Got Science?

I know, I know, I haven't posted in a while.  To follow up on my discussion of heat transfer, I'm going to discuss the question box from two weeks ago: Why is it more efficient to heat up your house with the fireplace rather than the oven?


The last time we talked about this, it was about jello and why it solidifies from the sides in, correct?  Jello solidified by cooling down.  However, in the case of the broken furnace in my house a few weeks ago, we weren't interesting in cooling the house down, but in warming it up.  The engineer and heat transfer specialist in my family happened to try to heat up the house by leaving the oven open. (Kids, don't do this at home.)  However, it didn't really work too well, judging by the lack of warmth whether or not it was on.  How come?

The oven is hot.  If at an average of 200 degrees Fahrenheit (I don't know how to put in the degree sign), the oven is still about four times the temperature of my house!  However, remember the jello is cooled down by convection?  Same case here.  Convection is, simply put (I'm altering it to this situation, this isn't the true definition) when the air warmed by the oven rises and moves to a different part of the house, allowing cooler air to take its place (and get heated up).  However, there are two types: free convection, when the air moves only because of temperature differences, and forced convection, when there is something to help move the air.  Ovens, since they are meant to cook food and not heat up the house, don't need to be transferring heat to all sorts of places, the heat stays in the oven.  When we keep it open, the hot air in the oven slooowwwly moves other places by itself, so it takes a long time to heat up a little bit of air- free convection.  So, to speed up this rather slow process, you would have to force the air to move faster- forced convection.  The best way to do this (and this is why your heaters work better than the oven, folks) is probably by some sort of fan. So, if our fan wasn't drying out at that time from the basement flood, we might have been able to use that to speed up the process.  (Alas, as luck goes, the basement flood got into everything, which caused the mess with the boiler and the furnace in the first place)  Also, another reason why our oven didn't work too well is because, if you notice, the walls of the oven don't stick out- they keep the heat in.  If something is designed to keep heat in, well, trying to get the heat out can be a nuisance.  Since the (hot) walls of the oven were all inwards, you would have to fan air in there to get more air to be warmed up by the oven walls- thus, a fan would be useful.

But what about the fireplace, you ask?  A fireplace, first of all, can get REALLY hot.  The fire itself can be a thousand degrees Fahrenheit or more.  Also, the fire in the fireplace heats up, well, the fireplace.  As the fireplace heats up, the surface area of the fireplace that's in contact in the air is much greater than the oven's.  Even though it's still free convection, there's a lot more area that air can get heated up, so more air is getting warm at one time.  Simply put, a fireplace is built to put heat out into the house; the oven is meant to keep the heat in.  Why didn't we simply use the fireplace?  I don't know, but according to my dad, it is messy to clean up the fireplace afterwards.  Although, if you ask me, I think that 59 degrees is a perfectly fine temperature for the house, especially since it's only for a day or two- and considering that our house, with the furnace, is usually around 65 degrees in the winter anyway.