Does this also apply to the 90% efficiency models that are vented vertically to the outside of the house? We just had one installed. Brand is Rheem. Should the return then be blocked off in heating months and opened for a/c, even with the newer models?

If it is a two pipe system then it is OK to have the return in the same room, and the return should never be closed off. The two pipe means one brings in outside air and the other vents the combustion gases, the fire box is sealed from the surrounding air. Not all 90% furnaces bring in the combustion air from a separate pipe. It should have two pipes. Mike

Where is the furnace located. Lets clarify 1 more thing. A furnace uses 2 kinds of air, (what you are treating and breathing) and combustion air, what you are burning. One cubic foot of gas requires about 14 cubic feet of air to burn properly. If the return air is to close to the furnace it can steal the air necessary for a good burn. It can also draw products of combustion from the furnace and send them into your breathing air. Now for the PVC pipes, one is for venting the flue gasses like acetc said, and the other is fresh air for combustion. I recommend using both pipes on every 90+, Rheem seldomn use a single pipe system, called concentric vent, but it can be done for a cleaner simpler termination outside the home. Where is you furnace located? Attic, garage, basement?

I don't know from your post, exactly what you are wanting to close off, but I don't think you want to close the return ever, unless there is something I'm not understanding from your post.

Hope some of this will help clear up what we are talking about.

The furnace is in the basement, and the house was built in 1939. I cut out a vent to the basement for heat and ac in one of the duct pipes a few yrs ago because it was cold in winter, and stuffy in summer in the bsmt. There is presently a dehumidifier running in the basement. It is a half basement, with half being a crawl space with plastic over the dirt.
Checked and there is only one pvc pipe for the exhaust going horizontally to outside of house. There is none for intake air. But there appears to be a pvc on top of furnace for an intake. It is an upflow model.
Thought that the air in adjacent attatched garage, and air from basement would circulate better if there was a cutout for return air in the basement. So I made one this summer, but can close it off during heating season, if that is recommended.

There is also a problem with condensation forming on the vent pipes close to the furnace, and dripping on the floor when the ac is running.
Thank you for your help, tom

First off if you do not have a pipe attached to your furnace to bring in out side air to your furnace I would highly suggest you install one, it sounds like the furnace is drawing air out of the room its in right now, some manufacturers( Carrier) do make a one pipe furnace but not intended for this type of installation, it would normally be installed in the attic. The concentric vent is a devise that both pipes coming off the furnace attach to so that you only have one roof penetration. The vent pipe may not be pitched and that is why it is dripping, it will drain condensate back through the furnace and out the drain line on the furnace when installed properly. Both pipes must terminate at about eight inches from each other.

Thank you for your help. It is not the furnace vent pipe that is dripping with condensation, but rather the supply pipes to the rooms, where they come from the pleneum and branch out to the rooms in the basement ceiling. About six feet away from the unit, when ac is on, moisture forms on the pipes going to the rooms and drips on the floor.

So, since there is no dirict outside inlet for comustion air for furnace season, should I have one installed? And if not, you say the return air vent in the basement should be taped off for the heating season. Thanks, tom

Condensation is forming on the ductwork because the realitive humidity in the basement is to high. You can insulate the ductwork, or add another dehumidifier. Also check the superheat of the A/C. It is possible the installer picked an incorrect blower speed or something causing the supply air to be to cold, there should be little more than an 18 degree split, see FAQ's on A/c section at top of main page. Using the A/c to cool and dehumidify the basement might cause the rest of the home to cool poorly, plus all the humidity hurts the overall efficiency.

Yes I would run a fresh air supply to the outside, like I said in an earlier post, a furnace requires a lot of air to burn fuel. You are using treated air, (air you paid to heat) to burn gas and throw away outside, it can also cause negative pressure within the home, pulling cold air through every crack and crevice of the house in the winter time.