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).
I perform a load calc on every install that I perform. Not a lot that I can do about the bad duct work from the 80's and the gravity systems. With that said I berform a static test on the existing equipment to see how the current air flow is. If air is low maybe change some duct work, turning vanes, variable speed motor, etc to get the proper airflow by utilizing the lower BTU of the furnace, as long as it works with the load calc.
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Last edited by jpsmith1cm; 12-27-2012 at 07:02 PM.
Reason: non AOP member
This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.
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Additional infractions may result in loss of posting privileges.
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...
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).
You do the right thing as you are obviously not smart enough to distinguish when a load calc is necessary or not.
You are right on all counts. A proper heat load separates the amateur from the professional and a condensing furnace will be the best buy for the residential heating for some time to come in most markets.
Another one who needs a pat on his back. lol... and some reading lesson combined with some abstract thinking.
BTW I do a heat load calculation on 95% of my installs.
Originally Posted by tostaos
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