Main Supply Too Small? Consequences?

Return air in furnace room is a big no-no. If you have any naturaly vented appliances in the room you could definetly have a negative pressure on them. Check around the drafthood on your water heater and see if there are any signs of back venting. Look for soot around the draft hood or in extreme cases even the plastic control valve will begin to melt. If the unit is in a wide open basement, the return air will probably not present too much of a problem. Rule of thumb in our area is no return air within 10 ft. of the furnace.

I assume the grill added at furnace was added because your return system was too small at installation. Is the blower door hard to remove when furnace is operating? Check opening behind return grills.The opening through floor and the grills any or all could be restricting the flow. How large is the return duct? How large are the returns inside the home? How wide is fin spacing?

I glanced over the postings about your questions. I really suggest:
1. First have a load calculation on your house to determine the heat gain and heat loss you have!
2. Have a trained contractor design and size your ducts per the loads.
3. Select the equipment per the loads.

People assume that more is better when it comes to air-conditioning. The consequences for selecting too big of equipment in air-conditioning is; Cycling of the equipment. This means that the equipment blasts cold air for short periods but never runs long enough to remove the humidity. This results in higher electric bills and lower comfort.
Also if the air handler is not sized for the evaporator, the air will be traveling too fast across the evaporator to allow the heat transfer from the air to the refrigerant. This would result in warmer air temperatures coming out of the supply ducts. Beside the noise from the wrong size of ducts that you have currently.
Good luck with your system. But I strongly suggest getting some help from a local reputable contractor.

Apologies for the radio silence - busy last few weeks with new baby!

Thanks to everyone for their input. Finally, with 6th contractor, received an analysis consistent with ideas and input from this great site!

First, to answer a couple of specific questions posted:

Black Adder - Thanks for the safety tip. I've seen that one posted here before. No worry in my case - furnace is only gas appliance and in wide open basement space.

hvacservice (et al) - Contractor #6 was only one that acknowledged return size/access problem. Additional return capacity and access will be added.

professor hvac - 2 of 6 did load calcs and actually came up with same result! Better yet, their results closely matched mine produced by HVAC Calc, which is one of the only consistent results of this whole process! Of these 2, 1 suggested adding 9-11 additional branches to my already undersized supply. Of course, he may not have realized it was undersized, since he never measured it! Anyway, that left only 1, which made the decision easy!

So, the final (and $$$) solution is that we're going to rework the ductwork (supply & return) and have it done correctly. Perhaps we're nuts to go to this extra expense, but we'll be in this house for another 15-20 years, and I don't want to be dealing with this problem beyond 2005!

Thanks again for the guidance...

Wow. Six contractors and only the 6th one sounds good based on building science principles. All I can say is, I'm glad it was only six and not more! It seems par for the course that 80% or more of the guys in the business, are not the ones an informed homeowner would really want.

Congratulations, and best of luck -- P.Student

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Originally posted by perpetual_student
Wow. Six contractors and only the 6th one sounds good based on building science principles. All I can say is, I'm glad it was only six and not more! It seems par for the course that 80% or more of the guys in the business, are not the ones an informed homeowner would really want.

Congratulations, and best of luck -- P.Student

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So the "uninformed" homeowners actually want them???LOL!!

I am embarassed to say, but hell yes. I lived in one house for 22 years and never knew the difference between a good pro and a hack. There were multiple occasions when I got pissed off because of poor results, but sincerely I did not know what to look for.

Looking back at my old records, back in the mid-1980's I paid for cellulose insulation in the attic, but said "no" to an offer for radiant barrier because I was unfamiliar with it. And infiltration was something I had only the dimmest understanding of.

I am convinced a person in business could survive indefinitely selling empty promises. Every day I hear radio ads promising big energy savings by replacing one's AC with a new Trane (as if), and energy savings from buying a filter which only has to be replaced once a year. By the time the homeowner learns it doesn't work for him, the check has been cashed and the business person seems to have developed a case of deafness. Sorry to say.

Best wishes -- P.Student