Thursday, June 14, 2012

Heat Pumps- Replace, Repair or Upgrade to Geothermal


The first sign of trouble was when I woke up one morning thinking that it smelled like rain. I was in bed with the air conditioning system on. I could think of several excuses why I might have had that thought and so ignored the first symptom and it would be a several more weeks until the heat pump failed. It was a relatively long and cool spring with nights in the 60’s cooling the house, but come the first 90 degree day  I knew my split system heat pump had failed.

My heating and cooling system like a lot of newer homes in northern Virginia is a split heat pump system that consists of an outdoor metal cabinet that contains the condenser and compressor and an attic unit that contains the evaporator coil and the air handler that sends the cool air through the duct system in summer and hot air in winter. In the heating cycle, the air-source heat pump takes heat from the air outside the home and pumps it inside passed refrigerant-filled coils. Inside the heat pump system are two fans, two refrigerator coils, a reversing valve and a compressor. The outdoor unit contains a coil and fan and the compressor. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling.

The effectiveness of a heat pump is based on the temperature difference between the source and the sink and which cycle it is in. Heat pumps are more effective for heating than for cooling if the temperature difference is held equal. This is because the energy used to power the compressor can be converted to useful heat when in heating mode and released into the house as extra heat. The condenser is normally outdoors and during the cooling cycle, and the compressor's dissipated work is not put to a useful purpose. Air heat pumps are best suited to relatively warm climates, such as the southeastern U.S. This is because when temperatures are low, a heat pump’s Coefficient of Performance, COP falls dramatically. According to the Department of Energy a7.5-ton rooftop heat pump that has a high-temperature COP of 3.0 can have a low-temperature COP of 2.0 or even lower. And at very low temperatures, a heat pump can require supplemental heat, typically in the form of electric resistance just to function further reducing effective heating efficiencies.

The most effective type of heat pump is the geothermal heat pump. In winter it collects the Earth's natural heat either through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond, lake or well. The temperature six feet beneath ground surface is cooler in summer and warmer in winter than the ambient temperature and fairly constant, but many loop systems are not installed deep enough in a suitable medium to maintain constant temperature, but nonetheless draws excess heat from the house and allows it to be absorbed by the Earth. I had always assumed that when the time came I would replace my heat pump with a geothermal unit.

Despite the fact that my heat pump system is under 8 years old and heat pumps should last 12-14 years, the evaporator coil corroded and leaked enough Freon (R22) that the system could no longer cool.  The corrosion of the coil was obvious upon inspection, but the Freon level had been fine 2 months earlier when the system had been serviced, so I was taken a little by surprise to find myself having to make the decision about whether to replace the coil, replace the entire system with an energy star heat pump or upgrade to a geothermal heat exchanger now. A heat pump should last longer than 8 years. This is the first major repair the system has required and I probably could get a couple more years out of the system if I replaced the coil, but there is no guarantee and the outdoor unit had started to show rust two years ago. If I replace the entire system, I will probably get another 8-10 years before I have any major problems.  

In truth we were never happy with the system; it could never keep the master bedroom cool in summer. The master bedroom has unobstructed southern exposure and though we installed drapery, window films and additional insulation as recommended by the Building Envelop Research of US Department of Energy Efficiency and Renewable Energy Unit, still the bedroom was never cool enough in summer. The attic, crawl spaces, and eves, were insulated with cellulose. The pipes, end caps, knee wall, sump pumps and all identified areas were sealed, while my energy bills were reduced significantly, I could not get the bedroom cool on the hottest days. In the winter the passive solar helps and I keep the house at 67 degrees Fahrenheit, which the heat pump has never had any problems maintaining. This is an opportunity to make sure that the heating and cooling system are sized and ducted optimally for my house and lot. The Manual J calculation showed my existing heat pump to be slightly undersized for the house.  The Department of Energy has lots to say about ducting problems with air handling systems.  In a typical home, about 20% of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts. The result is higher utility bills and difficulty keeping the house comfortable, no matter how the thermostat is set. The heating and cooling represent 40%-50% of power use in the typical American home.  An analysis of my electric bills showed that the heat pump operated on average about 7 months a year and that I spent about $1,260 annually operating the system. (My electric rates have been steady for over 5 years and my solar panels supply all my other electrical needs.)

 Most manufacturers advertise energy savings of up to 35%-75%; using an average existing system as a starting point and converting to a geothermal system. DOE states that with an energy star system,  it is possible to save 10%-20% of energy cost from an existing system, giving an implied savings of 15%-35% for a geothermal system versus a new energy star system. If I assumed that the geothermal heat pump would save me 50% of the electricity used for operating the heat pump that is about $600 per year.  There are several calculators on manufacturer's web sites to perform better calculations. I found the Bosch calculator and used it  for projecting savings from a geothermal system as compared to an EER 13 air to air heat pump.  The Bosch website calculator gave me a savings of about $971 with $295 of the savings from hot water heating using inputs for a well-insulated home in the Washington DC metropolitan area converting to a geothermal heat exchanger from a propane heated water and air heat pump. So my back of the envelope calculation was not a bad guess and the hot water heating cost is an important element in the cost calculation.

 Until December 31,2016 a 30% federal tax credit is available on the total cost of a qualifiedheat exchanger, reducing the capital cost. The largest hurdle to the widespread adoption of GHP technology is the one I am facing now- the capital cost for initial installation. The heat exchange loop portion of the GHP system can be half or more of the overall geothermal heat pump system cost (and equal to the total cost for a traditional furnace and air conditioner). However, the geothermal heat pump requires st least 75 feet of tubing (in my case either vertical wells or standing column wells) for each ton of size. The costs I have been quoted were $3,000-$4,000 per ton for installation of the heat exchange loop or well.  The difference in cost was the amount of damage that would be done to my garden. If indeed it is a 4 ton system that the house needs, the additional cost of the geothermal heat pump would be a minimum of $12,000 and could be as much as $16,000 plus any costs to reconfigure piping in my completely finished basement. Even with a 30% federal tax credit for the entire system the payback might take 10 or more years if the actual savings turned out to be 50% of the electricity used by the air heat pump system.

It now appears that this decision is a close call and I need to get detailed proposals to determine the actual cost, the damage to the house and garden, the Coefficient of Performance, COP, and Energy Efficiency Ratio, EER to obtain a better estimate of capital versus operating costs. Also, I need time to think about the benefits of an absolute reduction in energy usage while still maintaining my creature comforts. Installing the right size equipment for the home is essential to getting the best performance and comfort, and now is my opportunity to verify that the new system I install is sized correctly for the house and lot. A system that’s too large will not keep your home comfortable because of frequent ‘on/off’ cycling, but a system that is too small will not be able to cool the house on the hottest of days. Also the duct system which has already had all the leaks sealed needs to be evaluated for adequacy and optimal layout. The system selected will have an impact on reliability- at least according to Consumer Reports.  Finally, I need to make sure that the HVAC contractor I hire has insurance, contractor’s license without complaints, and good references for similar sized and types of projects. 

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