Runtime

How long a propane cylinder actually lasts

Nine hours at full output on a 48,000 BTU tower. Not a season, not a month, and the reason so many people are surprised is that nobody shows the division.

The two constants both have sources. Liquid propane is filled and billed at 4.24 pounds per gallon at 60 degrees Fahrenheit, the industry standard density. Energy content is 91,420 BTU per gallon at higher heating value, published by the U.S. Department of Energy — and higher heating value is the correct basis here because appliance BTU ratings are gross input ratings measured the same way.

Read down the 20 lb column and the picture is clear. A tabletop heater at 11,000 BTU runs for over a day and a half. A full-size tower runs for an evening or two. A 58,000 BTU fire bowl runs for about seven and a half hours. Output and runtime are the same number seen from opposite ends.

Three reasons, none of them a defect, and all of them worth knowing before you plan an evening around a cylinder.

1. 1Exchange cylinders are not full cylinders. A swap-cage cylinder is commonly filled to 15 lb rather than 20, because the exchange model prices convenience over volume. That is a 25 percent cut to every figure above — read the 15 lb column instead.
2. 2Cold reduces vapor pressure. Propane has to boil off inside the cylinder to feed the burner, and that gets harder as the cylinder cools, both from the weather and from the evaporation itself. In genuinely cold conditions a nearly-empty cylinder can struggle to maintain full output. This is also why a cylinder feels cold and sometimes frosts in use.
3. 3Rated input is full output. Every figure here assumes the control is wide open. Very few people run a patio heater flat out all evening, and turning it down stretches the runtime proportionally — half output really is double the hours.

There is no fuel gauge on a standard cylinder and the clip-on pressure gauges sold for the purpose are close to useless, because propane vapor pressure stays roughly constant until the liquid is nearly gone. Two methods actually work.

• Weigh it. Every cylinder is stamped with its tare weight — the empty weight — usually as TW followed by a number in pounds. Put it on a bathroom scale, subtract the tare, and the remainder is the propane. Straightforward and exact.
• Pour warm water down the side. The liquid propane absorbs heat, so the cylinder wall stays cool where there is liquid behind it and warms where there is only vapor. Run your hand down and you can feel the level. Crude, free, and surprisingly reliable.

The most common propane complaint is not cost, it is running out mid-evening. The arithmetic says a tower gets two or three evenings from a cylinder, so if you use the patio twice a week you are refilling monthly, and the refill never happens on a convenient day.

A second cylinder costs less than most accessories on this site and eliminates the problem completely: swap, and refill the empty one whenever you happen to pass. It is a duller recommendation than a bigger heater and it solves the thing people actually complain about.

And if the running cost rather than the swapping is the issue, the answer is upstream: size the heater properly so you are not buying 48,000 BTU for a patio that needs 4,000, or block the wind so the heat you buy stays put.