Q. We own a ground thaw unit that is specified to thaw up to 6000 square feet per application (1 foot of frost in 24 hours). But over the past two winters, we have averaged only 4000 square feet per application. The manufacturer of this unit says the process is to lay the hoses on a 2-foot spacing and cover it with a 6-mil poly vapor barrier followed by three layers of insulation blankets (in a crisscross pattern).
Our process differs only in that we will lay the poly directly on the ground, then the hoses on top of the poly, and then the three layers of blankets. This means that the hoses are clean when thawing is complete.
Salesmen and equipment reps have told me that we need to keep the vapor above the hoses in order for the ground thaw to be efficient. The purpose for the poly is to trap the moisture which helps the ground thaw. Our process still traps the moisture below the vapor barrier, and the blankets still drive the heat into the ground, and our hoses are clean at the end of the day. Is our process so much different that we are losing 2000 square feet of thawing?
A. We asked Mike Squire at Ground Heaters what he thinks about this. He notes that the answer is really a matter of preference and time/budget allowed to complete the project, but it is helpful to understand the ground thawing process. As the frost melts, the resulting water vapor condenses on the underside of the poly film and drops back onto the surface of the soil. This water is warmed by direct contact with the hose or the warm soil adjacent to the hose. Moisture is a key ingredient when conducting BTUs deep into the soil, so starting out with the warmest possible water makes practical sense.
But the other significant factor is thermal conductivity. While laying out the hoses for ground thawing, you might notice that the uneven soil contacts the hose in short segments of random length (at the high spots), which provides poor connection to the earth for the transfer of BTUs.
Squires recommends adding water until standing in order to maximize the hose-to-soil bond, which maximizes thermal conductivity. In addition, as the soil warms and softens, the hose will settle partially into the mud where BTU transfer will flow as fast as the soil type allows. Messy, yes, but very efficient. Placing the poly film under the hose will reduce this bonding or grounding to the earth by supporting the hose over voids in the soil and contact loss caused by wrinkles in the poly. This limited contact bonding will slow the transfer of BTUs to the frost and retard the thaw process.
So, you can have cleaner hoses at the end of the thaw, or you can complete the thaw up to 50% faster but you will have to clean the hoses afterward. Most contractors report that the more cost-effective method is to thaw faster and clean the hoses afterward, getting them off the thaw site sooner. (Ground Heaters can be contacted at 231-799-9600 or at www.groundheaters.com.