The First Law of Thermodynamics and Residual Oven Heat
January 28, 2019 9:03 PM Subscribe
I've often left my oven slightly ajar after using it in the winter thinking that I'm taking advantage of the residual heat to help warm the air in my house. But given that my house is a (relatively) closed system, the law of conservation of energy holds that the total energy in the house is constant: if I close the oven door, its heat will eventually radiate into the house anyway. So is there any reason to leave the oven door ajar while its heat dissipates?
Opening the door will release the hot air trapped inside the oven, which heats your kitchen up faster since convection is the most efficient form of heat transfer.
posted by btfreek at 9:14 PM on January 28, 2019
posted by btfreek at 9:14 PM on January 28, 2019
Assuming there's nowhere else for it to go, then yes, leaving your oven closed or open won't make a difference in the total amount of heat that makes its way into your house.
The exception might be if your oven is against an outside wall and can directly radiate away from the house that way. But that would keep your oven unusually cold when it's not in use.
posted by Hatashran at 9:27 PM on January 28, 2019 [8 favorites]
The exception might be if your oven is against an outside wall and can directly radiate away from the house that way. But that would keep your oven unusually cold when it's not in use.
posted by Hatashran at 9:27 PM on January 28, 2019 [8 favorites]
Check the adjacent cabinets carefully before leaving the over door ajar. We had cheap thermofoil coated cabinets, and the escaping head loosened and warped the external later so it peeled back on the corners most exposed to heat.
posted by NumberSix at 9:42 PM on January 28, 2019 [1 favorite]
posted by NumberSix at 9:42 PM on January 28, 2019 [1 favorite]
So is there any reason to leave the oven door ajar while its heat dissipates?
You get the local heating in the kitchen when you're there to take advantage of it.
posted by sebastienbailard at 9:57 PM on January 28, 2019 [16 favorites]
You get the local heating in the kitchen when you're there to take advantage of it.
posted by sebastienbailard at 9:57 PM on January 28, 2019 [16 favorites]
Your house is not a closed system! Heat is leaving through the windows and walls at rate Q_out. Heat is entering the house from the oven at rate Q_oven. The system is not at steady state, and heat accumulates at rate Q_oven - Q_out. Q_out is constant. Q_oven is small if the oven is closed and large if the oven is open. So heat accumulates more quickly if the oven is open.
posted by mr_roboto at 10:06 PM on January 28, 2019 [26 favorites]
posted by mr_roboto at 10:06 PM on January 28, 2019 [26 favorites]
If you enjoy the extra burst of warmth you get from opening the oven door, keep doing so. If you have thermostatically controlled heating in addition to your oven and what you're actually trying to do is minimize your total energy consumption, you'd be hard pressed to spot the difference between opening the oven door and not.
In theory the temporarily raised temperature inside the kitchen soon after the oven door is opened might cause a temporary increase in the rate of heat loss through any exterior walls and therefore a loss of overall house-heating efficiency, but in practice this effect would probably be swamped by the interior temperature's normal cycling between the minimum and maximum set by the thermostat.
Your house is not a closed system! Heat is leaving through the windows and walls at rate Q_out. Heat is entering the house from the oven at rate Q_oven. The system is not at steady state, and heat accumulates at rate Q_oven - Q_out. Q_out is constant.
This would be correct if the oven were the only heat source in the house, but this is unlikely to be the case. If there's a thermostat controlling a heating system specifically designed to warm houses rather than cook food, any heat the oven contributes to the kitchen will just cause a corresponding reduction in that contributed by the heater.
If the house heater is based on a heat pump, you want to let that pump provide the bulk of your heating; contributions from radiant elements in ovens or incandescent lighting actually reduce your overall heating efficiency. This efficiency reduction happens as soon as you use the appliance concerned to transform some electrical energy into heat; the rate at which that heat subsequently escapes into the interior of the house doesn't matter.
If the house heater works by burning a fuel or running a resistive electrical heating element then efficiency-wise it doesn't really matter whether the heat comes from there or elsewhere; it's all expensive.
If what you want is heat, any other form of energy can get transformed into heat energy with 100% efficiency - but heat pumps don't so much transform electrical energy into heat energy as use electrical energy to increase the quality (manifested as temperature) of existing heat energy drawn in from outside the building.
posted by flabdablet at 10:45 PM on January 28, 2019 [4 favorites]
In theory the temporarily raised temperature inside the kitchen soon after the oven door is opened might cause a temporary increase in the rate of heat loss through any exterior walls and therefore a loss of overall house-heating efficiency, but in practice this effect would probably be swamped by the interior temperature's normal cycling between the minimum and maximum set by the thermostat.
Your house is not a closed system! Heat is leaving through the windows and walls at rate Q_out. Heat is entering the house from the oven at rate Q_oven. The system is not at steady state, and heat accumulates at rate Q_oven - Q_out. Q_out is constant.
This would be correct if the oven were the only heat source in the house, but this is unlikely to be the case. If there's a thermostat controlling a heating system specifically designed to warm houses rather than cook food, any heat the oven contributes to the kitchen will just cause a corresponding reduction in that contributed by the heater.
If the house heater is based on a heat pump, you want to let that pump provide the bulk of your heating; contributions from radiant elements in ovens or incandescent lighting actually reduce your overall heating efficiency. This efficiency reduction happens as soon as you use the appliance concerned to transform some electrical energy into heat; the rate at which that heat subsequently escapes into the interior of the house doesn't matter.
If the house heater works by burning a fuel or running a resistive electrical heating element then efficiency-wise it doesn't really matter whether the heat comes from there or elsewhere; it's all expensive.
If what you want is heat, any other form of energy can get transformed into heat energy with 100% efficiency - but heat pumps don't so much transform electrical energy into heat energy as use electrical energy to increase the quality (manifested as temperature) of existing heat energy drawn in from outside the building.
posted by flabdablet at 10:45 PM on January 28, 2019 [4 favorites]
Yes, opening the door merely releases the heat faster. It will get out into your house one way or another, it'll just be more gradual if the door is closed.
However you might not notice it so much with the door closed. If the heat release is gradual, it will have more time to disperse evenly throughout the house rather than all piling up in your kitchen in a big warm lump. A gradual release may simply end up meaning that your furnace doesn't work quite as hard for a little while, since your thermostat is not going to tell it to turn on unless the temperature dips below its setting.
If you want to get the subjective experience of a nice burst of heat, opening the door would be the thing to do. If you just want to make sure that the heat gets used, don't worry about it.
posted by Anticipation Of A New Lover's Arrival, The at 2:30 AM on January 29, 2019 [3 favorites]
However you might not notice it so much with the door closed. If the heat release is gradual, it will have more time to disperse evenly throughout the house rather than all piling up in your kitchen in a big warm lump. A gradual release may simply end up meaning that your furnace doesn't work quite as hard for a little while, since your thermostat is not going to tell it to turn on unless the temperature dips below its setting.
If you want to get the subjective experience of a nice burst of heat, opening the door would be the thing to do. If you just want to make sure that the heat gets used, don't worry about it.
posted by Anticipation Of A New Lover's Arrival, The at 2:30 AM on January 29, 2019 [3 favorites]
As mentioned above your house likely is not a perfectly or perfectly enough closed system - what hasn't been mentioned is that sometimes, if you've made something delicious, the smell lingers in the stove. Leaving the door ajar helps fill the kitchen, maybe even a good part of the house with the smell of cooking. I see this as an advantage.
posted by From Bklyn at 3:13 AM on January 29, 2019
posted by From Bklyn at 3:13 AM on January 29, 2019
This thread is closed to new comments.
posted by shapes that haunt the dusk at 9:13 PM on January 28, 2019