3 Reasons Your Boiler Won't Fire

Posted on: 19 July 2022

Like gas furnaces, gas boilers must follow a particular sequence of operations when your thermostat demands heat. However, this sequence varies substantially from furnaces, and several additional components are in play on a boiler. Surprisingly, ignition occurs near the end of this sequence, and many other problems along the way can stop your boiler from firing up.

If your boiler won't fire, there's a reasonably good chance of a problem somewhere along this chain of operations. While there are many potential reasons for a boiler to fail, this article will discuss three of the most common underlying causes of ignition failure.

1. Low Water Cut-Off Failure

All boilers must avoid a condition known as "dry firing." Your boiler works by transferring combustion energy from its exhaust stream into the water that heats your home. While the purpose of running your boiler is to generate heat, there's also a secondary effect of this transfer: cooling the boiler itself. The water carries heat away from the boiler's heat exchanger, preventing it from overheating.

If your boiler fires with a water level that's too low, the internal temperature can rapidly increase to dangerous levels. The low water cut-off helps to prevent this condition by shutting the boiler down if water levels drop too low. However, a faulty cut-off switch can also stop your boiler from running. Since this is an essential safety item, it's important to replace a faulty switch rather than bypass it.

2. Low Water Condition

Of course, your low water cut-off switch may also stop your boiler from running if the water level in the tank is too low. A genuine low water situation usually means you have a leak somewhere in your hydronic system. Many modern boilers use automatic feeders that add water if the low water cut-off triggers, but some older systems may still utilize manual filling loops.

Note that you shouldn't ignore low water conditions even if they don't prevent your boiler from firing. Hydronic heating systems use closed loops, meaning they lose little water under normal operating conditions. If your automatic feeder needs to repressurize your hydronic system continuously, you likely have a severe leak that requires attention.

3. Flame Sensor Issues

Natural gas appliances prevent gas leaks by checking for the presence of a flame after ignition. If the burner in your boiler doesn't light, it's crucial to cut off the gas supply to prevent venting unburned gas into the combustion chamber. The flame sensor checks for the presence of an active flame and closes the gas valve if it doesn't find one.

However, like the low water cut-off, the flame sensor can sometimes fail. When this occurs, your boiler may briefly ignite before cutting the gas. As with the low water cut-off, you should never try to bypass a faulty flame sensor. Instead, contact a plumber to confirm the problem and replace the faulty sensor with a new one.

Reach out to a company like Mark McBride Plumbing to find out more.