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Intake air for 90 plus NG furnace

Fred RappuhnFred Rappuhn Member Posts: 104
My brother in law had a new 90 plus NG forced air furnace installed, old furnace was exhausted up the chimney. The installer neglected to pipe outside air to the furnace, he said "didn't need to, furnace sits in middle of basement". A PVC pipe that was supposed to carry out side air just runs into the open basement.
I thought this was odd, my furnace pipes outside air into the furnace.

Of course it was a after hours installer and no permits pulled .
Is the installer correct???


  • ZmanZman Member Posts: 5,734
    You should check the installation manual to be sure. Most manufactures will let you pull combustion air from the surrounding space as long as the space is large enough and louvers are installed so the air can move freely.
    It is not best practice but probably legal.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Leon82Leon82 Member Posts: 655
    It needs a certain amount of cubic feet of air to Operate that way, if allowed in the manual.
  • HomerJSmithHomerJSmith Member Posts: 878
    edited May 2019
    A 90% NG furnace is most certainly a category IV furnace pulling its combustion air from outside. Follow manufacturer installation instruction to the letter. There could be air/exhaust differential pressure problems affecting performance. Will it work that way, ya.

    Exhausting flue into a masonry chimney is suspect if that's what you're doing. The flue gas is about 120 deg. which is why you use a 3 or 4in pipe. The flue gas can cool and lack buoyancy in a large masonry chimney.

    Everything works until it doesn't.
  • Fred RappuhnFred Rappuhn Member Posts: 104
    Thanks for your input, the furnace is sitting right in middle of an open basement, so I would imagine it would not lack air , I just thought it didn't look right.
    The after hours install with no permits raised a flag with my thinking.
  • JUGHNEJUGHNE Member Posts: 6,783
    Biggest issue is did he run PVC piping up the existing chimney all the way to daylight?
    And is the water heater still vented into the same chimney?
  • Fred RappuhnFred Rappuhn Member Posts: 104
    Water heater is vented into chimney and new furnace has 3" PVC vent to outside
  • HomerJSmithHomerJSmith Member Posts: 878
    edited May 2019
    You definitly want to check for spillage at the W/H draft hood on a real cold day. A water heater temp output is probably in the 300 to 400 deg range which may chill down on a long chimney especially if it is a chimney on an exterior wall not feeding a fireplace or another appliance.

    Also, you should draw the combustion air from outside. The induced fan may pull negative pressure on the room and interfere with the W/H flue exhaust discharge.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,927
    @Fred Rappuhn
    after hours install or not he should have taken out a permit. he should have done a combustion test as well.

    As far as combustion air is concerned the old furnace took combustion air from the basement so your probably fine. The size of the basement with free access to the furnace determines if the furnace has enough combustion air available to it from normal leakage. Most of the new furnaces have the option of piped in from the outside or basement air. Check the furnace manual it will tell

    If this is a normal basement with no sheetrock and exposed foundation and open joists etc you probably fine.

    As far as the water heater vented to the chimney this could be an issue now that the furnace is not vented into the chimney.

    The chimney flue in square inches needs to be calculated. The water heater flue in square inches needs to be calculated. A 4 " round flue is 12.5 sq inches. A 3" flue has 7 square inches.

    This is called the "7 x rule " and the installer should be aware of this. So for"

    A 3 " water heater flue the area of the chimney flue cannot exceed 49 squae inches

    4" flue it cant exceed 87.5 sq inches
    John Mills_5
  • JUGHNEJUGHNE Member Posts: 6,783
    So the WH exhaust is "breathing" on the PVC pipe?
    Without opening a book MBG is that this is not good.
    WH exhaust gases are quite hot and will degrade the PVC eventually....IMO....then possible that the furnace will back draft out of the WH......CO!
    But the tail lights will be far gone by the time that happens.

    I believe action must be taken if this is the case.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,927
    edited May 2019
    The water heater is chimney vented . The new furnace is vented through the wall outside
    Fred Rappuhn
  • HomerJSmithHomerJSmith Member Posts: 878
    What I was kind of thinking was that the induced fan was "breathing" on the W/H draft hood.

    That's all right, I give a firm tail light warranty. When the tail light disappear so does the warranty. Besides which, all complaints are handled by our office in Budapest.
    Fred Rappuhn
  • JUGHNEJUGHNE Member Posts: 6,783
    So my bad misreading.....furnace is side wall vented....correct?

    The only main concern is the "orphan" water heater?
    Fred Rappuhn
  • HVACNUTHVACNUT Member Posts: 3,263
    See by me, it's not called an "after hours" job. It's called a "side job". There are many licenced pros here who supplement their meager wages with work outside the scope of their employer. Why work a Saturday OT (or not) for the boss when you can clear triple that with friends doing a good job and have a blast doing it. Of course the installer must do everything according to code and meet or exceed how he or she performs in the workplace. A written proposal should have been signed. And they absolutely should have had a combustion analysis done.
    And then there are of course the guys who should've never picked up a wrench in the first place.
  • lchmblchmb Member Posts: 2,983
    check the manual for clearances to windows/doors. The clearance for a direct vent and vent only appliance differ greatly
  • Fred RappuhnFred Rappuhn Member Posts: 104
    What really got me going with my BIL is how he was bragging how much $$ he saved because this guy was doing job "on the side" in the evening.
    And how he didn't have to pull a permit because he would say it was homeowner installed.
    I corrected him, It needed a permit, inspected and licensed installer.

    I am understanding what your saying about with out the help of the furnace venting into the chimney, the WH might not be able to draft properly
    And with the furnace intake within a couple feet from WH, the draw could suck flue gasses back into basement.

    But he saved $$$$ !!!
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,927
    edited May 2019
    @Fred Rappuhn

    Some locations may allow a homeowner to install, that varies but that usually does not relieve the homeowner from a permit. Most states with licenses require lincenses for those that "install for hire" or "offer their services to the public.

    In MA where I am the state allows homeowners to do their own wiring, they are still suppose to get a permit but some of the town inspectors won't issue homeowners permits

    Side job or not should have had a permit and been inspected. The liability is enormous if something goes wrong. And if disaster happened you BIL homeowners policy could refuse to cover any damage
  • John Mills_5John Mills_5 Member Posts: 935
    We pulled a job out Friday that was installed by a moonlighter. Some hackery but the reason it came out is it was so grossly oversized they couldn't take the air noise, lack of comfort and high humidity with the big A/C short cycling. Went from 120K 95% to 70K 95% and 4 ton A/C to a 2.5 ton. Costly lesson.

    You could have chimney damage with the water heater "orphaned" in the chimney. EBEBRATT-Ed gave good detail on the rules of leaving the WH alone.
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