My mom's Trane XL80 is blowing cold air. Not always, just sometimes. The first serviceman said it was a dirty filter. So, the filter was changed, and it started happening again. The second serviceman (different company) said it might need a fresh air intake. Mom's house is approx 90 years old, the furnace is in the basement, there is no fresh air intake. There is a 2'x5' grill in front of the furnace, there's wood panelling enclosing a 6'x6' space where the furnace & hot water tank sit.

What can be done to make sure this never happens again?
Do we need to put a hole in the basement wall to get this unit some outside air? How big does this vent need to be?

To start with do not add a fresh air intake to a 90 year old home. It most likely leaks too much air to begin with. I need more info to help further since I really doubt you have a 2 foot by 5 foot grill on the furnace. Another question, the 6ft X 6ft paneled room for the furnace, that is inisde the exterior of the basement, right?

A furnace needs a good supply of combustion air. If the furnace and hot water heater are sealed in a tight little room, it is possible the furnace is not getting the air it needs. The most cost effective thing might be to add an outside air intake. All the high efficiency furnaces duct outside air to the furnace. My 80% 80,000 BTU furnace needs 40 sqin of free area. The less efficient and bigger the furnace, the more air it needs.

If the furnace can draw plenty of air from the rest of the house through a large grill, then there is no need for additional air unless somebody has done an extraordinary job of retrofitting the house to tighten it up. It can be done. I have my crawl space so tight I did have to add a vet for combustion air.

If the furnace is getting plenty of combustion air, and the ductwork is open like it should be, then you need to look elsewhere for the problem. I would guess a bad temperature sensor or high limit switch. For more about them see the sticky at the top of the Heating forum, https://www.askmehelpdesk.com/heatin...tml#post268970

The grill in front of the furnace is 31"x63". The space housing the furnace & water heater that was panelled over is 63"x68". Last night, it started blowing cold, so I cut the power to the furnace (there's a wall switch to the right of the grill), waited 30 sec, then turned it back on. The furnace went throgh its start up sequence, then blew hot air. An hour later, (the next time it kicked in) it blew cold again. My guess was a control issue, since it can produced hot air, something somewhere it telling it not to from time to time.