Heat pump & supplemental electric heat

[palooka]

I have a heat pump with an Electric Duct Heater (EDH) as supplemental heat. Outside air mixes with return air, goes through the EDH first, then the Air handler. I've heard that the EDH should always be located after the heat pump coil but cant find out why. The heat pump is energized off of space temp at one setpoint and the EDH is activated off a lower SP(3 degrees lower). The heat pump is located in NJ, Design day is 10F.
What are the Advantages/disadvantages in locating the EDH upstream or downstream of the heat pump coil, if any?

[freonrick]

electric heat is always after the hp coil. if you heat the air before the coil you could cause the hp have too high of head pressure and trip head pressure switch.

[Dallas Duster]

do you know if it was designed to preheat the outside air?

[palooka]

I'm not sure if it was designed to preheat the outside air. It is mixing with return air so even on a 10F day the mixed air temp should be approx 45F. So that brings up more questions.
1) Is there a Min air temp that the coil for the heat pump should see?
2) In relation to the high head pressure response, Is there a Max air temp going to the coil? In this case the EDH does not energize until the space temp drops to 69F and then modulates to maintain 69F, so the highest air temp the heat pump coil should see is 69F.

[duke of earl]

Sounds like a preheater to me. A temperature controller measures the mixed air temp and energizes the electric heater to null the entering air temp. Nothing wrong with this as long as it is being controlled properly.

[cartercrew]

Sounds like a preheater to me. A temperature controller measures the mixed air temp and energizes the electric heater to null the entering air temp. Nothing wrong with this as long as it is being controlled properly.

I agree, this is more of an outside air pre heater than supplemental heat for the HP. As long as the HP controls are not bringing the electric heat in and the heater only energizes to bring the mixed air to 69, then high pressure won't be a problem. There is a potential for "cold air" complaints from the occupants if there is no supplemental heat after the indoor coil. When the unit goes into defrost, then the heat that is needed for that defrost is pulled from the indoor air, cooling it. If there is no heat after the coil, then there is nothing to reheat the discharge air.

As for the question of min and max air temp that the coil should see, if the air flow is correct, or at least close, I typically see temp rise of 30 degrees on most heat pumps at 45* outside. That gives about 220 heat for r22. I would say the minimum that you would want to see is about 65, because any lower and the liquid temperature will start getting too low to flash correctly through the metering device (kind of the same thing as low ambient conditions in a/c mode). The max air temp that you would want to hit the coil would be 100* in order to keep the discharge pressures below 300 psi (again, with r22)

The above is just my thoughts, and I have no technical references that I can sight to corroborate it.

[palooka]

I would have thought it was a Preheater also but the EDH controls off of Space temp also not mixed air temp. So if the heat pump can keep the space satisfied it will never bring on the EDH and it is possible that the air temp the indoor HP coil can see it lower than 65F.