Heat Pump temperature differential

[jweinberg]

Question....

Since a heat pump....when run in the heating mode....is basically a reverse Air Conditioner, does it make sense that the heat differential between return and supply vents should be similar to the differential found when it is run in the cooling mode (15 to 20 degrees)?

[Michael W. Gault]

Nope,

I typically find differentials as low as 6-7 degrees on an operational heat pump... Won't see much more usually unless you activate the "emer Ht"

[Bruce A. King]

If the heat pump is newer, seer 13 and up with variable speed and R410 the supply temp can be as high as 105 deg. with 70 return. 35 deg Diff with no aux heat strips!

I have two of them (bryant/carrier) and the heat strips are programmed to always stay off unless:
1. manually activated with aux heat mode
2. Outside temp less than 35 AND temperature lag of >2 deg.

The old crappy heat pumps are a thing of the past.

[JLilly]

There is basically no set right answer with delta T. As the outdoor temperature dips the DT will be lower. Like Bruce said with variable speed machines it’s even tougher to say what is correct. A lot of manufactures will have a chart on the inside cover that can give you an idea. I did have some old charts somewhere that gave you a ballpark answer. I’ll see if I can dig them up out of mothballs.

[Peter Foxe Smothers]

What about temperature rise on Aux heat with the heat strips running? or a gas aux heat unit?

[JLilly]

When I took Δ T as a tech I would disable the supp heat at the stat to ensure a true reading.

[Bruce A. King]

Originally Posted by psmothers

What about temperature rise on Aux heat with the heat strips running? or a gas aux heat unit?

Aux heat strips can be typically 5kw to 10kw, the amount of airflow will also change the heat output. Anywhere from 15 to 40 degree diff. could be normal depending on the system.

The best way to check the aux heat is to start with cold ducts so that any residual heat will not cause a false interpretation.

Gas aux heat (dual fuel units) will have a higher output.

[Peter Foxe Smothers]

So is it common for a newer heat pump unit to have a higher temp rise in HP mode than in Aux heat heat? Saw a newer unit today which in HP mode blew 105 and in Aux blew 92. Outside Temp was in the high 50's.

[Charley L. Bottger]

Originally Posted by psmothers

So is it common for a newer heat pump unit to have a higher temp rise in HP mode than in Aux heat heat? Saw a newer unit today which in HP mode blew 105 and in Aux blew 92. Outside Temp was in the high 50's.

Pete I find every different wiring combination under the sun concerning HP some right some wrong. The only true way to determine what you have running on a HP no matter what mode the stat is set too is to remove the cover if you are dealing with heat strips and place a amp meter on each strip I have found systems where the contractor wired the strips to run when ever the condensing unit was on line.

[Charley L. Bottger]

Originally Posted by cbottger

Pete I find every different wiring combination under the sun concerning HP some right some wrong. The only true way to determine what you have running on a HP no matter what mode the stat is set too is to remove the cover if you are dealing with heat strips and place a amp meter on each strip I have found systems where the contractor wired the strips to run when ever the condensing unit was on line.

I read my own post and it was a little confusing even to me.

Basically what I am trying to get across is if you are taking temperatures on a heat pump in the normal mode you should make sure that the electric heat strips are not activated also because some times they are if improperly wired and the best way to determine this is with a amp meter on the strips. When I am operating the the emergency heat mode I use the amp meter anyway to determine if all of the strips are operating. Does this make sense.

[Bruce A. King]

Even if wired properly the aux strips will come on if the temperature lags a few degrees. (unless a more advanced t-stat is present and programmed to keep the strips off)

Heat pumps will take more time to inspect.

[Charley L. Bottger]

Originally Posted by bking

Even if wired properly the aux strips will come on if the temperature lags a few degrees. (unless a more advanced t-stat is present and programmed to keep the strips off)

Heat pumps will take more time to inspect.

This is very true on the older stats not many left in use they used a mercury two stage bulb that you could pop the cover off and observe if the second stage bulb was activated. Newer digital stats no way to see. I still use the amp meter to determine if the strips are operating or not.

[Gary Reecher]

Originally Posted by psmothers

What about temperature rise on Aux heat with the heat strips running? or a gas aux heat unit?

The electric strips may or may not operate depending upon outdoor temperature and thermostat settings. I have seen some systems wired that only part of the heat strips will energize and the other part is wired with an outdoor thermostat that will only allow it to be energized until outdoor temperatures reach a certain point. A technician would have to adjust OD thermostat for testing then reset it.

Gas or oil furnaces would be a duel system and would be wired in such a way that if the thermostat shuts down the heat pump if 2 nd stage heating is engaged. For temperature rise you would use the information listed on the furnace data plate.

[Charley L. Bottger]

[quote=Gary Reecher]The electric strips may or may not operate depending upon outdoor temperature and thermostat settings. I have seen some systems wired that only part of the heat strips will energize and the other part is wired with an outdoor thermostat that will only allow it to be energized until outdoor temperatures reach a certain point.
Thats my point if as a HI you are recording HP temperatures you should know what you are recording and you can not tell by just looking at the system or by the wiring. The strips may or may not be on when you take the temperatures. Remove the normal excess panel on the strips and use the amp meter and you will know exactly what you are recording.

[Leo Bennett]

From the top, I'm a curious / data-driven home-owner. This message is a corroboration to Bruce King's 11/11/06 post about sizable temperature differentials for modern HP's.

My experience today: 60F ambient temp, 1-year old Coleman 18 SEER 2-stage Copeland-driven HP running on Hi stage; The return temp at furnace air filter = 64F, output temp taken in living space at the 1st wall-register beyond add-on HP atop oil furnace = 102F for a 38F differential. One mod I've done - I added a 2nd layer of insulation on the vapor line from the HP to the indoor coil, about an 18' run. I suppose this helps....

It's easy for me to monitor HP 1st/2nd stage performance + FF operation. I've installed at the T-stat a pair of LED status lights that confirms the FF operation and HP-2nd stage operation.