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Furnace Venting Screens

October 8, 2013
This is an email we got from listener George about the venting of a 90+ furnace , (Edited for length) Ron, The show ended last week with you suggesting taking the 90% furnace intake outside. I agree with most all the reasons you gave and mirror the points I made some time ago calling into the show with the exception for the outdoor screens. I think you need to give some fuller details if a homeowner were to attempt to do this. When terminating both intake and exhaust they should be: 1.) Within 18" of each other side by side to insure they are in the same "pressure zone". Each system should be separated by at least 36". 2.) The intake held 1' above the "anticipated" snow level. 12" in our area. Make the point if the intake is only 1' off the ground and the snow drifts it could get covered. If we get snow and the furnace won't start look outside before calling for service. 3.) They need to be separated 1' minimally vertically with the exhaust being above the intake so not to have the exhaust with lowered levels of oxygen to be mixed back in. The exhaust slightly warmer than the outside ambient temperature and would rise and could be drawn back in the intake. This could also cause icing should outside screens be applied. 4.) They both should pitch back to the furnace and drain out back at the furnace. On a pocket protector note, some higher efficiency furnaces vary inducer speed to meet demand. This means you don't always get the same "throw" away from the outside exhaust opening and why the exhaust needs to be above the intake. Even at lower demand the furnace should still operate at the point of condensation, which means there will be enough moisture available to ice over on a screen given a low enough outside temperature. All manufactures include internal screens (closed to the inducer housing if not part of it) usually plastic discs. Placing a fine mesh screen on the outside reduces the "net free area" and restricts flow thereby reduces efficiency. Insects may get in the combustion components but it's too acidic an environment so they won't stay there,  nest or get into the house and be blown out next time the inducer starts. There were some direct vent furnaces in the past where outside screen were used but they were not sealed combustion nor condensing. I know quite a few hvac installers that just keep doing what they where doing not knowing the difference. Still others did it, should I get a leaf or something, I don't need to take it apart at the inducer and still charge for a service call. Now I know you hate "we've always done it that way"! I Like, you deal with the code all the time. The fall back position is follow the manufactures specs. Lastly I'm including a 2-pipe venting jpg. For the audience, Most contractors attach the paperwork that comes with the equipment that may cover acceptable venting and homeowner maintenance somewhere close. It a good resource though most HO never look at it. mcb vent    

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