Why you should know your Heating and Cooling Load before replacing or installing your HVAC unit
It used to be that most homes were “leaky”, meaning not that the roofs leaked water, but that due to lack of air barriers, air from outdoors could easily flow in and out of homes even with the doors and windows tightly closed. With these construction methods, HVAC systems tended to be oversized because they were not only heating and cooling the air under roof (square footage of the home), they were conditioning the air that flowed freely into the home from the outside, too! Although the right way to size an HVAC unit is by performing a “Manual J” calculation for the home to determine the heating and cooling load, many HVAC contractors just go by square footage rule-of-thumb with some experience thrown in, usually by specifying larger units “just in case”.
Now that we’ve decided to keep outdoor air outside, with all its wildfire smoke, smog, humidity, dust and pollen, we’ve started making homes “tighter” by including air barriers and smart construction practices. We don’t have to condition “extra” air from the outside anymore! Just sealing crawlspaces and attics makes a big difference in most cases, because these are the main routes for the chimney effect that brings outdoor air in. This makes a big difference when you go to replace your HVAC unit–or buy the first one during construction of your home. Using just the rule of thumb may specify a unit that’s too large for your home.
Well, what’s wrong with having a larger unit? Isn’t it always wise to have “extra capacity” in case the summer is extra hot or the winter extra cold, or I have extra people staying in my home? In the case of HVAC capacity, this is not correct thinking. The reasons are that 1) you’re paying extra for a larger unit that you will not need 95% of the time (or even 100% if your home is more airtight than most), 2) when your unit is operating at its lowest load, it’s not being efficient. HVAC units are designed to operate most efficiently in their middle range, where they have adequate runtime to move sufficient air to get all of the home comfortable.
Here’s where, if you’re taking the time and money to “upgrade” things about your new or existing home, you should do the same with your air quality and pay a professional to design or renovate the HVAC system and size it correctly.
But–doesn’t the HVAC company do this? Sadly, no, most HVAC contractors are skilled in installation and sometimes skilled in troubleshooting, but HVAC design is not their forte. We can’t emphasize this enough, because although the contractor will probably do an excellent job of installing the unit, he will be hard-pressed to solve issues related to getting the house comfortable when the wrong size or type of unit is installed.
HVAC designers use a “Manual J” calculation, but they can’t blindly trust it. They run the calculation and observe how homeowners actually use the home. Manual J calculates the amount of heating and cooling your home needs at design conditions, which is how much to keep your home at 70 deg F when it’s 5 degrees outside, or 75 deg F inside when it’s 88 degrees outside. Are these the inside temps you like, or do you like them warmer or cooler? Other factors that can make the Manual J calculation seem “off” at first: actual insulation in an existing home has probably degraded over time. Also, most older homes are leakier than you think, and it’s an educated guess without a blower door test. The designer should also ask questions about your lifestyle, including cooking habits, and whether you like to turn down the temperature while you’re away and back up again when you come home, which requires more capacity.
Likewise, when comparing the theoretical load from the Manual J to what the existing system is rated for, the HVAC designer needs to figure out what the system is actually providing. If the existing system is huge (oversized) compared to what the Manual J calculates you need for your home, but doesn’t run all the time at extremely hot or cold conditions, then that confirms it is oversized–it runs even less (minimally) at normal conditions. Also, the ductwork might be small and undersized for such a massive system. That further confirms that if the home is comfortable, it’s not using the rated capacity of the existing system. Finally, if it’s possible to go up in the attic and verify what insulation is actually being used, the designer can adjust the inputs to the Manual J and get closer to a realistic load. Their job is to refine the calculation with real-world observations to specify a system that will meet your needs. (Manual J for Existing Homes: The Reality Check)
Finally, customers are learning about the flexibility of variable-speed systems and it seems like this means variable speed = variable capacity. This is true…to a point. The real limit sometimes is the space it’s installed in! Even variable speed has a minimum speed/size. In his recent article The Most Expensive Fixed-Capacity HVAC System, Dr. Allison Bailes gives the example of a 6000 Btu/hr heat pump mini-split that’s installed in a single bedroom. That bedroom only ever needs 500 Btu/hr of heating or cooling even in extreme weather, but the mini-split’s minimum capacity is 1500 Btu/hr. It’s way oversized because the load (the amount of heat or cool the space requires) is below the range of the unit that was installed, so even though the unit has variable capacity, it only ever runs in a fixed-capacity way, because the load is so small that runtime “never gets off the ground”. In order to be a good fit, this unit should have been smaller, or served several rooms with different fan coil units (to add more load). .
It’s good to ask if your HVAC contractor has the capacity to do Manual J calculations, AND to ask for references of other homeowners to see if they are happy with what they’ve specified and installed. A custom home deserves HVAC customized to it, not just rule-of-thumb, and the amount you spend on figuring out what you need is well-worth it in your comfort and peace of mind after it’s installed and running.
