February 4, 2014

How Much Does It Cost To Leave Our Door Open in The Winter?

I have some frugal habits that are pretty ingrained in my personality.    One of the habits is not to leave the door open when its cold outside.   Sometimes I see people who do not always close a door quickly when it is cold out.    Anytime anyone has a door open for longer than a few minutes in the winter I start to get uncomfortable about it.      And 'uncomfortable' is the right word to use too, it kind of makes me a little upset to be honest.  Its almost as if someone just lit a $10 bill on fire for no reason.   But it has occurred to me that I don't really know how much heat this wastes.   I mean I know it wastes heat, that much is common sense, but I don't know HOW MUCH heat is wasted.  

There are frugal things that I do and don't do based specifically on how much they cost.   So if I"m going to get upset about an wasteful habit then I should really at least have an idea of how wasteful it is.   I mean if it wastes $1 in heat every time someone does this and they do it every other day then that is $15 wasted heat a month.   But if it only wastes 0.1¢ then I'm getting upset about nothing really.   So the actual cost is pretty important to keep things in perspective. 

I couldn't think of a good way to calculate the heat wasted on my own.   I searched Google and came up with a site that had done the math   (I'm not sure if that information is from Questline or Rocky Mountain Power or both).    They figured the amount of air that would move over an 8x10 opening based on different inside and outside temperatures.   I'll look at the example for 40F outside temperature since I live in the fairly mild Northwest US and that is a more common winter temperature average for us.

Their figures are based on natural gas at $1.20 a therm and an 80% efficient heater.    Our heat is a bit cheaper than that actually I figured that our heat would be about 75% of their costs.    So when they figure that an open 8x10 door would cost $1.88 an hour our cost would be more like $1.41.   Furthermore they are using an 8x10 door and our door is only about 3' x 8'.   Our door is only about 30% as large.   The $1.41 cost for a 8x10 door would be proportional to a 42¢ /hr cost for our 3x8 door.    Translate that into a per minute cost and you're looking at 0.7¢ per minute for our front door and our heating system.

Bottom Line : It probably costs only about 1¢ per minute to leave our front door open.

That seems surprisingly low to me.   Is this right?   Did I make a math error?    I'm seriously asking... Assuming its right, then thats not nearly as bad as I'd have thought.

This actually helps me feel better about it.   I am not going to get all bent out of shape if someone occasionally wastes a couple pennies by lingering at the front door.   

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1 comment:

  1. In this thread http://www.eng-tips.com/viewthread.cfm?qid=110731 someone came up with a figure of 12 kWh. Using a therm-to-kWh calculator, your figure is something like 15 kWh. Of course, one or both calculations could be an order of magnitude off, and I wouldn't know it... :)

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