HVAC Engineering Blog
Technical articles on HVAC load calculations, Manual J, duct sizing, and equipment selection for contractors.
Is a 250 Gas Meter Big Enough for a 24 kW Generac Generator
Oct 18, 2026 · 14 min read
An AC-250 meter is rated 250 cfh at a 0.5 in. w.c. drop, and a 24 kW Generac Guardian burns 306 cfh at full load. Add up the house in cfh before you call.
Normal AC Pressures by Outdoor Temperature for R-454B and R-410A
Oct 17, 2026 · 13 min read
Lennox lists 336 psig liquid and 133 psig suction for its 3-ton EL16KC1-036 (R-454B, TXV) at 95°F outdoor. Read any gauge set against outdoor temperature.
Does an AC Condensate Drain Need a Trap and How Deep
Oct 16, 2026 · 15 min read
IMC 307.2.4 leaves the trap to the manufacturer. Goodman's AMST manual asks for a 2 in. drop and a 2.75 in. seal. Check it against the static at the pan.
How to Estimate Furnace Gas Usage From Heating Load and Degree Days
Oct 14, 2026 · 9 min read
Heat loss / design dT x 24 x HDD / AFUE / 100,000 gives therms. A 40,000 Btu/h Atlanta house comes to about 568 a year at 95% AFUE on a 65°F base, 296 at 57°F.
How to Convert Blower Door CFM50 to Manual J Infiltration
Oct 12, 2026 · 13 min read
Manual J doesn't divide ACH50 by 20. On ACCA's two-story sample house, its leakage-area method gives CFM50 / 10 for heating and / 22 for cooling.
Should You Lower CFM per Ton in a Humid Climate
Oct 11, 2026 · 12 min read
Lower CFM per ton for dehumidification? On a real 3-ton table, 350 vs 400 adds about 460 Btu/h of latent capacity and costs 2,400 Btu/h total.
What Friction Rate to Use for Flex Duct in Manual D
Sep 22, 2026 · 14 min read
Flex duct gets the same Manual D friction rate as metal, ASP x 100 / TEL, inside 0.06-0.18 in. wc/100 ft. Worked example, plus what 15% compression does.
Manual S Oversizing Limit for Air Conditioners Under Manual S-2023
Sep 21, 2026 · 14 min read
Manual S-2023 caps a single-speed air conditioner at 115% of Manual J total load, or 120% at 24,000 Btu/h or less. Read expanded data, then check.
What the A2L Refrigerant Transition Changed for Contractors in 2026
Jun 28, 2026 · 18 min read
EPA moved the R-410A install deadline in May 2026. Where the A2L switch stands with R-454B and R-32, the no-mixing rule, leak detection and what to install now.
Manual J vs Manual S vs Manual D and What Each One Decides
Jun 25, 2026 · 19 min read
Manual J vs Manual S vs Manual D: J finds the load, S picks the equipment, D sizes the ducts. What each ACCA calculation decides and the order they run in.
How to Size a Heat Pump and Find the Load That Sets the Size
Jun 18, 2026 · 17 min read
A heat pump has to cover two loads. Which one sets the size, how to find the balance point and how to size backup heat without oversizing.
How to Size HVAC for a 2,000 Sq Ft Home
Jun 17, 2026 · 13 min read
An ACCA Manual J walkthrough for a 2,000 sq ft house, with a worked example and the reason the square-foot rule of thumb oversizes it.
Run the calculation
The articles explain the procedures, and these pages run them in deterministic code. Whole-house loads are free to try without a signup; room-by-room Manual J and Manual S are part of the Pro plan.
- Manual J load calculator
Whole-house heating and cooling loads from a ZIP code; room-by-room loads are part of the Pro plan.
- Manual S equipment selection
Match the load to AHRI-certified capacity at design conditions, not at the rating point. Pro plan.
- Replacement AC sizing
For the changeout where the old nameplate is the only documentation on site.
- Retrofit sizing without a blueprint
Size from a walkthrough description when no drawings exist.
- Manual J for new construction
Preliminary loads during design development, before drawings are stamped.
- Permit-grade Manual J
Where sales-phase sizing stops and a certified submittal starts.
Full catalog on the tools hub, including Manual J pages for ten states.