With ground-source heat pump technology being a central focus of the U.S. Congress, allowing a 30% tax credit for “qualified” residential ground-source heat pump technology, many people are jumping onto the tax credit bandwagon for these expensive systems. The majority of the cost of a ground-source heat pump system is the earth-linked heat exchanger. The design of the heat exchanger can vary between something as simple as an immersed heat exchanger in a relatively deep pond or lake with continuous thru flow, or something as expensive as vertical bore holes drilled between 300- and 500-ft. deep per ton of refrigerant capacity. As any building contractor knows, drilling wells is not an inexpensive proposition. Hence, the need for tax credit subsidization to make these systems economically feasible.
We have a saying in the solar thermal business that goes something like this: “Insulation before insolation.” In other words, the basic load demands must be gotten as low as is economically feasible before applying alternative energy. Otherwise, the expense for the mechanical package, as well as the earth-linked heat exchanger will be even more expensive.
There are some that will argue that having a system with a base efficiency of 300% appears to look good on the outside, but when powered by an electrical system that is only 30% efficient (typical fossil fuel fired power plant seasonal efficiency) are you really reducing the carbon footprint of the dwelling by that much? But that is a whole other topic for another article.
According to a 2004 survey, the average new home is built at around 2,300-sq.ft. of living space, and the average peak heating demand is around 15 Btuh per square foot per hour, equating to an hourly peak heating demand of around 35,000 Btuh. If we were sizing a boiler, this would be our “net output” requirement for the space heating appliance. However, it has been my experience that with the lower heat source approach temperatures, you can only get around 80% of the rated output of a ton of ground-source heat pump out during design conditions. Therefore, the demand must be divided by 0.8 in order to properly size the heat pump to handle peak heating load conditions.
Some manufacturers and dealers of ground-source heat pump systems recommend installing an electrical resistance heater or other means of back up to handle those peak load conditions, shutting the heat pump down, and in the case of electric resistance back up, allowing toaster elements to carry the load during peak demand conditions. Although this may reduce the initial installation costs, and will work to get the dwelling through the typical short duration of the middle to central sections of North America dip to below design conditions, in some cases, this peak kilowatt demand results in residual energy charges (demand billing) that can affect the economics of the operation of the heat pump.
It is important that the consumer be made aware of these potentials and associated costs, so that they don't get a mid-winter surprise from their local utility. Granted, the basic equipment will impart a substantial kilowatt demand to the electrical grid, but the elements, at only 99% efficiency, will impart a significant demand that can upset the economics of the installation. Choose system sizing carefully with a lot of attention being paid to the details of distribution and back up or augmentation heating sources.
So, for the purposes of calculating first costs, using our above example, if the home is to be 100% heated with a ground-source heat pump, then we would take our base space heating load, (35,000 Btuh) and divide by 0.8 to get to the load factor that would allow the ground-source heat pump to carry 100% of the heating demands of the structure, which would result in a need of 44,000 Btuh. If we divide our new base load by 12,000 (the Btuh in a ton of refrigeration) then we see a need for approximately 3.64 tons of heating equipment capacity. Essentially, this will result in a need of three to four wells with a depth of 300- to 400-ft., depending upon numerous local geological conditions. At a current local cost of around $10 per vertical foot (includes grout, tube and placement) then the first costs for the earth heat exchanger is around $14,000. That is a big chunk of change, especially in our tight economy.
Tune in next month as we continue looking into the advances being made in this burgeoning field. Until then, happy New Year hydronicing.
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303-936-7606. All Mark Eatherton material on this Web site is protected by Copyright 2010. Any reuse of this material (print or electronic) must first have the expressed written permission of Mark Eatherton and CONTRACTOR magazine. Please contact via email at: [email protected].
All Mark Eatherton material on this website is protected by Copyright 2009. Any reuse of this material (print or electronic) must first have the expressed written permission of Mark Eatherton and CONTRACTOR Magazine. Please contact via email at: [email protected]