HVAC question

[Jeff]

Air conditioning/fans off this morning. Nothing running. I noticed air flow coming in through the floor vents by just putting my hand on the grille. The ducts, registers, air returns, etc. were sealed with mastic a few years ago so I'm thinking that the air flow is coming in from WITHIN the ducts. Kind of a back flow through the external venting of the heating/cooling system.

Our system: Heating; Trane high efficiency propane unit in the crawl space installed about 5 years ago. Cooling; Trane XL15i SEER, 3.5 ton AC with evaporator coil installed in June, 2011.

In retrospect, we used a lot of propane last winter at a 68-70 degree thermostat setting. It just seemed colder than we'd recalled from previous years. Maybe even a little draftier too.

Any thoughts on possible causes and how to diagnose them? Total replacement of the ducts is an option, but I'd rather not go there yet. Those ducts are about 22 years old so new stuff could be better. :dontknow:

Thanks as always.

 

[gator]

I'm not a HVAC guy but the first thing I would do is to turn the heating system on and see if you get heat. If you get heat then the problem is on the AC side. If the heat doesn't work either then I would suspect the air handler portion of the system. If I recall from a problem I had a few years ago there was a problem with a fuse in the air handler and if the blower wouldn't come on (blown fuse) then the neither the heat nor AC would turn on (some sort of interlock).

George

 

[Bigdog72]

I am assuming from your post that the system works and you are just concerned about air flow when air handler is off. Check to see if a duct has come apart, check to see how the air is getting into the crawl from other than the vents, and make sure your fan isn't set to "on" rather than "automatic" on your thermostat. Also, a critter may have chewed a hole somewhere in the duct but you really need to find the source of the air flow.

 

[RayH]

Are the fans etc not running because they failed to come on when the thermostat called for them or just because they are not being called for by the thermostat? If the latter, check the duct work for something that has pulled apart or torn. With the AC unit just installed a couple of months ago, something may have been knocked loose. Shucks. We lived in our last house for over a year before we realized one of the installed vents was not hooked to the duct work:embaresse It took just the right day with the breeze from the right direction to create a draft through it.

Good luck.
Ray

 

[SteveColes]

If I understand correctly, you weren't saying that it was broken. You were saying that when everything is turned off, there is air flow and you are wondering how to diagnose.

If that is the case and if you have access to all the ducts, the a "smoke" test is the easiest. I would start at the return(s) and see if there is flow in or out. Flow from or to the return could be normal, if there is a temperature differential between the return and any of the registers. But if I suspected leakage, the I would turn on the system and then use smoke to look for leaks.

 

[Jeff]

If I understand correctly, you weren't saying that it was broken. You were saying that when everything is turned off, there is air flow and you are wondering how to diagnose.

If that is the case and if you have access to all the ducts, the a "smoke" test is the easiest. I would start at the return(s) and see if there is flow in or out. Flow from or to the return could be normal, if there is a temperature differential between the return and any of the registers. But if I suspected leakage, the I would turn on the system and then use smoke to look for leaks.

Steve's assessment is correct: Nothing is broken; heat and ac work as needed. Just rogue drafts coming in through the ductwork. Nothing obvious from the crawl space and the supply/return ducts are insulated so the problem could be invisible at this point.

Sorry that my explanation was so confusing. :BangHead:

 

[junquecol]

Air can thermo siphon (hot air rises, cool falls.) I would check and make sure return was intact. Easiest way to do such is turn on fan only, then go outside, leaving door cracked. If you feel a rush of air out the door, then return has a leak.

 

[mkepke]

Steve's assessment is correct: Nothing is broken; heat and ac work as needed. Just rogue drafts coming in through the ductwork. Nothing obvious from the crawl space and the supply/return ducts are insulated so the problem could be invisible at this point.

Sorry that my explanation was so confusing. :BangHead:

Have you directly verified that the air handler and/or condenser fan are not running at the times you are experiencing these drafts ? The XL15 is a variable speed unit, so it may not be obvious.

If yes, I would check the registers in the house to map the 'worst offenders'. I would also pull the register covers (if so equipped) and see if any ducts have separated from the register boots. With a map of the offending vents, you might be able to discern if they all come off the same trunkline and trace that back to the source.

-Mark