Weil McLain Boiler Troubleshooting - Common Problems and Solutions

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Last updated: 
January 26, 2018

This Weil McLain boiler troubleshooting guide explores common problems and steps you can take to identify and possibly solve them.

Keep in mind the risks associated with boilers – severe burns, gas explosion and the risk of shock. Proceed cautiously with the diagnostic steps you take. If you believe there’s a risk of injury, leave the job to a pro. With that caveat out of the way, let’s explore troubleshooting a Weil McLain boiler.

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Weil McLain Boilers – Common Issues

Here are common boiler problems for this or any brand. In the next section are Weil McLain boiler troubleshooting tips the pros use to determine verify what is wrong.

1. No Heat At All 

A burned-out thermocouple is the top cause of a Weil McLain boiler not responding to the thermostat’s call for heat.

2. Little or No Heat

Insufficient water pressure will prevent the boiler from creating enough hot water to adequately heat your home. Adding make-up water will solve it, but the goal is to determine the cause of low pressure in the system. An early clue of low water pressure, besides insufficient heat, is the boiler making popping and/or rattling noises.

3. The Boiler Cycles On and Off Frequently

There’s something amiss if your Weil McLain boiler is turning on, running a short cycle and shutting off. The clues are hearing it cycle on and off, or you might first notice your home’s temperature being more than a few degrees higher or lower than the thermostat set point.

4. No Heat in a Zone / Too Much Heat in a Zone

When zone valves fail, they either won’t open to allow heat into a zone or they get stuck open and too much hot water enters the zone.

5. Carbon Monoxide Alarm Triggered

It’s wise to have a CO detector near your boiler and low to the ground since CO is heavier than the normal mix of air in your house.

Tools You'll Need for the Job

Depending on the specific problem you’re experiencing, one or more of these tools will prove helpful for Weil McLain boiler troubleshooting.

  • Your owner’s manual: Weil McLain boiler owner’s manuals contain helpful information about system operation, troubleshooting and problem resolution. That’s a good first place to start when something doesn’t seem right. Most Weil McLain manuals can be found online, even if the unit is 15+ years old.
  • Screwdriver set: If you’re going to be taking off the boiler cover, there will be Philips and/or flathead screws to remove.
  • Flashlight or headlamp: Light will help you see inside the unit and look for leaks at valves.
  • Handheld mirror: A flashlight and mirror are necessary to see many Weil McLain boiler model heat exchangers.
  • Voltmeter: This tool will prove handy when troubleshooting anything electrical in your home.
  • Set of small wrenches: More handy DIY homeowner tools!
  • Long lighter like those used for barbecue grills: This will be used to replicate a pilot light.

Weil McLain Boiler Troubleshooting

This section reviews common problems and Weil McLain boiler troubleshooting tips to determine their cause.

1. No Heat At All

If the electrical circuit for the boiler is on and the power switch on or near the boiler is on, crank the heat way up. If the boiler doesn’t start, it’s time to check the thermocouple:

  • Turn the gas cock on the gas line to the Off position, which is perpendicular to the direction the gas line is running
  • Remove the cover to boiler access panel
  • Locate the thermocouple, and take a picture of it with your cell phone or tablet
  • Remove the thermocouple by undoing the nuts at both ends
  • Connect the voltmeter to the wires extending from the thermocouple, and set the voltmeter to the proper millivolt setting
  • Hold the thermocouple with pliers, since it might get hot, ignite the lighter, and heat the thermocouple copper
  • If the voltmeter spikes, the thermocouple is working, and you can replace it using the picture as your guide; If it doesn’t, you’ve solved the issue: You need a thermocouple

2. Little or No Heat

Low water pressure is a common cause:

  • Check the pressure gauge on the boiler
  • If the pressure is too low, add water until the proper pressure registers on the gauge
  • Look for leaks at the system’s valves and connections that might be the cause of low pressure
  • If you find a leak or if the pressure drops again, call a service technician

3. The Boiler Cycles On and Off Frequently

When a boiler cycles on frequently, and your home is too cold or too warm, try these troubleshooting tips:

  • If the system is new, and the house is too hot, check to see if your home’s thermostat is installed on an outside wall, near a drafty window or by an entry door, and if any of those apply, it will have to be moved
  • If the system is new, and your house is too cold, your home’s thermostat might be installed in direct sunlight or near a heat source like the refrigerator or dryer and will have to be moved
  • When this is a new problem on an older boiler, increase the boiler’s upper-temperature limit, since it might not be making enough heat
  • If it doesn’t stop short-cycling, call a technician

4. No Heat in a Zone / Too Much Heat in a Zone

This is one of the easiest Weil McLain boiler troubleshooting issues to diagnose. Make sure the thermostat setting for each zone is where it should be, and if one zone is still too hot or cold, an electronic zone valve needs to be replaced.

5. Carbon Monoxide Alarm Triggered

The most common cause of leaking carbon monoxide is the plugging of the boiler heat exchanger fins with dirt and debris. If your CO alarm goes off, the safe approach is to open your home’s doors and a few windows to ventilate the area, evacuate the house, and call a boiler technician from outside. Here’s what the technician will do to check the heat exchanger:

  • Remove the boiler cover
  • With the aid of a flashlight or headlamp and a small mirror, the heat exchanger can be checked for debris or cracks
  • If dirt is the issue, the heat exchanger can be cleaned with a nylon brush and shop vacuum
  • If the heat exchanger is cracked, it’s an elephant sitting on the fence – It’s time for a new heat exchanger, and possibly for a new boiler

Caution Notes

Boilers present many significant dangers. Turn off power to the unit before opening the cover. This will eliminate the risk of shock. If you smell gas or oil fumes at any time, there’s a risk of explosion. Don’t ignite anything, and evacuate your household immediately. Call your utility company or emergency services from outside your home.

Finally, remember that carbon monoxide is deadly, so it’s important to have a detector near the boiler. If it sounds an alarm, turn off the boiler until you can inspect and repair it.

Weil McLain Boiler Troubleshooting Wrap-up

Weil McLain boilers are a reliable source of comfortable heat that should give you 15+ years of dependable service. If these troubleshooting tips don’t solve the problem or if you don't find your issue here, call a qualified Weil McLain technician. If these tips have been useful, perhaps sharing this post on social media will help others identify their boiler issues and avoid dangerous problems too.

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