Updated: Jan 21
My name is Kenny, and I am a mechanical engineer. You usually learn about a guy like me when the HVAC contractor pricing comes back at about...the entire construction budget. That's right about the time I get the phone call asking for the "value engineering" to begin.
Being in the HVAC world is tough because most solutions are lose-lose between us and the end-user. If we make the system cheaper, the end-user will inevitably be uncomfortable and scream "who the heck engineered this system!? They better pay to fix it!" On the other hand, if we design the ideal comfort solution, the end-user will proclaim, "these engineers are just over-engineering our system and trying to make us buy things we don't need! We're not paying those nerds to redesign this over-designed system!" (Don't worry, we don't charge change orders anyway.)
Very few people understand how to estimate the cost of their HVAC systems. Even from our colleagues that are peripheral to the process (developers, contractors, architects), we hear a few scary lines that put us in a tight spot. Here are a few examples of what we hear that end up offering false hope to you while tee-ing us up to look like the bad guys when our designs are complete:
"A five-ton unit should be plenty" (for literally every space in existence)
"The building already has HVAC, so we should be able to use what's there!"
"We assumed 400 square feet per ton, so that should be enough!"
"You can just put the fresh air on the return, that's the cheapest way to ventilate."
My technique for getting ahead of this little dilemma is always to remind our clients that "you can be as comfortable as you can afford," and here are a few key points to summarize the meaning:
So...How Comfortable Will I Be?
One the hottest days of the year, you will be able to get your space down to 75 degrees. On the coldest days of the year, you will be able to heat your space to 68 degrees.
That's the simple way to say it, but the secret killer in HVAC is moisture in the air. Removing moisture (dehumidifying) is complex and expensive.
Ideally, you want your space to maintain relative humidity levels between 40 and 50 percent. These are the humidity levels that generally promote a healthy breathing condition and reduce the likelihood of mold growing within your occupied space. The cheaper you get in your HVAC selections, the more likely you are to be exposed to high levels of moisture in your occupiable space.
You can add dehumidifiers after the fact, but generally speaking, these are only bandaids that will struggle to maintain cohesive climate control. The best thing you can do is to just buy the right equipment from the start.
Fresh Air is...Complicated.
The law requires that you ventilate your space. Ventilation is either the removal of harmful contaminants from the space by way of exhaust or the introduction of fresh air into space by way of mechanical ventilation.
Imagine the hottest, stickiest day of the year; now imagine opening a window using a fan to blow all that hot, wet air directly into your room. That is mechanical ventilation.
You can imagine how much energy it would take to literally cool down the outside and bring it into your space at a comfortable temperature. That 5-ton unit mentioned above suddenly became a 15-ton unit and quadrupled your mechanical budget. We've seen it happen in almost every restaurant, lab, church, bar, and night club project where the team has lower levels of experience in those areas.
Exhaust Ain't Cheap
The problem with the exhaust is that you can't just remove air or you'd suffocate all of your occupants. The code requires something called "makeup air," and it does exactly what it states: makes up the air.
This presents the exact same problem listed above: you are now bringing in air from the outside. The air from the outside is at the temperature you are trying artificially to modulate to create comfort.
It just ain't cheap.
Major Tip for Avoiding Mechanical Ventilation
Ask your engineer or architect if natural ventilation is an option. Basically, the code states that if your space has enough doors and windows that are accessible to the occupants, then there is a reasonable expectation that they will be used enough naturally to ventilate the space.
This is a major cost saver. You might find $50k-$100k in mechanical equipment suddenly no longer required for your project if you can use natural ventilation techniques. Sometimes it's cheaper to add an extra door to your building facade just to get yourself in a place to use this option.
Warning: sometimes, it's still appropriate to bring some level of mechanical ventilation into your space just to help pressurize your building and purge the stale air out. It's best to consult with your mechanical engineer (ideally, PermitZIP) about this topic.
Perfect Zoning is a Unicorn
Okay, let's just face it: you can't make everyone happy. It's best to start managing your expectations now on your project of general occupant comfort.
Have you ever been to a restaurant and complained that it was too cold? It's likely due to the fact the general manager is running around all day on their feet and to them, the restaurant is quite warm! The point is: everyone has a different perspective on what comfort means to them.
The only way to give everyone perfect comfort is to give each individual in the building their own zone control. The more zone control you have in your building, the more the HVAC costs go up. There is no unicorn solution for this that has a friendly price tag with it! You have to understand that if you "value-engineered" your system by removing VAV's, or additional units, or any other variant of zone control, then you will be sacrificing your ability to maintain control of your comfort.
We Try Our Best
We really do try our best. The general contractors, the sub-contractors, and the engineers all want to work to provide you the ideal solution. Just understand that perfectly cheap and perfectly comfortable HVAC are almost mutually exclusive concepts.
Lucky for you, PermitZIP will get our team on-site to your project for free, with no contract. We'll send you a quick summary of your building and the kinds of things you can expect that might have a major impact on your costs. Engineer Are Tools™...so use us!