A house can be defined as a single-unit residential building that can be of varying levels of complexity and is outfitted with plumbing, electrical, ventilation, heating, and A/C systems. Ducts can be defined as conduits or passages used in HVAC to deliver and remove air, including supply air, return air, and exhaust air.
We can contrast ducts and houses by looking at who/what inhabits them, air velocity, heat transfer, fenestrations, control of air direction, and volume control. People live in houses but not ducts, and we want lower air velocities in the house than we do in the ductwork. A house is also a place where we have to worry about multiple factors for comfort control, including radiant heat, and it has fenestrations that also contribute to comfort whereas ducts do not. Ducts, however, can control the direction of air whereas houses cannot. Ducts have intentional volume control, meaning that we can use dampers to control the volume of air intentionally; volume control in houses is rarely intentional (for example, opening and closing doors without any intention to affect the airflow).
However, ducts and houses also have a lot in common. They both have a relatively fluid operation, meaning that we can balance or control comfort factors. Houses and ducts also have the potential to affect airflow to the equipment. Both of them also need to be sealed tightly if we want to have control over the operation.
When we look at houses, we have to understand that there is a pressure envelope separating them from the outdoors, and fenestrations penetrate that envelope without actually affecting the pressure. However, routing ductwork can introduce another penetration that CAN affect the pressure and expands the pressure envelope; kicking the fan on will create a pressure differential. Air takes the paths of least resistance out of AND into a space, and the pressure differential is only part of the equation.
We can use manometers to measure pressure, and some of them have resolutions that can read Pascals, which are very small pressure units. In any case, the pressure outside the structure is our “zero” reference; everything we test inside the house will be in reference to that pressure. (WRT = with reference to.)
When doors close in the house, those respective rooms can go under positive pressure WRT the rest of the house and get too hot during the heating season; while that happens, the house goes under negative pressure and draws more air in. The room that’s under pressure will have air escaping to the rest of the house AND the outside, which cuts off the low-resistance path to the return in that room; the room will have more “dwell time” in that room, which makes the room feel “over-conditioned” and affects comfort in the entire home.
Register sizing and placement are also vital for indoor comfort, as you want the conditioned air to blow on surfaces and affect surface temperature.
Mechanical ventilation results in depressurization, and it can come from bath fans, kitchen hoods, and clothes dryers. Non-direct vent appliances (furnaces, water heaters, etc.) can also contribute to depressurization via the stack effect and must be taken into account. We can counteract those with makeup air (which should seek to neutralize the most-used exhaust), which can be interlocked with the exhaust. In an ideal world, the makeup air and mechanical depressurization would be equal, but that’s often not realistic. Makeup air should be filtered, and humid climates with makeup air units can benefit from dedicated dehumidification.
We can design makeup air to come in through soffits, and we have to be intentional about placement, as always. It’s best to make the filter as big as possible, too. We can also include dampers to balance the air; undersizing wouldn’t allow us to make situational adjustments.
ERVs or HRVs can also ensure a balance between the exhaust and intake air, but dehumidification can be a challenge in high-humidity markets. These are NOT intended for makeup air and are for ventilation ONLY.
When designing the ductwork in a house, Eric recommends avoiding pressurizing rooms. He also recommends central returns on rooms with low airflow per room and ducted returns on those with higher airflow per room. As always, ducted returns require the use of the proper friction rate. Calculating the pressure drop of the house-based return path and blowing air on surfaces (not people!) are more of his tips. On the house side, control radiant heat transfer (affected by room color) and temperature stratification.
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