AirWorks

Heating and Cooling Solutions



Water Heaters in Crawl Spaces

Dear Comfort Advisor,

The gas water heater in my crawl space has been nothing but trouble. I have had to relight the pilot many times, had a plumber replace the thermocouple, and eventually replaced the entire water heater. The pilot problems immediately returned with the new water heater! Any suggestions?

Dear Sir,

Water Heaters in Crawl SpacesFirst of all, the crawl space is a terrible place to install a water heater for any number of reasons. In fact, Montana code no longer allows this application because there are too many issues to contend with, one of them being combustion air. This may be where your problem stems from and while it is not unusual, it certainly needs to be resolved. You did not state if there was also a forced air furnace in the crawl space (electric or gas), but we will discuss both possibilities.

Pilot outages can certainly be from a mechanical failure of one or more of the water heater parts, like the thermocouple that senses the flame, the pilot assembly, or the gas valve. Generally these components are very reliable so any continuing problem with the pilot requires a look at the bigger picture. Any fossil fuel burning appliance consumes air when it operates. The standard natural draft water heater requires a fresh supply of air at the burner and a diverter on the top of the tank (the conical shaped fitting the exhaust pipe attaches to).

As a natural draft appliance, your water heater is relying on the buoyancy of the hot combustion gases to draw the exhaust up the flue. Hot combustion gasses rise in the flue because of temperature and density differences between the flue gasses and the conditions in and surrounding the flue. This is called the “stack effect.” The vent pressure created by these differences is slight and can be easily influenced by external factors. The most common factor is the lack of make-up air, which is simply a supply of fresh outside air to replace the air that has been used during combustion (or that has been pulled out via exhaust fans). A designated outside combustion air duct should be provided for the water heater. This duct, originating with a screened outdoor vent hood, should terminate near the bottom of the water heater. Generally, the pipe terminates in an open top container which reduces cold air spillage (commonly a five gallon bucket). Providing there is a dedicated combustion air duct, a properly operating water heater, and a properly sized vent system through the roof, everything should work well. If you have a forced air furnace in the crawl space, a major variable is introduced to the proper operation of the water heater.

As a result of poor installation, many duct systems leak air into and out of the crawl space. This leakage, especially on the return side of the ducting, will draw air from the crawl space as the furnace blower pushes air through the living spaces of the home. Because both the water heater and the furnace are fighting for air, the natural draft water heater will usually lose the battle. Air will be pulled from the path of least resistance; the water heater flue pipe that goes through the roof. The air drawn down the flue can extinguish the pilot flame, which would explain your frequent pilot outages. If the water heater is running, it can allow potentially deadly flue gas to be dumped into the crawl space (also known as back drafting). This contaminated air can then be drawn into leaks in the return duct and distributed to the living space. This, in turn, will increase the risk for carbon monoxide poisoning. This example is a worst case scenario but I have certainly seen it in many occasions.

Even without a duct system in the crawl space, the water heater should have a dedicated combustion air duct. Depending on how tight the home is, there are numerous exhaust points from the home waging the same battle for outside air; the clothes dryer, bath fans, kitchen exhaust, and wood stoves/fireplaces to name a few.

I would recommend that you call AirWorks to have one of our NATE certified technicians conduct a combustion test and to precisely measure home pressures to properly diagnose venting and infiltration issues within your home. Once these issues have been resolved your water heater pilot will stay lit.

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This post has 1 comment

  1. Thanks for this article it could save lives. This article points out how dangerous this practice can be Good job AirWorks..

    Jonathan Smith on December 16th, 2011


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