In Geothermal Designs, Don’t Succumb to Rules of Thumb
Abandon preconceived notions and carefully deliver properly designed loop systems
Generous federal tax credits, coupled with the desire to reduce energy costs without sacrificing comfort, have encouraged many homeowners to purchase geothermal heat pumps (GHPs) over the last several years. As customer interest has increased, numerous HVAC contractors have begun offering GHPs as a way to expand their businesses and give their customers another energy-efficient HVAC option.
While many of these contractors have undergone extensive training and took the time to properly design and install their GHP systems, others may not have been so careful. The result has been some disgruntled customers who say that their systems do not produce enough heat in the winter and/or cooling in the summer. In many cases, the poor design and/or installation could have been avoided if the contractors had taken advantage of the numerous training opportunities offered by GHP manufacturers.
DITCH THE RULES
Tom Niesen, division chair of HVACR at Gateway Technical College in Kenosha, Wisconsin, has taught the school’s associate’s degree program for 23 years as well as trained Modine Mfg. installers for four years. He said contractors must be willing to push aside their preconceived notions when designing geothermal projects.
“Many people entering the world of geo bring with them some huge baggage — dozens of rules of thumb they’ve gotten away with for years, if not decades,” he said. “For example, contractors will often ask a homeowner the square footage of the house and then apply some random multiplier to come up with a Btu load. They don’t understand that geothermal is a science and they have to know about the effects of rainfall, ground moisture, types of soil, the conductivity of rocks, and the quality and quantity of groundwater; and, they must be able to model heat flow sometimes 15 years into the future. Geo takes more than being good with your hands, being honest, and showing up on time.”
It comes down to proper design and installation, which are crucial, said Niesen, because many perceive geothermal as an experimental technology where one highly publicized mistake can wipe out 25 good installs.
“Typically, the mistakes happen on residential jobs, simply because of less oversight as well as code officials who don’t know what they are inspecting and homeowners who have nothing to compare a good geo install to.”
Some of the numerous rules of thumb contractors use, said Niesen, include:
• Using Btu per square foot for heating, Btu per square foot per ton for cooling, and/or cfm per ton for cooling;
• Always using 6-inch supply ducts and 8-inch return ducts;
• Doing the math, then adding a 10 percent fudge factor;
• Relying on an electronically commutated motor (ECM) to fix duct static problems;
• Designing ductwork for 0.08 inches water column (wc);
• Believing that leaky ductwork inside the thermal envelope doesn’t matter, because it’s free heat;
• For vertical boreholes, use 150 feet per ton; for slinky systems, use 600 feet per ton; and
• Believing that installing two 150-foot-deep holes is the same as one that is 300 feet deep, etc.
“Are these rules of thumb accurate? Rarely, if ever,” said Niesen. “There are too many variables. Even a job next door can vary greatly. The earth is very unpredictable, and no one has X-ray eyes to see down into the ground to know how it will transfer heat. On some jobs, a hole 20 feet away yields a completely different conductivity.”
As a self-professed “geo junkie” for 19 years, Dave Feyock, trainer, Bosch Thermotechnology Corp., travels throughout the U.S. performing training and diagnostic assistance. Incorrectly sized ductwork is one of the biggest problems he encounters, followed by the rule of thumb that states not to exceed the cooling load by more than 25 percent to handle the heating load.
“A home in western Pennsylvania may have a heat loss of 60,000 Btuh and a heat gain of 20,000 Btuh, and using this rule would imply the designed GHP would be 24,000 Btuh,” said Feyock. “In this case, we would have a 5-ton heating load and a 2-ton cooling load with a 2-ton GHP installed. Western Pennsylvania is heating dominant, and the results of this installation would be less than desirable, as the GHP would be severely undersized for the dominant load. This would result in high energy bills with the need to use supplemental electric heat or fossil fuel, as well as long — if not continuous — run time.”
Rules of thumb regarding the ground-loop system are also commonly — and mistakenly — used by many contractors, said Brian Urlaub, commercial sales manager for Enertech Global, who has been teaching geothermal courses for 13 years. “Many contractors believe there should be one loop per ton at 200 feet per ton depth, or, for horizontal loops, 500 feet per ton of pipe. This is not always accurate because ground conditions change, including soil type, moisture content, and depth (horizontals). Basically, the loop systems end up being installed without enough pipe/bore/trench, and the systems either run hot in the southern climates or cold in the northern climates.”
Better communication between the loop contractor or driller and the heating and cooling contractor could help improve the loop field design, noted Urlaub. “The HVAC contractor sells and sizes the job and then usually tells the ground-loop subcontractor something like, ‘I need a 5-ton slinky loop installed at this address.’ A 5-ton loop is really a rule of thumb, so, instead, the contractor should be saying: “I need a 5-ton loop, which consists of five slinky loops of 800-foot pipe in a trench that is 130 feet long and 7 feet deep. I want 10 feet between each slinky, and if the soil is dry when you start digging, then add an additional loop to make it a 6-ton.’ Better communication between these two individuals will make for a lot more success in the geothermal industry.”
Perhaps the most important part of any geothermal installation is the design of the system, and that begins with an accurate load calculation — a room-by-room calculation for zoned systems and a block load calculation for non-zoned systems, said Bob Pietrangelo, geothermal design, service, and installation trainer for WaterFurnace Intl. Inc. “It’s also a good idea to conduct a whole-house blower door test. Measuring how tightly a house is sealed will speak to the accuracy of the load calculation. It will also help determine if adding insulation and air sealing can increase comfort and reduce equipment size.”
The importance of conducting a load calculation cannot be overstated, as it will impact the loop design and flow center selection, said Pietrangelo. “If the loop is too long, the cost of the installation will increase; if it’s too short, the system will lose capacity and efficiency, resulting in uncomfortably high temperatures in the summer and low temperatures in the winter. And, without proper flow through the unit [1.5-2 gallons per minute per ton on open loops and 3 gallons per minute per ton on closed loops], the refrigerant will not properly transfer heat to and from the loop.”
The proper calculation of heat loss and heat gain is a crucial first step, and, if skipped or done poorly, the results can only be poor, said Feyock. “We urge contractors to perform this measurement on their own or with the assistance of a wholesaler, if needed. Once the contractor is confident the findings are correct, then look at the job and determine which equipment works best for the customer. These can include packaged or split water-to-air and water-to-water systems.”
Once the equipment is chosen (and sometimes even before this point), it’s necessary to look at the loop field options, said Feyock. “Loop lengths can vary widely and may be horizontal or vertical, depending on geographic location and soil moisture content. There are also vertical bores, horizontal bores, horizontal trenches, standing columns, open loop [wells], pond loops, and boiler tower systems. Each one offers unique features, but one is not inherently better than another. The best loop field option is the one that is installed properly, at the least cost, with the correct length of pipe with the proper water flow.”
The best way to ensure the loop field performance is to use software, which is often supplied by equipment manufacturers. This software can not only assist in sizing the GHP, but can also advise the contractor about the amount of loop needed based off of the most common soil profiles found in that specific area and the type of loop desired, added Feyock.
With the equipment selected and the ground loop sized, it’s time to select and size the pumping system, said Urlaub. “It’s also important to pay attention to water quality with closed-loop systems — that includes antifreeze guidelines in the northern climates. Pump failures are on the rise, and it isn’t related to poor pump quality — it’s due to water quality.”
Last, but definitely not least, make sure the ductwork is properly designed and installed, because if this is done incorrectly, the owners will pay through higher utility costs and decreased comfort, said Niesen. “In existing homes, the ductwork is often so poorly constructed and installed that it leaks 25-35 percent of its air from pickup to delivery. We go in and say, let’s rip out that old 10-SEER a/c and convert this home to geo. But, the structure is broken, and geo can’t fix it — in fact, it will be an expensive mistake.”
But, there is no reason to make that mistake if contractors take the time to obtain the proper training. “I’m not talking about a two-hour crash course,” said Niesen. “Start with the equipment manufacturer, then look for training at a local supplier, a post-secondary school, or trade organization. There are concentrated courses for plastic fusion run by the pipe manufacturers or tooling reps, and there are training opportunities in proper grouting, trenching, headering, duct fabrication, heat load, and duct sizing. Start with one and just keep going — the more you know, the higher your stock will rise in your company.”
Publication date: 2/15/2016