Upgrading My Heat Pump and Ducts

Though I had always assumed that when the time came I would replace my heat pump with a geothermal heat pump, that’s not what happened. I am replacing my air heat pump with another air heat pump, a more efficient one, and re-ducting the attic to create a more efficient and effective system. The cost to reconfigure my finished basement ($5,000-$10,000) and install either a vertical coil or standing column well ($12,000-$18,000) combined with technical difficulties, limited cost savings and the potential I might impact the drinking water aquifer or damage my garden ended my plans to retrofit a geothermal unit into my existing home.

Lots of things have changed since this house was built in 2004 (with builder grade system). First of all an air heat pump is usually a split heat-pump systems consisting of two parts: an indoor (coil) unit and an outdoor (condensing) unit. Both units are designed to work together.  Heat-pump systems manufactured today, by law, must have a seasonal energy efficiency ratio (SEER) of 13 or higher while my heat pump has a SEER of 12 and a HSPF less than 8. Seasonal Energy Efficiency Rating (SEER) or Heating Seasonal Performance Factor (HSPF) for heat pump systems are the efficiency ratings on heat pumps, the higher the SEER/HSPF, the more efficient the equipment. The SEER is measured in average Btu output over the season divided by the watt hours and is the standard measure of energy use efficiency. The Air Conditioning, Heating and Refrigeration Institute (AHRI), defines the method to measure SEER. AHRI was formed in 2008 by a merger of the American Refrigerant Institute and the Gas Appliance Manufacturers Association. Generally, the higher the SEER/HSPF of a unit, the higher the initial cost and lower the operating cost. For these new, high-efficiency systems to work properly, the outdoor unit and indoor unit must be perfectly matched, properly sized and correctly ducted to deliver the right air flow.

New Energy Star certified air heat pumps have minimum requirements of a 14.5 SEER, 8.2 HSPF and 12 EER or higher. Air heat pumps are available with Up to 20.5 SEER; and Up to 13 HSPF. Two-stage or variable cooling makes this possible.  The heat pump has a compressor with two or more levels of operation: high for hot summer days and low for milder days. Since the low settings are adequate to meet household-cooling demands on all but the hottest days, a multi-stage unit runs for longer periods and produces more even temperatures. Longer cooling run cycles allows a two-stage or multi stage heat pump to remove more moisture from the air and allows you to size the unit for the hottest day capacity without reducing efficiency. The indoor air handler (the fan) provides the energy to move air through the ductwork to the rooms of your house. The high efficiency units also have a variable speed motor that automatically changes speed based on air flow requirements to maintain temperature settings to eliminate the on/off cycling of the blower.

To properly size a system for a home there is Manual J from the Air Conditioning Contractors of America, ACCA. In truth what there really is are several computer models and an iPhone app available that does the calculations for you. The only problems is the input factors that impact the calculation include the climate; the size, shape and orientation of the house; the home's air leakage rate; the amount of insulation installed; the window areas, window orientations, and glazing specifications; the type of lighting and major home appliances; and the number of the occupants. Slight variations in the input assumptions get different results. In the model I played with, baseline inputs were available based on square footage, orientation, age of home and zip code and then adjustments could be made. The results were no better than my back of the envelope calculation, but I know my house, the square footage, orientation, the additional insulation and window films I installed and I figure that the heat pumps should be around 3.675 ton.  My existing heat pump turns out to be 3.5 ton.  Once the temperature reached 90 degrees in Virginia the heat pump ran continuously and could not keep the master bedroom or the bonus room cool and is probably one of the reasons why I am replacing an 8 year old system. The high efficiency two-stage or multiple stage heat pump allows me to oversize the unit slightly so that it can handle the hottest days without sacrificing optimal performance on more temperate days so the old rule that if a system is over sized, the system will cycle on and off too frequently, greatly reducing its ability to control humidity and its efficiency is no longer strictly true. If you are going with a multiple stage system round up.

An essential element to the efficient and effective heating and cooling of your home is the duct system, and there is the Manual D by ACCA intended to ensure a good design.  Many homes built after 2000 have flexible ducting and this could be a problem in the performance of your system. ASHRAE, founded in 1894, is the leader in research focused on building systems, energy efficiency, indoor air quality and sustainability.  ASHRAE sponsors research a various universities to advance the sciences of heating, ventilating, air conditioning and sponsored a series of studies between 2002 and 2006 that found that the airflow loss in flexible ducting in real world installations was 9-10 times the loss anticipated in the 1999 design standard used in Manual D in most homes built during the last building boom. In addition, the experimental results they also found that with compression ratios exceeding 4% (the minimum compression found in the real world), the duct performance varies considerably with slight variations in the installation. A low skilled, inexperienced or sloppy worker does a poor job that will impact the performance of your system.  

The ducts in my well insulated attic

An examination of my ducts in the attic found a poorly executed installation. I should not be surprised since several of the ducts were not properly attached to the distribution boxes when we first bought the home from the lender. I had the ducts sealed when I added additional insulation to the home. The flexible ducts in my attic are R-6 with a black vapor barrier. The flexible ducts consist of three layers an inner core of a metal helix encased in a plastic or foil film, and insulation layer and the outer vapor barrier jacket. While fully extended properly installed flexible duct can be as good at maintaining air pressure as a galvanized steel duct, performance deteriorates as the ducts sag. In all the real world tests there was some degree of compression or sag (more than anticipated) even in good installations. In poor installations there were sharp bends, excess lengths and significant restrictions due to squishing the duct into tight spaces. When the flexible ducts are compressed (or sagging) the inner layer crumples (it is a soft spring) and the helix pops out. Instead of smooth circular tube the flexible duct turns into a bumpy pathway for the air that causes turbulent flow and very significant pressure drop from the beginning to the end of the duct. In my case, almost no air flow in the bonus room.  The scientists at Berkeley Livermore Laboratory and Texas A & M found this effect to be orders of magnitude above the range provided in the ASHRAE design standards. The reason the drop was so great is that the ducts operate at very low pressure and small resistance due to fitting or duct friction can have a very big impact on flow. The scientists calculated pressure drop correction equations so that systems designer could correct for this effect.  

I did not even bother to look for a Manual D computer program. The solution to improving my duct air flow was simply to install galvanized steel trunk lines and distribution boxes, properly sealed with UL 181 foil-backed butyl tape and with R-8 (or higher) reflective insulation. The trunk lines will have straight runs and gentle curves to the distribution boxes, but I am going to use flexible R-8 to tie into the last foot of the vent sleeves (to avoid replacing all the boots) keeping the transition as smooth as possible. I am going to use reflective insulation at a minimum of R-8 to take advantage of what little boost I can get from the decreasing the emittance of the ducts. Radiant barriers on your ducts work in your attic to prevent some of the heat from the roof from being transferred into the ducts. The idea is to have the radiant barrier or coating reflect some of the heat of the attic space away from the ducts. Oak Ridge National Laboratory, ORNL, found in field experiments that radiant barriers installed in the attic could reduce air conditioning bills in the hottest parts of the country, so hopefully I will get some small boost from it. In addition, I will install a temperature controlled attic fan to reduce peak temperatures in the attic, but allow the attic to benefit from southern exposure heat gain in the winter. The new insulated and sealed galvanized ducts and new properly installed reflective flexible duct supply lines to existing registers will add several thousand dollars to the cost, but should significantly improve performance of the system and the galvanized steel portion of the ducts will last for decades. Total cost $16,400. After the work is done I will have to blow more cellulose into the attic to correct what has settled or was disturbed in the installation.

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.