Besides proper indoor air quality, comfort, and system sizing, there is one other important reason to size your heating and air conditioning equipment to the proper indoor design conditions: Residential Code! This is always a tough conversation with the homeowner. After all, the customer is always right, right? When I start to talk about state codes, I like to explain to those less concerned that this is the worst case allowed by laws in our state. Luckily for homeowners and quality contractors, the days 'that minimum' is getting better. The 2006, 2009, 2012, 2015, and 2018 International Energy Conservation Code sets the minimum indoor design temperatures in cooling and the maximum indoor design temperature in heating. Notice the years? This is not new information, it is just finally being enforced. For the most part, as soon as something is enforced in your state, that is when contractors "embrace".
IECC Section 302.1: Interior Design Conditions
- The interior design temperatures used for heating and cooling load calculations shall be a maximum of 72° (22°C) for heating and a minimum of 75°F (24°C) for cooling.
ASHRAE Comfort Zone
Why these temperatures you might ask? Please refer to the ASHRAE Psychrometric Comfort Zone. If you notice, there is no place in the Summer when sizing to 70F and any humidity level is comfortable. If your system was to actually reach 70F in a home during the summer, your customer will feel cold and clammy. You should always design to the ACCA Manual J cooling indoor design conditions of 75F and 50% Relative Humidity (RH). This will put you in the middle of that comfort zone with some room for "drift" on those above design days in mid August. If the unit cannot keep up, it will operate constantly, and it will still be comfortable in the conditioned space. Humans are much more sensitive to the relative humidity than the sensible temperature on the thermostat. ASHRAE was nice enough to publish some charts regarding their reasoning for 50%RH with respect to indoor air quality as well. Lo and behold the least amount of viruses, bacteria, mold, etc. grow between 30% RH and 50% RH! This is why we add humidifiers to forced-air furnaces in the Winter.
So, customer education to establish expectations is a must in today's equipment sales process. If you install a system to code, the very minimum required by state laws, it will not run the same as the old unit. We found it is much more efficient to properly size systems for homes, not homeowner lifestyles. Sure, you can give the customer what they want - but I am sure they would rather be comfortable, with a unit installed to code!