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Background: The HVAC unit I have serves both the 1st and second floor, but does a poor job of distributing heat to the 1st floor. The previous owner added a hydrodronic boiler and baseboards to the 1st floor, so the duct vents for the 1st floor are closed and sealed (with magnetic covers and some duct tape).

Problem: My HVAC unit is located on the 1st floor and when the upstairs T-stat calls for heat I can hear a fan kick in for about 15-20 seconds before the poof/rawr of the burner lighting. During this time there isn't a smell, but shortly after lighting another fan kicks in making a clacking noise (you can hear it here). A minute or two after this I start to smell something earthy akin to burning aluminium, or maybe a strange mold. The smell is most prominent just out side of the equipment room (even more than in the equipment room itself) and isn't nearly as bad on the 2nd floor. What might be causing the smell? the clacking noise?

Possibly related info: There seems to be two sets of fans associated with my heater. When it first kicks on, a fan, that I'm guessing provides the air supply for the burner starts up and begins drawing air though a grates on the side and top of the side of the furnaces burner compartment. About 45 seconds later another fan that has starts up, and begins circulating air though the system, however, at the same time, the same slots that were previously drawing air in now begins to blow air outward. Might this be part of it?

Video of one panel removed, before and after the second fan kicking in: https://photos.app.goo.gl/CgdhcCxrmfskzMjZA

And another clip better demonstrating the force of the air flow after the second fan kicks in: https://photos.app.goo.gl/pdEWUi4Q39MHwkLB6

The triangular grated thing next to the motor connects to the flue, and it's ability to draw in exhaust doesn't seem to be affected by the second fan (though this might change once the paneling is back on).

  • Probably unrelated, but any time there is a possibility of incomplete or improper combustion you should consider the possibility of carbon monoxide. If you don't already have a detector on the first floor near the HVAC unit (not too close to avoid false alarms, but not all the way across the house), install one. – manassehkatz Mar 29 '18 at 2:17
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I think you need to find your way into the heat exchanger and look for "foreign" matter to clean out.

Let me walk you through what is going on here. The initial fan noise you hear, is the draft inducer blower, and it does exactly what you suspect. It should take air from either your intake air vent, or older furnaces would use the ambient air of the room. After the ignition logic does it's safety thing, then the main burner kicks in with that big whoosh. The main blower will come on soon as well. At this point the flames are burning in a combustion chamber where the exhaust should go up your chimney or exhaust vent. You should not smell this for more than a minute if everything is drafting properly.

The heat exchanger is the above the flames, and the main blower moves return air over the other side of these channels and that is the heated air sent out the rooms.

Now when this metal heats up there may be some odors depending on how clean they are and what condition they are in. That would be in the air flow, so that your don't smell that everywhere is telling. A bad possibility would be a hole in the heat exchanger where combustion products could leak over. A CO detector is highly recommended.

  • I' definitely smelling this for more than a minute. I think the exhaust of the main blower is somehow pushing the flue exhaust (see the video I added). Also, I'm not sure if the clacking noise is an issue. – virtualxtc May 3 '18 at 18:01
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    The main blower should pull from your return ducts, and push to the output registers. The duct work should be closed system, but often their are minor holes and unsealed seams in the assembly. The combustion airflow should have a natural draw once the main flame is running. If the main blower is pulling too much on the furnace room, then both ductworks need to be tightened up. I'm not an experienced HVAC professional, so I'd encourage some other opinions here. – DaveM May 4 '18 at 1:25

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