Propane (liquid petroleum gas) must be treated with the utmost respect. It has 2.5 times the BTU value of Natural Gas; is heavier than air and odorless. If leaks it will pool like water and will explode it if contacts a source of ignition. Therefore never install a propane fired boiler below grade.
Never use a normally open vent valve (N.O.V.V) on a gas train with propane.
On atmospherics, the use of propane is a tricky because when the boiler cycles off there is always a chance that the flame will be sucked down one or more of the burners and will "candle" at the main burner orifice until the fuel left over in the manifold is depleted. There is a not problem with this as long as the candling ends before the next call for heat. If the candling is still on when the boiler relights - the flame will burn in the venturi and will produce magnum quantities of soot that will plug up the heat exchanger. For this reason we provide adjustable time delay relay(s) that must be adjusted by the start-up personnel to insure candling is extinguished before relight occurs.
Candling can also be caused by a sudden drop in manifold pressure when the boiler is firing. Normal manifold pressure for propane at the manifold is 10.5" to 11" water column; normal supply pressure to gas train is 14" w.c. The supply pressure can quickly drop if the pipe from the propane tank to the boiler freezes up. This is not uncommon especially in cold weather. For this reason, all RITE atmospheric propane boilers are equipped with a low gas pressure switch. set at 9" w.c. The only way to guarantee the line from freezing with a propane "Vaporizer" (sometimes two for redundancy) which is basically a heater that the propane supply pipe passes through through to keep it warm and maintain L.P. Gas in a vapor state.
RITE propane atmospheric fired boilers on on-off or low-high-low but not modulation change.
RITE propane power burner fired boilers can modulate and are not subject to "Candling".