Ambient Temp vs Exhaust Temp on Home Heat Ducts

http://www.pathnet.org/si.asp?id=205

from tables here you can see the furnace gives a 50-60 delta T and some of that is lost in the ducts so the ducted air is 40-50F above ambient.

A better insulated house can get by will lower volumes of lesser heated air

Skin temp is about 92 F in average conditions. Any air discharged below this temperature will feel "cold" even though it's warmer than ambient air. So, try to get your discharge temperature to at least 100 F. Check the ASHRAE heating guides for more info.

Guest, your question is "the differential," though to get this you have to measure both supply air temperature and room temp.

Since a heat pump is generally sized to supply ~90 deg F during heating operation, and your room will generally be kept at 72-75, the "differential" or delta-T is 15-18 deg F.

(Not 40-50, that's only with a gas furnace providing ~120 deg F supply air)

Since your skin is 98.6F, the supply air will still feel cool to your hand, though it may be capable of keeping the room warmer than comfortable.

btw - If you live in a very cold climate, where you expect temperatures below freezing, know that below about 35 deg F your air-to-air heat pump will not be effective, which is why all-electric units include an electric heating coil as well, sometimes tagged as "EmHeat" or "Emergency Heat" on your thermostat. This is what kicks in when the unit can't get any heat from the outside condensing unit (like a big hair dryer). Often, commercial or high-end units will include accessories to allow them to control whether the system even allows the outside unit to be attempted during cold weather, to save the unit from extra wear or damage when it is not going to be effective anyway.

Geo-thermal/Ground-source/Water-source heat pumps, by the nature of their use of the much higher temperature underground as the heat sink/source, do not have this problem, and keep on using the more efficient compressor system to do heat exchange throughout the winter without having to kick on a big electric coil, which is one more reason why they are so much more efficient as a system year-round.

"...so choice. If you have the means, I highly recommend picking one up." (Ferris Bueller, 1986)

If you wish to make your standard air-air heat pump more efficient, get your favorite mechanical contractor to install one of these for you: FanHandler
It will slow your system fan down to increase the delta-T until the coil is fully working, and slow it back down to minimum to keep the house air circulating full time, which does all kinds of great things for IAQ, including keeping your electronic air cleaner working (you do have one of those, right?) at a minimal electricity cost - usually the system as a whole uses LESS electricity for various reasons 'beyond the scope of this class'.