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Anyone ever use an AC as a heat pump?

E

Eli

Super Moderator | Elite Member
I was contemplating this the other day. We have an un-used 5500BTU AC sitting around.

With a little modification to the thermostat, I could just turn the thing around and use it to heat the house. 😀

It looks like their efficiency drops in colder weather, though. Hmm. Surely there are some ghetto engineers out there that have done this?

I'm sure it would still be more efficient than using the 1500W space heater... which, obviously, is 100% efficient... but heat pumps have the capability of being much more than 100% efficient.

The only problem would be dealing with icing on the evaporator... In freezing weather, just turning it off wouldn't work, and adding some sort of heating device would quickly eat up efficiency, tho it may still be more efficient than the space heater.

/ponders
/gets out calculator
 
Originally posted by: tasmanian
You mean like turn an ac backwards?
Click to expand...

Yep, exactly. That's all a "real" heat pump is, a standard air conditioner that works backwards.

In reality, an AC is a "heat pump" also - it "pumps" the heat from the inside of your house and deposits it outside.

A heating heat pump does the opposite - it pumps heat from the outside, and deposits it inside.

They have window-mount air conditioners that also double as a heat pump, but they're pretty expensive.

I don't think it would be too hard to modify an air conditioner to work fairly well as a heat pump. Like I said, you would just have to modify or re-locate the thermostat so the unit wouldn't turn off in the cold outside air. The real problem would be dealing with evaporator icing..
 
Originally posted by: Fullmetal Chocobo
You could get a dual Xeon (P4 class) system and have it crunch. Very good at heating up rooms.
Click to expand...

But that would be even worse than just using a space heater of the same wattage, since some of the energy is actually being used to perform work other than produce heat. 😛
 
Originally posted by: Eli
Originally posted by: Fullmetal Chocobo
You could get a dual Xeon (P4 class) system and have it crunch. Very good at heating up rooms.
Click to expand...

But that would be even worse than just using a space heater of the same wattage, since some of the energy is actually being used to perform work other than produce heat. 😛
Click to expand...

Yeah but not very much.
 
My old apartment had a brand new carrier heat pump system. As far as I'm concerned it sucked at heating air for a cold Illinois winter. Not sure if they aren't supposed to be used this far north but the cheap landlord did.
 
This will not work for long.

AC units are not designed to run with a low suction pressure. When the evaporator runs below the frost point bad things will happen. Even worse in the absence of a suction accumulator excessive liquid can flood back to the compressor causing serious damage to the valves.

A chill chaser to make up for a small differential possibly. That's what quartz heaters are for. 😉

 
Originally posted by: Rubycon
This will not work for long.

AC units are not designed to run with a low suction pressure. When the evaporator runs below the frost point bad things will happen. Even worse in the absence of a suction accumulator excessive liquid can flood back to the compressor causing serious damage to the valves.

A chill chaser to make up for a small differential possibly. That's what quartz heaters are for. 😉
Click to expand...

Hmm. If I'm interpreting this correctly, you're basically saying that if the evaporator becomes too cold, it will fail to.. evaporate the coolant, leading to compressor damage?
 
Originally posted by: Eli

Hmm. If I'm interpreting this correctly, you're basically saying that if the evaporator becomes too cold, it will fail to.. evaporate the coolant, leading to compressor damage?
Click to expand...

Yes this is why heat pumps have a large suction accumulator to hold excessive liquid so it does not reach the compressor. Also with the lack of a defrost cycle it would be hard to keep the evaporator ice free. Once the ice blocks the airflow suction pressure plummets and you have a flooded system all the way back to the compressor. This makes the compressor very unhappy!
 
Originally posted by: Safeway
Gas heater > anything electric.
Click to expand...

Fail.

A heat pump can be an order of magnitude more efficient than even the best gas heaters. Even a simple resistance heater is technically more efficient than the most efficient gas heater, though electricity is much more expensive than gas.
 
Originally posted by: Eli
Originally posted by: Safeway
Gas heater > anything electric.
Click to expand...

Fail.

A heat pump can be an order of magnitude more efficient than even the best gas heaters. Even a simple resistance heater is technically more efficient than the most efficient gas heater, though electricity is much more expensive than gas.
Click to expand...

Fail on you, for using end-point efficiency. Win on me, for being right.

(With Ruby's caveat, for when gas is unavailable or expensive due to infrastructure.)
 
I took five courses relating to building environmental systems, HVAC, infrastructure, et al, so I know quite a bit number and theory wise. I lack the practical knowledge gained through working in the field, since I've never worked in the field.
 
Without a reversing valve in the refrigerant line how is this going to work? It is a much more complicated conversion than you think and I doubt it is going to work with a standard AC unit as there a lot more difference between a standard unit and a heat pump than you would think.
 
Originally posted by: Ronstang
Without a reversing valve in the refrigerant line how is this going to work? It is a much more complicated conversion than you think and I doubt it is going to work with a standard AC unit as there a lot more difference between a standard unit and a heat pump than you would think.
Click to expand...

I'm beginning to realize that, but my idea was to reverse the AC unit, so the condenser was inside the house.

It sounds like it could theoretically work, as long as the outside temperature wasn't too cold, and you could prevent the evaporator from icing over.

Probably not really worth the trouble, but it would be a fun project.. lol.
 
Originally posted by: Eli
Originally posted by: Safeway
Gas heater > anything electric.
Click to expand...

Fail.

A heat pump can be an order of magnitude more efficient than even the best gas heaters. Even a simple resistance heater is technically more efficient than the most efficient gas heater, though electricity is much more expensive than gas.
Click to expand...

Depends on the gas heater. Unvented gas heaters are theoretically 100% efficient in that all the heat from combustion is delivered to the living space. Fresh make up air is essential to prevent O2 depletion which would cause carbon monoxide to be produced. Obviously we don't want this to happen. 😉

Burning gas also releases CO2 which can be undesirable in certain circumstances however with proper ventilation this is not an issue.

Unvented gas heaters also put moisture in the air. This boost in relative humidity makes you feel warmer at a given temperature and acts to counter drying out the air of conventional forced air heat particularly when the outdoor temperatures are at their coldest values.

Like the original idea unvented gas heating was never designed to a primary source of heat. The other advantage is these heaters will operate in the absence of electrical power.

RE: heat pump efficiency...

Yes they can be more efficient than 100% compared to a resistance element in btu/hr per W used. However your window shaker is going to be far less efficient in these conditions (ignoring the evaporator and slugging issues discussed earlier) due to the differences in metering device type when in the heating cycle and a compressor designed for higher head pressures, etc.

It's a fun project undoubtedly and thus we can learn about these things and have hands on experience working on them too. 🙂
 
At the level of basic theory OP is on the right track. A heat pump fundamentally is used to "pump" heat from a low-temperature zone to a higher-temperature zone, the exact opposite of what normally happens for heat flow. In doing so it consumes outside energy (electrical) to power the compressor which is converted to heat that is exhausted to the hot side of the system. So from the perspective of the hot side, the heat available is the electrical energy consumed PLUS the heat moved from cold to hot side, and the result is that the hot side receives MORE heat than just the electrical heat. Thus it appears to be MORE than 100% "efficient" in converting electrical energy to heat energy. In that sense it actually provides more heat to the room than an electrical heater of the same power consumption. Now, nothing is more than 100% efficient - the point to recognize is that the extra heat does not just magically appear - it is moved from the cold side to hot side.

AC efficiency really is related to heat transfer rate divided by electrical energy use rate - that is, BTU/hr divided by Watts consumed. As you might expect, this is highest when the temperature difference between "hot" and "cold" is small, so the compressor needs to "push" heat up a small step to the "hot" side. The larger the difference, the lower the efficiency of the AC. So even if it worked, efficiency (that is, really, how much "free" heat you pump on top of how much is just from electrical energy that runs the motor) would not be very good in the winter when the indoor "hot" side may be 30C (55F) degrees higher than the outdoor "cold" side.

However, the devil is in the details. There are many versions of heat pumps. They all differ at the core in what temperature ranges are the "cold" and "hot" sides operating at? The type of refrigerant and the pressures around the circuit are based on those answers. Then that leads to the design of the compressor, the two heat exchangers (the Evaporator and the Condenser) and their piping, and the control systems and their setpoints. A through-the-window AC is designed to operate with a "hot" side of from 0 to 50C (32 to 122F), approximately, and a "cold" side in a similar temperature range. NEITHER side can operate below 0C (32 F), both because of the refrigerant characteristics and because you can't allow the heat exchanger fins on either side to get plugged up with water frost. We also should recognize that the air temperature on the "hot" side must be LOWER than that of the compressed refrigerant circulating through the Condenser; otherwise there would be no heat flow into the "hot" air and no condensing action.

Bottom line, mounting a window AC backwards in the window and creating a thermostat system to work "backwards", too (so it turns on when the "Hot" side is too cool, rather than when the "cold" side is too warm) might only work over a limited range of temperatures. And it might break itself, because it is not designed to prevent it from operating under the wrong conditions that could be created that way. It was only designed to anticipate and protect itself from other conditions expected when it is used for its original purpose.
 
Originally posted by: Paperdoc
At the level of basic theory OP is on the right track. A heat pump fundamentally is used to "pump" heat from a low-temperature zone to a higher-temperature zone, the exact opposite of what normally happens for heat flow. In doing so it consumes outside energy (electrical) to power the compressor which is converted to heat that is exhausted to the hot side of the system. So from the perspective of the hot side, the heat available is the electrical energy consumed PLUS the heat moved from cold to hot side, and the result is that the hot side receives MORE heat than just the electrical heat. Thus it appears to be MORE than 100% "efficient" in converting electrical energy to heat energy. In that sense it actually provides more heat to the room than an electrical heater of the same power consumption. Now, nothing is more than 100% efficient - the point to recognize is that the extra heat does not just magically appear - it is moved from the cold side to hot side.

AC efficiency really is related to heat transfer rate divided by electrical energy use rate - that is, BTU/hr divided by Watts consumed. As you might expect, this is highest when the temperature difference between "hot" and "cold" is small, so the compressor needs to "push" heat up a small step to the "hot" side. The larger the difference, the lower the efficiency of the AC. So even if it worked, efficiency (that is, really, how much "free" heat you pump on top of how much is just from electrical energy that runs the motor) would not be very good in the winter when the indoor "hot" side may be 30C (55F) degrees higher than the outdoor "cold" side.

However, the devil is in the details. There are many versions of heat pumps. They all differ at the core in what temperature ranges are the "cold" and "hot" sides operating at? The type of refrigerant and the pressures around the circuit are based on those answers. Then that leads to the design of the compressor, the two heat exchangers (the Evaporator and the Condenser) and their piping, and the control systems and their setpoints. A through-the-window AC is designed to operate with a "hot" side of from 0 to 50C (32 to 122F), approximately, and a "cold" side in a similar temperature range. NEITHER side can operate below 0C (32 F), both because of the refrigerant characteristics and because you can't allow the heat exchanger fins on either side to get plugged up with water frost. We also should recognize that the air temperature on the "hot" side must be LOWER than that of the compressed refrigerant circulating through the Condenser; otherwise there would be no heat flow into the "hot" air and no condensing action.

Bottom line, mounting a window AC backwards in the window and creating a thermostat system to work "backwards", too (so it turns on when the "Hot" side is too cool, rather than when the "cold" side is too warm) might only work over a limited range of temperatures. And it might break itself, because it is not designed to prevent it from operating under the wrong conditions that could be created that way. It was only designed to anticipate and protect itself from other conditions expected when it is used for its original purpose.
Click to expand...

:thumbsup:
 
Originally posted by: Rubycon
Originally posted by: Safeway
Gas heater > anything electric.
Click to expand...

Not if you're far from a pipeline. 😉
Click to expand...

my house had a 30 yr old gas heater/ ac combo on the roof. heater was great, ac not so much. got a trane unit that had heat and ac but was electric. damn thing SUCKS below 35 degrees. it blows cold air, none of the 6 companies ive had out over the last 4 years have been able to fix the problem. each one had their ideas, implemented (and charged me) and failed. if i had the chance to go back in time and rethink that choice for the electric unit id stay with gas.
 
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