Properly sizing heat pumps is crucial to maintaining comfort and efficiency in all weather conditions for both heating and cooling. “Sizing” means matching the heat pump’s hourly output to a home’s heating (and cooling) requirements. While legacy fossil-fuel heating systems were routinely oversized without negative consequences, a heat pump’s output must be matched more closely to a home’s heating requirements. It’s a Goldilocks-and-the-Three-Bears challenge: not too big and not too small, but just right. Proper sizing requires accurate estimates of a home’s heating and cooling requirements.
The industry-standard method for sizing residential HVAC systems is to perform a Manual J analysis followed by Manual S analysis using ACCA-approved software. But there are several ways to do a Manual J that require varying levels of input detail. A simple “block load” analysis can be done quickly based on a satellite image of the home while a more detailed room-by-room analysis requires measurements of each room. Doing a careful, room-by-room analysis used to be quite time-consuming, but a recent innovation has dramatically reduced this time to a little as 15 minutes for a typical home. Here, we’ll discuss some recent observations on how this innovation impacts installers’ use of Manual J. The Manual J innovation relies on the LiDAR scanners that are now built into some tablet devices, such as Apple iPad Pro models. We’re aware of tablet apps by Amply, ConduitTech, and CoolCalc that make use of the LiDAR scanner and enable a user to scan the interior of a typical home in about 15 minutes. This makes it possible for trained HVAC personnel to complete a Manual J analysis while visiting homes instead of returning to the office and facing the time-consuming task of manually entering measurements into Manual J software. Recently, one of our volunteer coaches assisted a homeowner in Medfield, MA. The homeowner invited 5 HVAC installers to visit the home and provide quotes, and also asked each company to provide their Manual J analysis. All are members of the Mass Save Heat Pump Leaders Network (HPLN). Three of the 5 installers used a tablet app to perform a Manual J during their home visit, which is a big change from just a couple years ago. Only one installer provided a Manual J report on request, though it appeared to be a block-load analysis based on a satellite image. The other installers provided the homeowner, when asked, with the design heating load.
Unfortunately, even with the more detailed measurements provided by the LiDAR scanner, the design heating loads provided by the 3 installers who used tablet apps varied by 30%. The 2 installers who did not use tablet apps provided design heating loads that were over 50% higher than the lowest scanner-based estimate. Based on our experience, we don’t think this example is an anomaly. For some homes, the ranges may be even larger.
What accounts for this large variation in heating design load estimates? While some of the variation may be due to different assumptions about the heating design temperature, it’s likely that most of the differences are due to different assumptions about the non-geometric inputs to Manual J, such as infiltration rate (air leakage), R-values of walls and ceilings, and U-values of windows. Evidently, installers who use tablets to do Manual J still need to be careful to review the other input assumptions for each home.
To sum up, recently introduced Manual J apps for LiDAR-equipped tablets are enabling more HVAC installers in eastern Massachusetts to perform heating and cooling analyses on site. This is good news and should lead, ultimately, to more heat pumps being properly sized for homes. However, users of these apps still need to check all input assumptions to obtain accurate heating and cooling load estimates. Of course, we expect that consistency and accuracy will improve as HVAC installers gain more experience with these new apps.
