The thinking that slinky loops will let you get the same or more geo capacity from a smaller loop footprint is dead wrong. If you were to apply the laws of thermodynamics to ground loop design, it would be clear that there is a limited amount of energy available from a given area of earth, and a limited amount of energy that the earth can absorb. Cramming in more pipe by using a slinky pit will have a number of effects:
1. In heating mode, the loops compete with each other for the same thermal energy, interfering with each others ability to do the intended heat exchange. This results in very low temperatures in the loops in the centre of the pit, and lower overall loop temperatures. We’ve seen these loops running at 20deg F, abysmal performance. The earth is frozen and the moisture is crystallized, air pockets appear and the thermal conductivity nose dives. The system is forced to run on back-up heat, your operating costs go through the roof, and the high efficiency system you invested in becomes a black mark on our industry.
2. In cooling mode, the loop can’t dissipate the heat, so the loop heats up far above design temperatures. The slinky loop has hundreds of potential pinch points. When the pipe is softened by the high temps, and with 5 or 6 feet of earth on top of it, it tends to flatten at each pinch point, causing flow restrictions that decrease system performance even further. We use slinky loops in surface water loops (pond, lake, ocean), and in wet, swampy earth with high ground water flow. We don’t recommend this layout unless the conditions are ideally suited. The Geo Guy has measured a slinky in cooling at 123degF! The heat pump was groaning, continually on the verge of locking out or failing outright.
The tried and true horizontal loop system uses 2 straight pipes in trenches and requires way more space. Other established horizontal designs can use four or six pipes in a trench. Each time you add a pair of pipes, your trench length per nominal ton of capacity goes down, but your total pipe length and antifreeze volume must go up.
Another method is the “parking lot” loop – a pit loop using straight runs of pipe on 2’ centres. This will require a space typically more than twice the square footage of the building depending on the load and the ground conditions. If the loop field is large, temperatures in the centre may be low and impact the overall average temperature. This will be considered when geoexchange experts are evaluating design options.
The ideal design for your property depends on the space available, the ground conditions, and the cost of the system making the geo system site specific – one of the things that makes geoexchange so much fun and the reason we don’t get bored doing this over and over again!
The last word: DRILL! the Geo Guy used to be a proponent of horizontal systems but after a decade of doing geo, please drill! Horizontal loops are almost always in overburden which typically has a pathetic thermal conductivity, unless there is subsurface water flowing, they are still slightly influenced by seasonal temp fluctuations and they occupy a lot of real estate that could be better used.
Drilling, on the other hand, occupies almost no space, gets into the bedrock for great thermal conductivity, the deeper the holes the better, for more bedrock/less overburden exposure to the ground loop. The Geo Guy has designed and installed boreholes up to 600ft, usually with double U-tubes at those depths. Drill, baby, drill!
FLOW CENTRES or Ground Loop Pumps
There are basically 3 types of flow centres, the purpose of which is to pump the fluid and to provide valves to allow flushing of the system to remove debris and air from the ground loop pipes.
The “QT” or non pressurized which has the pump(s) embedded in a storage tank that automatically eliminates air and has some spare fluid available in the tank. They come with cast iron e-coated pumps of a few varying sizes.
The second type is the one shown in the accompanying drawing. They are for pressurized systems and are convenient to install. They come with cast iron or bronze pumps of varying sizes.
This drawing courtesy of Flow Center Products Inc.
This drawing shows a “Pressurized Failsafe Unit” which is an excellent and vital component of a ground loop pumping system. Most always when we go to do an inspection the first thing we hear in the mech room is the sound of air in the ground loop. It’s almost universal.
The irony is that the geo industry, pretty much the ENTIRE geo industry says go ahead and flush the ground loop to get the air out and you’re done. Well, they’re wrong! There is so much micro air trapped in a geo loop, it can never be removed adequately by “purging”. The tiny air bubbles will come out slowly and cause the pumps to cavitate and the system performance to decline. The PFU shown will prevent that.
The third type is the build it yourself type, which is the best IF you know what you are doing. You can spec whatever type of pump you want and you can build in automatic air purge and maintenance features. You can use large pipes and large valves which enables you to use smaller pumps which brings the system COP higher. However, it takes more time to build these things than opening the box of a ready made unit. There are example pics of these somewhere on this site.