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Old 05-23-2014, 09:54 AM   #1
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Manual J calculation… what does the resulting BTU value mean?


Let's say I do or have done a Manual J HVAC load calculation for my house and it says I need such-and-such number of BTUs for heating and cooling. Are that value the actual numbers of BTUs that must be supplied per hour to keep the temperature at the desired level, or is that the BTU output of the heating and cooling appliance I should be shooting for? It doesn't seem like it should be both because then the appliances would be running 24/7…

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Old 05-23-2014, 03:55 PM   #2
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Manual J calculation… what does the resulting BTU value mean?


when you do a manual J it will tell you how much heat and ac you need for each room. add those up and you get the total for the house and then you size that total with the equipment. There is a national chart called a MEAN TEMP. guild that will let you know the average temp. for each city in the US. Where I'm at its 85 degrees F. in the summer and 15 degrees F. in the winter. They use these numbers to size the system.

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Old 05-23-2014, 04:42 PM   #3
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Manual J calculation… what does the resulting BTU value mean?


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Originally Posted by iLikeDirt View Post
Let's say I do or have done a Manual J HVAC load calculation for my house and it says I need such-and-such number of BTUs for heating and cooling. Are that value the actual numbers of BTUs that must be supplied per hour to keep the temperature at the desired level, or is that the BTU output of the heating and cooling appliance I should be shooting for? It doesn't seem like it should be both because then the appliances would be running 24/7…
It should be both.

A properly sized A/C and or furnace should run 24/7 at design conditions. Which only happen a few hours a year compared to the normal average temps.
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Old 05-23-2014, 04:47 PM   #4
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Manual J calculation… what does the resulting BTU value mean?


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It should be both.

A properly sized A/C and or furnace should run 24/7 at design conditions. Which only happen a few hours a year compared to the normal average temps.
Ah, that makes sense, since the design conditions are "worst-case scenario" ones.
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Old 05-23-2014, 06:15 PM   #5
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Manual J calculation… what does the resulting BTU value mean?


Design conditions are not worst case. Just about a 2% of
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Old 05-26-2014, 01:44 PM   #6
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Manual J calculation… what does the resulting BTU value mean?


Design conditions are based on lowest and highest average temps for your area based on about a 10 year average, and then calculated based on about a 98% of the time model so for approx. 2% of the time you may have actual heat gains or losses that exceed your equipment sizing requirement, so you may be a little hotter than you would like, or a little cooler than you would like on those "worst case" days

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