hot air goes to cold air ??????

ScottMP

I lost the heating end of a job to another company that does mainly hot air. They offered the same basic hydro air system only mine included radiant for the front hall. slab on grade with 14 ft cielings. This other company is doing copper finned baseboard.

While walking around with the owners yesterday and waiting my turn, I heard the other contractor tell the owners he could'nt get to the front area with his baseboard. "We'll have some duct brought out from the 2nd floor." The owner said will the heat get from that high down to the front door. " Oh yea " he says( as I shake my head ) " Heat goes to cold and cold goes to heat "

???????????

As I looked at him with a blank stair, he says "I' got a book that explains it."

I think maybe he's reading it wrong. I let it go but I know I'll see him on the job again.

Anyone have an idea where he's getting this ????
Hot goes to cold and cold goes to hot ??????
Maybe he's thinking nature wants to equal things out but in
a 14 foot high vaulted ceiling ??

With out a cold air return by the font door, HOW is hot air going to sink 12 ft and mix with the cold at the front door.

Signed me Ready to be inlightened

Scott

<A HREF="http://www.heatinghelp.com/getListed.cfm?id=237&Step=30">To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"</A>

Timmothy S. Plennum

Hot to Cold

Hot air will always go to cold air, the same way if you open your windows on a cold day, all the heat from your radiators go right out the window. It may be tough to shoot that air down 14ft. but not impossible. Timmothy S. Plennum

DanHolohan

Hot goes to cold, sure.

But cold goes to hot? Huh?

Online Sheriff \"Murph\"

SCOTT

Explain to them about stacking , I believe some one has charts on floor heat compared to convection heat and forced air, best way to proove it would be to show them on an actual floor-heated job with a digital thermometer!!

Timmothy S. Plennum

I didnt even see that part. I never heard cold goes to hot, Maybe what he meant is that in A/C if the registers are up high the cold air does drop.Timmothy

Frank_3

I'm not questioning the comments about hot going to cold, I understand that's correct, but try this ...

1) Pick a flat calm day when it's really cold outside and really warm indoors

2) Open a window just a crack

3) Put your hand near the opening in the window.

Which way does it feel like the air is moving?

To me it always feels like the cold air is coming in towards the hot air. Now, again, I'm not questioning that hot air moves to cold but it sure doesn't feel that way.

Don't forget that your customers probably complain more about "feeling a cold draft coming into the house" and not about "feeling a warm draft leaving the house".

:-)

Dale

Think about a refrigerator

One of the classic applications. Heat goes to cold or the food heat would not go the kitchen air by way of the refrigerant. Cool air in the freezer section is controlled spilled down to the food part since it's a bit more dense. This is why the old fridges worked with an evap fan and why the bottom freezers and side by sides need one. With the tall ceilings on your job I hope the best window were installed or the homeowner will learn a physics lesson.

Bill Wright_2

There are a couple of reasons for this ...

Heat goes to cold. That's just the way it works.

Now it does this in one of 3 ways:

Conduction: Direct contact between the molecules of 2 objects at different temperatures (temperature is just a measurement of the vibrational energy of the molecules -- all molecules above absolute zero have some vibrational energy & therefore some temperature that can be measured). The molecules transfer their energy through direct contact ... this is the strongest mode of heat transfer.

Radiation: Energy transfer between two surfaces at different temperatures, through space (they are not in direct contact). The transfer is accomplished through electro magnetic waves -- similar to light but not visible to the eye (this is why there are themal imagers that can "see" heat).

Convection: Actually a combination of heat transfer & fluid flow. A fluid (such as air or water) in direct contact w/ a surface at a higher temperature will gain energy (heat) via conduction. As the air becomes heated, it becomes less dense (as the molecules start vibrating faster they spread out) or it becomes more "bouyant" ... causing it to rise. This in turn pulls fresh, cooler air into contact w/ the heated surface.

All this to say (sorry for the thermodynamics lesson), in your window analogy heat was being transfered in a variety of ways.

1.) If you shut the window & hold your hand about 2-3 inches away, you will feel the sensation of a draft. This is partly due to the air in contact w/ the cold window being cooled (giving up it's heat to the cooler surface), becoming denser (less bouyant) and falling to the floor -- creating air movement (the "cold waterfall" effect). But the sensation is also due to your hand transfering energy through space via radiation to the cooler glass. Your body doesn't know how the energy was lost ... it just was, so your nervous system tells your brain that "it must have been a draft".

2.) With the window open, there will be some flow out (& in) due to the density difference. Heated, bouyant air rises, but it also expands (not enough to "pressurize" the house) ... so it will try to get out through an opening creating air movement. Is it really a case of heat chasing cold? Not really, it is more a case of heated, bouyant air pushing against it's boundaries.

I think, in Scott's case, the only thing that is going to "motivate" that heated air to drop down 12-14 ft is a pressure difference (i.e. a fan of some sort). Heated bouyant air is sort of like releasing a bunch of Helium filled balloons in a house -- they are less dense than the surrounding air & end up stuck to the ceiling. Over time they lose enough helium (Balloons tend to be made of helium permiable rubber) that their density matches the surrounding air & now the weight of the balloon causes them to settle to the floor (similarly as the air molecules lose vibrational energy ... by transfering it to the ceiling ... they become less dense & settle down to the floor -- creating a modest draft).

Hopefully Scott can get his message across to these Home Owners.

Noel Kelly

Thermodynamics

I was in the heating business for years before I beame aware of the laws of thermodynamics. Let's face it, the rudiments of heat are not intrinsic to its application. By that I mean, we apply given formulae to particular situations.


And there is nothing wrong with this either. In fact, I still don't fully understand the laws - when I see equations I turn the page - but knowing something of the history of heat goes a long way towards understanding it.
At one time, it was thought that heat was an amorphous substance that existed amongst the atoms awaiting its release. One of the early thinkers, Count Rumford (of stove fame)experimented with boring out canon under water and was amazed when the water boiled. Where did this heat come from?

I recommend the book "Maxwell's Demon:Warmth Disperses and Time Passes: A History of Heat" by Hans Christian Von Baeyer.

While I do admit to being a little overwhelmed by the later chapters I have to say that the author gives a great account acount of the origins of thought with regard to heat.

After all, its all about the motion of atoms!

Bill Wright_2

Thanks Noel ...

for the book tip. I'll keep my eyes peeled for that title.

Mike T., Swampeast MO

Out of print

But available at amazon.com generally <$20

>$100 at bn.com (Barnes & Noble)

Sounds like a good book--gets very good reader review. Will be glad when my copy arrives...

Art Pittaway

Cold does go to hot

when it's the wallet that's cold and the customer that's hot for price and will listen to a warm load of BS. If you lose the job because the customer is willing to accept your competitors hot air then the blame has to fall on a customer who either doesn't understand the difference or is looking at price only. Either way the floor will be cold and the other guy will have Mr.HO's cash. Just my spin.
Art.

Steve Ebels

Sorry boys but this guys theory is wrong

Hot air does not go to cold air. They resist mixing until forced to do so by stirring them together. (I.E. a fan or blower). This room will never heat with supply ducts in the ceiling unless you use a constant fan scenario. Even then, it will feel drafty and cool. Hot air is less dense (therefore lighter) than cold (heavy) air. They will ALWAYS attempt to stratify. It's just simple physics. One of the immutable laws that cannot be broken. You can try to design around it as well as you can by using constant fan/ceiling fans etc. but all you wind up with is a cooling effect from the air moving across your body. The higher the temp difference between the supply and return air the more it will attempt to stratify. Air is a very poor conductor of heat. Consider that you can easily get as many btu's through a 1" hot water pipe as you can an 8X16" duct. Along with that, It takes over three times the electrical energy to move the same btu's in the duct as it does in that 1" pipe mentioned above. Even with the new variable speed blower drives.

Heat on the other hand. Heat energy. Radiant energy or whatever you want to call it DOES go to any object that is cooler than itself. Some form of radiant heat source is the ONLY way to make this room feel comfortable. Why? because it's using the physical laws of heat rather than trying to get around them or trick them. Ever wonder how the earth is heated by the sun? 93,000,000 miles from us? It is NOT forced air lads. It is pure radiant energy. If it works from that distance it'll sure work in that room.

I rest my case.

John T_2

I may be taking this wrong but......

How can cold go to heat if "cold" doesn't exist.

John T.

ScottMP

Thank You Steve

This is excatly my argument. I did'nt get into it with this guy in front of the HO. But there is NO way this foyer is going to be warm. I bid radiant for the floor and of course was the high bidder. The HO had allready been talking with the Warm Air Guys by the time my contractor asked me for a bid also. I really had no time to talk to these people before the locked in with the other crew.. .

Scott

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Bill Wright_2

Sorry Scott, but ...

it is the HO's loss -- not yours, there are plenty of other opportunities for you out there!!

I wonder what the HO is going to do w/ "all that money he saved"? ... probably buy a warm pair of slippers for wearing around the house!!

Steve Ebels

Wrong Tim

Radiant energy does not leave the room with the windows open. The cold air coming in momentarily exceeds the output of the radiator yes, but the radiators energy is still striking and heating objects in the room.

Case and point.

I did a 6000 sq ft auto repair shop with in floor radiant heating. They are amazed at how they can open the 12X14 overhead door and seconds (yes seconds) after it is closed the room feels as though it was never opened. They stated in their old building it took forever to heat back up after the doors were opened. Unit heaters and rooftop forced air in that building.

Jim Davis

Movement of Heat vs Movement of air

Simple explanation= cold absorbs heat(Heat loss/Heat gain)
An air conditioner doesn't cool the air, the freon absorbs the heat out of it. But the hot room does not suck the cold air from the ducts, the blower blows it in.
Air regardless of temperature moves in the direction of greater or lesser pressure. Cold air coming in the windows is because the pressure of the building is less negative than outside thus it is pulled in. At the same time it is absorbing the heat that is already inside which lowers its temperature.
Heat always tranfers to cold-Air always moves in the direction of the force.