Residential Furnace Blower Performance

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The objective of this study was to assess the performance of furnace blowers and the potential cost-effectiveness of setting performance standards and changing motor technologies.

In this study, a testing program was undertaken at Lawrence Berkeley National Laboratory and the PG&E test laboratory in San Ramon to compare the performance of furnace blowers. Five different combinations of blowers and residential furnaces were tested for air moving performance. The laboratory test results for blower power and air flow were combined with DOE2 models of building loads, models of air conditioner performance1, standby power, as well as igniter and combustion air blower power.

Energy savings and peak demand reductions in this study are based on replacing a Permanent Split Capacitor (PSC) blower, dominant in the market, with a Brushless Permanent Magnet (BPM) blower2. Annual energy savings for a typical three-and-a-half ton air conditioner with typical California ducts are 45 kWh. Peak demand reductions are 50 W per system, or about 0.13 GW statewide if all blowers were replaced. The numbers improve significantly for duct systems that match manufacturers' rating points. For such systems, annual energy savings increase to 153 kWh, and peak demand reductions to 70W per system, or about 0.18 GW statewide when all blowers are replaced. The payback time assuming no utility rebates for typical system is 13.5 years and for an improved system that meets manufacturer's specifications is reduced to 4 years. If fans were operated continuously—for mixing, filtration or ventilation purposes, the potential savings for BPMs improve even more, about 1800 kWh per year and the payback time is less about 4 months.

However, the benefits of the variable speed BPM motors were found to depend strongly on interactions with the rest of the thermal distribution system. The high air flow resistance of typical California duct systems almost eliminates the advantages of the BPM motors. As a result, BPM blowers may not be cost-effective because the potential improvements blower performance are restricted by the static pressures prevalent in residential thermal distribution systems in California.

It is possible to obtain better performance from BPMs and interactions with the rest of the HVAC system were found to be very important. Key areas for improvement include:

  • Use of larger or multiple returns
  • Use of low pressure filters – promote the use of 4 inch deep pleated filters Use of larger air conditioning coils to reduce coil pressure drop3
  • Use of more compact duct systems with shorter duct runs
  • Careful installation of ducts to reduce number of elbows and make duct runs as straight as possible
  • Encourage use of sheet metal duct instead of flexible duct

In conclusion:

  • BPM and PSC blowers have distinctly different performance characteristics that must be accounted for when proposing performance specifications
  • A performance specification should include both an air flow specification and a cfm/W specification.
  • One important utility issue with BPMs is their lower power factor. For PSC motors, the power factor ranged from 0.7 to 0.9, with the lower power factors at high-speed settings and high pressures. For BPMs the power factors range from 0.53 to 0.62 with higher power factors at higher speed.
  • As well as the external static pressure effects, the air flow patterns within blower door cabinets also affect performance.

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Lawrence Berkeley National Laboratory