Last updated on March 16th, 2024 at 02:10 pm

This is a loaded question and unfortunately is one I can’t answer without a lot more information. I can break it down for you as best I can generically and let you plug in the numbers yourself.

First, understand that all furnaces are not created equal. Some are energy star certified, some aren’t. Some are high efficiency units, some are not. Some are old, some are new. Some are sized correctly for the living space, some are not.

Let’s take the variables one at a time. First, I’m going to assume that the furnace is sized correctly. In other words, the furnace heat output (in BTU) is appropriate for the living space you want to heat. Is this a big deal? You bet it is. If a furnace is undersized, it will work more (possibly even overwork itself…yes its possible) and thus cost you more.

Obviously, newer furnaces will be more efficient than older units. As a general rule, the efficiency of a gas powered appliance increases by about 25% every few years. So furnaces made now are at least 25% more efficient than units made a few years ago. That doesn’t mean you need a new furnace every few years, remember. But the older a furnace gets, it starts to lose efficiency by the nature of natural gas combustion. There are many parts in a furnace that contribute to this – valves, nozzles, switches, piping, etc that all contribute to this. Plus, natural wear and tear and just plain old dirt/grime all play a factor.

High efficiency units are better at consuming power than others, but even high efficiency units do not utilize 100% of the fuel they are given. No gas appliance, furnaces included, will ever consume 100% of the natural gas and convert all of it directly to heat. There is inherent and built in inefficiency. That efficiency, or inefficiency, needs to be accounted for.

Here’s why this matters. Let’s assume your furnace is sized correctly and (we’ll use a nice round number, totally made up for illustration) your furnace has a btu rating of 100,000 btu. If your furnace is an energy star or high efficiency furnace, it may have an efficiency of 95%.  That means that it can convert 95% of the BTU input into heat with only 5% energy loss. So, 95,000 is the top heat output you can expect. Likewise, an 85% efficient unit will do 85,000 BTU and so on.

The last things you’ll need to figure out how much your furnace costs you is the amount of time your furnace runs, and the amount you pay per ccf (thousand cubic feet) – you can get this from your gas bill.

Once you have these figures together, its time for some math. The formula is:

(((btu/1031)/100)*cost per ccf)*hours

This formula assumes 100% efficiency, which you already know can never be the case, so it will need some “padding” to accommodate for the efficiency of the unit. That’s why the time the furnace is run is so important. It takes longer to heat an area because of the inefficiency built into gas appliances. So the more inefficient the unit, the more it will run, thus the more it will cost you. This equation illustrates this perfectly.

Let’s say that you pay $1.20 per ccf and your btu is 100,000, and the unit runs on average 3 hours per day.

(((100000/1031)/100)*1.29)*3 = $3.49 per day, or about $104 per month. This number doesn’t include delivery charge for the natural gas used, which comes to about $40 in most areas.

Incidentally, this is another reason why your thermostat is so important. Turning it down just a few degrees  will mean the furnace is on for less time. Let’s say that turning down the thermostat trims just 1/2 hour from the time the furnace runs. This means…

(((100000/1031)/100)*1.29)*3=$2.91 per day, or $87.30 per month. That’s a savings of over $16.

If you want more information about this, or want to play with this formula easily (and account for delivery costs), I would suggest looking into the Professional Home Energy Audit kit, available from http://www.energyaudits.net. The site and kit hasn’t launched to the public yet – so there still may be some site issues. However, the kit itself is an awesome resource – with excel calculators that help you find out how much money you’re losing on energy related problems.  It also lets you play around with the figures so you can see how much you could save by doing simple things like turning down your thermostat.

UPDATE: I would strongly advise you to hold off on the kit. For readers of this blog, I’ll be giving you a special coupon code in the next couple of days that will save 50% off the price.