How to Lower the PSI Pressure on a Weil McLain Boiler

Kevin Johnston

The Weil McLain boiler has an auto-fill valve that is designed to prevent the boiler from accumulating too much water pressure. However, you must set this valve to the proper limit or else it will not shut off the water supply early enough.

Relieve water pressure on a Weil McLain boiler before adjusting automatic settings.

Learn how to manually reduce the pressure before you work on the auto-fill valve. In addition, you must learn how to make adjustments to the auto-fill valve by doing so as you watch the pressure gauge.


When you drain water from the boiler, make sure you have a drain for runoff. If you do not have a drain, use a bucket in order to avoid getting the floor wet. Be careful not to turn the screw bolt when you tighten the lock nut, because this can change your pressure setting. The loosening and tightening directions may seem backward to you, but turning the screw counterclockwise actually reduces water flow, and turning it clockwise increases water flow. Turn the screw bolt slightly each time, counting half turns as you go so you can remember how far you have turned it in case you want to return to the original setting.

  1. Drain water out of the boiler's drain valve. This valve is located at the bottom of the boiler. The valve looks like a water spigot you would find on the outside of your house. Turn the handle counterclockwise to open the valve. Drain water until the pressure gauge reads 10 psi (pounds per square inch). Turn the handle clockwise to close the valve and stop draining.

  2. Close the drain valve. Watch the pressure gauge to see if the pressure begins to climb again. It may start climbing above 12 psi. If it does, you must adjust your auto-fill valve because it is set for too high a pressure.

  3. Locate the auto-fill valve. You will find it on the line that brings water in to the boiler from the city water supply or your well. It should be located two to three feet from the boiler, but some installers place this valve near the ceiling.

  4. Locate the bolt screw and lock nut on one end of the valve. Loosen the lock nut 1/2 turn with a wrench. This will allow you to loosen the bolt screw. Use a flat screwdriver to loosen the screw counterclockwise. Loosen it until you see the pressure gauge moving down to 12 psi. (Note that loosening the screw will reduce the water flow.) If the pressure goes below 12 psi, turn the screw clockwise until the needle rests on 12 psi on the pressure gauge.

  5. Tighten the lock nut with a wrench in order to hold the screw bolt in place and maintain the water pressure at 12 psi.