Fuel Usage During Summer Months

gdtrfd

I bought a house with an oil burner and indirect water heat a little over a year ago (forced hot water heat). We had a new fuel tank installed at the time and the furnace was serviced and they said it was 83% efficient. From June to Sept. of last year, with the thermostat turned low (so only using it for hot water) we used 27 gallons. For the same period this year, we used 120 gallons, using the same amount of hot water, no unusually cold days, etc.
Any thoughts on what would account for such a big discrepancy?

Jamie Hall

I would be very much inclined to look first at measurement variation in the amount of oil used. A "full" tank of oil isn't always the same volume. It is easy to have a 30 gallon discrepancy -- either way -- when "filling" a tank. My other thought is that using 27 gallons for four months is remarkably low. 6 gallons a month is about a bathtub and a half of hot water per day.

random12345

I agree with what Jamie said about the measurements. 27 gallons of oil for 4 months of hot water is fantastic. Even at $4.50/gallon that's only $121.5. I don't think even a tankless gas water heater is that efficient. I am personally interested in this topic because I've been thinking about getting an indirect, but I haven't been able to find any formal studies online looking at the energy efficiency of tankless coils vs indirect tanks on oil boilers. We have a tankless coil and used 120 gallons for 4 months over the summer keeping our megasteam at 180 F. 3 people. My sense is that indirect tanks are not much more efficient than coils. Also, I believe you have a boiler, not a furnace. That's only for forced air. Forgive my pedantry.

Robert_25

gdtrfd said:

I bought a house with an oil burner and indirect water heat a little over a year ago (forced hot water heat). We had a new fuel tank installed at the time and the furnace was serviced and they said it was 83% efficient. From June to Sept. of last year, with the thermostat turned low (so only using it for hot water) we used 27 gallons. For the same period this year, we used 120 gallons, using the same amount of hot water, no unusually cold days, etc.
Any thoughts on what would account for such a big discrepancy?

Something is inconsistent with your measurement. An typical oil boiler & indirect water heater will consume about 1 gallon of heating oil per day for an average family, which makes your most recent measurement of 120 gallons right where I would expect.

Hot_water_fan

120 gallons seems extreme for just DHW. I’m at 12 therms per month, so about 1/5th your btu usage per month with just a plain old gas tank, non-condensing. Nominally, a gas tank should be less efficient than an indirect.. Are you positive the heat wasn’t running? I’d also look for dripping faucets. Is water usage similar compared to last year?

Jersey2

I think the way you are measuring the oil isn't right. 27 gallons is nearly impossible low, and 120 is much too high. Can you use a measuring stick dipped into the tank instead of how much the oil man says he put in? Another way is to time how long the burner runs, and your nozzle size. For example a .75 nozzle running for two hours would use 1.5 gallons.
Edit- About the 120 being much too high, it might not be. Without knowing the number of people and how long they stay in the shower, 120 might be right.

Jersey2

random12345 said:

I agree with what Jamie said about the measurements. 27 gallons of oil for 4 months of hot water is fantastic. Even at $4.50/gallon that's only $121.5. I don't think even a tankless gas water heater is that efficient. I am personally interested in this topic because I've been thinking about getting an indirect, but I haven't been able to find any formal studies online looking at the energy efficiency of tankless coils vs indirect tanks on oil boilers. We have a tankless coil and used 120 gallons for 4 months over the summer keeping our megasteam at 180 F. 3 people. My sense is that indirect tanks are not much more efficient than coils. Also, I believe you have a boiler, not a furnace. That's only for forced air. Forgive my pedantry.

I think an indirect is a lot more efficient than a tankless coil, but I don't have data to quantify it. The difference is, your boiler has to maintain 180 degrees whether you use hot water or not, and a boiler loses heat relatively fast compared to a modern water tank. With an indirect, the water in the tank is heated to about 125 degrees, and then the boiler will sit and get cold as long as the water in the water tank stays hot. There is very little heat loss from an indirect tank. I had an oil fired boiler with tankless coil and switched to gas and indirect tank, and only used 36.596 therms in 4 months. I tried figuring out how much oil I used from last year but I didn't measure the oil frequently enough. My estimate is 126 therms or 90 gallons of oil in the 4 warmer months.

EdTheHeaterMan

gdtrfd said:

Any thoughts on what would account for such a big discrepancy?

@Jamie Hall is most acurate with the answer to your query.

I delivered oil in my early days. I know that when someone fills a tank by the vent alarm whistle. there can be up to 20 gallons difference in the amount of oil in a "FULL" tank. If you are filling an outside above ground tank without a vent alarm, then there may be as much as 35 gallons difference.

The economy gauge that most oil tanks are equipped with can read full when there is room to add 40 gallons of oil to actually fill a tank. But you never want to actually FILL a tank to the very top. You must leave room for expansion. If your delivery truck has 40°F oil in it and you deliver oil to the brim of the tank that is in a basement that is 65°F or the sun heats the outdoor tank to 80° the next day, there will be oil on the ground because the warmer oil will expand.

All that said, there is always the possibility that a deliver driver may be dishonest. Not saying that is what happened to you, Let's just say my father has fired at least 2 drivers in my lifetime, for doctoring metered delivery records.

The only accurate record of fuel usage is to use an average over several metered deliveries or to use a timer or hour meter attached to the fuel valve on the oil burner as suggested by @Jersey2 .

george_42

Years ago I had a boiler with a coil and put a meter to measure oil consumption and foundit used 1 gal of oil per day to maintain temp and supply hot water for family of 4 all summer long.

jpatric12

I have the same system as OP and was led to believe you could recoup the cost in the summer during the winter just through heating the house? Basically the hot water is a by product of what you would already be using to heat the home in the cold months

Hot_water_fan

@jpatric12 nah, you were misled. The winter heat isn’t free.