Gas water heater guide:
How to buy, install, inspect, troubleshoot & repair gas-fired hot water heaters. The articles at this website will answer most questions about all types of hot 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).
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The sketch at left shows the basic components of a gas-fired water heater and is provided courtesy of Carson Dunlop Associates. It's easy to identify a gas-fired water heater:
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.
Watch out: sometimes an electric or other additional 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 besides the first water heater that you see. Other hot water sources, such as a Tankless Coil for Hot Water, may also be present.
Gas-fired water heater Inspection Checkpoints, Defects, & Hazards List
Gas Fired Water Heater Chimney & Venting Defects
WARNING: no inspection checklist for any topic can ever be complete or comprehensive; important water heater inspection topics may be omitted as we're working on this section. If you have comments, questions, suggestions, Contact Us
Our photo (left) shows an LP gas fired water heater that caught fire. This outdoor water heater installation was observed at the Ex Hacienda, Gogorron, San Luis Potosi, Mexico. The complete absence of a venting chimney on this heater may have been a factor in its poor operation. The water heater inspection dog was a volunteer.
Check for proper exhaust venting of combustion products from your gas fired water heater. If necessary see these gas water heater venting guides:
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.
Short or missing gas fired water heater flue & chimney warnings
A gas fired water heater whose exhaust flue is too short may fail to develop adequate draft to vent the water heater safely and properly.
See your water heater's installation manual for the minimum allowable water heater chimney or flue height, and consider also the effects on water heater chimney draft of installations in cold or hot climates.
Outdoor Installations of Gas Fired Water Heaters
In our photo at left you can see an LP gas fired water heater installed outdoors beside a mobile home that we inspected in Monticello, N.Y. Notice (click to enlarge any InspectApedia image) that the water heater is not connected to its chimney, is installed outdoors with no protection from the weather, and is installed in a climate subject to freezing weather.
Also notice the old dead water heater leaning against its replacement. We figured that this water heater, if it's working at all, will soon join its sister. This is an improper and unsafe water heater installation.
Even a semi-outdoor installation such as the LP gas fired water heater shown at left can have problems. This water heater (inspected in San Miguel de Allende, Guanajuato, Mexico) is installed where it is completely protected from rain.
But the water heater was not adequately protected from periodic strong winds. The clue that led to this finding was the parade of clay floor tiles stacked around the water heater bottom in a (usually successful) attempt to prevent the wind from blowing out the pilot light for this water heater.
Watch out: extreme measures to protect a gas fired (pilot light type) water heater from being blown out by wind may risk blocking combustion air - resulting in dangerous, potentially fatal carbon monoxide poisoning even at an outdoor water heater if the heater is not properly vented and is installed where its exhaust gases may enter a building. See the water heater photo just above, outside the mobile home window, for an example of this hazard.
Blocked or missing gas water heater draft hood warning
On occasion we find items stored atop of a gas fired water heater, or as in the photo at below left, debris which has been drawn into and around the draft hood, interfering with proper water heater draft and risking unsafe venting of combustion gases from the building.
At the property where we took the photo at above left, 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.
In our LP gas fired water heater photo shown at above right, the installer simply didn't have room for the draft hood when the water heater was installed, so he just omitted it - leaving the draft hood atop the water heater but not connected - let somebody else figure out this installation he decided.
Without its draft hood a gas fired heating appliance cannot be expected to operate properly & safely. We discuss gas appliance draft hoods further at DRAFT HOOD, GAS HEATER.
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.
Leaky water heater tanks & other leaks at water heaters
In our photo at left a gas-fired water heater is leaking.
That puddle on the floor needs investigation. Look for leaks at valves and piping around the water heater, leaks in the water heater tank itself, and look for leaks at the pressure/temperature relief valve.
Plastic piping used for hot water distribution & connected to a water heater
In our photo at left a gas-fired water heater is connected to plastic water supply piping. The pressure rating of plastic water piping systems is usually specified in terms of the temperature as well as pressure in the piping system.
Watch out: hot water temperatures may reduce the permitted pressure rating of plastic water piping in some installations, depending on the brand and product type of plastic water piping installed.
The risk is a burst water pipe, scalding, or flooding in the building.
On some plastic water piping systems you may also find a hot water expansion relief valve installed to provide added protection to the system.
If plastic water piping is installed, such as polybutylene water supply piping, it should be at least 18" from the water heater itself.
Look for insulation that has been improperly added to the water heater tank - it may be unsafe and may violate the manufacturer's installation instructions and warranty.
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 (blue arrow in our photo at left)..
There should be no shutoff valve installed on the hot water line (red arrow in our photo) 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, LP / NG for details.
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. See this Carson Dunlop sketch of a vacuum relief valve [image].
Antiquated & Antique Gas Fired Water Heaters
At below left we show a working Standard Combination Gas FIred Boiler / Water Heater manufactured by Keystone Supply in Philadelphia, PA. At below right The antique gas-fired water heater shown at left is installed in the Gaudi Apartments in Barcelona, Spain. By modern standards this is an unsafe water heater.
Corrosion & leaks at water heater piping & connections
Our photo above shows corrosion at dissimilar metals at the top of a water heater tank. When the connections at the water heater top are sufficiently rusted and corroded the heater will be un-repairable.
Check that the temperature on your gas control has been set to a safe level.
see ANTI_SCALD VALVES & HOT WATER QUANTITY for details about safe hot water temperatures.
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
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
Check for Unsafe Venting or Inadequate Combustion Air: Dangerous Carbon Monoxide Hazards
Reader Question: soot formed at gas fired water heater vent or chimney
4/19/2014 (at WATER HEATERS - home) Dan said:
I have a propane water heater. It builds up a very thick layer of carbon inside on the burner area. It has done this now for 5 years in a row. I have replace 5 water heaters in 5 years.
My problem is I can not get anyone to properly fix my issue. I have had 3 different plumbers come out over the last 5 years. I have replaced the water heater every year now for 5 years in a row.
I have spent more than $9,000 on water heaters and repairs in the last 5 years. The propane company has come out every time and checked everything line presure is good presure regulators are checked, I also have a propane furance which uses the same propane feed and has no issues. I suspect contanimation in the propane tank but the propane company disagrees mainly because my furance has no issues. Is their an independent testing lab that I can have come out and draw a sample from my propane tank and test it?
Perhaps I should add that these 5 water heaters have all been different brands. I was thinking it might be a problem with the water heater itself but seeing as all 5 have all had the same problem with carbon buildup and we are talking about 2 inches or more in a year enough to plug up the vent stack which is powered by the way and that causes the shutdown of the tank. Sometimes the gas valve sticks because of the buildup which causes the shutdown
But anyway it is all the same problem excessive carbon and lots of it. And all they do is replace the water heater with a new one. Every plumber says they have never seen a water heater look as bad as mine does. They are all stumped and so am I. Quite frankly I am at the end of my rope with this.
Reply: watch out for potentially fatal carbon monoxide poisoning & watch for gas leaks
Dan I can only *speculate* as information is very incomplete in your question, but
WATCH OUT: it sounds to me as if there is a combustion air or venting problem that risks production of dangerous, even fatal carbon monoxide gas. [There may also be a propane gas leak hazard related to this problem - Ed.]
Normal burning of propane gas produces CO2 and water vapor if combustion is complete. If combustion is incomplete because of any reason the result is CO - carbon monoxide.
You should start with an inspection of the flues, vents, chimney connections, chimnney, and combustion air by a qualified expert, perhaps a chimney sweep and a qualified heating technician or by your local gas provider and building department.
Reader follow-up: ... the vacuum cleaner he was using exploded on him and filled my finished basement with a black cloud of carbon dust
The vent is PVC pipe it runs up 3 feet and out side of the house another 3 feet. It is 4 inch PVC and all of it was replaced last year including the vent cover on the outside of the house all done according to code and signed off by a building inspector because I demanded it last year from all involved parties. I did not want this to happen again but it did anyway this year. It is a powered vent with a presure switch that is supposed to shut down the water heater if it is not venting properly. That switch has never kicked out.
I can add now a horror story after the fact. The plumber came out to fix my hot water heater. The one I have installed now has a 6 year warranty I pay nothing for repairs or maintainence for 6 years.
And the install was certified by a building inspector as well to insure this warranty. So anyway the plumer comes out and I tell him if the tanks burner is full of carbon again then please replace it as its efficientcy will go down even if you do just clean it out. Then I went to work. I get a phone call 3 hours later. He cleaned it and well the bad news. I say what...
He says his vacuum cleaner he was using exploded on him and filled my finished basement with a black cloud of carbon dust. He called his insurance company and they sent out a hazmat team to clean out my basement.
Clean the furniture and the entertainment center equipment and several ultra high end computers systems all need to be opened up and cleaned the washer and dryer they opened up and cleaned out the motors and the circuit boards etc. 7 guys cleaned Friday and now today and all this week they expect to be done by this friday. So I said to the plumber wouldn't it have been cheaper to just replace the water heater like I asked. He said his boss told him the same thing.
So my wife is ready to sell the house we had to evacuate our house for Easter so much for our family get together. This is really getting to us. Everybody I have talked to about this says they have never seen a problem like this before.
Addendum I now have insurance adjusters that want to pull a sample of propane from my tank to have it tested.
It will be indeed interesting to check for contaminated propane. We don't know a thing about your fuel supplier, but once a year or so we do read about a fuel supplier, usually for heating oil not propane, who was fudging a bit on what they were delivering.
But one would think that if the problem were bad propane fuel, many other consumers in your area, served by the same propane delivery company, would be having similar problerms.
WATCH OUT: because of the risk of a fatal carbon monoxide poisoning problem the priority of attention should be
Where safety is concerned, a "sign-off" without being dead certain about the qualifications of the inspection person and the care of the inspection leaves me nervous. I've run into near-fatal safus involving too many assumptions and sign-offs.
Usually your best bet is to contact the company who made your heating equipment - they have a great interest in its safe, reliable operation.
Vacuum Cleaner Explosion Case Law
WATCH OUT AGAIN: your story of an exploding vacuum cleaner is a strong suggestion that un-burned combustible gas was being vacuumed by the service technician. Most vacuum cleaners use rather modest motors that spark continuously and will certainly ignite an accumulation of combustible gas. A gas piping error or leak in or near the heating equipment could explain this explosion of the vacuum cleaner.
Only if a technician were using a special explosion-proof vacuum cleaner (such as those made by HafcoVac, Nilfsk, Ruwac, or Minuteman, ) would might this hazard sneak by - even then leaving an explosion hazard in the building! This is not a theoretical hazard but a real one, exemplified by your own explosing vacuum cleaner report and supported by case law such as these vacuum cleaner explosion citations and I'm sure many others:
Thank You for the warning on CO. I do have a CO detector in the room with the water heater for just that reason. Got it 3 years ago. It has never shown any signs of CO over the 3 years it has been in operation.
And again I have had everything inspected last year when this water heater was installed. Vents connections all was told to me to be in perfect condition and my "problem" was solved I would not have this new water heater carbon up on me. That was said last year and I said we will see about that, I bet I will have you come out again next year to fix it again. He said at that time well we do cover it under warranty so if that worst case happens we will fix it at no cost to you. I said fine see you next year.
And here we are next year same problem. They have no idea what caused it, The carbon build up which shut down the water heater. It is an on going problem now 5 years running. Until my basement is cleaned out end of this week I will not be able to have anyone work on the water heater problem.
It is working now after he cleaned it. How well I do not know. I may be burning an orange flame right now for all I know. I can not insect it til the hazmat team is finnished cleaning the basement.
Dan the exploding vacuum cleaner detail argues for a gas leak, perhaps inside the device. So we've got two hazards to watch for: production of soot often means production of dangerous CO, and exploding vaccuum cleaner while vacuuming the heater argues for a combustible gas leak in the area where the vacuum was operating.
When we hear about a heating device that produces soot as you describe, it should be patently obvious, even to a beginner servicve tech, that just vacuuming out the soot is not fixing the problem, and the equipment is unsafe to boot.
keep me posted and stay safe.
Reader follow-up: Possible vacuum cleaner explosion due to carbon dust
I was told by the service tech and he told the insurance adjuster that he thought the carbon dust caused the vacuum to explode similar to a grain dust explosion in a grain elevator. He checked the gas lines in the house for leaks before he started the vacuum. And the gas was turned off at the tank at the time.
I had not considered that the vacuum explosion was because of a gas leak inside the house. I have never found a gas leak inside the house. And I have had this done 5 years in a row now. Every year they check my system for gas leaks before they install the new water heater. 5 water heaters in 5 years now.
The propane supplier last year installed all new presure regulators one on the tank and one on the house and tested the supply for leaks and presure. They tested it all again this year and found no problems or leaks. They tested the line presure at the water heater and the furnace both were good.
Well that's encouraging and says something about the limits of speculation about what's going on with mechanical systems when we're offsite and texting.
Still, we want to know why there is so much soot being produced. Generally it's a symptom of improper or incomplete combustion or improper venting - which is why I've been worried about system safety. When you report that the sooting problem has occurred across a number of gas fired appliances installed in the same location and venting through the same chimney or vent system, then one tends to suspect the fuel, combustion air or venting system rather than the appliance, though I suppose the same installer might be making a similar mistake across multiple installations.
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