Balance of System Components and Materials
The hydronic balance of system
An array is only as good as the parts around it. Typical components and their jobs:
- Expansion tank - absorbs volume change; size it for stagnation, not just normal operation.
- Pressure relief valve - the only safe over-pressure path; never valve it off.
- Air elimination or air vent - air pockets stop circulation and corrode loops.
- Check valve - prevents night-time reverse thermosiphon that cools the tank.
- Isolation and drain valves - service without draining everything.
- Strainer and flow meter - protect the pump and let you balance flow.
- Circulator and differential controller with collector and tank sensors - pump on at the set ΔT, off when gain disappears.
- Heat exchanger - separates glycol from potable water.
- Tempering/mixing valve - limits delivery temperature at fixtures.
- Vacuum breaker and drainback tank where the design requires them.
Sizing rules of thumb
Storage of roughly 1.5–2 gallons per square foot of collector for DHW; glycol expands a few percent between cold fill and operating temperature; cold loop pressure is commonly set between 20 and 40 psi per manufacturer instruction.
Worked example
A 40-gallon glycol loop expands about 4 % to operating temperature - roughly 1.6 gallons - but full stagnation vaporizes far more. If the expansion tank's acceptance volume is only 1 gallon, the relief valve will lift every sunny summer afternoon. Resize for stagnation volume, not normal operation.
Traps
Omitting the check valve and wondering why the tank cools at night; placing the tempering valve on the solar loop instead of the potable outlet; wiring a pump with no disconnect; using automotive hose or non-rated insulation that fails at stagnation temperature.