Thread: Hot to cold and vice versa

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Hot to cold and vice versa

I want to understand something once and for all about warm air and cold air.

Lets say you have 2 rooms next to each other that are complete boxes and a small opening (say a a square foot) in the wall that separates the rooms. One room is 70 degrees and the other is 45 degrees.

All other things being equal (humidity, dew point, etc), how would the air flow "feel" ? If I held a lighted match in the middle of the opening which way would the flame bend?

Toward the colder room?

Seems that when I've encountered this, I always feel the air flowing into the colder area -- bu then I read somewhere recently that wash hogwash.

I'm not an HVAC pro or anything -- just curious. THANKS.

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The air movement troughs the hole wod be negligable. Now if you cut two holes, one high and one low, you could have a fair amount of movement. The low hole would have air moving from cold to hot and the high hole would have air moving from the hot room to the cold room.

Remember that hot air is more dense and therefore "heavier", so it will seek its level between the two rooms and push the hot air to the top

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Heat moves to cold, but warm air is lighter then cold air....that's the way I always heard/learned it.
I think in your experiment, although I don't know how you could measure or see it....some warm air in the warmer room would move through the hole, at the top and proceed upward, as some cold air passed from the cold room, through the hole, and dropped. Think more about how air moves by convection under and around a radiator...
This assumes no air infiltration or exfiltration is taking place.
JMO

4. Since the flame tip is on the top 1/2 of the flame and the flame is fixed at hte base, and since it's heat would move toards the colder room, it would probably still bend ever so slightly towards the colder room. Ther should be movement of warm air through the upper 1/2 of the opening and cold air throgh the lower half. This assume there rooms are otherwise extremely well insulated so there is not other heat transfer or convection currents involved.

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Dan Houlihan explained how it "feels" to me this way.

because hot goes to cold your body heat is being pulled away from you causing you to feel like the cold is coming at you.

stand by a cold surface in a warm room. like a window. the feeling is that cold air is blowing at you . however more likely heat is being drawn away from your skin.

don't know if that helps but it helped me a great deal in understanding hvac.

6. depend on where the match is lit what temp that area is? if it s is 70F no flame bounce the 45F room would pull the 70F heat into the opening flame would bounce.same as entering and leaving your home on a winter day 20F outside you leave and get hit with the cold it seems....that is your body loosing heat walk in from 20F into a 70F house and the heat grabs you cause your body is pulling it.....there is no such thing as "COLD "it is just a word! they made up... it is defined as a lack of heat.if it is 20F outside there is still a lot of heat in the air why ......cause it isn't -200F.

7. Heat always flows from warmer to cooler. It does not care which direction that is. Air always flows from a high pressure to a lower pressure; so you can have cold air flow from one room into a warmer room if the pressure in the cold room is higher than the warmer room. Warm air will rise because it is less dense than cold air, so it is lighter. Air with more humidity is lighter than dry air even though water is heavier than air, which seems insane to common sense, but it's true; just look UP at the clouds.

8. Ok cooler room would have a slightly lower pressure, pressure and temp are directly related, and heat travels to less heat. So if one room has a slightly higher pressure and a higher temp any air flow would be from higher to lower. However if you were to hold your hand 3" from the hole on the warm side you would swear cooler air is being drawn in. That is just the air near the hole loosing its heat to the cooler air on the other side.

9. Don't forget to account for mechanically induced pressure differences overriding that "cooler rooms will have less pressure" thing. Or wind induced pressure differences.

10. Originally Posted by tipsrfine
Don't forget to account for mechanically induced pressure differences overriding that "cooler rooms will have less pressure" thing. Or wind induced pressure differences.
Wasn't part of his example but you are correct of coarse.

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Ok,here's my two cents worth. Start with the opening closed (how else will you get the two rooms to their respective temps?). Gently open. The colder air is denser, and so will tend to flow over the edge of the opening and sink to the bottom of the hot side (picture the kitchen sink overflowing onto the floor). This will displace air on the warm side, which will flow through the upper part of the opening and head toward the top of the cold side because it is less dense (picture hot smoke rolling out of the top part of a window with house on fire). So there will be convective flow in both directions. This would continue until the temps equalized. Since there is an opening between the two rooms (and initial conditions including static pressure are equal), no pressure differential exists, so there would be no pressure-induced flow.

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12. In a hypothetically perfect scenario both rooms being perfectly insulated and having equal mass of air, hot air would go into the colder room to until pressure was equalized. Here is another interesting thermo brain buster http://en.m.wikipedia.org/wiki/Maxwell's_demon

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