Why Vent Gas Hot Water Heater?

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    • #278326
      David Christian

      I recently replaced the flue pipe on my gas hot water heater. I am wondering why a gas hot water heater needs to be vented and my gas stove and gas oven do not have to be vented. Does the hot water heater produce more carbon monoxide than the oven?


    • #298293

      All dependent on BTU usage.

      Water heaters produce anywhere from 30,000 to 150,000, depending on size requirements (Residential, Commercial, Industrial Application)

      The comparison from stove to water heater is not similar in the amount of Carbon Monoxide produced.

      Make sure you used Galvanized Ductwork piping for your flue pipe, along with the number of screws per joint per inches of pipe size.

      EI 3″ – 3 screws per joint 4″ – 4 screws per joint

      Each fitting (flue 90) has two ends so a 3″ flue fitting would have 6 screws per fitting, 4″ would have 8.

      That is what inspectors are looking for upon inspection of a water heater.

      “Your best interest is secured by making the right decisions the first time.”

    • #298294
      Tony the painter

      Thanks Dunbar for explaining why I need to vent a hot water heater and don’t need to vent my stove or oven. Also thanks the information on installing a hot water heater flue pipe – looks like I need to add a few more screws at each fitting – I have 2 or 3 screws where the pipe connects to an elbow or another section of pipe. I used 4″ flue pipe.

    • #298295

      The whole and single purpose of the vent or flue as it is known for a gas water heater is designed to form three main functions which are (a) to allow the products of combustion which if everything is burning properly is carbon dioxide and water vapour, (b) to make the heated air from the flame draw up a tube (flue) allowing the baffle fitted in there to slow the flue gases and allow the maximum heat transfer through the cylinder walls into the water and (c) to ensure that the main burner can operate safely without baffling or smothering.
      The size of the flue is dependant upon the specifications provided by the manufacturer which at times will vary from model to model and size to size.
      An oven or cooker does not have to be flue vented as it is considered to be an “intermitent use” product whereas a water heater is operating at all times even if only at the pilot light level.
      A gas water heater should NOT produce Carbon Monoxide as a by product of combustion – if it does then it has not been set up correctly or properly serviced.

      Selgas Services Ltd
      Craftsman Gasfitters, Plumbers, Electrical Service Technicians

    • #298296

      Actually (a) should read at the end “to allow the products of combustion to safely discharge to atmophere.

      Sorry about this ommission I must have been asleep!!!!

      Selgas Services Ltd
      Craftsman Gasfitters, Plumbers, Electrical Service Technicians

    • #298297


      I could use some knowledge in this area, not afraid to ask either.

      I was upon the assumption that any burned gases in the use of a water heater creates the by-product of carbon monoxide, or CO2. I guess I don’t know the difference between dioxide or monoxide.

      Flue size when I was taught dealing with numerous types of water heater with combination 3 or 4 inch draft hoods, anything above a certain BTU has to be vented at 4″ minimum. Is it anything above 60,000 BTU that requires a 4″ flue?

      For example a 75 gallon gas, or 66 gallon gas high efficiency.

      The draft hood will accept 3 or 4 inch piping, but inspectors will fail if it is undersized.

      I always use a draft gauge when installing a new gas water heater to see if the water heater is getting adequate draw to which it is connected.

      In my area, 300 fpm’s is the required minimum, I prefer to see one run anywhere from 550 to 850 for satisfaction that all is well with the operation of chimney.

      I am going to memorize your response since I know from your postings and info that you know your profession well. In my novice years as a plumber, I use to install water heaters for Sears, Home Depot, and Lowe’s for a plumbing contractor, and I learned a great deal, but you can install a product everyday and still not know all about it’s workings.

    • #298298


      We must have had the same instructor or they read the same book beacause; that is exactly the same way it was explained to me years ago.

    • #298299

      Plumbill : Thanks for you kind words who said that Tradesmen thousands of miles apart don’t think similar on ocassions hehehehehe.

      Dunbar : I am happy to assist where I can and I too am not too proud to ask when I don’t know or am unsure of things – there is no shame in that whatsoever.
      CO2 is in fact Carbon Dioxide the product of what we breathe out each breath we exhale, yes in some cases very small portions (harmless quantites) of Carbon Monoxide is formed this is in fact true when any hydrocarbon is burnt – so it is nothing new.
      Not sure about your draught requirements where you are – down here it is not a specified item unless you are involved with inductrial gas burners and the like. For domestic type appliances a smoke test usually suffices for legal requirements. As far as flue sizes are concerned our regulations state that whatever the manufacturer specifies must be adheared to so the responsibility of flue sizes off appliances belongs to the manufacturer – unless one is combining a number of appliances together on the same flue line – then the rules change and that system is carefully laid out in our Standards to which we must work to here as they do in Australia and England as well.
      You are so right we can all install appliances and not know all the workings completely this is one way that we keep learning as we do our jobs.
      Thank you for your valued input into the discussion.

      Selgas Services Ltd
      Craftsman Gasfitters, Plumbers, Electrical Service Technicians

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