- Moisture
- — gr/lb
- Dew point
- — °F
- Wet bulb
- — °F
- Total heat
- — Btu/lb
- Moisture
- — gr/lb
- Dew point
- — °F
- Wet bulb
- — °F
- Total heat
- — Btu/lb
Read in the middle of the house, away from a register, a door, and the kitchen. Give the meter two full minutes to settle before you write anything down.
house and system
are keeping out
- Extra load
- — Btu/h
- Of your capacity
- — tons
- Latent share
- — Btu/h
- Water carried in
- — pints/day
A blower-door number is better than a guess. Without one: a tight newer house runs roughly 50–100 CFM of natural infiltration, an average older Florida block house 150–300, and a house with attic duct leaks or an unbalanced return can run far higher.
- Margin to dew point
- — °F
- Inside dew point
- — °F
Shoot the supply plenum, a register face, the line set where it enters, an exterior wall behind furniture, and the slab in a closet. Anything at or below the indoor dew point is making water right now.
| Inside condition | gr/lb | Dew pt |
|---|---|---|
| 75°F / 45% — dry, well controlled | 58 | 52 |
| 75°F / 50% — target | 65 | 55 |
| 75°F / 55% — acceptable | 71 | 58 |
| 75°F / 60% — mold threshold | 78 | 60 |
| 78°F / 65% — failing | 93 | 65 |
| Clearwater design day, outside | 153 | 79 |
Sustained indoor RH above 60% is the line where mold growth becomes likely on interior surfaces. It is a building condition, not a comfort preference.
| Station | Cool DB 0.4% | MCWB | DP 0.4% | gr/lb |
|---|
St. Petersburg – Clearwater is the documented station and carries the values used across the compendium. The rest are reference grade — confirm against ASHRAE Fundamentals Ch. 14 before they go on a submitted load calculation.
Enter any two properties. Everything else is derived.
- Humidity ratio
- — gr/lb
- Enthalpy
- — Btu/lb
- Wet bulb
- — °F
- Dew point
- — °F
- Rel. humidity
- — %
- Vapor pressure
- — in.Hg
- Spec. volume
- — ft³/lb
- Density
- — lb/ft³
What a one-degree wet-bulb error costs at these conditions.
- ±1°F WB
- — gr/lb
- ±1°F WB
- — Btu/lb
- As RH error
- — % pts
- As dew point error
- — °F
Carrier's fourth principle
Wet-bulb temperature depends on the total of sensible and latent heat and is independent of their proportions. Constant wet bulb is constant total heat. That is why capacity gets measured across the coil in wet bulb, while a dry bulb only tells you a story.
Both bulbs each side of the coil, plus measured airflow. The split comes from the enthalpy difference, not a rule of thumb.
- Total
- — Btu/h
- SHR
- —
- Sensible
- — Btu/h
- Latent
- — Btu/h
- CFM per ton
- —
- Moisture out
- — pt/h
- Delta T
- — °F
- Grains removed
- — gr/lb
- Total could be
- —
- As % of reading
- — %
| SHR | What it means here |
|---|---|
| 0.65–0.75 | Coil is working the latent load. Normal humid-day call. |
| 0.75–0.82 | Design range for a correctly sized Florida system. |
| > 0.85 | Little dehumidification. Airflow high, charge, or dry entering air. |
| < 0.60 | Very wet entering air, low airflow, or coil colder than intended. |
Dual constraint. Return and supply carry different friction rates because they carry different consequences.
- Round duct
- — in.
- Velocity
- — fpm
- Next stock size
- — in.
- At that size
- — fpm
- Width required
- — in.
- Aspect ratio
- —
Equal-friction equivalent. Keep the ratio at or under 4:1 — past that the friction penalty and the sheet metal both get expensive.
| Location | Target fpm | Max fpm |
|---|---|---|
| Trunk, supply | 700 | 900 |
| Trunk, return | 600 | 700 |
| Branch, supply | 600 | 700 |
| Branch, return | 500 | 600 |
| Filter face | 300 | 500 |
| Coil face | 450 | 550 |
Why the return is sized tighter
Return friction is charged against the blower before the air reaches a single fitting, and a starved return shows up as low airflow, a cold coil, and a humidity complaint. Return at 0.05 and supply at 0.08 keeps total external static inside the range the equipment was rated at.
Calculator returns galvanized round. Flex runs materially rougher — size from the manufacturer's rated pressure drop with the flex fully stretched, and treat any compression as a defect, not a tolerance.
R0 — life safety precedence
A2L mitigation outranks every other control rule. Where a comfort, efficiency, or staging decision conflicts with refrigerant detection and mitigation airflow, mitigation wins and the conflicting rule is abandoned, not balanced.
120°F attic hard stop
No diagnostic, brazing, or commissioning work proceeds in an attic measuring 120°F or above. Not a guideline and not subject to customer schedule. Reschedule to early morning, or cool and ventilate the workspace first.
| Condition | CFM/ton |
|---|---|
| Florida humid-climate default | 350 |
| High latent load / RH complaint | 325 – 350 |
| Dry-climate or high-sensible spec | 400 – 450 |
| Below this, coil freeze risk rises | 300 |
Manufacturer data outranks this table. Where rated airflow and a humidity complaint disagree, verify total static and the blower table before changing tap.
Lives in the Field Diagnostic app
R-454B handling, P-T charts, superheat targets, and subcooling verification are held in one place so a correction only has to be made once. Open the Action Air Field Diagnostic app, Tables and Charge tabs.
The safety rules above appear in both files on purpose — a hard stop should be wherever a technician is looking. Charge numbers appear in one file only, for the opposite reason.
For the day the probe dies, the tablet won't wake, or the reading looks wrong and you need a second path to the same answer.
Verify before you trust
A thermometer that reads two degrees high fails silently — it returns a confident number all day. Check it against a known point before it decides a compressor's fate.
| Check | Known point | Accept |
|---|---|---|
| Ice bath — crushed ice, distilled water, stir, wait 2 min | 32.0 °F | ±0.5 |
| Boiling water — sea level, Pinellas County | 212.0 °F | ±1.0 |
| Two probes, same air, settled 60 s | Agree with each other | ±0.3 |
| Wet bulb sock — clean, fully wetted, distilled | Reads below dry bulb | — |
| Manifold gauges against ambient saturation | Matches P-T at known temp | ±2 psi |
Distilled water only on the sock. Tap water leaves mineral scale that insulates the wick and drives the wet bulb high — which reads as more capacity than the coil is actually producing.
| Wick | Clean cotton, fully saturated with distilled water, no dry spots |
| Speed | 150–225 rpm on a 15 in. sling — about 1200–1800 fpm at the bulb |
| Method | Swing 20–30 s, read fast, repeat until two readings agree |
| Read | Wet bulb first — it starts climbing the moment you stop |
| Correction | Use 2755 − 1.28 t′ in eq. 24, not 2800 − 1.3 t′ |
| Never | In direct sun, near your own body heat, or in still air |
A stationary wet bulb without forced air reads high from radiation — Carrier measured the error at roughly 1.6 % of depression even on a properly swung sling, and far worse on a still one. That is the whole reason the correction exists.
| Component | Job | Reads wrong when |
|---|---|---|
| Evaporator | Absorbs heat, boils liquid to vapor | Airflow low, coil dirty, charge low |
| Compressor | Raises pressure and temperature of vapor | Valves leaking, windings shorted, liquid returning |
| Condenser | Rejects heat, condenses vapor to liquid | Coil fouled, fan weak, air recirculating |
| Metering device | Drops pressure, controls feed to evaporator | Restricted, bulb loose, moisture freezing |
| Filter drier | Traps moisture and debris | Restricted — look for a temperature drop across it |
| Suction | Discharge | Look at |
|---|---|---|
| Low | Low | Undercharge, restriction, or starved evaporator airflow |
| Low | High | Restriction between condenser and evaporator |
| High | Low | Compressor not pumping — valves or scroll |
| High | High | Overcharge, non-condensables, or condenser heat rejection |
| Normal | High | Condenser airflow or ambient recirculation |
Adapted from public-domain U.S. Navy rate training material, NAVEDTRA 14279 (Utilitiesman Basic, Vol. 2) and 14259 (Utilitiesman Advanced). Direction of a symptom, never a substitute for manufacturer data and a full set of measured readings.
Carrier's rational set, condensed to the relations that carry weight on a Florida job. Equation numbers follow the 1911 paper.
- e
- vapor pressure, in.Hg
- e′
- saturation pressure at wet bulb
- P
- barometric pressure
- t, t′
- dry bulb, true wet bulb °F
Use 2755 − 1.28 t′ for a sling psychrometer, which reads about 1.6 % shallow on depression from radiation. Aspirated and electronic instruments use the equation as written.
Constant wet bulb is constant h. Carrier's principle D, and the reason capacity is measured across the coil in wet bulb rather than dry bulb.
Specific volume v is computed at entering conditions. The familiar 1.08, 0.68, and 4.5 constants are these same equations frozen at sea-level standard air — they drift several percent at 90 °F and 150 gr/lb, which is most of the Florida cooling season.
The moisture deficit — how much water a given air stream can still take on. Read backwards, it is the drying capacity of supply air, and the number behind every post-remediation and post-flood dry-down decision.