CSLB C-36 Plumbing ContractorService, Repair, and MaintenanceHard
A plumber is servicing a hydronic heating system and notices that one radiator is significantly colder than the others, despite the boiler operating normally. The system is a two-pipe reverse return system. What is the most likely cause for this specific radiator's issue?
- AAir trapped in that specific radiator.
- BA blockage in the main return line to the boiler.
- CThe circulator pump is failing and not providing enough pressure.
- DThe boiler's aquastat is malfunctioning, leading to inconsistent heating.
Show answer & explanationAnswer & explanation
Correct answer: A. Air trapped in that specific radiator.
In a hydronic system, air is lighter than water and tends to accumulate at high points, such as within radiators. If a single radiator is cold while others are hot and the boiler is functioning, it strongly indicates that air is trapped inside that specific radiator, preventing hot water from circulating through it. The solution is typically bleeding the air out.
Why the other options are wrong
- B. A blockage in the main return line would affect the circulation to *multiple* or *all* radiators, not just one, and could cause the boiler to short cycle or overheat.
- C. A failing circulator pump would result in poor circulation and reduced heat in *all* or *many* radiators, not just an isolated one.
- D. A malfunctioning aquastat would lead to inconsistent boiler operation and affect the temperature of the water supplied to the *entire* system, not just one radiator.
Hydronic Radiator Air Lock
Air trapped inside a radiator in a hydronic heating system, preventing hot water circulation.
- Causes a radiator to remain cold despite system operation.
- Air is lighter than water and collects at high points.
- Requires bleeding the radiator to release trapped air.
- Common issue in many hydronic systems.
Memory trick: Air's light, but it blocks the heat's flight.