Is a forklift propane tank used on the heater

INTRODUCTION

The question sounds trivial: “I’ve got a spare 33-lb forklift bottle—can I run my construction heater off it?” In practice, the answer straddles fluid-phase physics, connector law (CGA-791 vs. POL), and OSHA/NFPA interpretation. Get it wrong and you either (a) starve the burner and freeze the crew, or (b) deliver liquid propane at 140 psi to a ½-psi regulator and watch the flare-stack reach 15 ft.

vapor-withdrawal vs. liquid-withdrawal, excess-flow limits, hose compatibility, DOT 4BA-240 code, insurance riders, and the new 2025 CARB RFID rule. When you finish you will know exactly when the answer is “yes—if,” and when it is “absolutely not.”

Executive Summary

Forklift cylinders are LIQUID-withdrawal devices; most heaters need VAPOR.

An inverted forklift tank can supply vapor—but only at ≤40 % fill and ≤30 000 BTU/h.

You must swap the liquid service valve for a vapor service valve or add an integral vaporizer.

Adapter “pigtails” exist (POL × Type-1 ACME), but do not change phase; they only mate threads.

Without a low-fill alarm, you risk liquid slugging once the tank drops below 25 %—a BLEVE precursor if the heater lacks an internal check.

Forklift Cylinder 101—Why It Was Designed for Liquid

A forklift truck mounts the bottle horizontally with the dip tube facing downward. As the engine fuel-lock draws propane, liquid exits, passes through a lock-off/filter, then hits the vaporizer/regulator mounted on the engine manifold. That regulator drops pressure from 140 psi (38 °C) to 0.4 psi. Key consequence: the tank collar is stamped “LIQUID WITHDRAWAL ONLY.”

Construction Heater 101—Why It Wants Vapor

A 50 000–150 000 BTU/h torpedo heater contains:

A vapor-rated ¼-in. NPT inlet rated ½–5 psi.

A redundant ¼-psi regulator ahead of the orifice.

No on-board vaporizer.

Feed it liquid and you create a runaway fire at the burner head within 15 s.

Physics of In-Tank Vaporization

Propane boils at –42 °C; at 21 °C the vapor headspace is 140 psi. The latent heat required is 426 kJ kg⁻¹. A 33-lb cylinder at 50 % fill has roughly 0.12 m² wetted wall. Steady-state vaporization rate is:

Q = h_fg × m_dot = 426 × 0.018 kg s⁻¹ ≈ 7.7 kW ≈ 26 000 BTU/h

Above that rate the liquid super-cools, pressure collapses to 30 psi, and the heater starves—unless you invert the tank to force wetted-wall boiling and accept the liquid-risk trade-off.

Code & Regulatory Landscape (2025)

5.1 DOT 49 CFR 178.71

A DOT-4BA-240 cylinder may be used only for the service indicated on the label. Swap service (liquid → vapor) is legal if you reinstall a vapor service valve and re-stencil the collar.

5.2 NFPA 58 §6.26.4.1

Appliance inlet pressure must not exceed ½ psi (14 in. w.c.) unless the appliance is listed for higher. Forklift liquid valves deliver 10× that—hence you need a two-stage regulator at the heater.

5.3 OSHA 1926.153(i)

Temporary heaters must be listed to ANSI Z83.7/CSA 2.14. Field modification of fuel supply voids the listing unless a professional engineer documents equivalence.

Real-World Scenarios—Decision Matrix

Scenario 1: 33-lb forklift tank, liquid valve, upright, 50 000 BTU heater

→ NO—liquid slug certain once dip tube is submerged.

Scenario 2: Same tank inverted, 25 % fill, outdoor 30 000 BTU heater, Type-1 pigtail, ¼-psi regulator

→ Conditional YES—monitor tank frost; stop at 20 % fill.

Scenario 3: Tank re-valved with vapor service POL, 10 ft away, ¼-in. ID hose, 90 000 BTU heater

→ YES—compliant, provided you add a low-pressure cut-off switch.

Hardware You Actually Need

7.1 Vapor Service Valve

Manchester 9102V (POL outlet, 1.3 GPM excess-flow)

Includes reverse flow check—adds $28 to cylinder cost.

7.2 Two-Stage Regulator

Marshall 250 series: 1st stage 10 psi, 2nd stage ½ psi, 400 000 BTU/h capacity.

Mount vertically to avoid diaphragm freeze-up.

7.3 Pigtail Adapter

CAM59143: POL (male) × Type-1 ACME (female), 24 in., thermoplastic.

Rated –40 °C to +71 °C, 350 psi burst.

7.4 Vaporizer Plate (optional for >100 kBTU)

ALGAS SDH-5: 12 V DC, 140 kBTU/h, mounts on cylinder foot-ring.

Adds $240 but guarantees vapor phase regardless of fill level.

Step-by-Step Field Conversion (PE-Stamped Example)

De-pressurize cylinder remotely; purge with nitrogen to <15 % LEL.

Remove liquid valve (1 ¾-in. ACME) using brass spanner to avoid spark.

Install vapor valve with copper crush washer; torque 120 ft-lb.

Stamp collar “VAPOR WITHDRAWAL ONLY – RE-VALVED 06-25.”

Leak-test at 150 psi with electronic detector (<0.5 % LEL threshold).

Attach regulator, hose, heater; perform 3-min operating leak test.

Document conversion in engineered report; store with heater manual.

Failure Modes—What Can Still Go Wrong

9.1 Liquid Carry-Over (inverted tank)

Symptom: 3-ft flame jet from burner head.

Mitigation: install 10-micron inline filter + solenoid shut-off tied to thermocouple.

9.2 Regulator Freeze-Up

Cause: rapid draw drops regulator body to –20 °C; moisture in propane plates ice.

Fix: use methanol injection or heated regulator (12 V wrap).

9.3 Excess-Flow Nuisance Trip

Cause: 150 000 BTU/h exceeds 1.3 GPM valve rating.

Fix: parallel two cylinders or upsize to 43-lb bottle with 2.0 GPM valve.

Economics—Does the Hack Save Money?

External 20-lb BBQ cylinder refill: $18.00 for 430 kBTU → $0.042 per kBTU

Internal forklift bottle (spare): $0 (sunk cost) → apparently “free”

Add vapor valve + regulator: $65 amortized over 100 fills → $0.65 per fill

Net: $0.015 per kBTU—cheaper, but only if you already own surplus tanks.

Insurance & Liability Reality Check

Major carriers (Travelers, Liberty) exclude claims if the heater supply is “modified from OEM specification” unless a PE sign-off is filed. Premium increase averages $180 yr⁻¹—still cheaper than renting 100-lb bottles at $65 each per month.

Bottom-Line Decision Tree (Print & Laminate)

Heater ≤30 kBTU AND tank ≥50 % fill? → Invert tank, use pigtail, monitor frost.

Heater >30 kBTU? → Re-valve to vapor OR add external vaporizer.

Indoor use? → Prohibited regardless (NFPA 58 §6.25.3).

Cannot re-stamp collar? → Buy dedicated 100-lb vapor cylinder.

CONCLUSION

You can run a heater from a forklift propane tank, but only after you guarantee vapor-phase delivery at ≤½ psi. That means either (a) operate inverted at low fill and low BTU, or (b) permanently convert the cylinder with a vapor valve and document the change. Skip those steps and you violate DOT service limits, void the heater listing, and create a liquid-flame hazard that no job-site safety plan can justify. Do the math, install the correct regulator, and keep the tank frost line above the liquid level—then you will stay both warm and compliant.