Trim the Fat Tuesday: The Furnace vs. Space Heaters

Now that our long autumn nightmare is over and we have heat again, we were curious about the financial upshot of sticking with oil.


The new furnace works much better and is much more efficient than the old one, thanks in no small part to fully insulated ductwork that is now installed properly – that is, the heat registers are under the windows instead of in the center of the house, where they did little good to anyone. 


The only thing that’s not entirely up to snuff is the ductwork to the second floor. These are quite small, and there are only two ducts to the whole upstairs: One in our room, and one that’s split between the kids’ rooms. It’s a good 5 degrees colder upstairs at any given time of day – and that’s actually an improvement.

But could we do better if we hung on to some of those electric space heaters we were using all through the fall? My thinking was that if programmed the thermostat to dial the nighttime temps way back to 57 degrees (our daily unoccupied temperature) and used the space heaters instead, perhaps it would be cheaper than keeping the whole house at 65 degrees (which means the bedrooms are only around 61 or 62 anyway.

Which is cheaper? Math time!

To measure how much energy we’re using to heat our home, it’s important to do this experiment at times when the outdoor temperature is the same for each test. Otherwise, it would take more or less energy to maintain any given interior temperature. I decided to figure out the average temperature, which I did by using the historic highs and lows for Newburyport from November through March. 

Our average winter temperature is 32.4, so I did my best to run my heat tests when it was right around that temperature. It took a few days to find a good window, and the kids did not enjoy the evening low temperature test, but I got it done.

First, the oil usage. The oil gauge atop the tank is pretty useless for measuring small amounts of oil usage, but if you know your oil nozzle size, you can make a much better estimate of how much oil you’re burning at any given time. Our new furnace uses a .7 gph nozzle, which means that it oil ran through it into the furnace for an hour, we’d have run through .7 gallons of heating oil in that time. 

Sort of. We have a variable speed motor now, which saves energy by running at lower speeds if it can maintain the house’s heat without kicking up to full blast. So if anything, my usage estimates will be on the high side.

Anyway, our nice new furnace definitely does not run for an hour straight. It cycles on and off, so to get the real figure for how much oil you use in an hour, you have to time when the furnace comes on and off during an hour time frame. The stopwatch on my phone came in handy for this – I sat by a heat register while watching TV and kept track of when it was on.

The Oil Results:

To keep the house at 65 degrees while it’s 32 outside, the furnace ran for 16 minutes. Divide 16 by 60 to get the percentage of the hour it ran (26.67%), than multiply that by .7, which is the number of gallons per hour of oil we use to maintain that temperature. In this case, .187 gallons per hour.

Considering an 8-hour nighttime window, we use 1.49 gallons of oil each night to keep the house at 65 degrees. Right now our oil costs just $1.99, so the total price is $2.97 per night.

Our current programmed sleeping temperature  is 61 degrees. The heat runs for 9.5 minutes per hour at that temperature, using .111 gallons of oil per hour. Total nightly price: $1.76.

To dial it back to 57 degrees at night, the furnace runs for just 4 minutes per hour, which uses just .047 gallons of oil per hour. Total nightly price: 74 cents.

The Electric Results:

It’s a lot easier to figure out how much electricity you need heat a room, since a space heater tells you exactly how much electricity it uses in watts, and your electric bill tells you exactly how much you pay for that electricity. 

Still, I found a great short cut to avoid having to figure out how long the heaters were running at different temperatures, etc. That would be more accurate, for sure, but only if the children keep their heaters at the (low, reasonable) temperature the parents think is appropriate (not a lot of evidence that this will happen). 

So this online calculator made it easy to plug in room measurements, tell how much insulation you have and a bunch of other stuff, and pow! The number of kilowatts you need to heat the room. Our total for the three bedrooms upstairs come to 4.117 kilowatts. We’re currently paying 18 cents per kilowatt hour for electricity (electricity and delivery charges), so 8 hours of electricity comes to a nightly total of $5.93. 

Even if you figure you can cut that cost in half once the room is brought up to temperature, it’s still $2.97 per night – the same as keeping the house at the daytime temperature, and nearly twice as much as turning the heat down to 61 at night.

So no, it’s not cost effective to heat the bedrooms with a space heat while turning down the furnace for the night – yet. It will be once the kids rooms are off the table, and it might be if the price of oil shoots up relative to the price of electricity. But for now, it’s cheaper to heat the house to 61 at night and hand the kids an extra blanket. 

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