RE: Is Manual J worth it?

I have been learning more and more about “proper” system design because the way i was taught was almost all rule of thumb stuff. I do mostly service and haven’t really done any full system designs. I recognize some short comings in some of the things i was shown and started looking into things like manual j and how to properly size a system if/when i need to do that.

The main point i am wondering about is for your average or even above average single family home, how much larger or smaller can you determine is needed from all the load calculations in manual j versus a simple equation or 1 ton of a/c per 500 square feet. If i oversize a unit by half a ton is that really going to make a noticable difference? 

Im asking this in hopes that i learn something, not so i can argue that the way i learned is better.

Andrew Souza Rookie Asked on May 17, 2018 in Design.
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3 Answers

Part of the answer here depends a bit on where you are located, more specifically the humidity in your region. One big disadvantage, among others, that many fail to consider is that humidity control in the home takes time. By over sizing equipment you might cool down the house quickly but it can drastically reduce the amount of water you are able to ring out of the air. Properly sized equipment will be able to meet the cooling load demands of the structure over a reasonable amount of time thus allowing you to condense more H2O from the warm moist air which will leave you with cool dry air which is much more comfortable the cool clammy moist air. Other considerations to think about is the wear and tear on motors, contractors and related components  the the extra start stop cycles will cause. As with many things in life it comes down to finding the right balance between getting the job done quick and shutting the system off and running the system for hours and hours and rarely shutting down. A manual J will help you find this balance. You are on to a good strategy with the 2 stage compressor however. Most systems I am familiar with usually end up with first stage being about 2/3 of the total capacity which with some careful planning in conjunction with a good manual J calculation can let you oversize a bit to have some extra capacity for the extreme hot days (Stage 2) but also allow for longer run times for good humidity control as well. A good thermostat would be a must if you are wanting to do that right.

Rookie Answered on June 7, 2018.
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