bleed a solar thermal system like a radiator?
No. A radiator sits below the fill point, so air rises to a bleed screw you can reach. A solar array is above everything else and full of glycol under pressure; the only reliable method is to circulate fluid with a filling station at a velocity high enough to carry bubbles out.
How much pressure should I use when filling a solar system?
Circulate at 3–4 bar during the purge for velocity, then set the static fill pressure to the design value — roughly 1 bar plus 0.1 bar per metre of static height, typically 2–2.5 bar in a two-storey house. Always stay well below the 6 bar safety valve setting.
Should the automatic air vent on the roof stay open?
Only during filling and purging. In normal operation close its isolating valve, otherwise a stagnation event vents steam and glycol through it, and the system loses both pressure and frost protection.
Why does air come back after a few weeks?
Either the loop is losing pressure through a small leak and drawing air in, or the collectors have stagnated and driven steam through the pipework. Check the pressure gauge cold, then look at the expansion vessel and the store limit setting before purging again.