Why don't we heat homes using air conditioners?

[Jocko Homo]

I got a chance to observe the vents of a gas furnace and was surprised by how warm the outtake vent is. The high efficiency furnace is supposed to be more than 90% efficient so why is the outtake air so warm? It eventually occurred to me that the outtake air can't be any cooler than the air in the house so that probably explains why it's so warm...

...but then another idea occurred to me. The outtake air can't be any cooler than the interior air as long as the heat exchange is passively done but it doesn't have to be. You could use a heat pump, like an air conditioner, to transfer even more heat... so why isn't this done?

If you live in an apartment that's electrically heated, why wouldn't you heat your home with an air conditioner facing the opposite way?

Thank you...




P.S. This is totally unrelated but I also noticed that the intake vent hardly sucked in any air while the outtake vent was blowing out a lot more. I wondered if this air was just being sucked in from the home but I now suspect that perhaps it just seems like a lot more air because it's that much warmer than the air being sucked in. Does this seem plausible?

 

[Dale]

This is exactly what a heat pump does.

 

[Jocko Homo]

This is exactly what a heat pump does.

Could you please elaborate? That seriously doesn't answer my question...

 

[Bill_K]

Google "heat pump" and you will see many advertisements for heat pumps that are designed to cool your home in the summer and heat your home in the winter.

 

[cjl]

Heat pumps are not efficient for large delta T, so in cold climates (where the interior of the house may need to be as much as a hundred degrees (F) warmer than the exterior), heat pumps aren't necessarily the best way to go. For milder climates, they are often used exactly as you suggest.

 

[dawin]

Could you please elaborate? That seriously doesn't answer my question...

It's funny you said that because it seriously does

 

[Drakkith]

I got a chance to observe the vents of a gas furnace and was surprised by how warm the outtake vent is. The high efficiency furnace is supposed to be more than 90% efficient so why is the outtake air so warm? It eventually occurred to me that the outtake air can't be any cooler than the air in the house so that probably explains why it's so warm...

Which vent are you referring to? The outtake vent in my home unit simply exits into the room with warmer air.

 

[Dale]

A heat pump is just an air conditioner run in reverse. They are the same thing. You ask why we don't heat homes using air conditioners, but we do and when we do we call it a heat pump.

 

[russ_watters]

I got a chance to observe the vents of a gas furnace and was surprised by how warm the outtake vent is. The high efficiency furnace is supposed to be more than 90% efficient so why is the outtake air so warm? It eventually occurred to me that the outtake air can't be any cooler than the air in the house so that probably explains why it's so warm...

The furnace flame burns at over 2000F so even if the combustion exhaust is 200F, it has still sent the vast majority of the heat into the house. But to get over 80% efficient, a furnace must take more energy from the exhaust and in the process, condense the water vapor in the exhaust. This requires a heat exchanger, a drain, a drain pump and exhaust piping that won't corrode (usually PVC) as well as ducted combustion supply. All of these features cost money so they aren't always done.

 

[Lsos]

Could you please elaborate? That seriously doesn't answer my question...

Your question of "why isn't this done?" is invalid because, well, it IS done. Hell, my home was heated by an air conditioner in reverse. It can be a much more efficient way of heating a space than by directly burning fuel or using an electric heater.

 

[rcgldr]

I've seen cheap ac/heater units that just reverse the flow for heat mode. One negative side effect is this increases the humidity in the room. I don't know if it can be enough to contribute to mold issues.

 

[russ_watters]

One negative side effect is this increases the humidity in the room. I don't know if it can be enough to contribute to mold issues.

1. No, it doesn't.
2. It wouldn't be a bad thing if they did, it would be a good thing (humidity is always too low in winter).

Not sure where you're getting that idea.

 

[rcgldr]

I've seen cheap ac/heater units that just reverse the flow for heat mode. One negative side effect is this increases the humidity in the room. I don't know if it can be enough to contribute to mold issues.

Not sure where you're getting that idea.

I recall an overly humid hotel room one time when I was working off site. Maybe it was just a faulty ac/heater.

 

[russ_watters]

In the spring and fall, when you don't use either the heat or air conditioning, the humidity can get uncomfortably high - that's the only thing I can think of. But air conditioning knocks down humidity and heat doesn't affect it.

 

[rcgldr]

In the spring and fall, when you don't use either the heat or air conditioning, the humidity can get uncomfortably high - that's the only thing I can think of. But air conditioning knocks down humidity and heat doesn't affect it.

In this case, the flow was simply reversed in a cheap window type ac unit, with the ac "cooling" the outside air, and the heat and much of the accumlated water vapor being directed. I assume there was a drip pan, but the amount of water being evaporated by the inwards flow was directing a lot of water vapor inside.

 

[gcracker89]

I got a chance to observe the vents of a gas furnace and was surprised by how warm the outtake vent is. The high efficiency furnace is supposed to be more than 90% efficient so why is the outtake air so warm? It eventually occurred to me that the outtake air can't be any cooler than the air in the house so that probably explains why it's so warm...

..but then another idea occurred to me. The outtake air can't be any cooler than the interior air as long as the heat exchange is passively done but it doesn't have to be. You could use a heat pump, like an air conditioner, to transfer even more heat... so why isn't this done?

The cost of manufacturing and operation would far exceed any recovery. But here is an idea for one of you guys,, just remember when you strike it rich to hire me or send me a check. A diode or transistor that transfers heat, like the coolers that can run Hot or Cold.

If you live in an apartment that's electrically heated, why wouldn't you heat your home with an air conditioner facing the opposite way?
Thank you...

Heat pumps are reversible air conditioners well that’s a little simple. The heat pump has a reversing valve that reverse the “Freon”
A/C units have two major Assemblies the Evaporator (inside unit) Condenser (outside unit)
In these are several basic items inside the assemblies:
Evaporator: a tubing coil, a blower, a metered orifice and a heat strip (for cold weather)
Condenser: a tubing coil, a blower, a pump and in the case of a heat pump a reversing valve.
The pump pumps Freon** to a high pressure gas which travels through the condenser where the gas condenses into a liquid and the heat is given off. The condensed liquid then to the evaporator thru a high pressure line thru the metered orifice at the evaporator coils where the Freon expands to a gas this reduction of pressure the Freon turns cold and absorb heat as it traveling through the Evaporator coil the cool Freon then returns via a low pressure line to the condenser as a low pressure gas and the process starts again.
The efficiency of Freon diminished as temperature drops somewhere around 30 degrees F. it becomes ineffective. There are more millions to be made find a gas that works at -30 F.
The heat pump has a reversing valve in the high and low pressure systems are now the gas expands outside picking up heat and condenses in the house.
** Freon generic term: Freon is Dichlorodifluoromethane (R-12), and was replaced by Chlorodifluoromethane Freon 22, which has been replaced by R-410A,




P.S. This is totally unrelated but I also noticed that the intake vent hardly sucked in any air while the outtake vent was blowing out a lot more. I wondered if this air was just being sucked in from the home but I now suspect that perhaps it just seems like a lot more air because it's that much warmer than the air being sucked in. Does this seem plausible?

Systems have to have balance, if you had more going out than coming in the windows in the house would suck in.

 

[Antiphon]

I'm picking a nit now.

Heat pumps are not air conditioners run in reverse; that would be a heat engine.

Air conditioners *turned around* are heat pumps.

 

[russ_watters]

In this case, the flow was simply reversed in a cheap window type ac unit, with the ac "cooling" the outside air, and the heat and much of the accumlated water vapor being directed. I assume there was a drip pan, but the amount of water being evaporated by the inwards flow was directing a lot of water vapor inside.

Window air conditioners don't typically bring in much if any outside air. That would be a big waste of energy. They just recirculate inside air.

 

[Dale]

I'm picking a nit now.

Heat pumps are not air conditioners run in reverse; that would be a heat engine.

Air conditioners *turned around* are heat pumps.

That is a well-picked nit . You are quite correct.

 

[Jocko Homo]

I'm picking a nit now.

Heat pumps are not air conditioners run in reverse; that would be a heat engine.

Air conditioners *turned around* are heat pumps.

That is a well-picked nit . You are quite correct.

Hey, you appreciate picked nits? How about this: air conditioners, even before they were turned around, are heat pumps... Okay, that wasn't quite as profound as Antiphon's nit but it's still true!


Okay, so what I've gathered is that are conditioners turned around are used to heat homes, which explains why I couldn't think of a reason why they wouldn't be. Thank you all for your participation!

I'm guessing they're not ubiquitous because the efficiency gain in harsh climates (like where I live) isn't enough to overcome the bother of the extra equipment needed...

 

[russ_watters]

I'm guessing they're not ubiquitous because the efficiency gain in harsh climates (like where I live) isn't enough to overcome the bother of the extra equipment needed...

That and natural gas is cheap. And the overall efficiency if you include the power plant isn't much better for a heat pump running on electricity (generated by a 40% efficient power plant, probably) than just burning natural gas is.

 

[pallidin]

Uh... wouldn't a simple electrical space heater be far more efficient than a reverse air-conditioner?

 

[russ_watters]

No, a half decent heat pump has a COP of about 3.5, that's 3.5x the heat output of an electric space heater for the same electrical input.

 

[OmCheeto]

I would ask the question; Why don't we heat our homes with our refrigerators?

Why do we dump all that energy out into our living areas, where it just rises to our ceilings, where it is doing no one any good(except for maybe warmth loving spiders).

Why don't we port all that hot air under our floors, to keep our http://www.urbandictionary.com/define.php?term=toesies" warm?

 

[TurtleMeister]

No, a half decent heat pump has a COP of about 3.5, that's 3.5x the heat output of an electric space heater for the same electrical input.

Heat pumps are more than 100% efficient?

http://en.wikipedia.org/wiki/Electric_heating#Electrode_heater

For an electrical energy customer the efficiency of electric space heating can be 100% because all purchased energy is converted to building heat.

 

[russ_watters]

Heat pumps are more than 100% efficient?

"Efficiency" is not a concept properly applied to thermodynamic devices that move heat energy around, only to thermodynamic devices that convert one kind of energy into another. In an electric heater, energy is converted from electrical to heat. In a heat pump or air conditioner, energy is not converted, it is only moved from outside to inside a house (or vice versa). So the concept of "efficiency" doesn't really apply. The somewhat similar thermodynamic concept is coefficient of performance (COP):

http://en.wikipedia.org/wiki/Coefficient_of_performance

 

[russ_watters]

I would ask the question; Why don't we heat our homes with our refrigerators?

Why do we dump all that energy out into our living areas, where it just rises to our ceilings, where it is doing no one any good(except for maybe warmth loving spiders).

Why don't we port all that hot air under our floors, to keep our http://www.urbandictionary.com/define.php?term=toesies" warm?

My refrigerator is on the first floor, so at worst it heats my second floor. Refrigerators put out a comparatively small amount of heat anyway.

 

[TurtleMeister]

"Efficiency" is not a concept properly applied to thermodynamic devices that move heat energy around, only to thermodynamic devices that convert one kind of energy into another. In an electric heater, energy is converted from electrical to heat. In a heat pump or air conditioner, energy is not converted, it is only moved from outside to inside a house (or vice versa). So the concept of "efficiency" doesn't really apply. The somewhat similar thermodynamic concept is coefficient of performance (COP):

http://en.wikipedia.org/wiki/Coefficient_of_performance

So the heat pump operation is similar to physically moving an object from one place to another? Except in this case the "thing being moved" is not an object, but rather it is heat energy?

 

[russ_watters]

Exactly!

[...although actually it does move an "object": the refrigerant. It's just that we don't care about the mechanical work being done to move the refrigerant.]

 

[pallidin]

Turtle, I'm losing something here{not from you}
An electrical space heater is FAR MORE EFFECTIVE than a reverse air-conditoner.

 

[pallidin]

Exactly!

[...although actually it does move an "object": the refrigerant. It's just that we don't care about the mechanical work being done to move the refrigerant.]

"It's just that we don't care about the mechanical work being done to move the refrigerant"

Really? Why not.

 

[Lsos]

Turtle, I'm losing something here{not from you}
An electrical space heater is FAR MORE EFFECTIVE than a reverse air-conditoner.

If the reverse air-conditioner has a COP of 3.5, then the reverse air-conditioner is far more effective. 3.5 times more effective, in fact.

 

[Antiphon]

The most effective heating system I've seen to date is a heat pump where the coils are buried underground. This is because it takes more energy to move a BTU from a cold place into the house than from a warmer place.

An electric space heater is 100% efficient but the measure is how much heat you deliver to the house for a given watt of input power. (By this measure, an electric conveyor delivering chopped wood into a fireplace if far better still.)

 

[Lsos]

So the heat pump operation is similar to physically moving an object from one place to another? Except in this case the "thing being moved" is not an object, but rather it is heat energy?

Yes. I like to use the train analogy. Imagine a train bringing coal to an electrical power plant. The energy contained in the coal is far, far greater than the energy that the train used to bring it. Yet, we don't say that the train is 100000% efficient, because it didn't create the coal...it simply moved it.

The heat pump performs the same function as the train. The energy is there, it simply brings it to where we want it.

Edit: an electric conveyor delivering chopped wood is yet another example :)

 

[russ_watters]

"It's just that we don't care about the mechanical work being done to move the refrigerant"

Really? Why not.

Cause it's just a fluid going around in a circle.

 

[TurtleMeister]

Yes. I like to use the train analogy. Imagine a train bringing coal to an electrical power plant. The energy contained in the coal is far, far greater than the energy that the train used to bring it. Yet, we don't say that the train is 100000% efficient, because it didn't create the coal...it simply moved it.

The heat pump performs the same function as the train. The energy is there, it simply brings it to where we want it.

Edit: an electric conveyor delivering chopped wood is yet another example :)

Good analogy.

I've come up with a thought experiment that has helped me understand the difference between the electric heater and the heat pump. Imagine two rooms, room A and room B, separated by an insulated wall. Both rooms are also insulated from the outside. There is a heat pump in the wall that separates the two rooms. Initially, both rooms have the same temperature.

Now, we want to increase the temperature in room A. So we turn on the heat pump. As the temperature increases in room A, it decreases in room B. But as the temperature difference increases we find that the average temperature of both rooms remains the same. So the electrical energy used by the heat pump is NOT being converted into heat energy.

However, if we have the same setup, except this time we use a space heater to heat room A, we find that as the temperature increases in room A, the temperature in room B remains the same. The average temperature of both rooms increases. So the electrical energy being used by the space heater IS being converted to heat energy.

 

[cjl]

Good analogy.

I've come up with a thought experiment that has helped me understand the difference between the electric heater and the heat pump. Imagine two rooms, room A and room B, separated by an insulated wall. Both rooms are also insulated from the outside. There is a heat pump in the wall that separates the two rooms. Initially, both rooms have the same temperature.

Now, we want to increase the temperature in room A. So we turn on the heat pump. As the temperature increases in room A, it decreases in room B. But as the temperature difference increases we find that the average temperature of both rooms remains the same. So the electrical energy used by the heat pump is NOT being converted into heat energy.

However, if we have the same setup, except this time we use a space heater to heat room A, we find that as the temperature increases in room A, the temperature in room B remains the same. The average temperature of both rooms increases. So the electrical energy being used by the space heater IS being converted to heat energy.

Well, almost...

The energy used by the heat pump will indeed go into heat energy, and the average temperature of both rooms will increase. However, the average temperature of both rooms will increase much less for a given delta t in the room being heated than it would with a resistive heater.

 

[TurtleMeister]

Well, almost...

The energy used by the heat pump will indeed go into heat energy, and the average temperature of both rooms will increase. However, the average temperature of both rooms will increase much less for a given delta t in the room being heated than it would with a resistive heater.

Yes, that is true. But the nice thing about thought experiments is that you can imagine such things as "perfect heat pumps". And the point of the thought experiment was to show the main difference between heat pumps and space heaters. Another reality of heat pumps, that I haven't quite figured out yet, is that the greater the difference in temperature (between room A and room B) the less effective the heat pump becomes. Eventually you will reach a temperature difference where the space heater becomes more effective than the heat pump. Does it have anything to do with the refrigerant, and the temperature difference between the evaporator and condenser coils?

 

[Lsos]

The difference in temperature is the equivalent of asking the train with the coal to go up a hill. The greater the temperature difference, the higher the hill. Eventually you get to the point where you're just better off screwing the train and digging for coal on the spot, or exploring other options.

 

[OmCheeto]

Cause it's just a fluid going around in a circle.

I didn't think you could have a refrigeration cycle with strictly a fluid. Should this be; "Cause it's just a fluid/gas going around in a circle." ?

I suppose a radiator/cooling system in a car is strictly fluid. But I don't think they refer to them as refrigeration units, even though they remove heat.

hmm... http://en.wikipedia.org/wiki/Refrigeration" ?:

Refrigeration is a process in which work is done to remove heat from one location to another.

Bah!

That means I'm a refrigerator...

 

[Antiphon]

The car radiator is convective cooling only. In my book, to be heat pump you'd have to involve a thermodynamic cycle resembling Carnot. Unlike a radiator, such a thing can move heat from a colder into a warmer area.

 

[OmCheeto]

The car radiator is convective cooling only. In my book, to be heat pump you'd have to involve a thermodynamic cycle resembling Carnot. Unlike a radiator, such a thing can move heat from a colder into a warmer area.

Not according to the all knowing, all seeing wiki:

http://en.wikipedia.org/wiki/Convection" is the movement of molecules within fluids (i.e. liquids, gases) and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids.

bolding mine

I don't know what planet you are from, but all of the automotive radiators* on my planet are solid.

*mostly made out of either copper or aluminum. both very good thermal conductors.

 

[russ_watters]

I didn't think you could have a refrigeration cycle with strictly a fluid. Should this be; "Cause it's just a fluid/gas going around in a circle." ?

Note the definition of "fluid" in your next post...

A car radiator utilizes both conduction and convection: Conduction is what transfers energy to/from the two fluids and the radiator and convection which mixes/equalizes temperature inside the fluid due to turbulence.

 

[Antiphon]

Not according to the all knowing, all seeing wiki:


bolding mine

I don't know what planet you are from, but all of the automotive radiators* on my planet are solid.

*mostly made out of either copper or aluminum. both very good thermal conductors.

I meant the heart of the radiator system of course which is a circulating convective fluid loop.

 

[OmCheeto]

Note the definition of "fluid" in your next post...

Gads!

I stand corrected.

 

[cjl]

Yes, that is true. But the nice thing about thought experiments is that you can imagine such things as "perfect heat pumps". And the point of the thought experiment was to show the main difference between heat pumps and space heaters. Another reality of heat pumps, that I haven't quite figured out yet, is that the greater the difference in temperature (between room A and room B) the less effective the heat pump becomes. Eventually you will reach a temperature difference where the space heater becomes more effective than the heat pump. Does it have anything to do with the refrigerant, and the temperature difference between the evaporator and condenser coils?

First off, even a perfect heat pump/air conditioner actually requires some energy, and will add that to the heat leaving the hot side. The amount of energy added to the warmer room per unit of energy used by an ideal heat pump is equal to the (absolute) temperature on the warm side divided by the difference in temperature between the warm and the cold side. This also shows that as the temperature difference increases, the maximum efficiency goes down.

As for why the pump becomes less effective (physically) with larger temperature differences? You're pretty much right about it being the temperature difference between the evaporator and condenser coils. To maintain a larger temperature difference between evaporator and condenser, the pressure difference between the two must be increased, which means that the compressor must work harder.

 

[TurtleMeister]

The difference in temperature is the equivalent of asking the train with the coal to go up a hill. The greater the temperature difference, the higher the hill. Eventually you get to the point where you're just better off screwing the train and digging for coal on the spot, or exploring other options.

Another good analogy Lsos.

As for why the pump becomes less effective (physically) with larger temperature differences? You're pretty much right about it being the temperature difference between the evaporator and condenser coils. To maintain a larger temperature difference between evaporator and condenser, the pressure difference between the two must be increased, which means that the compressor must work harder.

I'm thinking now that it's not just the temperature difference, but also the temperature of the cold side. The colder it is, the less heat energy there is to move. Which makes it more difficult. Similar to pulling a vacuum.

Edit:
On second thought, the vacuum analogy is probably not right. However, having less heat energy available to move may still have an effect on the effectiveness of the heat pump.

 

[pallidin]

Stop already.
A purely resisitive heating element is far more efficient than a two-stage mechanism.
A 1000 watt air conditioner in reverse is LESS able to produce useful heat as a 1000 watt resistive space heater.
Not to mention the fact that you don't have to have part of the resistive heater outdoors.

 

[russ_watters]

Stop already.
A purely resisitive heating element is far more efficient than a two-stage mechanism.
A 1000 watt air conditioner in reverse is LESS able to produce useful heat as a 1000 watt resistive space heater.
Not to mention the fact that you don't have to have part of the resistive heater outdoors.

Nope, nope, nope. Read the specs on any heat pump.

[though I don't know what you mean by a "two-stage mechanism".]

Try this one: http://www.residential.carrier.com/products/acheatpumps/heatpumps/performance.shtml

It has a cooling SEER of 16.5 and a heating HSPF of 9.5. That means:
In heating mode, it produces 2.8 watts of heat for every watt of input power.
In cooling mode, it produces 4.8 watts of "cool" for every watt of input power.

 

[Lsos]

A 1000 watt air conditioner in reverse is LESS able to produce useful heat as a 1000 watt resistive space heater.

Maybe you're right. However, an air conditioner in reverse doesn't PRODUCE heat, it merely MOVES it. And if your goal is to have a warm home, in a lot of cases it reaches this goal far more effectively.