In our continuing effort to advance sustainability issues, Manley Architects recently updated HVAC systems at our home occupancy location to geo-thermal. The geo-thermal heat pump system was installed by Hendrickson HVAC of Battle Ground, Washington and engineered by Hendrickson’s own Howard Eck.
Geo thermal heat pumps operate at as much as 70% greater efficiency than conventional air to air heat pumps. Here’s how it works – simply stated. Using the stable temperatures found from 4’ and deeper below grade, a geo-thermal heat pump system circulates a refrigerant - typically water with an anti-freeze additive - through pipes in the ground known as the “ground loop”. The refrigerant absorbs or sheds heat through contact of the piping with the earth. The heat pump compressor unit then borrows the ground's heat (or sheds heat from the home if in cooling mode) much like a typical air to air heat pump, but using the earth’s stable temperature instead of outside air to heat or cool. The department of Energy has a decent primer on the basics (Dept of Energy Geo Thermal). Our system includes a 3 ton, 2 stage variable speed Climate Master heat pump, served by 2,400 linear feet of horizontal ground loop. A “de-super-heater” system uses waste heat from the home heating system to heat domestic hot water for further energy savings.
Given our small, built-out existing urban site and it’s distinctly Columbia River Valley soils sub-strates, our project created some real challenges and called for true creativity in engineering.
Optimally on a small footprint site, ground loops for refrigerant water will be “vertical”, grouted into cased wells drilled into the earth from vertical to about a 20 degree angle from vertical. A notable example of a geo-thermal heat pump installation on a small urban site is the design for the “Red Steps” building housing a TKTS outlet at Father Duffy Square, Times Square New York (Green Buildings New York). The TKTS booth, which is housed in an enclosure beneath the red steps, is conditioned by a geo thermal heat pump system served by wells. Our new system originally called for three 250’ deep wells. Our driller encountered cobble at a depth of about 20’. The material encountered is typical of gradated river rock deposited by the Missoula Floods during the last ice age. With its round “bowling ball” shape, the drill bit bounced off the cobble like a pinball in an arcade; entertaining, but unproductive! Drill rigs capable of handling the cobble were not available before the next heating season; it was time for plan B.
For us, plan B was making a horizontal ground loop on a small site pan-out. To make this happen, Hendrickson HVAC used a slinky coil arrangement for the ground loop piping and embedded the coils in pervious concrete at a depth of 4’. The higher rating for thermal conductivity of the pervious concrete increased the exchange of heat versus direct contact with soil just enough to make the system viable.
Our system has been up and running since the end of October 2009. In that time we’ve seen our natural gas usage (water heating and other uses) cut by more than 50%. We’re still evaluating our electricity savings as weatherization of the existing structure continues, but the savings in electric utilities also appear significant.
Along with the energy savings, our local public utility, Clark PUD, offers a $2,000 rebate for installation of a geothermal heat pump. This is typical of programs offered by utility vendors nationwide. This when combined with existing federal tax credits and further incentives and tax credits recently proposed by the Obama administration make upgrades for energy savings and greater sustainability a choice everyone should consider.
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