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Plumbing Connections

4 min read

Connect a collector with cold water entering at the bottom and hot water leaving at the top, pipes that rise continuously to the tank, a vent or relief path at the highest point, and a drain at the lowest. If the collector can be shut in with water inside, stagnation turns it into a steam boiler: saturated steam at 150°C is about 4.8 bar absolute and at 180°C about 10 bar, which bursts garden hose, plastic pipe and most salvaged tanks.

Part of Flat Plate Collectors

Collector plumbing is the set of pipes, vents and valves that carry water to and from the absorber and decide whether a no-flow afternoon is harmless or dangerous.

The Layout That Works

Water enters at the bottom of the collector (cold, dense) and leaves at the top (hot, light), as the parent page directs. From the top outlet the pipe must rise continuously to the tank, with no dip in which water can sit and no crest in which air can lock. Put an air vent at the highest point, a drain at the lowest, and keep the pipe short and insulated. The wiki’s hot-water page explains why the loop must stay open to air at the tank (Hot Water Systems).

Pipe choice: salvaged copper, steel, or high-temperature plastic (parent page). Garden hose works temporarily but degrades in UV and heat (parent page). Avoid PVC for hot water (Salvaged Modern Pipe).

What Stagnation Does To A Closed Loop

Saturated steam pressure depends only on temperature. Steam-table values (standard handbook figures, recalled): about 1.99 bar absolute at 120°C, 4.76 bar at 150°C (4.8 rounded), and about 10 bar at 180°C. Atmospheric pressure is about 1.01 bar, so subtract it for the pressure above atmosphere (gauge). At the stagnation range of 120-220°C (The Greenhouse Effect in Collectors), a sealed loop of water would reach several bar above atmosphere, beyond what garden hose or most plastic fittings withstand. That is why the loop needs an open vent or a relief valve.

Worked Example: Pipe Growth

Copper lengthens by about 17 µm per metre per kelvin (handbook figure, recalled). A 3 m run heated from 20°C to 100°C grows by 3 m × 17 × 10⁻⁶ per K × 80 K = 4.1 × 10⁻³ m, about 4 mm. Hard-mounted at both ends, that growth bows the pipe or stresses the joint, so allow a bend or a flexible section and mount with clips that let the pipe slide.

What You Need

  • Pipe and fittings rated for the stagnation temperature
  • An air vent at the high point and a drain at the low point
  • A relief valve or an open vent that discharges somewhere harmless
  • Pipe clips that allow sliding
  • Insulation for the hot pipe

Method: Plumb, Fill, Prove

Step 1: Run the supply to the bottom connection and the return from the top connection to the upper part of the tank.

Step 2: Fit the vent. Never place a valve between the collector and the vent.

Step 3: Fill from the bottom slowly so air rises out of the top. Run the vent until it spits only water.

Step 4: Test for leaks for an hour at working pressure, then insulate.

Common Mistakes

  • A valve that can close the top outlet. One closed valve makes a boiler.
  • A sag or a high point without a vent. Air traps stop circulation.
  • Discharging the relief where people walk. Steam and hot water come out.

Safety

Pressure and steam from a sun-heated loop

Without flow, water in the collector reaches boiling and then steam pressure rises quickly: about 4.8 bar absolute at 150°C. A closed loop with no pressure relief is a pressure vessel heated by the sun, and a relief path is not optional. Route the vent or relief outlet to a place where discharged steam and scalding water cannot reach anyone, protect it from freezing, and test relief devices (Safety Valve, Pressure Testing). Do not add antifreeze that cannot tolerate the stagnation temperature; degraded glycol can become acidic.

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