A/C or heat pump???

We were faced with the same choice last summer when our 40 year old central AC finally decided to retire. At the same time we needed new ductwork so we decided to upgrade the ductwork and spend the extra dollars for the heat pump. We reduced our electricity usage and the house is far more comfortable than it was before. Today's high efficiency heat pumps really do save electricity and dollars versus those of just a few years ago. Our house was cooler and much more comfortable last summer and fall and was much warmer and more comfortable this winter. Our total electrical usage is down while our comfort level is up! The combination of a high efficiency heat pump and backup oil heat worked well all winter (natural gas is not an option in our rural area). We also have a pellet stove but wood pellets were in such short supply that we were unable to use that as supplemental heat instead of oil most of the time this winter.

We haven't yet gone solar to generate electricity but that is coming. Meanwhile our utility company offers a "renewable option" where we pay a slight surcharge to purchase 100 percent wind and other renewable sourced power instead of coal sourced electricity. To reduce our carbon footprint more we offset the carbon dioxide produced with the oil backup heat and the gasoline for our vehicles by purchasing carbon offsets through TerraPass (www.terrapas.com.)

This IS a Prius forum, so... Since trading a 2004 Nissan minivan (22 mpg average) in on a 2007 Prius we've reduced our gasoline usage and carbon output significantly! We're still learning to drive the Prius efficiently, but in the first 3000 miles of mostly highway driving we've averaged right at 46 mpg overall, and in the process saved 63 gallons of gas, $178 and released 1256 less pounds of CO2! We expect those savings to improve as the warmer weather is here and we learn driving techniques for better mileage.

 

<div class='quotetop'>QUOTE(barbaram @ Apr 6 2007, 01:16 AM) [snapback]418681[/snapback]</div>

It basically depends on how much you pay for gas vs. electricity, and how cold it gets during the winter. You'll have to determine how often the heat pump can heat the house for less than gas, and how quickly the savings will pay for the additional cost of the heat pump vs. regular A/C.

The best case for a heat pump is when winters are mild (near or above freezing most of the time). In these conditions, the heat pump is operating in it's most efficient conditions and can heat your house for less than gas. I've got a house in Las Vegas and there's no need for a gas furnace there - the heat pump is operating in an efficient temp range and can produce enough warm air to heat the house.

If it gets really cold, really often, then 1) the times when it costs less to heat your house with the heat pump vs. gas are reduced, and 2) you won't like that the colder it is outside, the colder the air coming out of the registers when using a heat pump. If it's really cold outside, and the heat pump can only produce say, 85 degree F air out of the registers, will that still help with your cold spots?

You'll need a pretty smart thermostat capable of controlling a heat pump with multistage heat to do things like lockout the compressor at really cold temps (when gas heating is cheaper), like a Honeywell TH8321U. Your A/C contractor should be able to help you with things like this, as well as determine if there is any benefit to a heat pump in your situation.

 

If cold spots are an issue, you need a multistage furnace fan--one that's always on at a low speed to circulate air and keep things even, and which can move to faster speeds as needed to increase heating/cooling capacity as called for.

 

are you guys talking about geothermal heat pumps or something else?

 

I lived in a house that had a heat pump and was backed up with oil heat as an alternative. I lived in Maryland at that time which was back in 1984. Since then moved out of state and then back to Maryland.

The thing is that once the weather gets below freezing, your heat pump is not going to do you much good because it draws in the outside air and uses it. The heat pump works well in the summer time for cooling but you really don't want it for heat in the winter time. The air maybe warm but warm is not gonna cut it.

I don't know if there are other alternatives but that's been my experience. We too had an attic fan before we went to a heat pump but it didn't do much good in the summer time when it got really hot, and the house was a rambler/ranch. It got awful hot. First year we were there, the 2 of us, we spend part of the time in the basement, (unfinished), trying to stay cool. Thank God for mother-in-law who invited us to stay for the day at her house with the AC.

Do your research really well before jumping the gun.

With the way prices are nowadays, anyone will sell you anything.

 

<div class='quotetop'>QUOTE(tripp @ Apr 6 2007, 10:27 AM) [snapback]418874[/snapback]</div>

Air-source heat pump.

 

Wrong question. First, do an energy audit, including at least one year of utility bills (electricity, natural gas, other). Then, determine the energy efficiency index (EEI) making use of heating-degree and cool-degree days for your location.

Looking at your utility bills will provide your household energy demand in Joules, which is then compared to the square area being heated and cooled. An EEI of >20, indicates the structure is a serious energy waster and there is a lot you can do to tighten up the house. EEI >10 <20, indicates there are several things you can do. EEI <10, indicates the house is fairly energy efficient.

It is great that you are looking into more efficient energy sources, but it makes no sense to "just go get more," and end up heating and cooling the outside. This is just "substituting oil for knowledge." You can put the money into a window one time, or throw the money out the window each month. Your choices should be energy efficient (high SEER rating) and you should make every effort to hold onto what you have (conservation).

For example, triple-pane windows make a huge difference, especially in an older structure. Gaskets behind switch and outlet plates (total cost ca. $20) is the equivalent of plugging a one square meter hold in the front door. Walking around with a caulking gun and caulking every crack you see provides another rapid payback.

Cooling - make use of an Earth Tube. A small PV panel that runs a slow speed fan that comes on at sunrise and turns off at sunset, can draw passively-cooled air through a tube buried in the soil. With an Earth Tube, some days you come home and swear you just stepped into a meat locker. Via the Earth Tube, air comes in at one end of the house and is vented out at the opposite side of the house.

Water heater - insulate pipes and the tank (our two tanks are wrapped four times). Use a combination slave tank (allows water to reach air temperature), then to a natural gas tank (less of a temperature increase) and make use of "instant" hot water at key points - bath, shower and kitchen.

When you decide on a unit (HVAC, water heater, etc.) get the highest SEER rating possible. This means you pay capital costs up front, but you reap savings through efficiency and enjoy a long, maintenance-free life for the unit.

To perform your own energy audit, make use of James Dulley's web site, http://www.dulley.com/. Compare your findings to those of your neighbors. You will find conservation and efficiency is the most cost-effective way to go with the fastest payback period. Another source is Amory Lovins and the Rocky Mountain Institute, http://www.rmi.org/

Enjoy your comfort and savings.

 

<div class='quotetop'>QUOTE(loveit @ Apr 6 2007, 10:51 AM) [snapback]418898[/snapback]</div>

A heat pump does not "draw in the outside air and use it". It's just an A/C that can run in reverse, thus moving heat into the house (winter operation) instead of out of it (summer operation). The outside air is just blown over the evaporator coils, it never goes inside the house. It is true though that when the outdoor temps drop, the coefficient of performance of a heat pump drops, as well as the temperature gain across the condensor (indoor) coil. And as I wrote before, a heat pump works quite well for areas with mild winters.

The absolute threshold below which you don't want to run a heat pump is around -18 degrees F. This is because the COP (coefficient of performance) of the heat pump drops below 1.0, meaning that pure (resistance) electric heat would be more efficient. You'd want to switch over to a gas furnace long before then though, because there wouldn't be enough temperature gain across the indoor coil at temps below freezing. For a newish heat pump in good condition, the number I hear thrown around is usually around 18 to 20 degrees F. That's about the lowest temperature you'd want to ever regularly experience without having your heat pump supplemented by an oil or gas furnace (most people pay enough per kwh to make using the resistive heat elements too costly).

I missed the part where the OP stated she'd be getting (some) electricity for no cost. That changes things quite a bit. Resistive electric heating of any kind is usually way more expensive than gas. You might want to keep your gas water heater and see how far the heat pump can go to provide heating, unless you are generating so much electricity you can run both the water heater and heat pump for free. Looking at the averages for Trenton, NJ on weather.com, I think you actually could run on the heat pump alone for much of the winter, and just supplement it with the gas furnace for the coldest few weeks of the year.

One thing you usually don't want to do with a heat pump is program your thermostat to drop the temperature while you're sleeping and then raise it right before you wake up. That causes it to run at the coldest part of the day, when it is least efficient. It's often better to just let it keep the temperature constant at night so it doesn't have to warm the house a bunch at the worst possible time for it to be running.

 

<div class='quotetop'>QUOTE(tripp @ Apr 6 2007, 01:27 PM) [snapback]418874[/snapback]</div>

I think the OP was referring to what really is a "reverse" cycle A/C unit.

Yes, heat pumps are more efficient than using resistive heat strips (I don't know the cost comparisons to natural gas though). Having lived in FL most of my life most everyone demands a heat pump (but not me anymore, see below). It is my understanding heat pumps are not recommended in the northern climes. You will experience many more nights and days compared to the southern states below or near freezing where a heat pump will be near useless compared to a good running modern gas furnace.

But heat pumps as compared to a straight cool units have their disadvantages. They cost lots more (about 50%), are more complex (requiring more service calls, lots of calls for stuck reversing valves at the start of the cold weather), are slightly less efficient than straight cool units of the same size, and the condensers tend to freeze up rendering them useless for heating even in above freezing conditions. In the Winter, a heat pump literally takes the "heat" from the outside and pumps it inside (for cooling your house it is the reverse, you are pumping the "heat" inside your house outside).

So for heating your house, of course, the condenser coils will get colder than the outside ambient temp, usually collect condensation, and then freeze rendering their heat transfer capabilities nil. Some condensers try to prevent a freezing condition through various methods but in my experience I usually notice my high end Trane heat pump units eventually turning themselves into blocks of ice futilely running throughout a 40 degree night (and usually the resistive heat strips kick in anyhow) to heat the house.

My next units (I have two central units, a two ton for upstairs, and a three ton for downstairs) will be straight cool. I'm done with the heat pump hype and hassle. I believe with cost savings on the new units, less service calls, it will still cost less to heat our FL house with resistive heat strips for the dozen or so nights we use the heat.

Not a real big fan of Carrier too. I always thought their construction (sheet metal, fit, etc.) was some of the worst of the brands out there. No matter what HVAC unit you get, make sure it is a scroll compressor. Much better (esp. in life cycle) than the older style piston types.

Rick
#4 2006

 

<div class='quotetop'>QUOTE(viking31 @ Apr 6 2007, 12:47 PM) [snapback]418975[/snapback]</div>

I haven't experienced any of the problems you've described. In fact, I think the only call I made to the A/C repairman in 14 years was to install a new heat pump to replace the downstairs unit that came with the house. I'm surprised that the price premium is 50% - are you sure about that? I never priced straight A/C units but I thought the difference was much smaller.

The appearance of frost, and periodic defrost cycles are normal for a heat pump in the winter. Having the coils turn to a block of ice is not normal at all.

http://www.hannabery.com/faq1.shtml

 

I just got back from Knoxville, TN and my Son-in-Law just installed a heating/AC system that has a compressor for AC and the heat pump, and a gas fired backup when outside temps are too low for the heat pump to operate effectively. The gas fired unit is 95% efficient. The combustion heat loss is so low that it doesn't need a conventional flue pipe - just a PVC vent pipe to the outside. If you need any information about the unit let me know.

 

We never had a problem with the heat pump. But like I said before, we didn't use it when temps were below freezing. We used oil instead. The air that came through when it was below freezing and the heat pump was on was just blowing warm and not hot air.

A humidifier will help keep things toasty also, but you don't want too much humidity.

 

<div class='quotetop'>QUOTE(priusenvy @ Apr 6 2007, 05:17 PM) [snapback]419025[/snapback]</div>

Sorry, I did not mean to imply that most heat pumps will eventually have some sort or failure. Most will run fine for their lifespan. But look in the HVAC catalogs and you will find a section with the reversing valves (it's a common failure item for heat pumps). They are about $100 to $200 depending on the size of the condenser. Again figure double the cost with the contractor and labor to install (sweat in the fitting, evacuate/charge the system).

A quick check with Grainger shows a 3 ton 13 SEER straight cool unit at $1,013.00. The unit with the same specs in a heat pump version is $1,586.00 (slightly more than 50%). Factor in the air handler at $686.00 (they both use the same one) and the difference is somewhat less than 50%. But still a large amount IMHO. And expect an HVAC contractor to charge double (and nearly all supply house will not sell these units unless you have a current HVAC contractors license).

You are correct. But here in FL it we just don't have enough cold nights for me to justify a service call. So will I get my condenser fixed to heat properly in the winter? Well that's if I trust the tech to correctly figure out the problem. And if he replaces only the parts that truly need replacing. And if he doesn't crack the schrader valve ever so slightly to get the homeowner to call next year complaining the unit doesn't cool like before... And if....

I trust the techs as far as I can throw 'em. And I'm getting old and the techs are getting fatter...

Rick
#4 2006

 

I don't understand spending the huge amounts of money to get a 95% efficient furnace (over an 85% unit) to use as BACKUP heat to a heat pump. That makes no sense.