Aha ! A heat pump. The plot thickens. Or at least gets more complicated.

Q1 : Where are you ? Heat pumps will only work to about -15C. Their efficiency decreases as you approach this temp. Most are sized for the house depending on cooling, not on heating.

Q2 : Are you sure it's a frost cycling type, or a frost sensing type ? Is there a cutout temp on the unit ?

Q3 : Heating capacity (from the data plate on the unit in the house) ? or from the owners manual ? Floor area ?

Q4 : Any other heating system besides the HP ? You said something about a furnace ?

Perhaps you could get your owner's manual in front of you and we could PM on this. Kinda sounds like you need to adjust the cutout temp and get some secondary heat.