Day & Night (BDP) BTU level

Have a Day & Night model #518A060CC and trying to determine BTUs produced from it, in order to know if resizing needed in addition to house. The house was built in 1979.

What is it? If that is a model for a condenser, you could take a stab at it being a 5 ton, based on the 060 in the model. ( 60,000 BTU)

If this is original, it is probably low SEER, and worn out after 28 years. This would be the time to condsider replacing it. Naturally, it will require replaciing the entire indoor unit ( cased coil, air handler, etc) because you cannot mismatch a new 13 SEER (minimum) with any existing equipment.

If that is just a gas furnace, I would guess 60,000 BTU input.

As far as the room addition, are you guessing at the requirements ( bad idea!) or will you have an HVAC contractor do a heat load calculation for you?

Bdp

Is this a gas unit or a heat pump? If it is gas, the BTU's will be on the rating plate. If it is a heat pump, the capacity should also be on the air handler model number plate.

I did a google search and a #518A060CC is a furnace circuit control board.

That was a good catch by peanut. Looking at the article, it is used in literally over a hundred different furnaces of different brands. You need to look for the equipment name plate, not the model number on a part.