Background: We purchased our home about 3 years ago. It was a significantly distressed forclosure, and as part of the purchase the seller agreed to install a new furnace. A Goodman GMS80904BN was installed in the unfinished basement in November 2008.
The furnace has operated fine, but I noticed when the outside temperature dropped below about 35-40 degrees (a common occurrence in CO!), I could hear a growly hum resonating through our ductwork when the draft inducer started. When I stood next to the furnace it sounded fine, so I wrote it off to being on of the many weird things that happen in neglected 30+ year old houses.
About a week ago we had our first cold snap of the year. When I started the furnace, I didn't notice the growly hum upstairs. When I came down to listen to the furnace, there was some vibration when the draft inducer came on, but it smoothed out and stopped after the burners fired. Expansion/contraction in the flue vent maybe? I wasn't sure, so I contacted the company that did our installation and they sent someone out to take a look.
The tech took off the access panel, checked my filter, and turned on the furnace. He said the draft inducer was working fine (it was a warm day, I expected nothing less), BUT that when the furnace was installed they used single-wall vent pipe and that was allowing condensation to form and drain back into the draft inducer motor. There is about a 5' run of horizontal (but sloped slightly up toward the ceiling) vent which joins the water heater vent, and then runs straight vertical to the roof (1.5 stories up). He pointed out something around the bottom of the draft inducer that might be corrosion, and basically refused to continue with any other diagnostic work until we replaced the vent with type b and replaced the gasket around the motor housing. He said the noise/vibration in the draft inducer motor was likely because it had to "fight" the condensation in the run of vent pipe before the furnace fired. He charged us a lovely fee for the trip and the "diagnostics" and recommended that we "renovate" the installation his company did only 3 years ago (at a cost of nearly 1/2 the original cost of the furnace).
So, finally, my questions!!
1) Was it reasonable to use single wall vent pipe in an unfinished basement installation? There are heat vents down here and the ambient temperature is typically 60-ish degrees during the winter.
2) If that much condensation is forming, would it be normal to see *no* signs of it in the actual vent pipes (no drips, no corrosion at joints, etc), and only see it in the furnace?
3) I've read that Goodman has had issues with draft inducer motors failing. Is intermittant noise/vibration that seems correlated with low temperatures super scary (like, replace it NOW) or can it be related to changing ambient temperatures and resulting expansion/contraction of the connected ductwork? Or is the tech right and it's just a matter of the motor having to work extra hard to clear the vent when it's cold outside?
4) The installation company has an A+ rating with the BBB so I was kind of suprised when their tech didn't seem to think there was anything wrong with telling us we need a significant amount of "renovation" and repair work only 3 years after his company installed the furnace. Is it unreasonable for me to expect that they stand behind their installation? If their installation was done in a way that it allowed condensation to cause significant damage to the furnace itself (as implied by their tech), should I expect them to fix it?
I know it is impossible to really diagnose my problems without being able to inspect the furnace, but I appreciate any insight you can offer!! I tried to be thorough but if I can add any more information please let me know.