Most older homes in Seattle were built with belt driven gas or oil furnaces. These old duct systems were sized for relatively low airflow and a relatively high discharge air temperature when compared to current furnaces. Most of these older furnaces had supply air temperatures exceeding 145 degrees. These older duct systems were sized in the pre-air conditioning days and the air returns are generally smaller and not large enough to add A/C or a heat pump system without major modification.
New gas furnaces and air handlers are designed to deliver a large quantity of air with a much lower discharge air temperature. Most gas furnaces discharge 100 – 125 degree air and most heat pumps supply 90 – 100 degree air, so more airflow is needed to supply the same amount of heat as an older system.
Having a more efficient blower design is nice but can lead to excessive air noise in the duct system and especially at the registers and air return grills. Your basic single stage furnace is the main culprit in this scenario. The thermostat calls for heat and the furnace supplies 100% heat capacity and 100% airflow on every call for heat.
The next, and better option, is a furnace that supplies more than one level of heat. Two stage gas furnaces usually come on at 60% heat capacity and 60% airflow on low stage and as long as the furnace is raising the inside air temperature, it stays in low stage. When the indoor air temperature starts to fall while in a call for heat, the thermostat kicks the furnace into high heat and high airflow. This is much better than a single stage, but is still less than ideal.
The best option is a furnace that can fire at a very low rate and supply a very low amount of airflow. York’s Modulating/Condensing (ModCon) furnace is fired at 35% heat and 35% airflow on the first call for heat. This is a very quiet and efficient operating condition and the furnace will only ramp up and produce more heat and supply more air if necessary. The blower motor is an ECM (Electronically Commutated Motor) and uses about as much energy as a conventional light bulb. In comparison, a conventional motor (PSC or Permanent Split Capacitor motor) can use as much energy as 10 light bulbs.
Heating with such a low gas-burning rate along with low airflow makes the system easily retro-fittable into any home, but especially in older homes with older duct systems. It circulates the air within the home almost continually (we actually prefer that the blower is always running) and makes the home much more comfortable and prevents those great big temperature swings that we have all lived with for so many years.
The same ECM motor design also allows us to program the motor to deliver the exact amount of airflow needed to heat or cool the home. We can add a heat pump or air conditioner to this system and provide better dehumidification in the summer and higher discharge air temperatures from a heat pump in the winter. Because the ECM motor is so inexpensive to operate, it is always advisable to add a high efficiency air filter in the duct system to improve Indoor Air Quality (IAQ). The ECM blower is almost noiseless and so quiet that it makes perfect sense to be constantly circulating the air within the home and remove impurities at the same time. The result is an even temperature from room to room and a much healthier living environment.
