Even with the downturn in our economy, geothermal sales have been relatively solid, and with a rebound in our economy, the pace is picking up rapidly. It is difficult not to be involved with this technology due to the market penetration that is being experienced.

Many contractors who sat on the fence for the past few years are now getting trained in order to keep their customers who are embracing the technology. Ten years ago it seemed the technology was part of a niche market with only expensive luxury homes being able to afford the products. Today, there is a significant change in that mind set and we are seeing more multi-family homes such as townhomes and condominiums, mid-range single family homes, and even high-rise buildings going geothermal.

As with any newer technology, there are bumps and hurdles along the way while contractors learn to adapt and familiarize themselves with these relatively new technologies. One area that needs particular attention is the duct sizing requirements.

Unlike gas furnaces or typical gas fired rooftop units, Geothermal Heat Pumps need to move much more air, in some cases over double the airflow of conventional equipment when trying to achieve the same heating capacities. The available external static pressures may also be lower than conventional equipment posing even greater challenges.

Duct design and duct efficiency play a vital role in the overall life and efficiency of the entire system.

As an example, a 5-ton gas/electric rooftop unit with 100,000 btu/h heating capacity would deliver approximately 1800 cfm in the heating cycle and 2000 cfm during the cooling cycle. It would be fair to assume that the duct system would be sized to deliver the 2000 cfm because of the cooling.

Most Geothermal Heat Pumps can produce between 8000 and 10,000 btu/h of heat for every nominal ton of cooling capacity. Assuming the rooftop unit is 80% efficient, the actual output would be 80,000 btu/h.

Even if we look at  a quality higher output geothermal units and wanted to match the capacity, we would require an 8-ton heat pump requiring 3200 cfm, that’s 38% more air flow!

GeoComfort engineering specifications reveal a typical GeoComfort eight-ton unit’s heating capacity at 30 degrees entering water temperature. The 30 degree entering water temperature is typical of a closed ground loop towards the end of the heating season unless there is a winter cooling load.

Another consideration when designing a system using geothermal is the amount of outside air that can be introduced into the system. While there is no set rule because it largely depends on the outdoor temperature, the mixed air temperature needs to be calculated and sized so that the air going across the condenser (air coil in the heating mode) does not fall below 60 degrees Fahrenheit while in the heating mode. Below that temperature, the refrigerant system is unstable and low pressure lockouts are likely to occur.  In the cooling mode, the system should be designed so that the mixed air temperature entering the evaporator (air coil in the cooling mode) does not exceed 85 degrees Fahrenheit.

Smaller residential systems up to six tons of capacity can accept a 6” diameter duct introduced into the return air system without heat recovery at outdoor temperatures down to approximately -40 degrees F provided the connection is far enough upstream of the blower to provide adequate mixing.

While most contractors tend to use .10” static per 100’ to size their duct systems, large residential systems can have very long runs when the measured length, equivalent length for fittings and equivalent length for the position of the take off on the duct system are considered. In many situations, the total equivalent lengths can approach or even exceed 200’.

Typically, a duct system is designed so that it does not consume more than .3” of the available external static pressure of the system. That pressure is allocated to both the supply and return air systems allowing .15” of static for each system. Some units can only afford to give up a total of .2” of static to the duct system meaning even larger ducts.

The better equipment typically offers electronically commutating motors (ECM) that are capable of maintaining a pre-programmed cfm through a wide range of static pressures, usually up to .7” S.P.

While this is a great advantage, we must allow for other pressure drops such as high efficiency filters that can consume up to 35% of the available static, even when they are clean.

While gas appliances are quite forgiving under these circumstances due to their wide range of operation, geothermal systems are not. Every cfm counts and extra care and attention must be given to the ducting layout and fitting design in the system.  

Additional friction loss must be allocated for acoustically lined ducts. Lining can add upwards of 30% more friction loss due to the roughness of the surface. Filters can add significantly to the pressure drop of the system. In many situations, doubling the filter surface area will improve the overall efficiency of the system and reduce the sound level in the process.

Heating equipment may not always make your duct system look good, but your duct system will definitely make the equipment look, feel, and sound good.