# Do you like 1.1xCFMxΔT for sensible loads?



## Dr. Barber (Jan 9, 2020)

...well, its nowhere in the NCEES manual (as far as I can tell)

Neither is 4,840xCFMxΔW for latent loads.

I guess if you work in HVAC you have these memorized, but these could be time savers for TFS folk who in previous test editions learned about them while studying for the test. Now, they either have to commit them to memory or deal without them.


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## OldSquaw (Jan 10, 2020)

Dr. Barber said:


> ...well, its nowhere in the NCEES manual (as far as I can tell)
> 
> Neither is 4,840xCFMxΔW for latent loads.
> 
> I guess if you work in HVAC you have these memorized, but these could be time savers for TFS folk who in previous test editions learned about them while studying for the test. Now, they either have to commit them to memory or deal without them.


These equations are in section 9.1.7 of the new reference manual.

Personally I don't use these equations. I prefer to start with the "grass roots" equations that have been burned into my brain (Q = mdot*cp*DT and Q = mdot*Dh). Plus I don't trust any convenient formulas from this manual. It's just easier to stick to what I know and spend a few extra seconds working out the units.


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## Dr. Barber (Jan 10, 2020)

OldSquaw said:


> These equations are in section 9.1.7 of the new reference manual.


Oops. I stand corrected. Thanks,



OldSquaw said:


> These equations are in section 9.1.7 of the new reference manual.
> 
> Personally I don't use these equations. I prefer to start with the "grass roots" equations that have been burned into my brain (Q = mdot*cp*DT and Q = mdot*Dh). Plus I don't trust any convenient formulas from this manual. It's just easier to stick to what I know and spend a few extra seconds working out the units.


Agreed 1,000%


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## OldSquaw (Jan 10, 2020)

I would actually suggest to other people taking the CBT test to try to solve problems with the most basic form of the equation you can think of that you know in your head rather than using the "time saving and convenient" formulas from this NCEES reference manual. There are just way too many errors to trust using those equations. When I have to use the "convenience" equations from the manual, I double check the units. Just another word of advice, there are a lot of equations in the manual that specifically say to use certain units. When you plug in those units they tell you to use, the units don't work out. So just do this on your own and don't trust anything NCEES says.


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## Pass the PE Exam (Feb 15, 2021)

Those equations can be a great way to save time if you are working at standard conditions, but you may get yourself into trouble if you're solving a problem with particularly cold air, for example. The standard 1.08 * CFM * dT style equations might lead you astray in that case. 

As was mentioned above, it's generally safest to start from the base equations and think through the process of incorporating the actual densities and other conditions from the problem.


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