How do I figure out the cost differential between turning on an inefficient furnace and using oil-filled space heaters?

the big deal is that this furnace also heats the upstairs unit of a duplex, which i also own and rent out to my brother in law.  the deal is that he spends maybe 5 nights per week at his girlfriend's house.  so, if i turn on the furnace, all of the heat that goes upstairs will be wasted. 

i'm interested in learning how to measure energy usage and learning how to determine the usage difference between running the furnace, which is about 50 years old and runs at about 50% efficiency and three pretty standard 7-fin oil-filled space heaters.  the flat in which i live is about 800 sq.ft. and the upstairs space is about 600 sq.ft.  the windows are old, but are covered with plastic shrink film for better insulation.

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kelseymh8 years ago
In some ways this is an easy problem; in others it's verycomplicated. It depends on what your measurable outcome (goal) is,and on what measuring tools you have. I'm going to make someguesses as to what you might care about, and outline how you could getthere. This is NOT going to be a "firstprinciples" calculation. There are just far too manythings you don't know, can't know, or can't control for that to be done.<br /><br />I'll assume you want to maintain your flat (downstairs) at sometemperature, T. You need a thermometer to measure thattemperature, and be sure you're maintaining it. Then I'll assumethat what you really care about is spending the least money on yourutility bill for this activity.<br /><br />First, you need to determine how long you have to run the furnace inorder to get to T, and how often it has to go on and off to maintainit. That means using a watch or clock to do the timing, and payingattention (keep a written log) to when it goes on and off after reachingthe goal. You probably want to run this part of the experiment fora couple of hours to get reliable data.<br /><br />Next, you need to know how much electricity or natural gas your furnaceuses during the time that it is on. For an electric furnace,that's just the wattage. For a gas furnace, it's more complicated,and you may have to figure it out by checking your gas meter at thebeginning and end of the experiment.<br /><br />Do the same experiment (on the next day or evening, to have comparableoutside temperatures) using the space heaters instead of thefurnace. Since these are electric, you just need the wattage ofthe space heater.<br /><br />For the electricity use, multiply the wattage by the time the deviceswere on (in seconds) to get joules (W = J/s). You may have toconvert joules to BTUs depending on what your utility company uses tobill you.<br /><br />Now you have the data to make a conclusion. Look at the unitprices on your last utility bill, and figure out whether the furnace orthe space heaters cost more during the course of your experiment.<br />
hobbssamuelj (author)  kelseymh8 years ago
that is excellent!!!

i moved into the house in december 2008 and have all bill stubs sincethat period.  i was going to just play out my hunch with the spaceheaters and setting the furnace very low and see what the differencewould be starting from december.  however, your method seems farmore precise!!!

thanks soooo much.

a piggy-back question:  with the electric space heaters listed at1500 watts, how does the energy usage for that unit change relative tothe high-medium-low settings and a thermostat that runs from levels 1 to6?  is 1500 the maximum usage or the minimum usage or somewhere in between?
I think (but I don't know!) that the wattage rating is maximum, notaverage.  The owners manual might have those details in it (lookfor something like "technical specifications"). 

The differences should be in the spacing and duration of on/off cycles;that'll reduce the average power draw (and why you may want to try tokeep track of those cycles for the space heaters, as for the furnace).
hobbssamuelj (author)  kelseymh8 years ago
i think i'm just going to buy an electric usage meter.  it wouldlikely be helpful for other stuff too.
kelseymh8 years ago
Since you own the duplex, you might consider installing a dual-handledduct valve between the downstairs and upstairs (assuming there just onemain duct that goes up and then branches out to the registers).  Bygiving it a handle both upstairs and downstairs, you can turn it offwhen you BIL is gone, but he can turn it back on when he returns.
Absolutely!  You can even get electrically operated diverters soyou don't have to have access to the ductwork to change the setting.