AccuServ Heating and Air Conditioning

A Single Boiler Test That Can Save Your Life

February 5, 2020

Back in 2016, a family left their three-year-old daughter at home with their nanny. The house was quiet, and nothing seemed amiss. As the three-year-old girl slept soundly above the boiler room, a sudden explosion rocked the house.

The boiler, located in the basement, exploded, blasting all the flooring on the first floor and cracking several walls. Though the child and nanny were unharmed, it gave the family the scare of their life.

What Happened? 

A follow-up report on this video sheds some light on exactly what happened.[1] No, it was not the vicious attack of a hostile neighbor; it was the failure of two essential safety components. What appeared to be a bomb blast in the video was an exploding boiler.[2]

The Aquastat and Relief Value 

Every conventional boiler comes equipped with an “aquastat” and a “pressure relief valve.” The aquastat is essentially a thermostat that tells the burners when to come on and off.

The relief valve releases excess pressure build up in the boiler. During high levels of pressure, the relief valve will open and allow that pressure to escape safely.

Typically if the aquastat fails to shut off the burners, the pressure relief valve will release the excess pressure out of the system. Likewise, the aquastat automatically cycles the burners on and off, so the system will never reach dangerously high pressures even if the pressure relief valve fails.

The real danger is when both the aquastat and pressure relief valve fail at the same time. In this case, the aquastat allows the burners to run continually, and the excess pressure has no way of escaping the system. The system keeps building up the pressure and, like a balloon with too much air, eventually pops. Or, in the case of a boiler, the system explodes.

Now your boiler has just transformed into a homemade bomb.

This video from Myth Busters shows a similar scenario with a water heater.[3] With all the safety devices removed, this ordinary water heater turns into rocket, launching 500 feet in the air!

The Test that Can Save Your Life 

As a homeowner, you have limited access to the tests you can conduct to ensure you boiler is safe. Without the proper tools and expertise, you won’t be able to know the overall condition of your boiler.

However, this 5-second test will allow you to find out if you are safe from excess boiler pressure.

As both components must fail at the same time, you only need to make sure one of these is working. Unfortunately, the aquastat will require several tools and an understanding of electricity to diagnose. Therefore, you will have to test the functionality of the pressure relief valve.

Locate the relief valve on your boiler – it will be a valve with a lever on the top which you can pull for testing purposes. Pulling the lever upwards should release a stream of water, indicating that it is working: automatically releasing excess pressure from the system. If this test checks out, your boiler won’t be exploding anytime soon.

What If It Fails? 

However, if no water comes out, your relief valve is broken. This scenario is dangerous, and you should call a professional to have it replaced right way.


Important: a working pressure relief valve doesn’t ensure safety. It might keep you safe from your boiler turning into a bomb, but you could be exposed to many other hazards. For example, your boiler could be producing dangerous levels of CO and aldehydes or be leaking gas. Performing annual maintenance checks ensures your boiler is safe and fully functional for the upcoming heating season.

You should get your boiler checked every year to prevent hazards. If you call a proper HVAC company, in addition to checking your boiler, they will also look for other common but dangerous home heating risks, including carbon monoxide[4] and frozen pipes[5].


The chance of both your aquastat and pressure relief valve failing at the same time and causing your boiler to explode is low. However, do not confuse “low” with “impossible.”

Taking a few seconds to ensure at least the pressure relief valve is working can spare you from ending up in the news as the latest home tragedy.

Stay on the safe side and perform the test today. Better yet, contact your local HVAC company to provide a full safety inspection.