Three Most Common Mistakes Made by Geothermal Designers

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Working in the ground source heat pump (GSHP) industry, you will find yourself answering the same questions with each new customer. Get used to it. If you work in the industry long enough, you will be asked every question under the sun. How will this system heat my home when the soil temperatures are only 50°F (10°C)?  What if I want to keep it warmer in my house? Do I need to buy an air conditioner too? How does a heat pump work? The list goes on and on.

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The truth is, the average homeowner doesn’t understand how their central air conditioner works (or the refrigerator in their kitchen for that matter). They just know it works. When you mention the term “geothermal heat pump” or “ground source heat pump” to your prospective customer, all of a sudden you find yourself in uncharted territory. Then the questions begin.

Homeowners ask these questions because they want to understand what they are getting for their extra dollar. You are asking them to spend a lot more of their hard earned money than they would have to spend on a conventional heating and cooling system. Because of the price tag, the homeowner will ask questions to get comfortable with the idea of purchasing such a system, that they aren’t buying into some new fad that will be here and gone before they know it. Most importantly, they want to get the sense that you know what you are talking about and that they can trust you with their money, especially when they are spending it on something they don’t completely understand. These systems are a complete unknown to a vast majority of people, although that is changing.

Even if you are able to land the big sale, an improperly designed or installed system can be huge detriment to your company (and to our small industry as a whole). Most experienced contractors and designers have made their mistakes, learned from them and moved on. But everyone needs to start somewhere. An expert in anything was a beginner in that subject at one point in time.

In my opinion, three of the biggest mistakes that a new designer can make are:

    1. Underestimating the importance of accurate peak heating and cooling load calculations
    2. Not giving proper consideration to the many options that are available and
    3. Overcomplicating matters.

1. Underestimating the importance of accurate load calculations

If you’re involved in this industry in any way, you already know that the biggest hurdle we must overcome is the up-front cost associated with GSHP systems. Because of this, we need to be accurate when sizing and selecting equipment.

It is not uncommon to see a natural gas-fired furnace with a 60,000 Btu/hr output rating down in the mechanical room when a mere 30,000 Btu/hr heating output is required. The thing is, it really doesn’t cost that much extra to install a furnace that is twice the size it needs to be.

The price difference between putting a 30,000 Btu/hr furnace and a 60,000 Btu/hr furnace is probably a few hundred dollars. Because of this, it is common practice to size conventional HVAC equipment based on rule of thumb principles. Such principles minimize the amount of up-front time and investment on the part of the contractor to size and select the equipment while still being able to install a system that will serve its purpose.

If you install a 5-ton unit coupled with a “5-ton” ground heat exchanger where only 2.5 tons is required, the installation cost will be much higher than necessary. Severe heat pump oversizing can cause a number of issues related to comfort, efficiency and equipment life expectancy, but the main concern is the fact that the customer will be paying much more for the system than necessary, which will dramatically affect the payback period on their investment.

Even if you’re able to sell a grossly oversized system, you might say, “No big deal, I was able to sell a larger system than necessary. My bottom line looks pretty good”. But when the homeowner realizes they aren’t as comfortable as they were hoping for and they aren’t recouping their initial investment as quickly as they were promised, they’ll tell their friends that geothermal isn’t all it’s cracked up to be. The same goes for severely undersized systems.

Properly sizing equipment to deliver maximum performance and occupant comfort while keeping costs in check is the only way to keep our industry moving in the right direction. The importance of high quality and great value cannot be stressed enough. If you stick to the principles of proper system design, it’s unlikely that you’ll ever be the cheapest bidder but I guarantee that many homeowners will choose quality over price. It is your job to inform them of the value you bring to the table that the other guys don’t.

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2. Not giving proper consideration to the alternatives

It would be difficult, to say the least, to install a horizontally-trenched or horizontally-bored ground heat exchanger (GHEX) in an area where granite exists at the surface. It could be cost prohibitive to install a vertically-bored GHEX in an area where drilling through a highly fractured rock formation that produces a lot of water would be part of the process, especially when ample land area is available for the installation of a horizontal loop field.  Limiting factors aside, any type of closed-loop GHEX (vertically-bored, horizontally-bored, or horizontally-trenched) can be designed to function properly in any type of soil for a given location. As far as the heat pump is concerned, it really only cares about the water flow rate through the coil and water temperature entering the unit itself. If you could design any number of different GHEX configurations, all with the same pumping power requirements at a given flow rate while delivering the same water temperature to the GSHP, system performance (efficiency, capacity and resulting electricity bills) will also be the same.  When it comes to closed-loop GHEX design, the options are limitless. A four-pipe horizontally-trenched system can be designed to deliver the same performance as a vertically-bored system. A system that uses a slinky pit can be designed to deliver the same performance as a horizontally-bored system. A vertically-bored GHEX with standard grout can be designed to deliver the same performance as one that uses thermally-enhanced grout. Granted, for most situations, one configuration will require installation of more pipe material and have higher pumping power requirements than the next, but you get the point. Whether dealing with highly conductive limestone versus dry sand or a vertically-bored GHEX versus a horizontally-trenched GHEX, etc., as long as factors that are critical to design are properly accounted for, your system design will work.  The goal in GHEX design is to deliver a certain level of system performance for the lowest up-front cost to the customer. Any designer who can strike that balance will be two steps ahead of his competitors.

3. Overcomplicating matters

GSHP systems are extremely reliable, yet very simple systems. Their reliability is due to a number of factors. A packaged GSHP unit has a relatively easy life because it spends the entirety of its existence tucked away in an indoor mechanical room. It won’t be exposed to adverse weather conditions, dust or salt-laden air or have to deal with the problems caused by grass clippings being blown into the coil during weekly lawn mowing occurrences in the summer.

The true beauty of these systems though, is in their simplicity. When you get down to it, we are dealing with a fairly straightforward process of moving heat from one place to another. Because of this, there aren’t that many complicated components that make up the basic, residential GSHP system.

However, all of the benefits that these systems have to offer can quickly be negated by complex (and inherently confusing) design or controls. When supplemental heating is relied on frequently due to daytime setback on programmable thermostats or improper fresh air system design, operating costs will be higher than they could be. When circulating pumps are oversized (even when VFDs are used), pumping efficiency along with overall system efficiency suffer. If a unit short-cycles frequently due to improper sizing or control, its service life and ability to maintain the comfortable levels inside the home can be reduced.

Conclusions

The best commercial HVAC system currently possible is a simple one-heat pump, one-loop, one-pump GSHP (that looks very much like a residential system).

When not implemented properly, complicated controls and piping/pumping arrangements can quickly compromise the many benefits that GSHP systems have to offer. Also, it usually goes that the higher the level of complexity, the higher the price tag will be for your system.

Another thing to keep in mind is that as more complexity is introduced to a system’s overall design, the harder it will be for the installing contractor to match the intentions of your original design.

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