Who builds these
About these HVAC tools.
These tools are built by Watson Consulting, a custom development organization. The HVAC apps and the free browser tools on this site are built and maintained here by Mike Watson — one shop, four apps, no venture money and no sales team. This page exists so you can see what the numbers are built on before you trust one in front of a homeowner.
The short version
Field calculators that work with no signal.
Every tool started from the same complaint: the software that does this math properly is desktop software, priced for an office, and useless in a crawlspace. So the calculations run on the phone. Nothing waits on a network, nothing needs an account, and the arithmetic does not change because you drove out of coverage.
There are four apps — a Manual J load calculator for the US, a CSA F280-12 heat loss calculator for Canada, duct sizing and static pressure tools, and A2L charging and charge-limit checks — plus a growing set of free browser calculators and technical guides that need no download at all.
- Offline first. The calculation engine ships inside the app. Mechanical rooms, new builds, and rural service calls are the normal case, not the edge case.
- Assumptions on the page. When a tool defaults a friction rate, an orientation, or an equipment rating, it says so in the result rather than burying it. A number you cannot explain at the door is worth nothing.
- Scope stated plainly. The free browser Manual J tool is a whole-house block load from eight inputs, and it says that in the output. It will not pretend to be a room-by-room design.
- No data collection in the calculators. Projects stay on your device. See the privacy policy for what each app does and does not touch.
Sources
Where the numbers actually come from.
None of this is proprietary. It is published methodology and public data, implemented carefully — and you should be able to check any of it.
- US design temperatures. 99% heating and 1% cooling dry-bulb values from the ENERGY STAR County Level Design Temperature Reference Guide, with ZIP codes mapped through the US Census ZCTA gazetteer. When a ZIP cannot be matched exactly, the tools tell you whether the value came from a reference city or a three-digit regional prefix instead of quietly guessing.
- Canadian design temperatures. HOT2000 weather station data covering 403 stations and 1,651 forward sortation areas, so a Canadian job is sized against Canadian conditions rather than a converted US table.
- Load methodology. ACCA Manual J 8th Edition methods in the US app, CSA F280-12 methodology in the Canada app, ACCA Manual D methods for duct sizing and static pressure.
- Refrigerant data. Pressure/temperature tables with bubble and dew points handled separately for glide blends, and lower flammability limits from ASHRAE 34-2022. Charge limits follow the UL 60335-2-40 and ASHRAE 15 family of methods, including mitigation cases.
Two things follow from that list. First, the tools use standards methodology and are never described as approved or certified by ACCA, CSA Group, ASHRAE, or UL — those organizations have no involvement with this software. Second, where a manufacturer’s chart or nameplate exists, it wins. The apps say so on the result screens, not just in the fine print.
How it is checked
The web tool and the app run the same engine.
The free browser Manual J calculator is not a simplified rewrite of the app. The calculation engine is extracted from the app’s source at build time, and an automated test runs both across dozens of input combinations and climate zones on every build. If the browser number and the app number ever disagreed for the same inputs, the build would fail.
That matters more than it sounds. A tech who gets one answer on the web and a different answer in the app has no reason to trust either. The engine also carries a version number, and a set of pinned reference calculations has to reproduce byte for byte before any change ships — so a refactor cannot quietly move your load by a few thousand BTU.
The same principle runs through the rest: the duct tools validate forward and reverse sizing against each other, and the charging tools separate bubble and dew rather than averaging them into one column that will fight you on subcooling.
Contact
Real email, answered by the person who wrote the code.
Bug reports, wrong numbers, and “this assumption does not match how we do it here” are all welcome — several shipped changes started as a one-line email from a tech. Reach us at mike@hvacloadcalculator.online. If you can include the inputs you used, the fix usually lands in the next build.