Want to convert our geothermal open loop to horizontal closed loop. Heat pump manufacturer has a publication that assists in deciding what type of loop, length, and selection of circulation pump. It briefly discusses depth of horizontal loop, and provides a graph showing greater depth a more steady state temperature is experienced annually. While searching for trenchers I could rent, the rental price between one that digs 60" versus 70" was practically the same. Am I understanding correctly then, that the 70" depth trench is preferred?
My heat pump manual says that its operating temperature range for closed loop configuration is 25-110 degrees F. I think this refers to the fluid temperature exiting the heat pump. I live in Dayton, OH. Would burying the loop field deeper open the possibility for a lesser ratio of antifreeze to water in the loop fluid?
25-110F range refers to entering water. Leaving water would be lower in heating. With antifreeze the unit will protect itself (lockout) at 15F refrigerant temperature entering the coax coil. In a no flow condition (pump failure) your loop fluid will get there quickly. It would be prudent to antifreeze to 15F.
Probably the biggest impact on entering water temperature (EWT) once you are at or below ~6' deep is the amount of loop in contact with earth plus the moisture available in the soil. And it isn't just the amount of loop in the ground but also the proximity of the loops to each other - the farther you can get the loops from each other and the farther you can get the supply pipe from the return pipe on each loop, the better the earth will be able to transfer heat to the loop pipes. With a trencher, it would be better to run the supply and return lines for each loop in different trenches separated by several feet rather than run both lines in the same trench (i.e. run the supply line out at the bottom of the trench, then back fill a foot or so and loop the return pipe back over the partially backfilled earth). It would also be better to keep the loops separated as far as possible, i.e. 10' separation would be better than 5' separation. If you have enough loop and it is sufficiently spaced and you have sufficient soil moisture, it would be unlikely that your Entering Water Temperature (EWT) would drop much if any below 30 deg F due to the phase change from water to ice that occurs in the water in the soil surrounding the loop pipes at 32 deg F. Here is what Climatemaster says about freeze protection: "Freeze protection should be maintained to 15°F below the lowest expected entering loop temperature. For example, if 30° F is the minimum expected entering loop temperature, the leaving loop temperature would be 25-22°F and freeze protection should be at 15° F (30° F - 15° F = 15° F)."
A deeper loop will raise the EWT which is a good thing. It would be prudent to keep the antifreeze level the same as has been stated above.
