Reviewer
Marcus Reilly
EPA 608 Universal, NATE-certified, 14 years HVAC
Marcus is a residential and light-commercial HVAC service technician based in the Midwest. He holds EPA Section 608 Universal certification, which is the federal credential required to handle refrigerant on any equipment type from window units through high-pressure commercial chillers, earned by passing the proctored Core exam plus all three Type I, II, and III specialty exams under 40 CFR Part 82 Subpart F. He is also NATE-certified in air conditioning and heat pump service, the Traditional Pathway track that requires the Core exam plus a 100-question specialty exam, with biennial recertification through 16 hours of continuing education.
Day to day, Marcus runs service calls on 2-to-5-ton split systems, mini-splits, and gas furnaces. The math on this site is the math he uses on the truck: subcooling on a TXV unit, superheat on a fixed orifice, PT chart lookups for R-410A and R-454B, line-set sizing on long-run installs, and load calculations when a homeowner wants to know whether their existing equipment is the right size before he writes the quote. He has worked through three refrigerant transitions in his career (R-22 phaseout, R-410A standardization, and now the A2L move to R-454B and R-32), which is why the refrigerant tools on this site are the ones he checks most closely.
Marcus reviews the system sizing math (BTU sizer, heat pump sizing, furnace sizing, mini-split sizing, line set sizing), the refrigerant tools (PT chart, superheat, psychrometric), and the dew-point math that drives condensation troubleshooting. He flags any tool that defaults to a value he would not accept on a kitchen-table walkthrough.
Tools reviewed by Marcus
Every tool listed below was reviewed by Marcus Reilly against the published code, standard, or manufacturer data it implements. Marcus signs off on the math, the inputs that drive it, and the edge cases the result panel flags.
Pro tools
How BTU Size uses reviewers
Every tool on BTU Size credits a named technician with field experience in the relevant trade. Reviewers check the math against published code (ACCA, ASHRAE, NEC, NFPA, IECC), against manufacturer engineering data (Carrier, Trane, Mitsubishi, Owens Corning, Slant/Fin), and against the edge cases that show up on real service calls. See the full reviewer roster and methodology.