A propane system serves a 90,000 BTU per hour furnace, a 40,000 BTU per hour water heater and a 65,000 BTU per hour range. Using a heating value of 2,500 BTU per cubic foot for propane, what is the total demand of the system in cubic feet per hour?
- A52 cubic feet per hour
52 cubic feet per hour is what you get by leaving the range out of the total, dividing 130,000 by 2,500. Every connected appliance counts toward system demand.
- B62 cubic feet per hour
62 cubic feet per hour comes from omitting the water heater, dividing 155,000 by 2,500. Dropping an appliance from the tally is the most common arithmetic slip in gas sizing.
- 78 cubic feet per hour
- D195 cubic feet per hour
195 cubic feet per hour is the natural gas answer, using roughly 1,000 BTU per cubic foot. Applying the natural gas heating value to a propane system inflates the volume by about two and a half times.
Why C is correct
The total connected load is 195,000 BTU per hour, and dividing that by the fuel's heating value of 2,500 BTU per cubic foot converts it to 78 cubic feet per hour. Volume demand is always energy demand divided by heating value, so the fuel type decides the divisor.
What this question is testing
This tests whether you can total connected loads correctly and whether you know that the BTU-to-volume conversion depends on the fuel, not on the pipe.
On the job
Tank and regulator sizing runs on cubic feet per hour, but every appliance nameplate is in BTU per hour, so the conversion happens on every propane job. The reverse error is just as common: quoting a propane system's demand to a natural gas utility. Remember that the maximum demand of a system is the sum of the connected appliance inputs, without a diversity factor, unless the code or the utility explicitly allows one for a specific occupancy.
Memory technique
Add every nameplate, then divide by the fuel's heat per cubic foot.
Exam tip
Natural gas is about 1,000 BTU per cubic foot, propane about 2,500. Check which fuel the question names before you divide.
Where to look it up
Maximum gas demand is IFGC Chapter 4, Section 402.2, and the sizing tables that follow are split by fuel. UPC jurisdictions use UPC Chapter 12 or NFPA 54 tables, which are organized differently.