Natural-Gas / Propane Conversion (Input and Orifice)
What changes when an appliance is converted between natural gas and propane.
Example
You enter
- Appliance input (BTU/hr) 100000
- From heating value (BTU/cf) 1030
- To heating value (BTU/cf) 2500
- From specific gravity 0.6
- To specific gravity 1.52
- From manifold pressure (in w.c.) 3.5
- To manifold pressure (in w.c.) 11
You get
- From-fuel flow 97.09 cfh
- To-fuel flow 40 cfh
- Orifice area ratio (to / from) 0.3699
Checked against published worked examples from: BCcampus Open Education (Camosun College authors).
Details, formula, and sources
ScopeThe required volumetric flow for each fuel at the same input (cfh = input / heating value) and the orifice-area ratio from first-principles orifice flow (Q proportional to area x sqrt(pressure / specific gravity)). Field orifice drilling is generally prohibited; install the listed manufacturer conversion kit.
Formula or tablecfh = appliance input / heating value for each fuel; orifice flow Q ~ area x sqrt(pressure / specific gravity), so holding input the area ratio = (cfh_to / cfh_from) x sqrt((p_from / sg_from) / (p_to / sg_to)).
Source and editionFirst-principles orifice flow holding appliance input (public); the NG/LP heating values, specific gravities, and manifold pressures are editable defaults.
Free accessThe orifice-flow relation is public physics; the manufacturer's listed conversion kit governs.
What governsEstimate. AHJ and licensed professional govern.
API field namesappliance_input_btuh, hv_from, hv_to, sg_from, sg_to, p_from, p_to, cfh_from, cfh_to, area_ratio
- Defaults NG/LP heating values 1030 / 2500 BTU/cf, specific gravities 0.60 / 1.52, manifold pressures 3.5 / 11.0 in. w.c., all editablepublic fuel-gas data
- Orifice model flow goes as area x sqrt(pressure / specific gravity); the area ratio holds appliance input across the fuel changefirst principles
- Field drilling field orifice drilling is generally prohibited; install the listed manufacturer conversion kitmanufacturer instructions