A bypass radiator, specifically in a hydronic heating system, doesn't refer to a specific type of radiator but rather describes the function of a bypass valve or circuit installed to ensure proper system operation, particularly during pump overrun and zone valve closure. It allows water to continue circulating when other zones are shut off.
Understanding the Need for a Bypass
Modern hydronic heating systems often use zone valves to control the flow of hot water to different areas (zones) of a building. When a zone reaches its desired temperature, its zone valve closes, stopping the flow of hot water to that zone's radiator(s). However, the boiler and pump may still be running for a short period (pump overrun) to dissipate residual heat and prevent damage to the boiler heat exchanger.
How the Bypass Operates
The bypass circuit, usually positioned upstream of the zone valves, provides an alternate path for the water to flow when one or more zone valves close. Here’s how it works:
- Zone Valves Open: When all zone valves are open, the water predominantly flows to the radiators in each zone, as this path offers less resistance. The bypass valve is usually partially or fully closed during this time, or a differential pressure valve will remain closed with sufficient system pressure.
- Zone Valve(s) Close: As zone valves close due to thermostats reaching set temperatures, the resistance in the primary circuit increases. This pressure change forces water to flow through the bypass circuit. This can occur via a pressure-activated differential bypass valve, or a manually adjusted valve.
- Pump Overrun: During the pump overrun period, when the boiler is still producing heat and the pump is still running, the bypass circuit allows the water to circulate continuously, preventing pressure build-up in the system and safely dissipating the excess heat produced by the boiler. This prevents the boiler from overheating and potentially being damaged.
Types of Bypass Systems
There are two main types of bypass systems:
- Manual Bypass Valve: A manually adjustable valve is installed in the bypass circuit. The installer sets the valve opening to provide sufficient flow when zone valves are closed. This type requires careful balancing during installation.
- Automatic Bypass Valve (Differential Pressure Valve): A differential pressure valve automatically adjusts the flow rate through the bypass circuit based on the pressure difference between the supply and return sides of the system. This type is more responsive to changes in the system and typically requires less manual adjustment.
Benefits of a Bypass
- Prevents Boiler Overheating: The primary function is to provide a path for water to circulate during pump overrun, preventing the boiler from overheating when zone valves close.
- Reduces Pump Strain: Provides an alternative flow path, reducing the strain on the pump motor when zone valves close.
- Quieter Operation: Can reduce noise in the system, particularly "water hammer," by preventing sudden pressure changes.
- Balanced System: Helps maintain balanced flow and consistent temperatures across the system.
Example Scenario
Imagine a home with two heating zones, Zone A (living room) and Zone B (bedrooms). The boiler heats water, and a pump circulates it through the system.
- Initially, both Zone A and Zone B thermostats call for heat, and their zone valves are open. Hot water flows to both zones.
- The living room (Zone A) reaches the set temperature. Its zone valve closes.
- The pump is still running, and the boiler might still be producing heat due to pump overrun. Without a bypass, the water flow would be completely blocked.
- The bypass circuit opens, allowing water to continue circulating from the boiler, through the bypass, and back to the boiler. This prevents overheating and maintains a steady flow, even though Zone A is no longer calling for heat.
- When Zone B reaches the set temperature and its valve closes, the boiler shuts down entirely.
