Why are burner block valves not throttled?

• A. It could flood out because it has to be fully open or closed
• B. They damage the valve seats
• C. They would cause backflow
• D. It would introduce air into the burner

Answer: (A)

Burner block valves are meant to provide a tight shutoff, so the system can be either fully supplied or fully stopped. If you try to throttle such a valve, you end up in a partial open position where fuel can accumulate and flow irregularly toward the burner. That partial flow makes it easy for fuel to puddle or flood the burner area, and when ignition is attempted, unburned fuel can flood the burner and create a dangerous, unstable flame or ignition failure. The safe, reliable operation relies on using the valve as a switch—completely open for operation, completely closed for shutdown—while metering and flow control are handled by other equipment designed for that purpose. The other issues like seat wear or backflow can be concerns in some systems, but the primary safety reason here is the risk of flooding the burner when the valve is not fully open or closed.