Quote Originally Posted by tostaos View Post
Let's look at my situation two weeks ago. I have a 3.5 t AC that is running just fine and doesn't need replacement, which makes it irrelevant that I don't know if the AC is slightly oversized or not. I will need a 4 ton blower. I have 2 sizes of furnace available, a 80k and a 100k btu. I know that the 80k furnace is large enough as the old furnace heated the house just fine. So for what reason would I run a heat load calculation that will tell me that the optimal size would be a 70k furnace?
It tells more than this.

1. Heatloss tells me what capacity I need for heating...
2. Heatloss tells me what amount of air flow I need...
3. Heatgain tells me what amount of airflow I need...
4. Sensible HG tells me what capacity I need for cooling...
5. It helps me determine how much fresh air I need...
6. It tells me the square footage and approximate leak rates I have...
7. It tells me the correct size of the duct system...

And it takes me just one hour to accomplish all this...

Even if I determined that the AC system is oversized, I still would install the correct furnace capacity for the house (output of heat and airflow regardless of
the AC system). I would not size a furnace based on the size of an existing AC system; this would be a no no for me and for the customers.

The existing system does not dictate what should be installed!! (I hate guessing: my warranties can not exist with chance).