Sizing and Designing Your Forced Air Ducting System


We can help you design or evaluate your air ducting for use with a geothermal heat pump, whether you are building a new home, or replacing your existing fossil fuel furnace. Most forced air ducting systems are designed to be used with fossil fuel furnaces, which output air at about 130 - 160° F. A water-to-air geothermal heat pump outputs air at about 95 - 100° F, meaning it is more efficient, but requires more airflow for proper operation. Your air ducting must be able to provide this higher airflow, efficiently, and without being noisy. Additionally, the duct design should allow the proper amount of air to reach each area of your home.

Geothermal air ducting distributor The total air flow through the geothermal heat pump/air ducting must be at the geothermal manufacturer’s rated CFM (cubic feet per minute), and at the rated static pressure. If the air ducting is too small, the air will move too slowly, causing inefficient geothermal operation. Simply increasing blower speed will not solve the problem, since the extra blower energy use will lower the system's efficiency. The only way to get the correct amount of air flow, with acceptable energy use, is to have large enough air ducting.

The air ducting system should also be quiet. The blower noise, or any noise that the geothermal heat pump makes, must not travel through the air ducting system. Using vibration dampers on the supply and return air ducts or plenums, and lining these areas for 5 feet in each direction with an approved sound deadening material, will usually prevent this. Also, room air registers should be sized large enough to let the air flow quietly through them. You shouldn't be able to hear your geothermal heat pump running; you should only know it is operating by the very gentle movements of your curtains.

Finally, the air ducting must provide the correct air flow to each room. Air flow is affected by the air pressure in the ducting, which is determined by the length and size of the duct, and the speed of the blower motor. Also, every room should have a return air register, or else air flow will be interrupted when the room's door is shut. For the best control of room air flow, an automatic duct damper system should be used. Automatic duct dampers open or close air ducts as a room's air flow requirements change, according to the room's thermostat. They allow each room to be kept at a few degrees warmer or a few degrees cooler than the main system setting, if the occupant desires it. They also automatically adjust for the effects of the sun shining on one side of the home, or different amounts of air infiltration from the wind, so that each room stays at the desired temperature.

Air ducts that are installed in unconditioned spaces will need to be insulated to the most recent building codes for their locale. Most new insulation codes require that air ducting in unconditioned spaces be insulated to at least R-11, and be air tight. If air ducting is installed in attic spaces, those spaces must be ventilated so the air temperature will not rise higher than a few degrees above outside temperature. If you are going to install the geothermal heat pump unit itself in any of these unconditioned spaces, you must make sure it is rated for it.