Heat Pump Amp Draw Question

newtoazJanuary 16, 2010

Hi folks:

My first post here, thanks in advance for any help. My wife and I are in the inspection stage of a 5 year old house in Phoenix, and I'm not sure how to read the amperage draw from the name plate. They are both Aire-Flo AFHEAT12B-36 units, 1 for each floor.

I did very basic tests in AC mode, checking compressor amps after running them for 30 minutes and measuring the return and supply air temp differentials. Both seem good - 11 to 12 amps and 15 degrees F differential. The nameplate shows 17.9 RLA and 90 LRA.

My question has to do with heat mode. At start up, amps on both units were about 12, and steadily climbed to 21 amps within 10 minutes. One unit reached 23 amps in 5 additional minutes, then dropped to 0 with a quiet hiss from the compressor which continued to run for about another 5 minutes at (I assume) no load. The other unit ran at 21 amps until I shut it off at the thermostat. I shut down the 23 amp unit, left it for 1/2 hour and repeated the test with the same result. Both units delivered plenty of heat into the house while the compressors were running.

I'm not familiar at all with heat pumps, but 21 to 23 amps on a 17.9 amp system doesn't seem right. The thermostat was still calling for heat (I set it to 80) when the compressor cut out. If anyone can confirm this is likely a defective unit, I'll call in a tech to run proper tests, but I'd like to at least throw it out to you experts for an opinion.

Thanks again for any suggestions.

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creek_side

RLA is rated load amps. It is used for wiring and circuit protection sizing calculations. It is not the same as FLA (full load amps).

RLA varies with load. It is not a constant number. One "authority" states that as long as the current does not exceed RLA/0.71, it is not excessive. Others say the RLA cannot be used to troubleshoot or evaluate HVAC equipment.

The units are probably fine.

    Bookmark   January 16, 2010 at 8:52AM
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creek_side

My post could have been worded better. RLA doesn't actually change. It is a rating. The running amps change with load.

    Bookmark   January 16, 2010 at 8:56AM
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garyg

The data plate for my 2 year-old heat pump says 13.4 Rated Load Amps.

Amp draw is based on ambient temps. The technical operating data for my heat pump states that 13.4 amps are drawn at 35F ambient. A low draw occurs at -10F (7.5 amps) and a high draw of 19.3 amps at 65F.

I'm going to guess that you are good to go.

In cooling mode, my RLA occurs at 115F ambient.

This was a good question.

Best of luck with the new home.

Take care.

    Bookmark   January 16, 2010 at 9:01AM
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newtoaz

Thanks creek side and garyg. Good information, and I hope you guys are right because I really don't want to get into a conflict with the seller. I've always thought of RLA to mean 'run load amps' and LRA was 'locked rotor amps'. This one would draw 90 amps if the compressor seized, but I'm not sure where the manufacturer came up with 17.9 amps as a run load. I would think they would state the maximum amps the unit would draw under normal operating conditions, which in the case of AC might be about right, but not for heating. I suppose 23 amps to heat a single floor is still more efficient than a 15KW electric furnace which would take 63 amps.

So what do you think about one of the compressors shutting down while still calling for heat? I guess I should have let it run at no load for a while longer to see if it kicked in again instead of shutting it off.

    Bookmark   January 16, 2010 at 10:14AM
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brickeyee

RLA is rated, and as mentioned above the circuit wiring is sized to 125% of this number.

LRA isLocked Rotor Amps and is used to size the overcurrent protection for the circuit.

When an induction motor start up it pulls current that easily approaches the LRA since the rotor is not turning yet.

This is often the cause of the dimming in other lights that briefly occurs when the unit starts.
If the POCO feed does not have enough excess capacity, the voltage falls slightly for a few cycles of 60 Hz until the rotor in the compressor comes up to speed.

If the unit nameplate or installation instructions specify a circuit size, those numbers are used.

many electricians size the wires for the breaker for a compressor, but this is not required.
The wires are 125% of the RLA.

The unit will only draw this much current when it is loaded to full capacity, so the current will be lower on all but the worst days of heating or cooling.

    Bookmark   January 16, 2010 at 10:29AM
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countryboymo

Are you sure the compressor shut down or the blower fan on the unit shut down? Usually on defrost mode the fan shuts down and the compressor continues to run but reversed in a/c mode to clean any frost off the coil. If this did happen it should have went back to normal in 10 minutes or so and the fan kick back on.

    Bookmark   January 16, 2010 at 10:30AM
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newtoaz

Yes, the compressor ran for another 5 minutes before shutting off and leaving just the fan running. Kind of confusing because my clamp meter was reading 0.0, I would think even with no load it should still read a few amps. The fan and compressor sounds are distinctive, so I'm pretty sure I'm right.

    Bookmark   January 16, 2010 at 10:56AM
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brickeyee

" Kind of confusing because my clamp meter was reading 0.0, I would think even with no load it should still read a few amps. "

Once it is of the current flow had better be 0.0

    Bookmark   January 16, 2010 at 2:22PM
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