I just had mine serviced. Make sure you clean the filter regularly and depiending on unit my technical said that sometimes a sensor needs to be changed out especially with the snow we had.
Ee had the same problem when first installed. The blower was set too high. A technician came and reset it. The blower output was so high and fast the air passing through the heater did not have time to heat. It was an obvious installation mistake and Brennan reset it at no charge.
Check the settings on your thermostat
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