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Discussion Starter #1
I'm looking into getting an AC system (haven't had one since I've lived in CA).

I haven't found a whole lot of info on the differences between the 2 systems re: efficiency. I have gas heater now, so heat isn't much of a concern unless a heat pump is more efficient.

Does anyone know the efficiency differences between a heat pump and a straight AC system when talking about cooling a house down? What about heating with heat pump vs. gas unit (I have a single wall unit that does a decent job of heating my whole place)?

For the record, my place is a very small, two story, free standing house. I was looking at dual-zone split system (where the condenser is inside and the evaporator outside - or is it the other way around?) - something like this AC unit or this heat pump. Thoughts?
 

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I was looking at dual-zone split system (where the condenser is inside and the evaporator outside - or is it the other way around?)
It's the other way around; at least when cooling your living space is concerned.

A heat pump has the ability to work in reverse so to speak. When you want to heat your house in the winter, it acts like an air conditioner trying to cool the world outside your house, dumping the heat into your living space. This works well in areas that the winters are mild. As the exterior temperature drops a heat pump starts to really loose its efficiency/capacity.

xtn
 

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If you are going to stay long enough to have a decent pay-back period, consider geothermal. (If not ignore this),

It's a heat pump that uses the ground instead of air to pull or push heat from or into. So when it's 104 out there in LA, an air-sourced system is trying to reject heat into hot, not-very-dense air, which is tough to do. Efficiency sufers and I hear electricity is expensive out there. The same type of system rejecting into the ground faces below ground temperatures which are naturally cool (like 60 degrees). Imagine how well a normal AC would work if you turned it on when it was 60 degrees out. It can do this in reverse to heat in the winter. Here in Ohio it gets pretty darn cold out, and air-sourced heat pumps have to quit but ground sourced units don't.

Our system has a SEER of around 20.

Yes, it's expensive to install and you might not have enough yard.
 

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Discussion Starter #4
If you are going to stay long enough to have a decent pay-back period, consider geothermal. (If not ignore this),
You know, I'd never heard of this sort of system until I started researching this topic today. The people who were talking about the geothermal heat-pumps were from Ohio. hehe. That is an interesting idea, though my house is sitting almost directly on very hard bedrock. Wonder if that would have any effect on it. I've also heard price of admission is something around $25K.

An interesting idea, though.
 

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Our whole system was less than $25K, and we checked every checkbox on the option list:

horizontal directional drilling (no trenches in yard)
dual zones wi zone controller and new duct work
hot water (means free hot water all summer)
top-of the line two stage compressor

We've had it over two yers now and like the fact that it cut our heat bill in half. Ours is a Water Furnace.

You'll want to have an actual installer go over how the loops get put in. Anything else might just be guessing. We found out a good deal about strata and such and wound up forced into horizontal. Works perfectly, and less damage to the yard, but was more expensive.
 

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+1 on the geothermal system if you can afford it and have the real estate to install it. I want install one for my next house.

As for conventional heat pumps, remember that since you will be using the condensing unit more months out of the year (for both cooling and heating), it will wear out quicker than a conventional straight cooling unit. Gas furnaces typically outlast the A/C system, so the overall life of your system is greater that a heat pump system in that case.

Also, consider that the air temperature of the heat pump supply air is less than that of gas furnace supply air. Some people don't like the perception that the system is blowing colder air, even though the space will be the desired temperature. If you live in a mild climate, this isn't that much of an issue.

Heat pumps that use an air-cooled condensing unit typically cannot extract enough heat from the outside air once ambient temperature goes below ~40 degrees F or so. At that point, electric strip heaters are called into service, which can cost big bucks. Again, it depends on your climate as to whether or not this is a factor. If your heating season is relatively short and mild, then no big deal.

Just my two cents...
 

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Discussion Starter #7
Thanks for the response - I hadn't thought of the appliance's life cycle.

I'm more interested in cooling than heating as I've got the heating part down. Heating was more of a curiosity & thinking ahead to higher natural gas prices, but what I'm really after is this: Is a heat pump equal to, better than, or worse than a standard AC system when it comes to cooling a house down?
 

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A heat pump, when in A/C mode, is an air conditioner and works the same way. Same refrigerant, same thermodynamic principals. What makes it a 'heat pump' is the ability to reverse the flow of the refrigerant via a crossover valve. Therefore it has the ability to either cool the inside of the house (and give off hot air outside) or heat the inside of the house (and give off cool air outside).

If you are strictly concerned about using it for A/C purposes than the differences will strictly lie in the efficiency values between specific models, manufacturers, etc. A heat pump will probably cost you more than a comparable A/C due to the crossover valve, among other things so if you don't plan on ever using it to heat your house, then probably cheaper to go straight A/C.
 

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+2 on the Geothermal, and +100 on the Water Furnace brand. I have had mine for almost 4 years now. It was put it after two years of 500 month propane bills thru the winter. I paid 13000.00 to have it installed but I did a lot of the interior work myself. The contractor did all the piping and the loop system out in the yard. It paid for itself in about 2 1/2 years. The only problem I have had was a blow fuse because of a faulty pump, and that was taken care of under warranty. Water Furnace has an excellent warranty. It does an excellent job cooling the house in the Summer also.
 

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If you are strictly concerned about using it for A/C purposes than the differences will strictly lie in the efficiency values between specific models, manufacturers, etc. A heat pump will probably cost you more than a comparable A/C due to the crossover valve, among other things so if you don't plan on ever using it to heat your house, then probably cheaper to go straight A/C.
+1 on this. This crossover (or reversing) valve is a feature that can give trouble as well. If cooling is your main concern, go for a straight cooling unit with the highest SEER you can afford. The Trane XL series is an excellent unit, as the condenser coil surface area is larger than those of many of its competitors and is located behind a protective cover. The compressor (last time I checked) is based on the old GE residential unit (Trane bought them out some years ago) and is pretty rugged. My two units (2 and 2 1/2 tons, 12.5 SEER) are 13+ years old and have hummed along during long, hot Georgia summers with very few issues. These units are quiet and have a good reputation for reliability. At the time I bought mine, 10 SEER was the standard, so higher numbers than I have are the way to go.

+2 on the Geothermal, and +100 on the Water Furnace brand. I have had mine for almost 4 years now. It was put it after two years of 500 month propane bills thru the winter. I paid 13000.00 to have it installed but I did a lot of the interior work myself. The contractor did all the piping and the loop system out in the yard. It paid for itself in about 2 1/2 years. The only problem I have had was a blow fuse because of a faulty pump, and that was taken care of under warranty. Water Furnace has an excellent warranty. It does an excellent job cooling the house in the Summer also.
One more thing to remember about the geothermal units is that the compressors live inside your house vs. outside. There's no noisy condenser fan like outdoor units, so noise isn't a big concern. The main advantage for reliability would be that the units are not subjected to the extremes in temperature that outdoor air-cooled units are.

Regardless of the unit you pick, make sure than you use a contractor that can provide load calculations to confirm that your unit is sized correctly. I had this done, checked his calcs, then had one of the engineers in my group who specialized in HVAC to re-check them. One of my neighbors, who relied on his contractor to get it right, wound up with units that were undersized by 1/2 to 1 tons per floor! Only when he sold the house to an engineer did the problem get resolved. He checked it out and found the problem, as the upstairs unit would never cool off the house on a "design day" (extremely hot weather).

That being said, it is important to get it just right, as an over-sized unit can be just as bad. It may run just long enough to satisfy the temperature set point, then shut off. On humid days, the moisture won't be removed from the space, as the total volume of air isn't recirculated past the evaporator coils long enough to remove the proper amount of moisture. The unit comes on, then quickly shuts off. Repeat. This cycling can also shorten the life of the unit. Either way, make sure the unit has some reserve to pull down temperature quickly on a hot day.

Again, just my two cents...
 

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That being said, it is important to get it just right, as an over-sized unit can be just as bad. It may run just long enough to satisfy the temperature set point, then shut off. On humid days, the moisture won't be removed from the space, as the total volume of air isn't recirculated past the evaporator coils long enough to remove the proper amount of moisture. The unit comes on, then quickly shuts off. Repeat. This cycling can also shorten the life of the unit. Either way, make sure the unit has some reserve to pull down temperature quickly on a hot day.
Certainly true that you do not want to substantially oversize. But if your load calculations indicate, for example 4.3 tons required, by all means pick the 5 ton unit not the 4 ton unit.

xtn
 

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Certainly true that you do not want to substantially oversize. But if your load calculations indicate, for example 4.3 tons required, by all means pick the 5 ton unit not the 4 ton unit.
+1. Absolutely correct. As I said, go for some additional oomph for pull down on really hot days. For instance, the calcs for my upstairs unit came out to something like 1.506 tons, so I went for a 2 ton unit and have no regrets.
 

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Dual Zone and Split system are two different things. Dual Zone means you have either two completely different systems (one upstairs one downstairs) or one system that has a electromechanical damper which can shift the air from downstairs to upstairs (this is usually not very efficient) or both. If your heater was installed with A/C in it's future, it will be sized (airflow) with the ability to put the evap. coil on top of it. If it's not too old this would be the better option then a heat pump. If you have an older house, the ducts will not be sized correctly for A/C especially if the heater is on the first floor or under the house. Everthing depends on where you live. I'm down the road from you in Pasadena (my business) and it's hot all summer. With energy going up you would save on a two system deal. This can utilize your old duct work, by capping it off between the floors and putting a new system in the second floor. You really want A/C vents on top and heating on bottom. You possibly can leave your old unit to do the heat and put in all new ducts for A/C this is common. Also a self contained unit could be used for the second floor. You have, unfortunately many options, and they all depend on the layout of the house. For me, out here heat pumps are a wobbler, we really don't need all that much heat in the winter and remember heaters are built to last 20 years or more, heat pumps 10-12.
 

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I'm looking into getting an AC system (haven't had one since I've lived in CA).

I haven't found a whole lot of info on the differences between the 2 systems re: efficiency. I have gas heater now, so heat isn't much of a concern unless a heat pump is more efficient.

Does anyone know the efficiency differences between a heat pump and a straight AC system when talking about cooling a house down? What about heating with heat pump vs. gas unit (I have a single wall unit that does a decent job of heating my whole place)?

For the record, my place is a very small, two story, free standing house. I was looking at dual-zone split system (where the condenser is inside and the evaporator outside - or is it the other way around?) - something like this AC unit or this heat pump. Thoughts?
You might want to check out Mitsubishi Mr. Slim units. They are available with heat and cooling or cooling alone. Some units are available with an Inverter-driven DC Compressor. These units have a SEER of 16.0.

Mitsubishi Electric - HVAC Advanced Products Division: Products: Homeowner
 

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Ductless split units like the Mitsu Mr. Slim models noted above can be good applications for buildings that are difficult to retrofit with ductwork/conventional split DX units. They're more the norm overseas, but have been used in this country for some time.

The facility where I work has ~9 million square feet of space under roof, with all types of applications from window units to large centrifugal chillers. We have installed Mr. Slim units for years, but the story I hear from our maintenance guys is that they aren't very long-lived and are difficult to work on when something goes wrong. The units are very compact and efficient, but that compactness makes them a pain to work on. Also, can you readily get service for them through residential HVAC service companies? Depends on the area, I suppose. To me, this would be a last resort.
 

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Ductless split units like the Mitsu Mr. Slim models noted above can be good applications for buildings that are difficult to retrofit with ductwork/conventional split DX units. They're more the norm overseas, but have been used in this country for some time.

The facility where I work has ~9 million square feet of space under roof, with all types of applications from window units to large centrifugal chillers. We have installed Mr. Slim units for years, but the story I hear from our maintenance guys is that they aren't very long-lived and are difficult to work on when something goes wrong. The units are very compact and efficient, but that compactness makes them a pain to work on. Also, can you readily get service for them through residential HVAC service companies? Depends on the area, I suppose. To me, this would be a last resort.
+1
Fantastic for unusual applications, a room with no windows, thick walls, other problems. Never meant to cool a whole house. Plus as the photo shows they hang from the ceiling, extremely unsightly.
 

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That being said, it is important to get it just right, as an over-sized unit can be just as bad. It may run just long enough to satisfy the temperature set point, then shut off. On humid days, the moisture won't be removed from the space, as the total volume of air isn't recirculated past the evaporator coils long enough to remove the proper amount of moisture. The unit comes on, then quickly shuts off. Repeat. This cycling can also shorten the life of the unit. Either way, make sure the unit has some reserve to pull down temperature quickly on a hot day.

Again, just my two cents...
+1 Cycling too frequently also increases the cost of operating the system. The high inrush current that occurs when a motor is started causes an increase in power usage.

Ductless split units like the Mitsu Mr. Slim models noted above can be good applications for buildings that are difficult to retrofit with ductwork/conventional split DX units. They're more the norm overseas, but have been used in this country for some time.

The facility where I work has ~9 million square feet of space under roof, with all types of applications from window units to large centrifugal chillers. We have installed Mr. Slim units for years, but the story I hear from our maintenance guys is that they aren't very long-lived and are difficult to work on when something goes wrong. The units are very compact and efficient, but that compactness makes them a pain to work on. Also, can you readily get service for them through residential HVAC service companies? Depends on the area, I suppose. To me, this would be a last resort.
I appreciate this information. I have a neighbor that is thinking about having three of these units installed to cool/heat his house. It's a 57 year old Cape Cod style house that would be extremely hard to have a ducted system installed. He has a 1 year old heating system in the house (gas fired steam) and I have suggested that he consider a cooling only system. There are at least four HVAC contractors in the area that install and maintain the Mr. Slim units. He is going for a multi unit installation so that he only has to heat or cool the area of the house that he will use (he is 84 years old and lives alone). I will pass along the information you posted on the service life of these units.

BTW: Do the Mr. Slim units you spoke of use Inverter-driven or conventional compressors (or both types). I'm curious about how much more efficient the inverter driven units are and if they have more service related problems.

+1
Fantastic for unusual applications, a room with no windows, thick walls, other problems. Never meant to cool a whole house. Plus as the photo shows they hang from the ceiling, extremely unsightly.
JoshS

Installing multiple units allows for zoning the areas to be cooled or heated. That way a person that has no need or desire to heat or cool the entire house can do so. Doing this with a more 'conventional' system might not be practical because of the previous discussion about properly sizing the unit.
 

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JoshS

Installing multiple units allows for zoning the areas to be cooled or heated. That way a person that has no need or desire to heat or cool the entire house can do so. Doing this with a more 'conventional' system might not be practical because of the previous discussion about properly sizing the unit.
First these units don't heat. See my previous posts, I suggested doing multizone units. The cost of putting one in every room would be astronomical and would not pay back over the life of the units as some you would barely ever use (bathrooms, guestrooms). A/C Another one of my myriad occupations (Engel air in Philly, EL PAYNE, BEVERLY HILLS A/C man to the stars), I never, ever have seen a house done in this manner. It would be completely unsightly and as others have posted an extreme last resort
 

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+1. Absolutely correct. As I said, go for some additional oomph for pull down on really hot days. For instance, the calcs for my upstairs unit came out to something like 1.506 tons, so I went for a 2 ton unit and have no regrets.
On the west coast you don't have to worry about oversizing and too many companies do. It does burn a hell of a lot of extra energy, but you never have freeze ups or high humidity. Now on the east coast, the num nuts who over size (and sometimes customers relatives tell them we didn't know how to size properly) get to have water streaming down their windows as the unit cools the humid air before the evap. cooler has a chance to remove the humidity, so their windows do that trick for them. You have a damp cold house, totally unbearable.
 

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First these units don't heat. See my previous posts, I suggested doing multizone units. The cost of putting one in every room would be astronomical and would not pay back over the life of the units as some you would barely ever use (bathrooms, guestrooms). A/C Another one of my myriad occupations (Engel air in Philly, EL PAYNE, BEVERLY HILLS A/C man to the stars), I never, ever have seen a house done in this manner. It would be completely unsightly and as others have posted an extreme last resort
My neighbor is having three units installed not one in every room. I have talk to him about all of the points you have made. He has a PhD in mechanical engineering and fully understands what he is doing. He worked in the aerospace industry and is one of the engineers that designed the Lunar Excursion Module for the Apollo program when he worked for Grumman.

It’s his money, his house and therefore his decision to make. The man is 84 years old and is not concerned about pay back over the life of the units since his life expectancy is less than the life expectancy of the units.
 

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