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I've heard before that it's a bad idea to use an oven to heat your house, because it's inefficient and carries the risk of carbon monoxide poisoning. But what about leaving the oven door open after I'm done baking, to let the heat out into the room?

Edit: to clarify, the oven is off. It's definitely a bad idea to leave the door open with the oven on!

  • Are there any health risks when the oven is off?
  • Is this bad for the oven?
  • Would it actually be more energy efficient to leave the door closed?
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4  
Where would the heat go if not into the house? –  gerrit Mar 16 '13 at 17:10
    
@gerrit, the heat would go right up to the ceiling, where you won't feel it (unless you have a second storey) –  Highly Irregular Mar 16 '13 at 20:50
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If you're concerned about carbon monoxide, I assume you're talking about a natural gas/propane type oven, not an electric one? –  Flimzy Mar 22 '13 at 21:44

3 Answers 3

up vote 11 down vote accepted

Efficiency

The inefficiency you're referring to is probably not the efficiency with which the gas is burned (a separate issue), but instead is likely to be because the air heated by an oven does not mix well with air in the room, so convection causes it to rise straight to the ceiling.

This doesn't tend to make a cold house feel much warmer (especially if the ceiling is not well insulated).

The same problem exists with electric oil heaters, or other types of heater that rely on convection to distribute the heat.

A second reason not to use your oven specifically for heating is that you can use less heating if you have the heater close to where the heat is needed. Unless you have a particularly well insulated house, you're unlikely to get much value from heating the kitchen while you're in a different room.

Thirdly, an electric heat pump can put up to around 4 units of heat into your house for each unit of electricity used. This is far more efficient than using any type of electric resistance heating, though comparing it with other types of heating (efficiency and sustainability) is not just simple maths; for example, if you live in a forest you're probably going to have a free, renewable source of firewood for a wood burner.

If you really have a need to heat your living area with an oven, try pointing a large fan at it to distribute the heat through the room and avoid all the hot air sitting up by the ceiling.

Health Risks

If you've turned the oven off already, the only significant added health risk is likely to be the risk of burning yourself on the open oven door. If the oven is on though, then you may be at risk of carbon monoxide poisoning, especially if the oven is usually vented. Efficiency while the door open is lower in some models (eg this one), which may mean production of carbon monoxide is much higher when the oven door is open because the oven wasn't designed to operate that way.

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Thanks for the answer! The convection thing was mainly what I was worried about, that all the warm air would go up to the ceiling. Not trying to use the oven as a "primary" heat source, just trying to get as much heat out of it as possible if I'm going to be using it anyway. But from the sound of it maybe it would be better to leave the door closed and rely on radiation. (But it's getting to be summer here now, so I guess the issue is moot til next year...) –  Yumecosmos Apr 24 '13 at 19:28
    
This is all good information, but I'm not certain it directly addresses the question of leaving the oven open after heating. In this case the oven' heat has bee produced no matter what, so the question is more about how best to utilize it's heat. I can infer that the answer would be to keep it shut, as a slow steady 'heat source' is more likely to provide efficient heating then a fast burst of higher heat. A simple addition to this answer to address that would make it even higher quality. –  dsollen Dec 8 at 14:49

First, given that the heat has to go somewhere, I dont think you will get more heat transferred usefully to your house from opening the oven door. You may get less for the reason that the hot air may rise as a mass (rather than trickling out) and thus have less impact on the rest of the air.

As some of the others have said the issue efficiency-wise will have to do with hot air pooling in the ceiling. The best way to prevent this is with a fan of some sort, to keep the air moving and mixing. You may find that use of a fan by itself captures a fair bit of your cooking heat and that this helps.

As a warning however, I have noticed a few major problems with re-capturing electrical cooking heat. In particular if the heat is used to boil water, I have found that the opposite of what I expected to happen in fact did --- the house was made colder by the added hot steam, compared to not cooking at all.

My working theory is that what happens is that the steam condenses on the windows and other surfaces that are not as well insulated. They deposit heat there, and then evaporate slowly pulling heat from the room to do so. They then condense again, depositing this heat wherever the house is least well insulated.

So pay attention, experiment, and be careful.

As far as health hazards, once you have turned off the oven, the gas has to go somewhere. It's going to go into your house whether you leave the oven door open or not. Again, my gut feeling says that you will lose more than you gain by opening the oven door, but I can't be sure.

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I am not sure about steam, but in general, naturally using cooking (and other) heat can only help. I’m on the tight budget right now and saving money where I can, so there were couple of nights in the last few days where the temperature dipped below freezing, but I had not have to turn on the heat in the house, and got away with the sweater, heated blanked, and cooking, doing dishes or laundry later in the night to use that heat, and it helped pretty well, I think. It also helps that I am sandwiched on the middle floor of the apartment building, so I let other suckers heating my home. :D –  theUg Mar 20 '13 at 22:26
    
whenever you convert any substance from liquid to gas you 'lose' heat, as it takes a certain energy to convert between phases. for every gram of water converted to steam 540 cal of energy is consumed. That water will eventually escape the house as water vapor, and thus you lost 540 cal of energy per gram in the boiling. It wouldn't make it colder, but it would limit the amount of heating gained to make it inefficient. –  dsollen Dec 8 at 14:55

I wanted to add a sort of laymen-term answer to your specific question. Other addressed in general the practicality of using a stove to heat the house, but you specifically asked about after cooking, ie once the oven is already hot and you need to know how best to utilize that heat.

The most efficient approach is to keep the oven door closed, in fact the better you could insulate the oven the more efficient it is. The reason comes down to what Highly Irregular already said. If you leave the door open so much heat released at once will tend to rise straight up and out of the house.

If you allow the door to be closed the heat will dissipate much slower. This will allow more time for the warm air to circulate and spread throughout the house. Thus more of the heat will be kept in the house to warm it rather then immediately escaping. The better insulated the stove was the better this conversion would be. Of course a metal pipe running from the stove to your room would do an even better job heating your room, but would probably be a bit of a hazard so I don't really recommend it :)

I personally like to turn my oven off before the recipe says to. For the last 5-10 minutes I let the oven cool, but it will still be almost the right temperature for the recipe. Your food will come out the same, but you avoid a few minutes of heating the oven with heat that will be partially-wasted. Though rather or not this helps depends on rather your oven happens to be heating at the end of your recipe (the oven will turn on and off intermittently to keep the ideal temperature, so half the time you won't notice any difference turning the stove off early.

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