Amperage for two stage heating with heat pump


Old 12-05-10, 06:38 AM
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Amperage for two stage heating with heat pump

GA foothills USA. I have a heat pump with a two stage compressor outside and variable speed motor inside. I have lots of electrical engineering experience and a data logger with one exterior temp sensor and two clamp-on amp sensors. I designed and built the structure myself, but not the HVAC.


Ammeter readings show my heat pump going from heat-not-on to one-compressor-stage-operating to inside-heat-strips-on as temp falls from moderate to cold over two nightfalls. I never see amperage evidence of the outside unit operating at a second stage before resorting to heat strips. I see only 0.256A (off?) or 4.8A (on?) outside, and 1.6A (fan?) or 24A (heat strips?) inside. Am I correct in believing I should see two different amperages for two different compressor stages?

(Note that my clamp for each system, inside or outside, is in the breaker box and on only the black wire. I haven't confirmed the same simultaneous readings on the white wire, but they really should be. The systems should be using the two phases equally (110+110=220V) and not unbalanced with current down the safety ground.)


I'm individually reading the amperage going to both the outside and inside units. Based on amperage, I find no evidence of two stages outside. For example:

Nov 25, 2010:

Day - as high as 62degF, assume heat off (tight energy efficient home with human and computer heaters inside), 0.256A outside, either driving always-on electronics outside, or ammeter not well calibrated. Doesn't matter. Essentially OFF.

Night - as low as 52degF, 4.8A outside generally cycling every 20 minutes, but spending more time on as temp falls. Makes sense.

Only problem is that I expected to see TWO different amp readings, such as this 5A and then also 10A, or this 5A and also 2.5A. So it seems like only one stage of heat came on. PERHAPS it was the first stage, and the second stage was never called for.

HOWEVER, Nov 30, 2010:

Day - as high as 60degF, falling to 52degF by 8pm. 4.8A outside occurs as described for Night on 11/25. This is consistent.

Night Part 1 - fallling to 44degF, eventually get to where 4.8A is on full time, not going off for at least 4 hours (9:30pm to 1:30am).

Night Part 2 - by 1:45am 43.5degF. Outside amps remains 4.8A, but inside amps jumps from 1.6A (assume fan operating amps) to 24A (assume heat strip).

So, the outside unit never changed amperage, but the heat strips got turned on inside. Note that these two days fully covered the range from no heat to strip heat, I *should* have seen both heat stages operating, but I didn't.

Shouldn't I indeed see an amperage difference on the outside unit as it switches from stage one to stage two prior to going to strip heat (stage three)? I suspect that I *should* and that the thermostat or something else is incorrectly configured. I'd like to confirm my assumption about seeing the two compressor stages on the ammeter before moving forward.

Thanks very much in advance for your advice.
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Old 12-05-10, 04:09 PM
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Researching further, I find the technical ref manual for my ASZ16024 outside 2-ton unit:

I find on pages 24 & 25 expected AMP readings for low and high stage operation, respectively. In low stage, for temps between 40 and 60 degF, the amperage is 5.0 to 6.3A. In high stage, for the same temps, the amperage is 6.3 to 8.0A.

I see 4.8A, which is reasonably consistent with low stage. This manual suggests, as I expected, I should see another 1 or 2A at high stage, and I never see that.

Right *now*, 39degF, 5.0A outside, 23A inside, 250VAC multimeter reading.

This MUST mean heat pump is only running in low stage, yet heat strips are on inside.

I next ran the tstat through test mode. In heat stage one mode, the compressor outside had 24VAC on the Y terminal as expected. In heat stage two mode, the compressor outside still had 24VAC on Y, but did NOT have the expected 24VAC on W2. I later (now) realize W2 is something else, on a defrost control board. Elsewhere in the outside unit is a Y2, fed from Y(hp) of the inside unit. While Y outside is fed from the Tstat and common wired to Y1(tstat) of inside unit. When tstat energizes Y, it goes to both inside unit Y1(tstat) and outside unit Y. When tstat energizes Y2, it goes to inside unit Y/Y2(tstat). I *assume* the inside unit board forwards the Y/Y2(tstat) signal to it's Y(hp) signal, that then goes to the outside unit Y2. I need to confirm this last part. However the manual (link above) on page 57 section 4.2 says 'the VSTB will supply 24Vac to "Y/Y2" at the heat pump.' But there's no "Y/Y2" signal coming out of the VSTB going to the heat pump outside. There *is* this "Y" signal under a "Heat Pump" heading, that I've called Y(hp), and *this* does go to the heat pump "Y2" terminal outside. Again, I need to check to see if this is ever or always happening. It's sure not sometimes happening, or I'd be seeing an amperage change!

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