Heating Oil Usage Rate: How Long Will Tank of Oil Last?
InspectAPedia^®  -      

• How do I find the rate at which my heating equipment is using-up heating oil?
• How long you can heat your building with a known amount of heating oil in the tank
• How to determine how fast your heating oil consumption is likely to empty the heating oil tank
• How to find and read the oil tank gauge
• Types of oil tank gauges to measure how much oil is in the oil tank
• Questions & answers about how to determine the rate at which home heating oil will be consumed

Question: How long will a quarter of a tank of heating oil last?

Also see HEATING COST SAVINGS METHODS and OIL TANK GAUGES.

The thermostat and the temperature in my house is always kept at 68 degF.

If my oil tank gauge reading is 1/4 of a tank, how long will that last???

We have no other heating source, ( I'm comfortable).

Betty

Reply: It Depends ..

We cannot answer accurately how long your quarter of a tank of heating oil will keep your home heated from just the information you provided, but we can list the facts you'd need to know and how you could estimate or find out the answer:

What Determines How Long a Tank of Oil Will Last?

How long a tank or part of a tank of heating oil will last at a building depends on the following variables:

• The size of the oil tank and how much oil it contains at the time that you ask this question. You said that your oil tank gauge was at 1/4 full. If the oil tank were a perfect rectangle, that would mean that if your tank was a 500 gallon tank, you have 1/4 x 500 or 125 gallons of oil remaining. But since oil tanks are often oval in cross section or round, the oil tank gauge is not precise, and it is giving the accurate reading of how much oil is in the tank only at 3 points: when the oil tank is full, half full, and empty. Details are at OIL TANK GAUGES.
• The setting of the room thermostat - setting to a higher temperature makes a big increase in the rate at which heating oil will be consumed. See THERMOSTATS.
• The outdoor temperature - colder outside temperatures increase the rate of heating oil consumption if other factors listed here remain unchanged. Because outdoor temperature varies widely during the day and season, oil companies use a better measurement, heating degree days (HDD), that describes the heating load that your heating system will have to meet.
  
  Heating degree days or HDD are defined using a base temperature at which it is assumed that no heat would be required, typically 65 degF. or 60 degF. For an accurate HDD measurement, the observer records the outdoor temperature minute by minute, or every half hour; but some computations simply use the average temperature for a given period. The average outdoor temperature for a given period (one degree day) is subtracted from the base temperature (say 65 degF). If (65-Avg.Temp) = 0) then that was a "zero" degree day and no heat was required. If (65-Avg.Temp) is greater than zero, that number represents the number of heating degree days for that period.
  
• Outdoor wind conditions - wind significantly increases the rate of heat loss from a building, depending on building insulation and draftiness. Omitting wind consideration is a shortcoming of simplistic use of HDD or heating degree days to figure heating load.
• Building insulation level - how well the building is insulated is a major factor in determining its rate of heat loss and thus its rate of heating fuel consumption. See INSULATION INSPECTION & IMPROVEMENT.
• Building air tightness - a drafty building loses heat significantly faster than a tight building, even if the drafty building is "insulated". See AIR LEAK DETECTION TOOLS and AIR LEAK SEALING PROCEDURE.
• Oil burner nozzle size in GPH (gallons per hour) of oil delivery - oil burner nozzles deliver oil into the oil burner combustion chamber in a very fine spray to aid in combustion. Every oil fired heating appliance includes a data plate that includes the range of oil burner nozzle sizes or flow rates that will work properly for that appliance. Typically residential heating oil burner nozzles deliver oil at rates from .65 gph for a small or highly efficient system to 2 gph for an older or larger capacity (in BTUs) heating appliance. Details about selecting the proper oil burner nozzle are at A Total Look at Oil Burner Nozzles provided courtesy of Delavan.
• Oil burner fuel unit pumping pressure: oil burner nozzle sizes assume that oil is being delivered to the nozzle at a particular pressure, say 100 psi or 125 psi. If your heating oil technician has changed the oil delivery pressure then the oil delivery rate of the nozzle will also be changed accordingly. Oil burner nozzle companies such as Delavan and others provide charts of oil burner nozzle patterns and size recommendations for different oil burner models and pressure rates.
  
  For example, according to Delavan's "Fuel Nozzles for Oil Burners", the following simple equation and example relates oil flow rate and oil burner fuel unit pumping pressure when a 1.00 GPH oil burner nozzle calibrated for 100 psi is used at 125 psi of oil delivery:
  
      F¹²⁵ = 100 * (125/100)^.5 = 1.12 GPH
• Oil burner efficiency and state of tune - the efficiency of your heating equipment can vary widely by design, from say 65% efficient for an old antiquated out of tune oil burner, to better than 95 % efficient for a high efficiency heating system that is properly tuned. Just cleaning and tuning the heating system alone makes a huge difference. When we were in that business we found that we could improve an old oil burner's efficiency rating from a start of 68% up to 78% or better by cleaning and careful tuning and oil nozzle selection. That's 10 absolute percentage points, or a 14.7% improvement in heating efficiency. See OIL BURNER INSPECTION & REPAIR.
  
  Heating system efficiency numbers can be defined as the portion of each dollar that you spend on heat actually delivers heat into the building as opposed to sending it up the chimney or flue. A heating system that is 85% efficient means that for every dollar that you spend on heating fuel, 85 cents of that dollar sends heat into the building and 15 cents of that dollar goes up the chimney as waste.
• Oil burner cycling rate - it's a bit more subtle, but most heating systems take a few minutes after start-up to reach peak operating efficiency. So if your heating oil burner is turning on and off rapidly (short cycling) you are wasting energy and the controls may not be set properly.
• Other building heating energy efficiency factors - see ENERGY SAVINGS in buildings

How to Make a Rough Guess at How Long the Oil Tank Will Last

You can make a very rough guess by noting how many minutes per hour or day your oil burner is running. Oil burners use a spray nozzle that delivers oil at some flow rate in gallons per hour, typically on a home, around .8 to 1.7 gallons per hour.

On your boiler or furnace will be a data tag that gives the maximum recommended flow rate in gallons per hour. With the gallons per hour (GPH) and minutes of run time you are observing you can multiply GPH x (minutes of on time per day / 60 minutes per hour) = number of gallons of oil being consumed a day.

Of course this varies widely as weather and temperatures and house leaks vary, as we outlined just above.

So if you know your oil tank size (say 500 gallons) you can make a very rough guess at how much oil is in the oil tank - say 125 gallons. I'd round down to 100 to be on the safe side since the tank is probably round or oval.

Example of Calculation of Days Supply of Heating Oil Remaining in an Oil Storage Tank

G = Gallons of oil remaining the tank

GPH = oil consumption rate when the oil burner is running, in gallons per hour - the largest number on your oil heater's data plate, or the actual GPH number for the oil burner nozzle actually installed on your oil burner (usually this is smaller than the data tag maximum)

MPH = minutes per hour that your oil burner is running, averaged over 24 hours per day, from observation

GPD = (GPH x MPH / 60) = gallons of heating oil used per day

Here is an example using some sample numbers: 100 gallons of oil in the oil tank and a 1 GPM oil burner nozzle.

G=100 gallons of heating oil in the tank

GPH = oil burner nozzle deliver rate in gallons per hour (from data tag on oil heater or number stamped on oil burner nozzle)

MPH = 15 minutes per hour that the oil burner is actually firing (from observation)

30 (minutes of "burner on time" per hour) x 24 (hours in a day) = 720 minutes of burner on time per day

GPD = 1 (GPH) x (720 (burner on time per day) / 60 (minutes per hour)) = 12 gallons of oil used per day.

100 G (gallons of oil in the tank) / 6 (gallons of oil used per day) = 8 days of heating oil supply remaining

Watch out: calculating the number of days of heat that oil in your tank can provide is very rough, since outdoor temperatures, wind, and other conditions keep changing. Bottom line: if your oil tank shows 1/4 full or less and it's during the heating season, you ought to call your oil company and ask for a delivery soon.

In an emergency if you are out or almost out of heating oil and your oil company can't make a delivery soon enough, the oil company can send a technician who can bring a 5-gallon container of oil to pour into your heating oil tank, or you can yourself purchase diesel fuel (in an emergency only) at a gas station and use that.

If you run out of heating oil, re-starting the oil burner may require a service call from your heating company as it may be necessary to bleed air out of the oil piping in order to properly re-start the oil burner.

Extreme danger: Double watch out: if you are purchasing fuel to use in your heating oil tank, be sure it's heating oil or in an emergency, diesel fuel or kerosene. If you put other flammables into your heating oil tank you are likely to blow up the building and kill everyone.

Your Oil Company Knows How Long Your Oil Tank Will Last

If you are on automatic oil delivery the question of how long the oil tank will last is easier to answer - your oil company will have computerized data showing your home's oil consumption rate as a function of "degree days" - a rough measure of how many hours at what outdoor temperature your home is being exposed to winter weather. They can tell you your home's energy consumption rate more accurately.

Your oil company can provide a stick, a folding rule, or even a string and weight that can be placed into an oil tank to locate the bottom of the tank and to determine the level of oil in the tank.

The depth of the oil in the tank is measured by marking the top of the tank on the stick or oil tank gauge, then placing the stick into the oil tank and withdrawing it. The oil level seen on the stick is compared with the distance from bottom of the stick (bottom of the oil tank) to top of the oil tank (which we marked on the stick).

In the old days people kept an oil tank stick that was already marked and calibrated to tell them how much oil was in their tank. Today if we use a folding measuring rule or a generic "stick" to "stick the oil tank" to check oil level, we need to know the volume and shape of the tank as well as the depth of oil on the stick in order to calculate the number of gallons in the oil tank accurately.

In the photo our client is discovering a surprise buried oil tank at a farm we were inspecting.