# HVAC vs. TFS exam



## justdoitNG (May 31, 2017)

Need some help..I took HVAC exam in April and failed ..got 53/80.

For work, I am a mechanical engineer working as a construction manager..Since I deal with some HVAC at work, I thought it will be more relevant..For my 2nd try in Oct, I want to be better prepared:

1. Should I give another try HVAC next time? I do not have access to ASHRAE handbooks and nor am I familiar with it. Is it feasible to buy them (older versions are cheaper) and familiarize next 2-3 months? I thought I can do without them..probably I was wrong.

2. Should I switch to TFS. While preparing for HVAC exam, I had some issues with units in fluid problems (eg. like gc constant) and also I noticed TFS questions can be either in SI or US units. I wasn't comfortable solving fluid problems. But if it is more suitable for my case, I am ready for it as well..

3. I took the School of PE classes (there is free enroll for next time as well)

4. I have been out of school for 10+ years..

Any kind of advise in choosing HVAC vs. TFS will be greatly appreciated.


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## A Random Guy (May 31, 2017)

Unless you do HVAC design for a living, I would do TFS. I looked at all three mechanical exams (by that I mean I bought all three NCEES exams) and initially decided on the machine design. I studied for the machine design for a few months (probably 80 hours total). The week before the deadline to register, I decided to work through the TFS exam. I was able to solve those problems much quicker than the machine design, so I switched and registered for TFS the day before deadline. I put maybe 120 or so hours into studying (which was mostly running through various versions of the NCEES TFS exams). I passed TFS easily.


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## Viper5 (Jun 1, 2017)

If you are deciding between the two, I think HVAC would be a better choice.  The pass rate is almost 10 points higher and you were already very close.  Maybe not the best metric in making a determination but just my 2 cents.  Just make sure you understand the concepts and do lots of practice problems.


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## justdoitNG (Jun 1, 2017)

Thanks guys for the valuable inputs..waiting for some more responses please!!..


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## amats42 (Jun 1, 2017)

I think deciding the topic is a very personal choice, but if avoiding buying materials is your top priority TFS is a great choice. You should already have the MERM, most tables are available for free online, you don't _need _to buy the unit conversion book (but it does help and save prep time), and I didn't open any college textbooks during the exam (passed).

In my opinion TFS and MDM are the core mechanical topics while HVAC seemed too new to me. I don't work in HVAC so I didn't want to waste the time learning something new for a test. Now in your case it sounds like you have studied a lot for HVAC and purchasing ASHRAE handbooks is all that is holding you back so I guess it's a decision of study time versus money.


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## apford (Jun 1, 2017)

If you do choose to go the HVAC route, doing it without the ASHRAE texts is probably a big mistake.


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## Audi Driver P.E. (Jun 1, 2017)

Stick with HVAC. Get the ASHRAE books, pronto.


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## justdoitNG (Jun 1, 2017)

Thanks for your advise..

just to break it down..its a choice between *familiarizing between ASHRAE books vs. reading 3-4 new chapters (for TFS )* in MERM like power cycles, compressible flow etc. that were not part of HVAC syllabus..and after that multiple passes of reviewing the material and solving problems..

Is it difficult to familiarize ASHRAE books for someone who has never used them ..nor needs them at work? I can probably get older versions of ASHRAE handbooks for little over $100..


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## Audi Driver P.E. (Jun 1, 2017)

Not at all.


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## eref2009 (Jun 1, 2017)

I took HVAC-R past april and failed 50/80. I did not have ASHRAE books with me...I have purchased old version of 2009 Fundamentals and 2010 Refrigeration books to study

next time in october. But i am wondering those two books would be enough??


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## Maddog10 (Jun 2, 2017)

I'm not all that familiar with HVAC, but in my mind TFS is easier to prepare for conceptually because on the Thermo side so much of it relates to change in enthalpy. You have to have a deep understanding of concept, but on the surface nearly everything relates to delta h. Compressor work, turbine work, heat in/out, it's all related to enthalpy change across a piece of equipment.

As for Gc conversions, one tip that was given to me was to work with specific weight (gamma) instead of density (rho). This eliminates the need for Gc conversions in nearly every scenario. The only criteria is that to do this the problem must be dealing with traditional gravity values here on earth (32.2 ft/sec^2 or 9.81 m/sec^2). If acceleration of gravity changes, then specific weight and density are not equal. lbm must be equal to lbf for the concept to work.

Proof of Concept

Specific Weight (lbf/ft^3) = Density (lbm/ft^3) X Gravity (ft/sec^2) / Gc (lbm-ft/lbf-sec^2)

Thus, specific gravity equals density times 32.2 and then divided by 32.2, which gets you right back to the original density value except in lbf instead of lbm. If you just assume density to be specific weight from the beginning, it saves you the hassle of the conversion.

It's kind of tough to come to grasp with and trust that it works early on, but do a few examples and you'll feel good about it.

Work a standard manometer problem, but instead of using pgh (density x gravity x height) and then having to do a Gc conversion to get to lbf, just use the equation Gamma X h where Gamma equals the density but in lbf. It works, and is much faster with no conversion.


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