What you describe suggests that your furnace is grossly oversized for your house, and/or you with a marginal air-flow situation in your house, with the furnace producing heat faster than the ventilation system can remove it, and this could prove to be a problem for you in other ways. You need to investigate and correct that. It could be as simple as replacing a filter, or it could involve reworking some ductwork. In the worst case, you need a smaller furnace.

Here is what is happening. Your furnace has what is called a "high-limit" switch in it. This is a heat sensor that monitors the temperature inside the furnace. If the temperature gets too high, the high-temp limit switch turns the furnace off as a safety measure. The furnace will then not recycle until it cools down.

So, when you close off those registers, you wind up with sufficiently restricted airflow out of the furnace that it is overheating. Since you have one room that is getting too hot and you can't close it off and have the furnace work properly, this means you need more ducts and more registers in other parts of the house. Or else you need a smaller furnace. Or you have an obstruction in some other part of your ductwork. You could also make some changes to the furnace that might help a bit, though they won't deal with the basic problem.

First things first. Most such obstruction problems come down to a clogged furnace filter. This doesn't sound right for your problem, but it is where you start anyway. Try changing the filter.

When you close off that one room and open all registers in all other rooms, do you feel decent air flow out of ALL of those registers? If so, then you don't have any obstructions but you need to downsize the furnace, or add more ductwork and registers. If any of those registers doesn't flow well, then you need to check that branch to see if it is either obstructed or plumbed in a fashion that it won't conduct any air. If there are no obstructions and some registers still have inadequate air flow, you might try installing an inline fan in that branch to force air into the branch.

If all the registers flow but the furnace still shuts off, you might try speeding up the furnace blower. This will move more air and might deal with your problem, but it will cause the blower to work harder since you obviously have a pressure buildup in the furnace. This may shorten the blower life.

You also might pull some new registers from the furnace to places in the house that might need it. This might be easy, or it might be so hard that it proves impractical.

If none of these things work for you, then you could need to replace the furnace with a smaller unit. As an alternative, you might be able to block off one of the burner rails to reduce the amount of heat the furnace produces, but you certainly should NOT undertake something like this without first consulting face to face with a furnace expert.

Another fairly common problem (which isn't the problem you have, most likely) is that the return air system is undersized for the furnace, resulting in a furnace whose ventilation system is air starved. The solution to this is to enlarge the air return. I once lived in a house that had originally been wired with electric baseboard heat but had later been retrofitted with a gas furnace. The return air system had been routed through walls and through one downstairs closet, and it was much too small. When I realized the problem, I made a new cold-air return in the basement, put a filter in the opening, then put a louver in the door to the basement. This made the furnace draw some of its air directly from the basement, with that air being replenished from the living spaces via the louver in the basement door. It worked just fine, and was far, far better than remodeling the house to increase the fully enclosed cold air system.

About the Author: Jim Locker is a technical guy who has done a lot of real estate investing and landlording. The experiences he writes about and advice he gives are either first hand, or in answer to specific questions posed by others. He is commonly known as jiml8 around the internet.