Gas and Oil Home Heating Furnaces - Goodmand GMV95 Closet installation and t-stat questions

I live in a common tract home built in the early 60's on the coast in So. Cal. Am replacing old updraft gas furnace with a Goodman GMV950740CXB Two Stage Variable Speed 95%. The furnace closet in these tract homes, had a bottom in them that the furnace sat on - and drew the air to be heated from a grill just outside the closet. Inside the closet, a 6x12 plenum that fit between the studs and ran from about a foot off the closet bottom up into the attic....I presume to draw air for the firebox. The exhaust stack went up thru a wire mesh covered hole 8X12. Obviously, utilities were so cheap back then, that they didn't worry about cold outside air moving around in that closet at will.
Now, with the new 95% furnace, I can run the exhaust up thru the old exhaust pipe - with a 2" pvc pipe....to above the roof...AND bring in the air for the firebox with a 2" pvc pipe....from the attic. Question is: Can I rip out that old duct plenum that fit between the studs and drywall up that area....and can I re-drywall the ceiling of the closet...leaving just the 2 - 2" holes for the fresh air intake and exhaust? Also, what Brand and Model thermostat should I buy?

Hopefully I understood your info. It that plenum was just there for fresh burner air, and you are replacing the old unit with a 90+ unit, then you are naturally receiving your intake air from you pvc pipe, hence not needing combustion air like on the old unit. Make sure you read your install guide as to length of pipe and termination styles once you exit roofline, as if not done correctly you will have performance problems. If pvc exceeds xxx amount of length to reach roof or you have too many 90 deg elbows in your run, you will need to upsize the pvc pipe. A picture always helps, so if you can post some, would be nice.

You are safe to close up those openings only IF you DO NOT have a gas fired water heater in the same area of the furnace..

As Chi said, make sure you follow the PVC sizing chart. Sounds like you may be fine with 2" since I figure you have a 45,000 BTU furnace.

As for the t-stat, I'd look into the Honeywell VisoinPro TH8320 t-stat. Make sure you/they move the jumper on the board for two stage stat.

If you have high humidity in your area, then upgrade into the Honeywell IAQ t-stat. The IAQ is able to slow down your blower, and also if pulling new t-stat wire will be an issue,, the IAQ will only need to use 3 wires rather than 6 wires.

You can close off the area above the heater ONLY if you add a vent to the door of the closet. California requires fresh air to be required in an attic or a clost if that is where a gas fired heater is. You can close the space over the top if you leave a 4" by 6" vent, then you would not need to add a vent to the door. Also, you must maintain 2 inches clearence around the heater around all sides.

I'm a licenenced California contractor and I got nailed for it on my second job 10 years ago.

Slooo, They even have to have the opening in the room on a sealed 90% equipment?

90+ furnace is a sealed combustion chamber...no need for surrounding air. All combubustion air is supplied and vented thru pvc piping. Can seal all up unless you have a water heater there like Jay said.

Yup, everyone understood my situation properly...& that is great. Yes, plenum WAS for fresh burner air...no water heater in closet.
B-4 I got back to this thread...I called our local B&S Dept. & got to an inspector...& he said exactly what y'all did!!! Then I get home and see your answers - thanks !!!
He said, I can rip out old "FBA" plenum...run exhaust thru roof..and I can draw "FBA" from attic...both per mfg instructions...as old unit did...attic is extremely well ventilated.

YES, we DO have high humidity in our area. Does everyone agree on the Honeywell IAQ t-stat. I like the idea that it is able to slow down the blower.

Old t-stat wire IS OLD - and pulling new t-stat wire will NOT be an issue - so...I'll pull the six wire....

Would that change t-stat recommendations- do ya'll agree with Jay?

Sloooo- You say 2" - Even if the mfg says 0" sides and 0" back - 3" on front"??

YES, we DO have high humidity in our area. Does everyone agree on the Honeywell IAQ t-stat. I like the idea that it is able to slow down the blower.
I am guessing you are having A/C? If you are not doing A/C the IAQ will be an over kill, or if you find that the IAQ is more than you are willing to spend, then look at the VisionPro, and add a basic humidistat on the return ductwork.

Old t-stat wire IS OLD - and pulling new t-stat wire will NOT be an issue - so...I'll pull the six wire....When you do buy the wire, they come in a group of 2,5,and 7. Get the 18/7 t-stat wire. The IAQ will only use 3 wires, but won't hurt to have extra behind the wall. but from the ECM to the furnace, you may need 6 wires. The ECM mounts near the furnace, so won't need a lot of wires.

Sloooo- You say 2" - Even if the mfg says 0" sides and 0" back - 3" on front"??
Check with your building safety inspector on that.

I'll check with the building inspector on the clearance.

Nope...fortunately no need for A/C here !!

Jay, you said "The IAQ is able to slow down your blower," - The unit I am getting is the 2 stage - variable speed model.
So, if you say the IAQ can do that - how does it know.
I will buy whichever t-stat you recommend. I'd just like it to be programmable.
Do you know the mfg part#
I thank you in advance for your help.

The slowing down the blower is for the A/C set up. Since you don't have or need of A/C, the IAQ for sure is an over kill.

Look into the Honeywell VisonPro TH8320.

How does the IAQ know when to slow down the blower? Say you set the dehumidity level at 45%, and if the home gets over that set point, it will slow down the blower to allow the air go over the A/C coil slowly remove the humidity. When the humidity is below set point then the blower goes back to it's full speed to cool down the house.

Thanks Jay....I'll check into that unit...you and everyone has been very very helpful and I thank ya'll for that.

One last thing...This furnace has a variable speed blower...can I assume that the internal furnace controls - know when to and how much to speed up and slow down the blower...and that the thermostat does not do that...???

The board will control the speed.. The t-stat just tells what it do.

Could change the speed on the board to make sure you got the right temp rise.