HVAC Design

The most important thing about an HVAC System is that it is a system. And it is part of another system - the building itself. A mentor says that building spaces are like big ducts with people in them. Another old adage: You can't optimize for one part of the system. The whole system must be considered.

The HVAC industry has long been ignored by the design industry, and especially by residential design & construction. HVAC systems had to be optimized because the houses they were installed into were not considered holistically.

HVAC Systems were therefore oversized because of the need to simplify HVAC design and installation to keep up with demand. Rules of thumb became mantras passed on over decades. Most HVAC systems focus on keeping homeowners as warm or cool as they want to be and as quickly as possible. No HVAC Installer wants to get called back because their system doesn't keep their customers comfortable.

But over the decasdes, houses have become tighter. Homeowners could not afford the energy penalties of the old ways of building. It turned out that the rules of thumb that kept HVAC installation as simple as possible, but not simpler were too simple. Being able to cool down or warm up a space quickly has some bad effects on the space and the people in it. Temperature isn't the only thing that the HVAC System has to control. Moisture is very much a design load that needs to be calculated. In decades past, the moisture wasn't a problem. Buildings were very leaky and building materials were more robust. Those days are over.

Humid spaces are not comfortable spaces. For example, because homeowners have only been provided with a way to control temperature, they are generally unaware that when they have the temperture set to 68° in the summer, that the reason they still feel too warm is because the air is still too damp. The thermostat is a switch. On and off are the settings. Air that is too damp doesn't allow your skin to evaporate sweat that forms when you are too hot. These are symptoms of an HVAC system that can’t handle the moisture load in your home.

HVAC units are regularly oversized. Think of your oven or the burners on your range. They work well in helping you and your family cook and follow recipes because the burners match either the coil size that the electricity needs or the gas orifice size used with the gas pressure.

Sure, a LARGER coil will cook the food faster. But since cooking involves both chemical reactions and physical laws, that isn’t going to help. Plus, you’re probably not going to want to eat a turkey cooked in 10 minutes.

An OVERSIZED HVAC system WILL get to the set point on your thermostat much quicker. But those same physical laws are at play here. There is another important factor that isn’t on the thermostat- it’s the relative humidity inside the home.

That number ONLY comes down when the HVAC runs for a while. Since most HVAC Systems reach the set point earlier in that cycle than the building calls for, the HVAC System shuts off, and the relative humidity simply floats back up to the level it was at during the last cycle.

Adding a dehumidifier can help. But it shouldn’t try to make up for all that moisture. The right tool for the job is often a combination of a smaller HVAC system AND a dehumidifier.

What about the winter? Many HVAC Systems are blamed for making the building too dry. Occupants have to deal with the effects by using more moisturizers and often resort to humidifying the building. We have seen that a hoiistic design of the the building enclosure and the HVAC Systems meant to condition it will yield a comfortable result.

HVAC Systems Should Allow Occupants To Use The Whole Building

The temperature and the humidity should not be wildly different in one part of the building compared to another part of the building unless we are talking about a walk-in cooler. The second floor should not be many degrees warmer than the first floor simply because warm air rises.

In short, the HVAC system must be designed for the house and specifically the building enclosure it is meant to condition.

It also needs to bring in enough fresh, filtered air for you and your family to breathe and get pollutants out of your house as efficiently as possible.

Proper HVAC Design is not common. Uncomfortable, unhealthy buildings are becoming more common. Building systems should be optimized for the occupants. Occupants should not have to conform to the systems. If that were not true, what would be the point of "roughing it" when camping?