Assessing ductwork for heat pump installations

By Gord Cooke

Every day more and more compelling support and resources are available to help the HVAC industry to move as many buildings as possible toward heat pump focused space heating and cooling. Let’s focus our attention in the residential sector where, according to the 2021 Statistics Canada Census, there are almost 15 million private dwellings in Canada.

Sixty-four per cent of those are single detached, semi-detached or row houses. In that same census, approximately 51 per cent of households reported they had a forced air furnace for primary heating. Thirty-eight per cent of all households reported they had a central air conditioning system, and only six per cent reported they had a heat pump. This represents a tremendous opportunity for the HVAC industry.

Certainly, there are barriers or challenges in many of those applications, some perhaps daunting enough to exclude a small number of homes for heat pump consideration. However, it is important for all professional HVAC contractors and their supply chain partners to do thorough assessments in each potential application and, if needed, take remedial action to correct or overcome any barriers that would disqualify a home for a heat pump installation. One of the potential barriers could be the existing distribution system having shortcomings that could lesson the overall effectiveness of a heat pump.

Existing distribution systems

The introduction of a heat pump into a distribution system that has served a natural gas or oil furnace is facilitated by these basic measurements: the external static pressure on the return and supply side; the pressure drop across the filter currently preferred by the homeowner; the airflow of the existing furnace; whether the furnace will remain in place at its range of fan speed selections; and the temperature rise and fall across the furnace and evaporator coil in heating and cooling speed.

If the existing furnace is to stay in place as the air handler and back-up heating source for a new “add-on” heat pump, the pressure and airflow measurements help ensure the proper selection of the heat pump size and the best match up for the indoor coil. If a new heat pump air handler or even a new fuel fired appliance acting as the air handler is to be installed, these same pressure and flow measurements indicate the current capacity of the duct system and would indicate if there is a need for modifications or additions as required.

Static design pressure

In my experience the most-commonly used total static design pressure across the air handler has been 0.5 in. of water column (w.c.). New furnaces and air handlers are capable of delivering their design airflows at static pressures approaching 0.7 in. w.c. If the measured static pressure across the existing furnace or air handler duct work is less than 0.7 in. w.c. the higher airflows required of larger capacity heat pumps can often be accommodated without major duct modifications. However, you will encounter distribution systems that will need modifications to accept the higher flows and meet the higher comfort expectations of homeowners.

For example, the supply air temperatures off heat pumps is typically lower than what customers were used to with their fuel fired appliance and the airflow delivered to individual rooms may need to go up to deliver adequate heating. This prompts action on at least two fronts. First, duct leakage studies indicate that even in properly installed sheet metal duct systems 20 to 30 per cent of the total system airflow is lost through duct work leaks before reaching supply outlet grilles and as much as 50 per cent of return airflow is drawn through duct leaks rather than from the return grilles.

In the heat pump switch we are most concerned by the supply side losses. You want more efficient and precise delivery of air to rooms and spaces. By applying the aerosolized duct sealing technology you can expect supply side duct leakage to drop to under five per cent. This is a helpful enhancement to ensure the best possible air distribution for the new heat pump system.

Care should also be taken to ensure adequate delivery of air to individual rooms. In your initial assessments of a home’s readiness for a heat pump, after completing the pressure and flow tests at the furnace, measure the supply airflow to critical rooms such as the primary bedroom, kids’ bedrooms, kitchens, and rooms over garages. Specifically measure flows in rooms that homeowners have identified as not meeting comfort expectations.

Duct modifications

There will be some applications where more extensive duct modifications are required to accommodate the additional air flow of heat pumps. Use the initial pressure measurements noted above and then take additional measurements along any accessible ducts to indicate significant pressure losses of fittings or transitions to focus on.

Duct distribution challenges are one of a number of possible barriers to the widest possible adoption of forced air heat pump systems in existing homes. Professional HVAC contractors will use simple pressure and flow measurements to diagnose and resolve these issues so as to not miss out on this opportunity to reduce greenhouse gas emissions.

Case in point

For example, the existing furnace is found to be delivering at the highest fan speed setting 900 cubic feet per minute of air (CFM) and at that speed the total external static pressure across the system, including evaporator coil and the current filter in addition to the supply and return ducts, is found to be 0.6 inches of water column (in. w.c.) or 150 Pascal (Pa). In order to maximize the heat capacity of the heat pump, you would like to install a three-ton heat pump system. The nominal airflow requirement for this system would be 1,200 CFM and there is a legitimate concern about the ability of the distribution system to handle that extra 300 CFM of flow. A closer look at static pressure measurements reveals that the supply air duct side is only responsible for a pressure of 0.08 in. w.c., while the return duct is 0.15 in. w.c. and the filter has a pressure drop of 0.20 in. (as shown in Figure 1). The filter and the return duct account for 0.4 in. w.c. or almost 60 per cent of the pressure drop across the whole system, while the supply side of the duct system is well below a typical design pressure of 0.15 in. w.c. It’s always easy to blame the filter choices made by homeowners, but let’s recognize the filter aisles at the big box stores are dominated by filters with excellent filtration efficiencies but really high pressure drops. Look too at the location and size of the filter and the return drop. A 16” x 25” filter has a surface area of 2.8 ft² and is best suited for up to 1,000 CFM of air. The new 1,200 CFM heat pump is better suited to a 20” x 25” filter to keep the face air velocity of the filter below 350 fpm. Relocate the filter to the drop and create a filter cabinet that accepts a 20” x 25” by at least four-inch-thick pleated filter. Your material supply partner can find you a MERV 8 or better filter with a starting pressure drop of approximately 0.1 in. w.c. As you re-make the return drop, use a long sweep elbow (A) to connect to the air handler return or if space is a constraint install turning vanes in a squared off elbow (C) to reduce the pressure loss at this critical location. By remediating the return side of the system, you can reduce the pressure loss by as much as 0.15 in. w.c. in this example and that static capability now becomes available for increasing the flow through the supply side.

Gord Cooke is a professional engineer who has spent 35 years helping builders and HVAC contractors implement innovative technologies into high-performance homes. Gord has particular expertise in IAQ and air flow management in houses, and can be contacted at [email protected]