turn off heat pump when cold outside, emergency heat only???

I had a tech come out today to look at my Rheem heat pump that was icing over. He determined that it needs a new board and sensor which he had to order. So I’ll be using emergency heat for a few days. But he also said when it is cold, below 30, I should turn off the heat pump and just use the emergency heat since the emergency heat will come on anyway and no reason to run both the emergency heat and the heat pump. I never heard this before and wonder if anyone here has an opinion about this? I always thought that even when it was cold outside the heat pump contributed some. I live in northern VA which has typical mid Atlantic winters.

 

I let my heat pump run down to 15*. There is still heat in the air below 30*.

 

If the defrost board and sensor are bad then the condenser will not be able to go into defrost mode if it needs to do so. If the temperature is cold enough the condenser will ice up and without a defrost cycle the condenser fan will not be able to pull air through the iced up coil. Some thermostats allow you to lock out the heat pump condenser at the desired temperature setting and only use heat strips for heating.

 

Most manufacturers' specifications show their heat pumps are effective down to 10F or even lower.
I'm near Boston, so experience colder weather than you, and have my Low Temperature Cut Off temperature set at 5F so the compressor runs down to that temperature.
I have the Auxiliary heaters start coming on at 25F for comfort, though in good times the heat pump alone could do the job down to about 20F.

Performance problems seem unavoidable when the weather is humid (say 75% Relative Humidity and up) or snowing, especially wet snow, and the ambient temperature is near or below freezing. These seem to be the worst conditions for the outdoor coil icing up very quickly - so that a defrost is needed every 40 minutes or so.
Because the defrost cycle is so annoying I often force the auxiliary heaters to come on more and sooner, and if the weather is bad just give up and switch to emergency heat.

It's curious that the heat pump designers are unable or unwilling to produce a heat pump that doesn't rely on kicking into house cooling mode in order to deal with the defrost issue ... a real kludge if you think about it.