The first part to understanding any aspect of the geothermal industry, whether it be marketing, sales, design or installation is to understand how the technology works and it is designed. By understanding the design process, even if you are never going to do design yourself, you will better understand how the technology is sold, will be confident when speaking to customers, will know what information needs to be collected on a site visit, and what potential leads have greater potential then others. If you plan to be working on the installation side, understand design will give you the knowledge to understand the different parts of an installation and what those may cost. Understanding the design process is needed for anyone working in geothermal.
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Here are the four basic steps to geothermal heat pump design. For this article, we’ll focus on a single family residential building.
- Heat Loss/Gain Calculations
- Size Heat Pump
- Size Loop Field
- Size Air/Water Distribution Center
Step 1. Heat Loss/Gain Calcuations.
The industry standard for residential buildings is the ACCA Manual J. According to Ryan Carda, not spending enough time on heat loss/gain calculation is one of the top mistakes that geothermal designers will make.
Let’s focus simply on heat loss for this example and not get into cooling. The ACCA Manual J will provide you with both a block load for the entire house and a room by room heat loss estimate. 16 to 23 BTUs per square foot per hour is a standard rule of thumb. The main drivers are climate location and building quality, though a building site can also have a small impact particularly if the building is looking to harvest passive solar.
Therefore we can assume that a building that is 1,900 square feet in Maine could have a peak heat loss of 20 BTUs per square foot per hour. Meaning our maximize heat loss, what we’ll use to size our heat pump with, will be 1900 multiplied by 20, equaling 38,000 BTUs per hour maximum heat loss.
Step 2 Size the Heat Pump
We need to produce 38,000 BTUs/h to meet the heating demand and will use this number to select a heat pump that can meet the load. 12,000 BTUs per hour is one ton of capacity, so we will need a 3 ton unit. Keep in mind that the nominal rating for heat pumps from different manufacturers will differ and not all 3 ton heat pumps will produce exactly 36,000 BTUs per hour. However, this is a solid estimate. You’ll need to check the product specification sheet for each manufacture to find heat pump that meets you heating demand.
Step 3 Size the Loop Field
The key variables when sizing the loop field are water temperature and the amount of water. For this example, we will only be discussing closed loop systems. It is standard practice to size a loop field for a minimum entering water temperature (EWT) of 30 F in the worst case scenario.
The main considerations that drive how many feet of bore you’ll need or feet of horizontal tubing is the deep earth temperature in your region, soil characteristics and site characteristics. In northern, heating dominate climates it’s a standard rule of thumb to need between 150 and 200 feet of vertical bore per ton. We specified a 3 ton unit in step two so we can assume we’ll need 600 feet or bore (200 feet times 3).
When sizing the dimensions of the loop itself, you’ll want to keep in mind the flow rates that the heat pump will need. This will impact pipe dimensions and the circulating pump that you size to circulate the fluid. Each ton of heating capacity will need 3 gallons per minutes (GPM) of flow. 3 tons equals 9 gallons per minutes.
Step 4. Size the Distribution System
Industry standard is to use ACCA Manual D to size duct the duct system if you are using a water to air system. You will also need the ACCA Manual J room by room heat loss calculation to size the ducts. 400 cubic feet per minute (CFM) per ton is the standard amount of air flow needed. Thus, we’ll need to supply 1200 CFM (400 times 3 tons) to this house. Also, you must heat each room proportionally. Thus, if the bathroom accounts for 10% of the heat loss of the whole house (see ACCA Manual J room by room) then you will need to supply 120 CFM to that space (1200 multiplied by 10%).
Advanced Topics that we did not cover.
This was a quick overview of the basics on design and did not cover many more advanced topics. A couple, but not all, include:
- Small Unit Vs Large Unit. Many times during heat pump selection you will have a heat loss of a house that is between a smaller unit (3 or 4 ton) and a larger unit (6 ton) and you need to make a decision between installing a small unit that will need to use electric heat at times, but have a smaller and cheaper loop field or a larger unit that will be able to produce plenty of BTUs but have a large and expensive loop field.
- Ground Loop Trade Off. The ground loop is the most expensive part of the geothermal installation and reducing its length will decrease installed costs substantially. By using thermally enhanced grout, designers can increase the thermal conducitivty of the loop, allowing it to extract more BTUs from the ground faster, meaning less feet of bore needed. However, thermally enhanced grout is expensive so its key to understanding the trade off between more grout and less feet of bore.
- Typical Installed costs. Geothermal installed costs vary greatly depending on the market, new construction versus old construction, or if the system is using a vertical or horizontal loop field.