The Real Reason for HVAC Design — It's Not Sizing
Many people seem to think HVAC design means you get a load calculation (Manual J in the ACCA protocols) so you know what size system to put in. Hey, that's a great start. It's way better than just using a rule of thumb or Manual E (for eyeball). You don't really want to end up with a ginormous oversized air conditioner like the one above. (But I'm going to be a heretic here and suggest that oversizing isn't as bad as some think, or at least not in one of the ways commonly believed. More on that in a future article.)
But there's so much more to real HVAC design than simply finding out how much heating and cooling a building needs when it's at design conditions. And we might as well start with the fact that my first statement is incorrect: The load calculation does not tell you what size system you need.
Load versus capacity
In an article I wrote last year, I went into detail about the difference between getting the load calculation results and sizing your heating and cooling system. You have to factor in the type of equipment you're using, the efficiency of the equipment, the breakdown of the cooling loads into sensible and latent, and the difference between your design conditions and the conditions at which the equipment was rated. (Sensible load is related to changing the temperature; latent load is the part involved with removing moisture.)
But it really boils down to a difference between load and capacity. Heating and cooling loads are the amount of heating and cooling in BTU per hour (Watts for most places outside the US) that a building needs. Capacity is how much heating or cooling a piece of equipment can provide. Just remember that loads have to do with the building and capacity has to do with heating and cooling equipment.
So, the load calculation is the first step. It leads to sizing but doesn't give it to you right away. (Be careful reading those Manual J reports!) The second step is equipment selection (Manual S in the ACCA protocols), where you take into account those factors I mentioned above. It's an important step and more involved than just reading the load off the Manual J report.
But even that isn't the most important part of full HVAC design.
The dominance of distribution
Calculating the heating and cooling loads is the easy part. Even selecting equipment is straightforward. Once you know the loads and have made decisions about the type of equipment (furnace & AC, heat pump...), getting the right capacities isn't hard. But unless you're using ductless mini-split heat pumps (a great choice, by the way), the next step is in many ways the most difficult and the one that dooms many projects. That is, designing the distribution system to make sure the house gets the right amount of heating and cooling delivered to the rooms. (We focus on air distribution at Energy Vanguard, so if you're doing hydronic heating and cooling, you should talk to someone like my friend Robert Bean at Healthy Heating.)
Designing a good distribution means looking at a lot of variables:
- Placement of supply and return vents
- Location of air handler
- Framing obstructions
- Types of fittings
- Type of design: trunk-and-branch or radial
- Location of ducts (conditioned or unconditioned space)
- Proper air flow, both total through the system and the amount to each room
See my series on duct design to get a feel for it. There's a lot that goes into it!
When you do it properly, you get a true system. Many homes just get a bunch of components that appear to be a system but really aren't. Sadly, the bar is really low for heating and cooling systems. Since so few systems get true design, not many people know what they're missing. If the house stays relatively warm in winter and cool in summer, it gets over the low bar.
But here's what a good HVAC system provides:
- Indoor air quality
- Humidity control
- Quiet operation
As I write this, I'm sitting in our new office in Decatur, Georgia. We have our own system for the office, which is great. We don't have to fight over the thermostat with other businesses in the building, as we did in our previous office. But the system is oversized. It's loud. And it short cycles. We get blasted with hot or cold air for a few minutes. Then it goes off until the office starts getting a bit uncomfortable in the other direction, when it kicks on again.
The good news for us is that we've gotten permission from the building owners to replace the system we have with a Mitsubishi system consisting of a ducted mini-split for the two back rooms and wall-mounted ductless units for the front. We're also getting an Ultra-Aire ventilating dehumidifier for the office. Stay tuned for an article about the installation once we get it all done.
The real reason
So, back to the original question of the real reason for HVAC design, you can see now that it's a lot more than just proper sizing. That's certainly important. But more important than proper sizing is making sure the HVAC is a true system, not just a bunch of components pretending to be a system.
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