Propane tanks and outdoor temperatures

You ever have one of those jobs where nuisance heating system lockouts drove you crazy?

The issue was with a propane-fired boiler and, of course, the lockouts were happening when we were swamped with no heat calls and frozen pipes! Upon arrival, resetting the boiler’s control produced a smooth light-off and seemingly perfect operation. The gas pressure was a steady 11-inches water column and the combustion analysis was spot-on. The next morning presented the same issue. Gas lines sized properly? Check. Running low on propane? The tank gauge indicated the 250-gallon tank was about a third full.

Perhaps the regulator was defective, but common sense said no and, after all, why would the boiler operate all day and some unknown portion of the night if the regulator was defective? It had to be operating for most of each night because the phone calls were coming in around 7:30 a.m., and the indoor temperature at their thermostat was in the mid-60s, and with the bitter cold blast of winter we were having, that meant the heat could not have been off for very long. A real head-scratcher until digging deeper into potential causes, and, as it turned out, it wasn’t a boiler malfunction.

Propane begins to vaporize at minus 44° F, and although our nights were dipping below 0° F, the outdoor ambient air temperatures were well above the vaporization temperature. Quite frankly, if we ever saw temperatures approaching minus 44°, we’d be moving!

Digging deeper, I totaled up the demand-load the propane system would most likely see between 5 p.m. and 8 a.m.: water heater — 50,000 Btu/h; boiler — 140,000 Btu/h; kitchen stove — 80,000 max, but more likely 50,000 if the oven and two burners are operating simultaneously; and a gas grill they stated they were not using due to the bitter cold overnight temperatures. A single 250-gallon horizontal propane tank was installed, and they were set up on an automatic refill with their supplier. The total load was potentially 240,000 Btu/h, but who is cooking at 4 or 5 a.m.? Let’s agree the actual likely load during the overnight hours was 190,000 Btu/h if both the boiler and water heater burners ran simultaneously.

In order to determine the root cause, it was necessary to determine the actual vaporization rate for the 250-gallon horizontal propane tank when outdoor ambient air temperature dips below 0° and we were seeing lows of minus 5° overnight. What I learned was the exposed surface area of the liquid propane affects the vaporization rate, as does the amount of propane inside the tank, and outdoor temperatures when they fall below 0°. Sherlock Holmes, I’m not, but persistent investigative digging does pay off in the end.

The formula for determining the capacity of any propane tank in Btu/h is:

D (diameter) x L (length) x K (factor determined by percentage of liquid in the tank) = Btu/h capacity. 30 (diameter) x 92 (length) x 70 (K factor at 1/3rd full) = 193,200 Btu/h.

It looks like we have a small cushion of 3,200 Btu/h. However, once you go below 0°, you have to add in a de-rate factor, which is 0.75 for minus 5° F, and that, my friends, reduces our Btu/h output to 144,900 Btu/h, and why we were seeing the nuisance boiler lockouts. If you check the hourly weather forecast in your area, you’ll find the lowest ambient air temperatures typically last for just a few hours and occur just before sunrise. Another lesson learned in the school of hard knocks. Thus endeth the free service calls for these nuisance lockouts!

The customer had several options: Turn off the water heater at night by setting it to pilot only, a royal pain in the buttocks; add a heated blanket to keep the propane tank warmer, which I wasn’t exactly thrilled about by mixing a potential spark-generator via its thermostat; increase the tank size; or get a buried tank, which would allow ground temperatures to keep the tank warmer.

They decided their best option for the next few nights was turning the water heater to pilot and plan on installing a 1,000-gallon underground tank before next winter. Truth be told, they were pushing their luck with their potential total Btu/h load, and the larger tank would provide ample capacity. The larger tank also gave them the option to shop around for the best pricing, providing they own the tank. Leasing a tank locks the customer into the propane company, and since propane, unlike natural gas, is an unregulated fuel, the propane company can charge whatever price they want!

The proof is in the pudding: We had a few more bitter cold nights that dipped to minus 5° and no more nuisance lockouts, so the calculations proved to be correct. If that call was taking place at this moment in time, with inflation and current energy policies driving up the cost for oil and propane, I’d be recommending high-efficiency inverter-driven heat pumps, many of which can provide their nominal (if rated for 24,000 Btu/h heating, they deliver the same) heat capacity at minus 5° and hyper-heat models that can do so at minus 15°.

The advantage of installing inverter mini-split heat pumps is they can keep their existing propane heating system as a backup and mini-split zoned heating/cooling helps to further reduce energy consumption while reducing the operating cost, too. When the propane water heater needs to be replaced, a heat pump water heater can easily take its place. DHW represents the second-largest source of energy consumption in the home, and lots of electric power companies offer rebates, which takes some of the sting out of the installed price for your customers. Find rebates and incentives for your state and area here: www.dsireusa.org.