Why is that one radiator cold despite a working boiler?
In short: When a single radiator fails to heat a room, the culprit is rarely the boiler itself. Troubleshooting a forced hot water system requires understanding how pressure, air, and specialized plumbing components like diverter tees work together to move heat.
In a forced hot water system, water is heated in a boiler before traveling through pipes to radiators, where it releases heat into the room before returning to the boiler to repeat the cycle. This continuous loop is driven by an electric circulator pump. If the entire system lacks heat, this pump is a primary component to test. However, if only one radiator is cold, the issue is often more localized, such as trapped air or a closed shut-off valve.
The mechanics of water movement often rely on subtle pressure changes. In 1950s-era homes, for example, plumbers sometimes used a diverter tee setup. This configuration uses a single combined supply and return line from the boiler, where the diverter tee restricts flow at one end to increase back pressure, forcing water through the first pipe and into the radiator. To maintain the necessary system pressure, a pressure reducing valve is used to lessen the incoming city water pressure before it enters the boiler.
System stability also depends on an expansion tank, which acts as a shock absorber for pressure. These tanks typically feature a top portion filled with water and a bottom portion containing air under pressure, separated by a neoprene diaphragm. If this diaphragm fails, it can cause significant system issues. To diagnose a single failing radiator, one should check the air valve for excess air or ensure the radiator's shut-off valve hasn't been closed for service or zoning purposes.
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Sources
- Summary by Intellect · source: How to Diagnose Problems with a Hot Water Heating System | Ask This Old House
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