Inlet min rating less than manifold for LP?

ChrisJ

I'm a bit confused as to how this makes sense.
This is on my dad's 2006 forced hot dust furnace.

LP gas Minimum inlet pressure is 8" but the proper manifold pressure is 10" ? Is Rod Serling going to show up?

Tim McElwain

The minimum inlet pressure is shown at its absolute minimum for safety purposes. In other words if the inlet is less than 8" the unit is unsafe (rule is no more than a 2" drop in pressure) based on a set requirement for outlet pressure. The required outlet pressure is 10" if it drops down to 8" due to low inlet pressure it is unsafe and may delay on ignition. The rating plate is correct as written.

ChrisJ

Tim McElwain said:

The minimum inlet pressure is shown at its absolute minimum for safety purposes. In other words if the inlet is less than 8" the unit is unsafe (rule is no more than a 2" drop in pressure) based on a set requirement for outlet pressure. The required outlet pressure is 10" if it drops down to 8" due to low inlet pressure it is unsafe and may delay on ignition. The rating plate is correct as written.

Hi Tim,

Thanks for responding.

Why are the rules different for natural gas on the same system? Why isn't NG allowed to drop slightly below manifold pressure as well?

Tim McElwain

The rule for a two inch drop applies to both LP and natural gas. Typically out of the meter on natural gas you will have 6" to 7" W.C. at the inlet of all equipment we ideally would have the same 6" to 7" W.C. pressure if piped sized correctly.

Most natural gas valves the outlet pressure out of the valve is 3.5" W.C. there is an allowable 1" drop through the valve (there are actually charts for every gas valve made which show actual drop based on btu demand) the 1" drop added to the 3.5" W.C. gives 4.5" W.C. most valve manufacturers round off the minimum inlet pressure to a natural gas valve as 5" W.C. so the is the minimum inlet allowed there is your two inch pressure difference 7" W.C. to 5"W.C.