Guide to Gas-fired Hot Water Heaters - Inspection, Diagnosis, & Repair
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The articles at this website will answer most questions about electrical water heaters
as well as many other building plumbing system inspection or defect topics.
This page contains links to in-depth articles on inspecting, testing, and repairing problems residential hot water heaters of all types, including their parts, controls, and alternative sources for hot water as well as tips for improving hot water temperature, hot water pressure, and hot water quantity.
Our photo above shows an antiquated gas-fired water heater (white unit in left foreground) and just for fun, we've included an old Range Boiler water heater hidden back in the corner (silver unit behind the gas fired water heater).
Inspection, Diagnostic, & Repair Guide to Electric Hot Water Heaters
How do we Identify a Natural Gas or LP Gas-fired Water Heater?
The sketch at left shows the basic components of a gas-fired water heater and is provided courtesy of Carson Dunlop. It's easy to identify a gas-fired water heater:
if you can find your water heater tank at all, take a look at what pipes and wires are connected to it.
A gas-fired water heater will have two water pipes connected to it (cold-in and hot out), and a pipe or in some jurisdictions a flexible copper tube bringing LP gas or natural gas to the heater.
The gas fired water heater will have a flue or "chimney" which vents combustion gases outside, either by connection to an existing chimney or flue, or in the case of power-vented and high efficiency gas water heaters, flue gases may be vented outdoors by a plastic pipe.
You'll also see a thermostat and gas valve control which combines a temperature setting with a safety device that senses the presence of a gas flame. (If there is no flame the control will turn off the flow of gas to the appliance.) Lots of newer gas fired water heaters will also include a flue gas spill switch at the draft hood atop the water heater.
But watch out: sometimes an electric water heater is installed as an additional or backup hot water source, so you'll also want to see if you have other hot water heating equipment installed, such as a Tankless Coil for Hot Water.
Here are a few things to check on your gas-fired water heater:
(WARNING: This is an INCOMPLE LIST and important water heater inspection topics may be omitted as we're working on this section. If you have comments, questions, suggestions, Contact Us.)
Gas Fired Water Heater Chimney & Venting Defects
Check for proper exhaust venting of combustion products from your gas fired water heater.
Old chimney warnings: Where a gas fired water heater is connected to a large masonry chimney it may not be able to develop sufficient draft to vent properly. The result could be dangerous spillage of flue gases into the building.
Shared flue warnings: Where a gas fired water heater shares a chimney with other heaters such as a boiler, venting of the water heater's combustion products may be unreliable, depending on whether the other larger appliance is operating or not.
Some jurisdictions do not permit sharing a flue between a gas fired appliance and other fuels such as oil; other communities may permit shared flues provided the gas vent is properly connected: usually entering the main chimney flue below rather than above the more powerfully-venting oil fired appliance.
Blocked or missing gas heater draft hood warning: on occasion we find items stored atop of a gas fired water heater, or as in this photo, debris which has been drawn into and around the draft hood, preventing safe venting of combustion gases from the building.
At the property where we took this photo, the occupants had kept a number of large, hairy, shedding dogs in the basement utility area and their hair had clogged the water heater vent.
Check for evidence of flue gas spillage at the gas fired water heater's draft hood by looking for burn marks, rust, soot or debris.
The melting pipe insulation in our photo at left indicates that flue gas has been spilling from this water heater.
The burn marks in our right hand photo show a serious flue gas venting problem with the gas fired water heater.
If flue gas spillage detection sensors (spill switches) are required in your jurisdiction they must be properly installed, usually at the gas vent draft hood.
(On some gas fired appliances such as furnaces or boilers additional sensors may be installed at or close to the gas burner as well.)
Our photo at left shows that we found a flue gas spill switch lying atop of the gas fired water heater - it had never been installed.
Dangerous carbon monoxide warning & soot from gas appliances: If any gas fired appliance is producing soot it should be shut down immediately, inspected, and repaired before use. Soot from a gas appliance, or high levels of indoor moisture condensation that are traced to that appliance, are indicators of improper venting of combustion products and risk the production of dangerous, potentially fatal carbon monoxide gas.
Other Gas Fired Water Heater Defects
Check for proper combustion air supply for your gas fired water heater. Failure to provide sufficient combustion air is dangerous: lack of adequate combustion air will cause a gas burner to produce dangerous, potentially fatal carbon monoxide gas.
Check the pressure and temperature relief valve on your water heater: look for evidence of corrosion, leaks, improper installation, etc. A missing, modified, blocked, or leaky pressure relief valve is extremely dangerous and can lead to a catastrophic BLEVE boiling liquid vapor explosion that can cause severe damage or even fatalities at a building. See Relief Valves - TP Valves
Look for evidence of leaks in the hot water tank, or mechanical damage, or improper installation. For example most water heaters are intended to be installed in a vertical position. Installing a hot water tank horizontally or in a hole in a crawl space is likely to cause early failure of the heater, violate the manufacturer's guidelines, and may be dangerous.
In our photo at left an oil-fired water heater and an indirect-fired water heater are installed in tandem. That puddle on the floor needs investigation.
Check the settings of the water heater temperature: See Temperature of Hot Water is Too Low. On a gas fired water heater the heater temperature control is usually on the gas valve itself.
Look for insulation that has been improperly added to the water tank - it may be unsafe. See Insulate Hot Water Tank?
Check the water piping and control valves connected to the water heater for leaks, support, and for proper location of shutoff valves. An improperly installed shutoff valve on a water heater can be very dangerous, risking an explosion. Usually the "hot water tank shutoff valve" is installed only on the cold water pipe coming into the water tank.
There should be no shutoff valve installed on the hot water line leaving the water tank.
Check for Gas Leaks: In addition to a characteristic odor that you may smell associated with LP or natural gas leaks, a prolonged gas leak may leave telltale dark stains on the gas piping or control. See Gas Leak Detection for details.
If plastic water piping is installed, such as polybutylene water supply piping, it should be at least 18" from the water heater.
Vacuum relief valves are required at water heaters in some communities, to protect against collapse of the water storage tank and to prevent backup of hot water into the cold water piping should cold water pressure drop. Here is Carson Dunlop's sketch of a vacuum relief valve.
Gas Water Heater Controls & Operation
Check that the temperature on your gas control has been set to a safe level.
Check the Sacrificial Anode & Dip Tube of Your Water Heater Tank
The dip tube on many water heaters functions as a sacrificial anode, as we show here. By constructing the dip tube of a metal which is more readily corroded than the steel of a steel hot water tank, the anode protects the hot water tank from early failure due to corrosion. Here Carson Dunlop's sketch shows the location of the sacrificial anode on an electric water heater.
If your water supply happens to be highly conductive or corrosive (see WATER TESTING GUIDE) then the dip tub/sacrificial anode in the water tank may indeed corrode away until it leaks (dropping the hot water temperature) or disappears entirely.
If your hot water smells like rotten eggs, you should definitely check the condition of the sacrificial anode on the hot water heater, no matter what kind of water tank you've got installed.
For details of how to inspect or replace the sacrificial anode or dip-tube on a water heater, please see Water Heater Anodes & Dip Tubes
Check the Gas-fired Hot Water Tank Temperature and Pressure Safety Relief Valve
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Guide to Gas-fired Hot Water Heaters - Inspection, Diagnosis, & Repair
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