I live in an older house (circa 1941) with gas forced air heat, and air conditioning. Most rooms in the house have at least one supply register and one return. Returns tend to be on outside walls and supplies on inside. A few rooms (kitchen and bathroom) have only a supply.

I am planning on finishing the basement, which currently has all the ductwork hanging below the overhead floor joists. Where possible, I'd like to run ducts through the joists rather than under.

As part of this, I realized today that this would be much easier if I was able to disconnect a few of these returns (permanantly). They run through awkward spaces...

I plan on cutting in at least one or two new supplies and returns in the (newly) finished basement once I'm done.

What kind of problems would closing off a couple of returns cause me?

Close off no returns unless you supply the same square inches opening on the new returns.
If you do this you will have no problems. If you just close them off you could create a static pressure problem with the furnace blower.

Thanks for the quick reply. Let me ask, then -- what if I supply even more area of total return... even more than I have in supply total area?

When I got a new furnace and A/C installed two years ago the salesman who sold them to me mentioned that if I add some large returns in the basement (as "big as you can" were his exact words), the a/c can help de-humidify it in the summer.

What if I supply even more area of total return... even more than I have in supply total area?


No problem with more


When I got a new furnace and A/C installed two years ago the salesman who sold them to me mentioned that if I add some large returns in the basement (as "big as you can" were his exact words), the a/c can help de-humidify it in the summer.

A good performing return system is very important to the proper operation of the equipment.

Sorry, one more question: Is this likely to cause poor heating or cooling in the rooms where I have disconnected the returns?

Yes

You asked the question I gave the answer. There is no need to elaborate on this subject. It will cause poor performance to shut off registers.