What if the Furnace Runs Dry?

Everything eventually fails. In the name of safety, we try to account for as many failures as we can, because things end in tears when we don’t. One of the scarier scenarios to consider is when a furnace loses it’s water supply. You might think that the furnace would never run without water, but that’s often not the case. You might also thing “how could it possibly run dry? It’s got an automatic feeder and a pressurized water supply.” And we’ll have to answer, “everything fails eventually.”


Cutting Out the Supply

As we’ve brainstormed around the office, there’s at least half a dozen ways that a furnace’s water supply could be cut off, that might not prevent the system from operating. It is terrifying how many of these failures can happen in common, every day circumstances.

  1. Pump Failure
    The well pump to the facility shorts out or has it’s power supply cut off.
  2. Water Main Failure
    The city water supply is cut off due to construction, age, you name it.
  3. Facility Pipe Failure
    A technician closes a valve they shouldn’t, a main supply pipe rusts, hard water clogs a pipe.
  4. Internal Leak/Failure
    A leak inside the furnace prevents water from remaining in the boiler.
  5. Shorted Pressure Sensor
    A short causes a sensor to read incorrectly, signalling the automatic feeder to not-run
  6. Failed Water Feeder
    The feeder has seized up or otherwise just doesn’t feed.
  7. Internal Blockage
    Hard water builds up inside the furnace, preventing water from flowing.


In the course of our research, we even turned up a case where a boiler had developed a leak, produced steam, and had a temperature sensor error such that the system could not detect and respond to a failure. It operated at a perfect balance where nothing blew up but nothing worked right in ordinary usage either. When the system was taxed slightly, a fresh inflow of water apparently managed to trigger a steam explosion, destroying an entire chunk of the building and HVAC setup.


What Normally Happens?

When a furnace runs without water, the temperature is going to go through the roof. It’s almost like running your car without radiator fluid. Things will start going downhill fast. That’s just the nature of the technology. Excessive Heat = Disaster.

In the case of boilers, there’s at least two things that will happen. As the burner runs, the interior heats up. Without water, the interior temperature can easily soar towards 1000 degrees. The steel making up the furnace will likely begin to weaken around 800 degrees. Hot metal is weak and relatively easy to bend and deform. This type of failure can lead to any number of bad outcomes.

The facility could catch fire, with the flames getting through a melted casing. The furnace could conceivably melt, collapsing into a pile of flaming slag. The burner could run for hours at a time and ultimately lead to intensive wear and catastrophic failure elsewhere in the furnace.

The second thing that will likely happen is an explosion if things go unchecked. If water is introduced to the furnace without letting it cool, the water will flash into high pressure steam and destroy everything in sight. There will be metal flying through the air, inch thick shards of steel turned to daggers in an instant. It’s quite possible the roof, floor, walls, will be demolished in the process. We’re talking temperatures that would incinerate an egg if you tried to cook on the furnace.


Preventing Failure

Modern furnaces have a number of sensors that endeavor to detect these problems early. Roll out sensors can detect if the furnace is about to catch the building on fire. Temperature and flame sensors can try to keep the burner reigned in. For water shortages, we turn to a device known as a Low Water Cut Off (LWCO). These detect and measure the actual water inside the furnace using probes, floats, and other mechanisms, within the furnace itself.

When water gets to be too low, the LWCO doesn’t trigger more water to come in, as a water feeder would. This works on the assumption that no water is available. If no water is available, it must force the system to shutdown. Generally, this works by cutting off the burner’s fuel supply. Like many safeties, this is intended to grind the system to a halt. No fuel means no chances for a catastrophe.

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