Furnace replacement - proper vent size

[tunacan]

I am taking bids to replace a 25 yr old Bryant furnace (75,000 btu upright, cracked heat exchanger) in a two story, 775 sq. ft. RENTAL UNIT(existing two ton AC). I have been advised to replace the coil when I install the new furnace. Begging your indulgence, I have several questions.

1. Should I replace the coil now? What about the prospect of later having to replace the outside condenser with a model that uses the new refrigerant (410A?). Will my new coil be able to use the new refrigerant?

2. One bid proposes a Lennox 70,000 btu Merit series, pricing both a G40 and G50. I am seeking guidance about which model to use for my rental property which I hope to own for another 5-10 years - the price difference is about 20%.

3. A second bid proposes a Rheem (RGPN-05) 50,000 btu furnace. I was wondering why the difference in btu's (the residence is very tight, well insulated - Tulsa, Oklahoma climate).

4. I have had excellent experiences with my Rheem units in my three rentals and my residence (the Bryant furnace has been a headache). Are there any recommendations as to Lennox v. Rheem?

5. The Lennox bid wants to replace the vent. The existing unit has a 4" vent that runs for about 18" (one 45 degree turn) and then transitions into a 5" vent that runs vertically through a well insulated attic for a total length of about 6 to 8 ft to the top or the roof line. A downstairs hot water tank with a 4" vent taps into the 5" double-walled section before reaching the attic space.

The second bid (Rheem RGPN-05) wants to tap into the existing 5" double walled vent. I think he said he would run a new 3" double wall out of the furnace for about 18" to the existing 5" double-walled section up where the hot water tank vent tapped in.

I suspect the concern is condensation. If I opt for the Rheem unit, should I be concerned about using the existing 5" vent pipe - is it too large? (I think he would run a 3" vent into the existing 5" doublewall).

Thank you ...

[jwiehagen76]

The first thing i would suggest is having a load calc done on structure. As for the coil i would try to save it unless it has leaks or bad drain pan. It has been my experience with anything that old on a retrofit opening up a system could lead to problems. A new coil rated for 410 can be flushed and used from 22 to 410. That also is a practice that i do not use (do not need any headaches from some one being thrifty!) Sounds like the vent would be ok but that would have to be determined from site inspection.

[tunacan]

Thanks for your reply. Am I correct to understand that your experience suggests replacing the coil (which has no problems that I know of) might actually cause problems?

[jwiehagen76]

I would try to reuse coil without opening the system.

[BaldLoonie]

775 sq ft in a mild climate needs the smallest furnace you can get. Those proposing bigger don't care about doing it right. As for the vent, I'm not picturing too well what you have. Have the dealer show you the code book on vent sizing to prove what he is doing is right.

[tunacan]

Thank you. The furnace is in an upstairs closet and it has a 4 inch vent that runs about 18 inches (with a 45 degree bend) before transitioning into a 5 inch dual walled steel vent that rises vertically through the ceiling and roof. The hot water tank is tapped into the larger 5 inch vent before it goes through the ceiling. One bidder mentioned the vent should be replaced with a smaller size so that condensation wouldn't take place as the hot air escaped, causing rust. The other thought the existing 5 inch vent was okay to be used again.