7 Basic HVAC System Design Concepts

by
H. Kent Craig




Designing HVAC (heating, ventilation, air conditioning) systems has sometimes been compared to a mystical science, part alchemy, part religious ritual. Nothing could be further from the truth. Truth is, designing residential and commercial HVAC systems is nothing more that learning a series of principles of physics, combined with an understanding of certain real-world mechanical application concepts. Below, I present what I consider to be the seven most important concepts of HVAC system design, meant for the entry-to-mid level HVAC designer and/or design-build contractor.


1): You design HVAC systems for people living inside buildings, not for the buildings alone.


It's easy to get carried away sometimes and try to nail every single comfort engineering principle down to the last 1/4" of friction loss or inch of duct, and forget about the whole point of why you're designing the given system in the first place. The reason that someone is paying you to design the system isn't because they want a building heated and cooled, but because they want the people who live inside that building on whatever schedule to live within a narrow band of temperature and humidity ranges. The reason for that is because if you're designing a system strictly to maintain a temp and humidity range without any consideration to human comfort, then that's not HVAC system design, that's refrigeration system design as in a refrigerated warehouse or such, and that's a whole different topic.


You must be empathetic to those who will occupy the space that you're trying to temp and humidity control via mechanical means. Visualize the end uses of a given space, and try to imagine what you would or wouldn't like regarding airflow patterns and such if you had to work or live in that space, and adjust your system design accordingly.


2): You can't fit a 24" deep duct into a 23" space, or, it's the coordinated space, stupid.


If you don't remember but one concept of the seven presented here, please remember that all trades must share in the coordinated space overhead above dropped ceilings. More specifically to the title, you really shouldn't try to fit a 24" duct into a 24" or 26" or even 30" ceiling freespace, because you know if nothing else the drop-in lighting fixtures will occupy several inches of available depth.


No one likes to go back and redesign and "pancake" existing duct runs even wider and flatter and thinner, just to make room for gravity piping or conduit or whatever might conflict with your duct runs, so please be cognizant of allowing room above and below for the other trades, and you'll save yourself some time and aggravation.


3): Be as fair as you can with and to all manufacturers.


We all have our preferences for equipment and distribution component manufacturers, and there's nothing wrong with that. Over the years, we've learned that this manufacturer makes a slightly or significantly better product for that particular application in that circumstance, or this manufacturer consistently screws up delivery times.


On non-critical applications, be as generous as you can in spec'ing as many manufacturers as possible as equals in the specifications. Doing so will, in the end, save the owner money.


And please allow me one plea that I know manufacturer's reps will say "hallelujah" for. Unless you have a strong, deep preference or prejudice for or against one particular manufacturer, when listing as-equal manufacturers in the specifications, mix the listings of them up from time to time, i.e., don't just list all the approved equals the same way all the time.


I suggest this because many salesmen get paid a higher commission or not depending on how their line is listed, e.g., if their product is listed first in the specs and the successful contractor buys from them, they might get a 15% commission; if they're listed second on the same job in the same specs, then their commission might drop to 12%; then to 10% if they're listed third, and so on. It might not be fair, but it's the way it often is. By mixing the preferential as-equal listings up from time to time, you help insure a more competitive environment, and ultimately save your clients, the owners of the buildings you're designing the systems for, money.


4): Dig deep into a given piece of equipment's technical specs, and find nuggets of information that might save the owner and/or contractor money.


With manufacturing and distribution channels ever evolving and changing like they are, this becomes an even more important part of HVAC designing, as time goes by. Of course, all manufacturers are going to support what they sell, they have to by law anyway, but that's beside the point.


If one manufacturer's coils have a slightly thinner tubing wall thickness than another, is that a good thing or a bad thing? It's a good thing if it saves the owner some money initially, but does it, is there a cheaper initial cost, or is the cost the same as the competition? Does one manufacturer use motors that have some weird voltage requirements that are satisfied by an onboard proprietary transformer that would be a pain to order and install in aftermarket service, and if so, why? Is one manufacturer ISO-certified and another not, and if so, does that make any difference price-wise or sales-presentation-wise to justify any price differential? Does one manufacturer use primarily American-made valves and components in its units and another not, and if so, how does that affect what you specify on any federal government job where a "Buy American" boilerplate clause will be in the RFQ?


5): Use nature to help your design.


Assuming that you get the architectural and other plans towards the end of the design and engineering process so that you can fill in the HVAC system blanks, most of the time you'll have a full set of site plans, landscaping plans, etc., to refer to.


What's cheaper, adding 3,000 btu/hr capacity to a system via increase in mechanical capacity, or planting a 6" trunk-diameter deciduous tree strategically placed to kill that much thermal gain during the summer and add that much said gain during the winter? Don't tell me it's not your place to think about factors like this and suggest improvements in site or building design, it is!


What's cheaper, having a building place just-so on a site plan which will shoot thermal gain through the roof because of maximum exterior panel southside exposure, or asking the architect politely to possibly check his ego ever so slightly and maybe turn the building just fifteen to thirty degrees to help minimize thermal gains along flat wall exposure?


What's cheaper, submerging part of the building inside a full basement, or conditioning the same space above ground? Depends on the usage and other factors, doesn't it? It's still your job to think about all factors which affect overall system design and therefor cost to the owner, and pass along suggestions when and where appropriate that will save the owner money ultimately.


6): Have pity on the poor SOB's that have to come behind you and service the system you design.


Ever tried to get to a chiller system display panel that some idiot by design stuck in a corner inside a hidey-fence just inches away from said, where you have to stand on your head and still use an inspection mirror just to see some indicator lights? What about designing a system where the largest part of the chiller is 42" wide but the largest door to the mechanical room is a standard 36" in width? No? Then have you ever put the boiler access area in direct conflict to the required freespace area in front of a panel? No? If you've never had a change order come back at you, or never had a facilities maintenance person curse you silently under their breath, then sure, I'll believe you.


It doesn't cost you and won't cost the owner a dime extra to be nice to the folks that will have to keep your system up and running after the warranty and/or liability period has expired. In fact, thinking ahead to how the system will actually be serviced in the future will not only save the owner money in the longer run, but usually will result in economies of installation in the shorter run, since all systems have to be beta-serviced and T&B'd etc. before the C/O can be issued.


7): In the end, only complaints or the lack of them matter.


Between your bosses and the owner's reps and the municipal inspection departments and everyone else, while you'll probably show that you're human and make some mistakes and not design a perfect system. Whatever you end up putting to plans will be a working system that everyone will accept if not be 100% happy with, or it'll keep coming back to you for multiple revisions until you do get it right.


Once the system is up and running, that will be the proof of your professional pudding. Is the system you designed noisy as a Trinidad steeldrum band, because you didn't think further ahead enough and used fittings more suited to high-pressure duct runs than low-pressure, or because you spec'd plain duct hangers instead of vibration-isolation type? Do your airwash patterns dump air on the vertical directly onto workers at their desks, or does it flow more or less horizontally and naturally over them as they face the supply registers? Did you think to try to isolate non-code odor areas like the breakroom or artroom by using localized exhaust tied into the greyair/bathroom exhaust duct system? Do all areas get at least some eddy current suckthrough in the air return patterns, or will the occupants feel that there are dead spots within the building?


More than common sense, more than established engineering and design principles, more than even merely putting yourself in the place of a given building occupant while designing, embracing a total HVAC system zone of awareness means empathetically embracing the veiwpoints, needs, and concerns of all parties involved, from the owner that ultimately pays you, to the maintenance person who perhaps silently compliments you on designing a system excellently enough to where he doesn't have to scratch his butt with his elbow to keep his system running effectually.




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