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Question
Gas vs Geothermal for heat?.

OK. We have a Geothermal system with forced air, humidifier. We have forced heat (never on), and a 4 zone radiant floor system which heats the house. We have basement, main floor, and upstairs with a 4 zone radiant floor system. The Geothermal unit heats a GEO-Stor (50 GST) 50 gallon storage tank via a single grundfos circulating pump. Then 4 Grundofs ups15-58fc distribute the hot water from the Geo-Stor through the radiant floor.

Works quite well, but the energy cost in the winter is starting to get out of hand.

We  pay $.112/KWH, and about $9/mcf. I think I did the math right, and I should be able to save 50% by switching to Natural gas for heat.

My question. Thought was to put a tankless water (like Takagi TK-4-IN) heater at the outlet of the Geo-stor storage tank (and between the tank and the 4 pumps) and turn the geothermal unit off. Maybe even put some valving in place to bypass the storage tank. That way I stay flexible if prices swap again.

Thoughts?

Answer
At $.112/kW your  electricity come to $32.81/mmbtu and if you pay $.90/therm that is $9.00/mmbtu. This is a fairly common comparison where natural gas is available. However, you have a geo-thermal--more correctly--a ground-source heat pump and should be enjoying a uncertain coefficient of performance (COP) in the 2 to 4 range. Meaning, that your real cost-per-unit of electricity should be half to a quarter of your initial cost.

Doing the math then, with a conservative estimated COP of 2, the cost of electricity should be $16.40/mmbtu...not so good. If your system were perfectly installed and sized to the load, with the perfect emitter (radiation) for instance, a properly insulated, uncovered, radiant slab, you may expect a COP of 4, in which case, the cost of electricity should drop to the equivalent $8.02/mmbtu. This number assumes a lot. An expert technician can test the efficiency of your current ground source heating system and give you the real numbers. This is the only way to know for sure, and why I have long been a skeptic of the application of ground source heat pumps for residential applications in cold climates, where natural gas is available.

Where low temperature radiation is employed e.g. radiant floors, ceilings, walls or even properly sized wall-hung panel radiators, a natural gas-fired condensing boiler is almost always the best investment. Tank-less water heaters are just that, water heaters, and for the most part are not a good match or in any way superior to a properly sized condensing boiler purpose-built for low temperature heating systems such as radiant floors.

http://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/ground-source

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Morgan M. Audetat

Expertise

Radiant panel heating design, including floors and ceilings, European wall panels and snow melting for residential and light commercial buildings..............................................................-Master of Steam and Hot Water,City of Minneapolis....................................................--- Master Plumber, State of Minnesota...........

Experience

30 years..... Charter Board Member - Radiant Panel Association, former manufacturer of condensing boilers, former national distributor of radiant floor products, son and apprentice of mechanical & plumbing contractor. Current owner---www.BadgerRadiantDesigns.com---Minneapolis mechanical/plumbing contractor specializing in hydronic based, integrated HVAC systems. Radiant floor heating/cooling. Snow Melting. Condensing Boilers. Indirect water heaters. System design, consulting and technical training world-wide. Licensed contractor, designer, installer and consultant.

Education/Credentials
B.S. University WI 1981 CONTINUING EDUCATION: Viessmann Condensing boiler and Solar water heating certification 2010, N.D.S.U. Lead Worker RRP certification 2010, Knight condensing boiler certification 2009, Wrightsoft Manual J CAD certification 2009, RPA Designer & Installer Certification 2008, Nate Hydronic/Forced Air Certification 2008, Uponor/Wirsbo advanced design school certification 2007, Buderus Wall-hung certification 2007, Power Limited License (low voltage controls) 40 CE credits 2005, Basic Hydronic Certification IPEX & Northern Alberta Institute of Technology 1998, Charter Board Member - Radiant Panel Association 1994, Residential Off-Electric to Hydronic Conversion Heating School Canadian Hydronics Council 1994, B&G Little Red School House 1993, Rood Utilities (now Auburn Technical Institute) Oil Burner School 1993 ,Tekmar Controls residential and commercial 1993, Division Manager and Advanced Hydronic Seminar Instructor for the first Exclusively Hydronic Radiant Floor Distributor in the USA 1990, Hydronic Radiant Heating Association Workshop participant with Richard c. Bourne, PE spring 1988, Master Plumbing, Hydronic, Solar Course Red Rocks Community College 1987.

Awards and Honors
2009 System Showcase Award - Radiant Panel Association

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