/ Field knowledge · conversions
A rule of thumb is not a load calculation.
A rule of thumb is not a load calculation. Cooling that “always” runs 1 cfm/ft² will be wrong on a Miami glass box and wrong on a warehouse. These identities are sea-level textbook factors used to check whether a number even belongs on the sheet — not to size a coil, a pump, or a chiller. 1 refrigeration ton = 12,000 Btu/h. Sensible air at sea level: Btuh ≈ 1.08 × CFM × ΔT (°F). Standard water: Btuh ≈ 500 × GPM × ΔT (°F). Air changes per hour: ACH = CFM × 60 ÷ room volume (ft³). Energy efficiency ratio (EER) and coefficient of performance (COP): COP ≈ EER ÷ 3.412.
Educational field identities. Confirm density, glycol, altitude, and wet-bulb before anyone treats the result as a design. This page is original Masterbuild education. It is not a reproduction of any vendor “green sheet,” pipe-sizing table, or psychrometric chart, and it does not replace ASHRAE load methods or the PE of record.
- Sensible air (sea-level dry-air identity): Btuh ≈ 1.08 × CFM × ΔT. The 1.08 comes from 0.075 lb/ft³ × 0.24 Btu/lb-°F × 60 min/h. Humid Miami air and high altitude change it. Use 1.08 only as a sea-level check.
- Standard water: Btuh ≈ 500 × GPM × ΔT. 500 is 8.33 lb/gal × 60 × 1 Btu/lb-°F. Glycol, brine, and non-standard density need a fluid correction — do not leave 500 on a 30% propylene-glycol loop and call it done.
- 1 refrigeration ton = 12,000 Btu/h = the heat to melt 1 ton of ice in 24 hours. kW/ton ≈ 12 ÷ EER. COP ≈ EER ÷ 3.412. EER ≈ 12 ÷ (kW/ton).
- ACH: CFM × 60 ÷ volume. This is a room-air-change identity, not IMC Table 403 outdoor air and not a kitchen capture rate.
- Thumb ranges are a warning, not a design: office/school/warehouse/hospital cooling cfm/ft² and heating Btu/ft² vary a lot. If the only basis on M-601 is a thumb, send the actual load. Masterbuild will not stamp a thumb.
On the set: show the identity you used (1.08, 500, 12,000) and the ΔT. If glycol or altitude applies, say so. If the coil leaving-air is wet, total cooling is CFM × 4.5 × Δh (enthalpy) at sea level — that Δh comes from a psychrometric state, not from a ΔT guess.
Sized from “1 cfm/ft²” and “2.4 gpm/ton” with no load
Those thumbs assume a ΔT and a climate that may not be this project. Miami latent, glass, and 24-hour occupancy blow them up. Use the estimator as a sanity check, then send the room areas, occupancy, and envelope so the PE can run a real load.
Related: Conversion screen · outdoor air: 401.4 · hydronic flood/weather: duct exposure · refrigerant class: ASHRAE 34 / EPA 608.
What to send
The number you are trying to check (cfm, gpm, tons, EER), the ΔT or room volume behind it, and whether glycol or a non-sea-level site is in play. Do not send a screenshot of a vendor green sheet as the design basis.
If the schedule was filled from a thumb
Send the areas and the occupancy. Masterbuild will say whether 1.08 / 500 still has no ΔT on the sheet — or the tonnage still has no load behind it.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. 1.08, 500, 12,000 Btu/h per ton, ACH = CFM × 60 ÷ volume, and COP ≈ EER ÷ 3.412 are sea-level textbook identities for education only. They do not size equipment, do not set a glycol correction, and do not replace an ASHRAE load. This page does not reproduce third-party green sheets or psychrometric charts. This page is not legal advice.