Arkansas HVACR NewsMagazine March 2024

HVACR NewsMagazine March 2024

Tech News

Pressure Relief

near the exhaust fan, and you could bring in very hot, humid attic air. Bath fans and range hoods running continuously or on timers are examples of exhaust-only ventilation.

Some homes, especially airtight ones, also need pressure relief from one-way exhausts, like kitchen range hoods, since those homes may have airtightness blower door scores of under 400 CFM. Building codes currently state that a home only needs make-up air when it has an exhaust hood bigger than 400 CFM, which is why you don't listen to code when it comes to this stuff; you listen to industry professionals. Even once that has been figured out, you will need to think about how the pressure relief will affect the dilution air you bring in and how humidity control, filtration, and circulation will be affected.

Supply Only (Positive)

Supply-only ventilation happens when you bring air into a space through a duct. The air usually comes in with help from the air handler or furnace and negative pressure in the return (if the duct is tied into the return duct or plenum) or a dedicated powered fan (if tied into the supply). As its name suggests, this strategy puts the house under positive pressure (probably not much though). A balanced ventilation strategy has an equal amount of exhaust air and intake air. We usually achieve balanced ventilation with a single machine with two separate motors, such as an energy recovery ventilator (ERV) or heat recovery ventilator (HRV). ERVs exchange sensible and latent heat between the intake and exhaust air, and HRVs only exchange sensible heat between the two sources. In other words, ERVs buffer both temperature and humidity, whereas HRVs only buffer temperature. This short podcast explains a bit more about how HRVs and ERVs work. We have a few different ways to bring in outdoor air, including HRVs and ERVs. However, bringing in safe dilution air isn't as simple as installing an ERV and calling it good. We need to make sure that the incoming air meets the following criteria: Balanced Bringing in Outdoor Air Safely

Ventilation Strategies

Ventilation comes in a few different forms, and each one has a different effect on the building envelope.

Exhaust Only (Negative)

Exhaust-only ventilation happens when we exhaust air from the space without bringing any back in. It's the least expensive ventilation strategy. Utilizing exhaust-only ventilation is better than having no ventilation whatsoever, but you don't have a lot of control over where the "fresh" air is coming from. That air will come in through cracks in the building's envelope