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back drafting problem

Bob HarperBob Harper Member Posts: 732
You're trying to vent an atmospherically vented appliance through a 3 ft. vent and wonder why there's no draft? Now, you're going to add a 90 ell along with a 5 footer? Why the 90?
Thos gas pressure numbers don't make sense. Please clarify are you talking about manifold pressures and when you say "3 inches", do you mean 0.03 inches w.c.? What are your inlet gas pressures?
All the makeup air in the world won't cure a short, cold stack.
With the room full of warm air, the stack effect in the room makes the room a better chimney than that crummy 3 foot pipe.

Rudy, I hate to say it but if you're calling 4" B-vent expensive, you're in way over your head.


  • airmanairman Member Posts: 10

    Hello all on the "Wall",

    My old gas fired Modine unit heater that I had hanging in my garage for probably 20+ years, and faithfully served me without fail (till now) bit the dust. I purchased, then installed the modern direct replacement for it today and now it keeps tripping the safety switch on top. The flue gases seem to be spilling into the space and not to the out of doors. The unit, a Modine model # PD50AA0111 is a natural gas, gravity vented, 50,000 btu, your garden variety unit heater. The unit I took out was a Modine PA50AB (pretty much identical, just older). The venting consists of 2, 4" inch single wall 45 degree ells connected to a three foot piece of 4" B vent (with lots of pitch) which is sticking outside two feet with a termination cap/fitting on the end (I know, kinda redneck but it worked). The garage is a 20 x 30 block wall building with an insulated roof, insulated doors and window which I would consider somewhat tight construction. The weather today was without any significant wind so I don't believe this was a factor. The unit will fire, the fan will start, it runs for maybe 7-10 minutes, then the safety trips. I lit a match to check draft and its back-drafting. The flue is not blocked. I don't necessarily want to purchase a elbow and a length of B vent if it won't work (on a guess), meaning by increasing the height. I just don't know why I never had a problem with the last one. I've also tried it without the termination cap too. If I have to increase the height I will. I just don't want to "try it" (guess) and have it not work.

    Any ideas?

  • rumn8rrumn8r Member Posts: 104
    Propane or Natural gas??

    If you have a natural gas unit in a propane setting, it WILL back draft.

    THe life you save may be your own...Roll out switches tripping mean SOMETHING. Do not reset and ignore, or you may wake up dead:-(

    And THAT is not a good way to start the day.

  • airmanairman Member Posts: 10
    roll out?

    Thanks for the reply ME.

    The heater is listed and labeled for natural gas, and the gas valve is also. Is it possible the orfices on the manifold are for LP, and if so, how would you tell? I don't see any markings on the outlets.

  • Combustion Test

    Rudy the only way to determine what is going on with your unit heater is to have a contractor who has a combustion analyzer along with the proper training test it.

    They should be able to help you remedy your situation & give you some answers to your questions.

    You don't know how weird it is giving someone named Rudy advice on combustion testing, I am used to it being the other way around.
  • rumn8rrumn8r Member Posts: 104
    If that were the case...

    You'd have a different type of complaint. The orifi for LP are MUCH smaller than natural gas, and the unit would be under fired and would be short cycling.

    The LP vs NG was just a quick gut check on my part, and a typical common mistake that kills people. You do need to get someone in there to have it checked and adjusted.

    You may also be dealing with a combustion zone depressurization situation, which can also kill you. In any case, keep it turned off until a pro with proper instrumentation can check it out.

    If you can't find a pro via the Yellow Pages, try going to and see if there is a certified pro in your area.

    In leiu of that, try the Find a Pro here (top of page) and see if you can find someone to help you. THis is serious business. Treat it accordingly.

  • airmanairman Member Posts: 10
    Combustion Test

    Thanks for the reply David. A combustion analyzer would be nice, but I've already determined that its back-drafting. You see, I've never had a problem with the exiting venting configuration. Although, not ideal, with just a pitched piece of B vent it worked for 20+ years this way. I'm going to attempt to increase the flue height by adding a 90 degree ell and then adding a 5 foot piece to it vertically. This "should" work. I just don't want to pay for B vent for its very expensive, if I don't have to, or it won't work. My next fix (although even more $$) would be to add a exhauster to it.

    nothing is easy anymore*

  • TechmanTechman Member Posts: 2,144

    Is it possible the new heater has a spill switch and the old heater did not?And the backdraft has been there all along?
  • EdEd Member Posts: 284

    Vent plugged w/bird nest? If your doors are open & not venting, I would check the flue for a blockage. The other possibility is overfired due to high manifold pressure, or wrong orifices(too large).
  • EmpireEmpire Member Posts: 2,343
    Vent termination???

    Are you terminating out the side wall? Modine will void warranty if so and further more it is violating national fuel gas code. Only a induced draft unit can be side wall vented.

    Mike T.
  • MitchMitch Member Posts: 549
    I typically use the Hot Dawg

    sealed combustion unit.

    Better air to breath if working in the garage.

    Less chance of contamination problems.

    Eliminates draft issues especially if a door is opened during firing, draft always seems to go negative when that happens.

    My bet is from your description of venting and lack of vertical rise the problem always was there just unnoticed.

    Mitch S.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Robert O'ConnorRobert O'Connor Member Posts: 722


    I have determined that the orifice used is for natural gas, and its drill size is 30, so I'm "cooking with gas". I have also checked my gas pressure with a manometer and during the call for heat it registered almost 3" inches of water column. I have also checked the pilot flame which was burning just right by appearance as well as hooking up the manometer which was right on 6" inches. I'm not using a U-tube type but rather a meter As far as the "combustion zone depressurization situation", I'm not real sure I know what you mean but can only assume you mean some condition within the garage that would be pulling these gases out of my heater. The garage is real tight. The windows are double pane and I never open them and the garage door has a real nice seal on the bottom. Bottom line is not much air is entering or leaving the space forcibly.


    You are correct, I just checked the old unit and it does not have a spill/blocked flue safety on it. Now I can unequivocally confirm my wife's believe that I'm a dopey SOB, in more than one way and that condition "may" have been recognized this whole time.

    Ed, The vent is whistle clean and I (see above) checked for over-fire. Thanks for your reply

    Mike T,

    I may have to install a fan but I'm going to attempt to extend the vent height first based on the expense and the infeaquncy of its use.

    Mitch S,

    I would consider your reply to be the best solution unfortunately I'm committed to making this work so I'm going to try to first try and extend the vent vertically 6' feet and then test the results. If this works with single wall, I'll be switching it to B vent and securing it to the roof., or, I'm going to have to purchase a fan that will assist these stubborn vent gases.

    Thanks everyone for all your replies and I'll post back with the results.

  • rumn8rrumn8r Member Posts: 104
    Too tight may be the problem...

    If you are combusting, and using inside air, and there is no place for air to be made up from, the next best thing to releave the lack of combustion air is the hole in the roof that the flu gases are SUPPOSED to be going out of. You need at least 1 square in per 4,000 btuH input in the way of combustion air. Open a window and try it again.

    COmbustion zone negative pressurization could be coming from any one of a hundred different sources, including but not limited to stack effect, exhaust fan operation, wind loading, and more. THe draft for the appliance is supposed to be operating at .02" W.C. negative pressure. Anything that can generate negative pressures greater than that will cause back drafting, including the combustion process.

  • Larry WeingartenLarry Weingarten Member Posts: 1,173
    Would it be...

    ...a useful test to open those windows a crack and see if it has any effect on your heater's drafting? If the garage is too tight, that would tell you. Adding 6' of vent (and possibly some source for combustion air) would likely be the next step.

    Yours, Larry
  • Robert O'ConnorRobert O'Connor Member Posts: 722

    ME & Larry,

    I've "tested" the new configuration, that of an elbow and two, two foot pieces of single wall, with, and without the termination cap, and then with three, two foot pieces and either way its worked wonderfully. The unit heater is finally venting (drafting). I've recognized the longer the vent height, the better the draft effect. I've also (after reading your post) tried this configuration with and without the door and window opened to see how this would affect/change draft. It would seem as if it had little change. Once the vertical rise was made, the unit worked remarkably well. I have just did a little light reading on combustion air and if I may ask 1 more question? My garage is 20' x 30' with a vaulted ceiling (averaged at 12'). Would the calculation be.. 20 x 30 x 12 = 7200 cubic foot using a valve of 50 cubic feet per 1,000 Btuh (50 x 50 (50 representative of 50,000 Btuh unit heater) = 2500 cubic feet). If this is correct, am I correct to "assume" that I can use "all" the combustion air from within the space? If the old unit wasn't making me sleepy enough I most definitely must remember to hold on to this Fuel Gas Code for those nights I battle insomnia.


    I have most certainly been using this (old) appliance all these years without the where-with-all and or benefit of correct venting. I also learn from my mistakes and am not ashamed to say it was installed incorrectly all this time. I've checked the gas pressure on the outlet port of the gas valve while it was firing and have checked with Modine and they claim 3 & 1/2" inches is ideal at the outlet (mine was 3) port while firing. The pilot reading is also consistent with the manufacturer's spec. B vent is expensive. Why would I spend $120 for all the "right stuff" if its not going to work (not to mention the time). To correct the problem completely I would have to of bought 2 B vent directional fittings, 1 three footer, 1 elbow, 1 six footer, termination fitting, bulkhead combustible clearance pass-through, and a riser type clamp with bracketry to hold it fast to my roof and if it didn't work, then what? I'm a bit frugal but not cheap. As a matter of fact after all this I'm pretty much hating the way its going to look when finished and am considering Mitch's idea of the Hot Dawg (this is why I vented the thing like this is the first place). Now I'm probably "out of my head", but certainly not over it. I originally didn't want to go through the roof because for one, it would be a place that water would collect and get in, two, the location is in a valley of the roof and close to the edge, and three the main reason was the roof was slate. Thanks for the reply Bob.

    Anyone wanna buy an "almost" new unit heater?

    Thanks to everyone who replied!

This discussion has been closed.


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