Is airflow in modulating ducted heatpump lower or higher than conventional furnace?
I’m trying to square two sentiments from GBA articles pasted below (I’ve seen similar sentiments expressed numerous times and they appear to somewhat contradict):
From Jon Harrods piece (https://www.greenbuildingadvisor.com/article/191335)
Compared to oil and gas furnaces, heat pumps require more airflow per Btu of heating capacity. An older natural gas furnace might be perfectly happy with 100 cubic feet per minute (cfm) per 10,000 Btu of rated input; a high-efficiency condensing furnace needs about 150 cfm per 10,000 Btu. But heat-pump air handlers typically require 300+ cfm per 10,000 Btu of nominal output. This is because heat pumps deliver cooler air, typically around 95°F, compared to the 105°F-140°F air typical of a fossil-fuel furnace. Each cfm of air contains less heat energy, so more airflow is needed.
From Patrick McCombe’s piece (https://www.greenbuildingadvisor.com/article/high-tech-heating-and-cooling-systems)
Quieter operation. Inside, the system is running at a lower speed, so there’s no woosh of air rushing through the registers.
I have seen Patrick’s sentiment expressed elsewhere where I have interpreted it to mean that a variable capacity heat pump will have more steady airflow but less velocity*. Is that incorrect? Is he just saying that on low load days the airflow won’t be as high as on high load days… but that on high load days the airflow requirements are actually higher?
*Of course, larger ducts could reduce velocity needs while maintaining CFM, but that has nothing to do with variable speed equipment, only the duct design, so why mention it in relation to the equipment?
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