Thursday, July 5, 2012

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.


  1. We need to replace our 15 year old heat pump and are considering a 2 stage. We live in Phoenix AZ and will need the high speed often. One HVAC salesman said we can't use our existing galvanized trunk ducts from 1977 because they won't all enough pressure to be created for system to drop to slow speed. Does that sound right to you?

    1. A technicians tool to estimate your ducts is a tool called a duckulator. This video by Tad could give you a way to estimate what you need. There is an app you can put on your phone. I used it backwards to redesign my ducks to to up my tonnage a bit. Get another proposal. You might want to consider a variable stage.

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    3. this is a good explanation. You can redesign your ducts by utilizing the trunk line and changing the laterals. I find a rigid trunk works best to deliver consistent air flow. Do not be talked into a daisy chain flexible duct system.