The thermocouple on my furnace just went bad. While the repair man was in my house to fix it, I asked why is it colder upstairs than downstairs. He reply was, because you do not have a cold air return upstairs. My house was built in the early 1900's. Do I really need a cold air return upstairs and If I do how many and how large?

How many stories is the house?

You have to have a cold air return but more specifices as to how many you have now and there size and placement.

The house has 2 stories with one cold air return downstairs in the middle of the house. The house is a row style home that is about 1300 sq ft.

Yes you need the cold air return. The service person who was there gave you a bad answer.