The efficiency rating on that Raypak heater is 82% unfortunately I do not have dB rating.
In regards to the heat pumps.... Heat pumps draw in the outside air to heat the water so as the outside temperature drops the efficiency and effectiveness of the heat pump drops, typically all of the performance data on the heat pumps is set at an outside temperature of 82 degrees. So even if the unit can work at the lower temperature they will be far less efficient then when operating at warmer temperatures.
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