HVAC sizing

I have found conflicting information for the rule of thumb for sizing HVAC systems. can you please guide me what ARE/NCARB is looking for?
Area/600 = # of tons required to adequately cool (multiple googled sources energyvanguard.com)
Area/300 = # of tons (Mike Newman threw it out there in one of the videos but i’m not sure he was being literal or not)
Area/500 = # of tons (in a rehab project where the heat loss may be higher) Ballast
Area/1000 = # of tons (in a well sealed new construction) Ballast
4 sources, 4 different answers.
Also what’s the formula if we are given volume of the room to calculate cooling load?

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I have the same question.

@coachmarkbailey care to provide some awesome wisdom?

@coachmarkbailey @Coaches can any of you respond to my questions regarding HVAC and story drift please. My exam is on May 30th this Sunday.


So a couple of numbers to consider when looking at HVAC systems.

for Heating calculations you are looking for BTU’s, cooling loads are calculated in terms of tons (this is because it is based off of the old practice of using tons of ice in lieu of air conditioning)

For every ton of cooling there is 12000 BTU - so for example if you have a 48000 BTU cooling load, you would need at least a 4Ton unit to adequately handle that load.

The best way to go about doing this would be to figure your heating or cooling loads (which will be calculated in BTU as an R or U value which has units of (ºF*ft^2/BTU) which will get you loads in BTU that you can then size your heating system or convert to tons by dividing by 12000.

the idea of dividing the area by a value is hard to give direction to as the factor that you divide the area by varies by location, and there are so many different factors with in.

The formula for heating and cooling are virtually the same, as both use the change in temperature (Delta T) the area of the wall assemblies, and the R values (or U values) to determine heat loss in terms of heating or heat gain in terms of cooling since cooling cannot be created.

Hopefully this helps. I would also recommend watching the video segments regarding Heat load calculations within section 1.2 for more guidance on figuring the heating loads.


I passed my last exam, PDD. Thank you for all your help.


Congratulations Najia! What were some of the primary resources you studied?


It was all over the place. This was the third time I tested PDD. My primary problem was how to read the test. I have 8+ years of practical experience and that helps and hinders, if that makes sense. Do as many practice exams as you can from as many sources as you can. Then research the questions in depth, even the incorrect choices. Use the sources they have listed in the NCARB handbook. You’d be surprised how many answers in other outside sources are contradictory. You might think you’re right and you might be, but if it’s NOT the answer NCARB Is looking forward it’s not going to be marked correct. That’s what I mean by learning to take the test. Ask the coaches here to clarify if you find contradictory answers. Practical experience helps you visualize the problem but can trap you into thinking too practically. It doesn’t translate well into test responses, in my opinion. Read the question, choose the best response for the question asked, or in some cases, least bad response that would make you cringe in the field. Stick to the resources listed in the handbook. Those are the answers they are looking for. Good luck.


That is awesome, congratulations!

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