Temp rise on heat pumps

[White2Blue]

Good afternoon Gentlemen,

I am a junior tech who is struggling to understand temp rise on heat pumps. First and most importantly I would kindly like to know the general procedure to inspect temp rise on heat pumps, if there is any. I have asked around in my company for the general procedure, but it seems like I am beating around the bush.

When I go to homes, I bump up the tstat one degree to confirm 1st stage, this is when I will also get the temp rise (which I have been told that it should be 20 degrees, just like temp drop but only the opposite, fair enough). The only problem is that, everytime I do this the heating elements come on, and I able to tell this because my temp rise is no longer 20 degrees rather it spikes to 40 degrees.

I know tstats could be configured so that if I bump up the tstat 3 degrees the elements kick in, but that rarely happens...everytime Iam bumping up the tstat 1 degree, the heating elements are coming on...and ofcourse, most folks do not know how to operate the heat pump, they are just happy they are getting heat....my real question is, " what should be a temp rise on heat pumps, and why are these thermostats I am running into have auxiliary heat turned on everytime I turn on the tstats ? "

Anyhow, if someone can explain me a bit about this confusion, it would be a great help. Thanks for your time.

[Pnelting]

You could always temporarily disable the heat strips to keep them from energizing. When your done hook em back up.

[CincyHVAC]

I was thinking along the same lines. On a call for heat, the wiring logic must be calling for one bank of resistance heat to energize.

Normal supply temps for heat pumps are going to be in the 90*s depending on return temps, cleanliness of the system, and proper refrigerant charge.

[rundawg]

this is when I will also get the temp rise (which I have been told that it should be 20 degrees)

It depends on the outdoor temperature.

At 65°f OAT, the Delta T could be as high as 30°f.

At 0°f OAT, the Delta T could be as low as 9°f.

The expanded performance data for the unit will list what the Delta T should be for each 5°'s of outside air temp from about 65°f down to below zero.

[Ed Janowiak]

Good afternoon Gentlemen,

I am a junior tech who is struggling to understand temp rise on heat pumps. First and most importantly I would kindly like to know the general procedure to inspect temp rise on heat pumps, if there is any.

As said above disable the aux heater by removing the wire that active them at the t-stat or hair handler ( I always found doing it at the t-stat was easier, side note: leave your car keys at the stat so you can"t leave with reconnecting the wire).

Run the heatpump for at least 10 minutes, then measure your split:

CFM x ∆T X .317 = Watts output

Watts output x 3.412 = BTUH

A stating point for the output should be 47°. Thats where you should be near the nominal sizing of the heatpump...... IE: A 3 ton HP will make about 36K BTUH worth of heat (this is in place of having actual application data for the system you are checking).

If you want to determine the CFM moving over the system, before disconnecting the aux heat at the t-stat run the aux heat by itself at the same fan speed as the heatpump will operate at and record the ∆T after 5 mins of operation and use the following formula:

Amps x volts / ∆T x 3.16 = CFM

Do this 10 times and you'll have the entire routine down to about 30 minutes to really calculate a value that means something.

The reason you get no help from anyone on what a ball park ∆T is for a HP is because there is none on an air source unit. The goal posts keep moving as the outdoor temps move. Calculating the output and estimating the target is the only was to do it .

[White2Blue]

Thanks guys !

After doing some reading, I also concluded that the thermostat and wiring has a big role in how the heat pump works. I am goint to try all these methods.

[White2Blue]

Okay,

so i ran a service call on a heat pump today, and you bet I stayed there way longer than I should have.

I took off the wire from the tstat and ran the heat pump, my delta t was only 14 ! outside temperature at 70 F.
low side pressure - 110 psi
high side pressure - 190 psi
Indoor temperature - 72

Is low side little high here in this situation ? And the temp rise ? Shouldn't that be higher ? However, once I connected the tstat back for auxiliary, then the temp rise jumped to 30+

can we also calculate super heat/subcool in heat mode too ? This is my first winter season !

And in Auxiliary mode, only two heating elements came on instead of three ? Is that the norm ? Thanks guys

[beenthere]

Your temp rise is too low.

If its an R410A system. Both pressures are low.

Not unusual for only 10KW to be connected on some units.

Could also be that the stat is 3 staged, and wasn't sensing a cold enough indoor temp to bring on the last 5KW.

[pageyjim]

Okay,

so i ran a service call on a heat pump today, and you bet I stayed there way longer than I should have.

I took off the wire from the tstat and ran the heat pump, my delta t was only 14 ! outside temperature at 70 F.
low side pressure - 110 psi
high side pressure - 190 psi
Indoor temperature - 72

Is low side little high here in this situation ? And the temp rise ? Shouldn't that be higher ? However, once I connected the tstat back for auxiliary, then the temp rise jumped to 30+

can we also calculate super heat/subcool in heat mode too ? This is my first winter season !

And in Auxiliary mode, only two heating elements came on instead of three ? Is that the norm ? Thanks guys

Where are you measuring the temps at? It should be at the a/h. The hp could be operating ok but you are losing it in the ductwork.

[hvacrmedic]

Okay,

so i ran a service call on a heat pump today, and you bet I stayed there way longer than I should have.

I took off the wire from the tstat and ran the heat pump, my delta t was only 14 ! outside temperature at 70 F.
low side pressure - 110 psi
high side pressure - 190 psi
Indoor temperature - 72

Is low side little high here in this situation ? And the temp rise ? Shouldn't that be higher ? However, once I connected the tstat back for auxiliary, then the temp rise jumped to 30+

can we also calculate super heat/subcool in heat mode too ? This is my first winter season !

And in Auxiliary mode, only two heating elements came on instead of three ? Is that the norm ? Thanks guys

Assuming that this is an R22 system, the low side pressure is way too high.

[pageyjim]

Okay,

so i ran a service call on a heat pump today, and you bet I stayed there way longer than I should have.

I took off the wire from the tstat and ran the heat pump, my delta t was only 14 ! outside temperature at 70 F.
low side pressure - 110 psi
high side pressure - 190 psi
Indoor temperature - 72

Is low side little high here in this situation ? And the temp rise ? Shouldn't that be higher ? However, once I connected the tstat back for auxiliary, then the temp rise jumped to 30+

can we also calculate super heat/subcool in heat mode too ? This is my first winter season !

And in Auxiliary mode, only two heating elements came on instead of three ? Is that the norm ? Thanks guys

I'd be curious what the amps were on that compressor. Maybe inefficient compressor?

[Paul Bee]

Rundawg is correct, delta T in heat mode also includes the heat of compression and the heat added by the compressor motor. In cool mode this is rejected at the condenser ( ever heard the ambient + 30* rule of thumb for condenser air off temperature?) Remember though correctly sized ductwork and proper airflow play a big part in this number.

[White2Blue]

I thought the temp rise was low and the pressures for low side to be high too, now that I am starting to make some sense of heat pumps I will start to inspect them thoroughly, as I am noticing the installers or the initial builders/contractors are not properly wiring/installing these machines. And I feel so bad for the customers as they have no clue whats going on with there electric bills, they are happy campers as long as they are getting heat.

The compressor was amping at 6.9amps (I think this is sufficient for a goodman), and yeah the temp rise was also taken at the a/h. The customer told me she previously had air flow problem in her bedroom so the company added an additional return right beneath the a/h, since then she says the air flow has improved.

she had a white rogers tstat, where the white(heat) was connected to W1 and jumped to E terminal. I guess normal setting for a heat pump ?

I checked her filter, it was clean. This was R22 system.

Beenthere, you are right. Her two 5kw elements were connected to one sequencer (which were amping at 19.2amps) and the one that was not responsive was connected to separate sequencer, hence it wasn't sensing enough cold air to energize the third element.

BY the way she did have a variable motor also, maybe this has something to do with the odd low side pressure ?

You guys rock ! thanks.

[beenthere]

In heat mode. The outdoor coil is the evap. So the VS blower inside isn't causing the high low side pressure.

[Kevin O'Neill]

Outdoor Temp Indoor Temp Rise
65 degrees 33 degrees
60 degrees 31
55 degrees 29
50 degrees 27
45 degrees 25
40 degrees 23
35 degrees 21
30 degrees 19
25 degrees 17
20 degrees 15
15 degrees 13

This is a generic chart. If you have an oversized indoor unit, the temperature rise will typically be a couple of degrees cooler. Different brands vary some as well. But it is a good starting point for a heat pump with the strip heat help off.

Note: There does not seem to be a way to make the columns line up right.

[White2Blue]

Yeah the concept of heat pump is neat, in heat mode outside unit is Evap. Yesssir !

Thanks for the chart Kevin. This absolutely will help me diagnose heat pumps better (with elements off). I just got to learn so much its unbelievalbe ! Infact there is so much to learn in this industry, I love it!

Next is to master boilers...we don't have many here, well I say maybe 25-30 percent, but everytime I get a call to do a pm on a boiler it makes my day

[amd]

The problem with relying on temp rise alone is that the airflow greatly varies - you can't assume that the installer set the blower speed correctly.

Superheat/subcooling, compression ratio, indoor airflow, and amp draw theoretically can tell you exactly what's happening in the system.

[hvacmac7]

if you take ampdraws, pressures with panels off of condensing unit,ive gotten some crazy pressures and draws.air flow across coil is important right?pros i am asking your opinion

[White2Blue]

hvacmac7, you are right. once the panel is off, the pressures do generally go high. I never take pressures with panels off. I think its the air volume that goes through the coils is not enough and hence impacting the pressures in the coils and amping of the compressor.

*air flow and air volume are two different measurements*

[CincyHVAC]

if you take ampdraws, pressures with panels off of condensing unit,ive gotten some crazy pressures and draws.air flow across coil is important right?pros i am asking your opinion

Think about it. Where does the airflow go when you take the panel off? Why do you want the airflow to draw across the coils? Remember what is required for the vapor-compression cycle to work.