Hello. I have a two story home with no basement where there is a thermostat on the upper floor to control the upstairs' room temperature and a second thermostat on the ground floor to control the ground floor's room temperature. The home is heated with a single gas furnace located in the attic using a centralized heating system. I have no problems with the temperature control on the upper floor. However, on the ground floor, after the room is warmed to the target temperature, instead of shutting off, the furnace blows cold air for several minutes, cooling the room down before shutting off. I don't know why this is only happening on the ground floor. I've replace the thermostat unit with a new one and it's still doing the same thing. Can you please explain why this is happening and how I can fix it? Is this normal?

My guess is that the furnace is programmed to run for a few minutes to push the remaining hot air out of the ventilation ducts, but since the furnace is in the attic and providing heat to the ground floor, with the furnace no longer producing heat, the air is cool by the time it reaches the ground floor. Does that make sense, or am I totally wrong. In any case, how can I fix this problem? Thank you.

Since you have a zoned system I highly recommend you call on a professional for an On Site survey of your system. The reason I say this is that a single zone system with the new style equipment can be difficult to work on but when it is a zoned system it can be a much bigger problem.

Why you replaced the thermostat I have no idea but before you keep replacing any more expensive parts I suggest having the company you call make some adjustments to the system. There can be many different problems causing the exact problem you are having now from a damper end switch, time delay relay, bad fan control etc.

Now if you want to tear into the problem I can try to help but that help will be very limited since I am not there to actually see what is happening or test all the devices.


You will also need a good quality meter as used in the HVAC business and know how to use it properly since all diagnosis will depend on it. You will also have to list all the manufacturers names and exact model numbers of all equipment Thermostats,furnace,dampers,damper controls etc. Then try to locate all the manufactures install instructions for the equipment. This can be looked up using Google as a search engine. I have some info here but more than likely I will have to rely on you for most of the info.

When you have all of that together post back on this thread and we will see what can be done.
If there was a easier way to do this I would be glad to tell you but without doing it properly all will be guess work.

If you attic install looks like this it will be a lot easier to work on.