Water Heaters come in many different shapes and sizes and use a variety of ways to heat the water. We will focus on residential water heaters in the 40 to 50-gallon capacity using natural gas or electricity as the heat source and supplying enough hot water for a family of 5 in 2 to 4 bath homes.
Although there have been some recent upgrades and modifications in the latest breed of water heaters, electronic pilot light, vapour containment barriers, vortex technology dip tubes, etc. we will concentrate on the bulk of water heaters that are out there in the middle of their lifespan expectancy. When we discuss repairing water heaters, let’s cover a few of the components that are fundamental in most present-day water heaters.
Let’s begin with a natural gas water heater. All water heaters using gas as a heat source with have a readily accessible code approved ball valve type shut off valve. Note that I said readily accessible. That means that the valve is located in a place that can be seen and turned off at first glance. It doesn’t mean a valve that is out of reach behind the water heater and thus cannot be turned off. Attached to that valve should be a flexible code approved gas supply line no longer than 48″. If you are going to be soldering above the gas flex, make sure to cover it with a towel to keep flux paste from spattering onto the gas flex line. Upon contact, the flux will eat through the flex line and cause a gas leak.
How to repair a water heater that won’t stay lit … Common causes are faulty thermocouple, faulty unitrol valve, improper venting, improper supply air, gas supply column range not adequate or undersized due to additional gas appliance add on and failure to upsize gas system for increased Cubic Feet Per Hour BTU gas sizing requirements.
We will address some of these more common causes such as the faulty thermocouple scenario but first, let’s integrate a few of the components to get you acclimated to what can cause a water heater not to stay lit.
The gas flex line attaches to a flare fitting which is threaded into the unitrol valve (the brains of the water heater). This device contains a number of safety devices that prevent a runaway water heater from wreaking havoc. Unitrol Valve Temperature Safety Fuse, Thermocouple sensor, Unitrol Gas shut off mechanism, and Temperature and Pressure Relief Valve. All are important and provide an essential function in sustaining a safe operating condition for the water heater. The unitrol valve contains a thermostat which is adjustable by the user to set the water temperature to the correct setting. The unitrol valve is also the first line of defence in shutting down the water heater automatically. There is a temperature safety fuse located in the unitrol valve that extends into the tank of the water heater monitoring temperature. If the water temperature becomes too high inside the tank, the unitrol valve will automatically shut down. There is also a safety built into the unitrol valve that prevents gas supply to the burners anytime the thermocouple fails to supply a small millivolt signal to the unitrol.
This thermocouple is an electromechanical sensor that when heated produces a small amount of voltage to the unitrol valve. This constant millivolt signal tells the unitrol valve that a pilot light is currently lit inside the water heater. Whenever the pilot light flame extinguishes, the millivolt signal is lost in turn forcing the unitrol valve to shut off the main gas supply to the burners of the water heater. Thus, when a thermocouple breaks down, no millivolt signal is sent to the unitrol subsequently preventing the main gas supply from entering the burners.
You can test a thermocouple by using a voltmeter and a lighter. Place the flame under the sensor and touch the test leads from the meter to the inside and outside of the other end of the thermocouple. If you see no voltage present, replace the thermocouple. If you see voltage present, replace the thermocouple and the unitrol as well.
The thermocouple can have a standard thread or a reverse thread so make sure you take it with you when purchasing the replacement thermocouple and carefully compare the threads that so you purchase the right part. As for the unitrol, you want the correct replacement part so turn off the gas supply and disconnect the flare connection at the nut of the flex line to the unitrol valve.
The procedure goes something like this. Turn off the gate or ball valve at the incoming water flex line at the top of the water heater. Turn off the water service to the home and connect a hose to an outside hose bibb, drag the other end of the open hose to the sidewalk that is lower than the bottom draincock of the water heater and turns on the hose bibb and leaves it on. Turn on the hot and cold control knobs of the low presiding faucets in the home. Now connect a garden hose to the hose bibb at the bottom of the water heater. You are going to open up the drain cock and drain the water out of the water heater. Disconnect the hot side water connector at the top of the water heater, the water should go to the faucet and hose bibb you have previously opened in the house and the yard. This will remove the vapour lock that was preventing the water from draining out the water heater drain cock and outside hose bibb.
Once drained, disconnect the pilot gas tube, and the old thermocouple nut and igniter if present from the unitrol and turn the unitrol valve counter-clockwise using both hands if you are strong or using a pipe wrench if you need a little help. The unitrol is designed to unscrew from the water heater tank. When you unthread it all the way out of the tank, place it in a bag and take it to the store for a proper identification and matching replacement. Use Teflon tape 5 turns around the male thread of the new unitrol valve temperature sensor probe followed by a light coating of Teflon paste onto the threads before you reinsert the new unitrol 3/4″ male end with probe sensor back into the old tank. This should go several turns until it feels really tight. Make sure you have it in there real snug. Now connect up your new thermocouple the same way that you disassembled it. Reconnect the pilot gas tube. Reconnect the electronic pilot push button wire. Reconnect the hot flex line to the top of the water heater. Now turn on the water to the house and purge the lines of air. Then turn on the ball valve at the top of the water heater. Even more, air will come out of the hot side of the faucets in the home. Shut off the drain cock and disconnect the garden hose from the water heater. Once the air is completely purged in the water heater, and you see a steady stream of water coming from the hot side of the running faucet, close the faucet handle. And shut off the drain cock on the water heater.
Now, reconnect the gas flex line to the unitrol valve and turn on the gas shut off valve. Spray soapy water on the gas fittings and check for bubbles, retighten if necessary until leaks are stopped. Now turn the control setting on the unitrol to pilot and depress the control button as you continuously press the igniter button repeatedly until you see a flame in the window or a blinking electronic light. This usually takes about 2 minutes due to the time it takes for the gas to displace the air in the gas supply line from the shut-off valve to the entrance to the Pilot gas tube inside the tank. Once you see the flame or the blinking light count off 60 seconds before releasing the depressed pilot button. Now, rotate the control know to from pilot to ON. Then slowly turn up the temperature setting on the thermostats dial to between A and B and wait 45 minutes to get a valid reading of temperature.
A number of things to keep an eye out for on your water heater:
Regular inspection of the following items. How is the connection of the venting from the top of the draft hood of the water heater to the inlet of the vent connection extending through the roof? Is it connected properly? 3 screws per connection and securely heat taped where there are gaps? Does the vent have a constant gradual slope upward at all times? Does the T and P valve work when you test it? Can it be opened and does it shut off when releasing the test handle? Is the drain line connected to the Temperature and Pressure relief terminating between 6″ and 24″ above the floor? Is it piped outside per some local plumbing codes? Are there two earthquake straps properly lagged into studs with the bottom strap at least 4″ above the top of the unitrol valve? Is Gas Supply 48″ or less in length? Is the gas shut off valve a ball valve approved gas appliance shut off valve and not the old pot metal gate type? Do you have a drip pan installed under the base of the water heater to mitigate water damage in the event of a pipe burst? Have you maintained your water heater with an annual flush to remove sediment to prolong the life of the water heater? Have your anode rod inspected and replaced every 3 years to extend the life of your water heater!