My proposed system can't power my AC

[Frankman60]

The label on my AC compressor says 208-230 volts. I’m not sure how many volts it actually takes to spin the rotor but I know that when I tried it without the Sure Start I heard the relay clicking but that was all. At the time of the test, the house load was about 2 kWs.

Even if the compressor is 208 volts, or 28 kWs, isn’t that too much for 2 PWs that have a surge rating of 14 kWs (7 kWs each)? If the Sure Start device reduces the amps by half, that would still be 14 kWs which would be at the maximum surge capacity of 2 PWs, assuming not other house load being backed up.

Double-check if it's 240 volts. My labeling says HVACs motors run at 208 volts on 240V circuits. So the 135A LRA @ 208V is 28.08 kW, and the PWs should only see it as a load of 117A. Split to two PWs, that's 58.5A each.

58.5A is of course too close to 60A limit (for 2 PWs), but then the SureStart should have lowered the LRA enough without causing any other electrical damage. 58.5A / 2 (for 2 PW) / 2 (50%) = 14.625A per PW with the SureStart.

 

[NuShrike]

The label on my AC compressor says 208-230 volts. I’m not sure how many volts it actually takes to spin the rotor but I know that when I tried it without the Sure Start I heard the relay clicking but that was all. At the time of the test, the house load was about 2 kWs.

Even if the compressor is 208 volts, or 28 kWs, isn’t that too much for 2 PWs that have a surge rating of 14 kWs (7 kWs each)? If the Sure Start device reduces the amps by half, that would still be 14 kWs which would be at the maximum surge capacity of 2 PWs, assuming not other house load being backed up.

Looking at this, the minimum voltage on a 240v system at the motor terminals should be 208V.

Yes I made a mistake. It's 58.5A total after SS reduction, and so 29.25A each PW. Right at the edge of the surge capacity, and much above the continuous at 21A.

Just can't back-up the AC with 2 PWs at all without a brown-out.

I'm actually in a similar situation with my HVAC but at LRA 105A. Still had the PWs installed with the A/C not backed up -- but prewired on the subpanel to move the breakers over whenever I can get the HVAC upgraded to something modern with a much lower LRA.

 

[clos000]

This is what the Tesla Engineer/Designer was asking for from my HVAC project manager:

>>
Typically for an AC unit to be in a back up circuit with a Powerwall a few things need to be certain 1) the type of compressor - we prefer it to have a scroll compressor so we can use a hard capacitor (surestart) to lower the in rush current; 2) the LRA needs to be <28a per PW. With the current set up of 2 PW the LRA needs to be less than 58a.
>>

 

[hodad66]

You have two Powerwalls right? If you do, it is just below the maximum 56 amps that @clos000 reported for two PWs. Have you tried a simulated grid outage to see if you new Lennox AC compressor will start? Are you also using a Sure Start with your new AC compressor?

We did a grid outage test for 27 minutes, but in retrospect, I'm not sure that the compressor
kicked back in during that time. DUH I should have lowered the thermostat for the test but
so much was going on that I was just happy to see everything running. Should be interesting
once I get my PTO.

 

[Frankman60]

I know what you mean about so much going on during a grid outage test! Please let us know if your new Lennox compressor can turn on during a grid outage without causing a low voltage situation in the rest of your home. But if the compressor starts and there is low voltage, that should be totally correctable with a Sure Start given that the LRA on your new compressor is so low. BTW, how many tons is it and what was the cost?

We did a grid outage test for 27 minutes, but in retrospect, I'm not sure that the compressor
kicked back in during that time. DUH I should have lowered the thermostat for the test but
so much was going on that I was just happy to see everything running. Should be interesting
once I get my PTO.

 

[NuShrike]

This is what the Tesla Engineer/Designer was asking for from my HVAC project manager:

>>
Typically for an AC unit to be in a back up circuit with a Powerwall a few things need to be certain 1) the type of compressor - we prefer it to have a scroll compressor so we can use a hard capacitor (surestart) to lower the in rush current; 2) the LRA needs to be <28a per PW. With the current set up of 2 PW the LRA needs to be less than 58a.
>>

SureStart is NOT hard-capacitor. There's some crossed wires there, that I've seen common among my local HVAC contractors.

 

[hodad66]

I know what you mean about so much going on during a grid outage test! Please let us know if your new Lennox compressor can turn on during a grid outage without causing a low voltage situation in the rest of your home. But if the compressor starts and there is low voltage, that should be totally correctable with a Sure Start given that the LRA on your new compressor is so low. BTW, how many tons is it and what was the cost?

2 ton 9K

 

[hodad66]

OK, I did an off grid test for about 30 minutes & lowered the thermostat.
The AC cycled perfectly and even my UPS units seemed to forgive the
interruption. They both signaled the loss of power but then turned off as
the Powerwall replaced grid power.

 

[Dan123]

OK, I did an off grid test for about 30 minutes & lowered the thermostat.
The AC cycled perfectly and even my UPS units seemed to forgive the
interruption. They both signaled the loss of power but then turned off as
the Powerwall replaced grid power.

What was the state of charge (percentage) of the powerwall at the time of the test? Was solar system supplying power?

 

[hodad66]

What was the state of charge (percentage) of the powerwall at the time of the test? Was solar system supplying power?

When the grid goes off my inverter resets so no solar for 5 minutes and the PWs
were at 75%. When the solar came back on a bank of clouds had moved in.......

 

[Pale_Rider]

Got my 3 PWs installed this week. Was able to start up both of my ACs and even run my wall charger at 48A once the ACs were running. No soft starts were needed. My LRA is 75 on one unit and 67 on the other unit. I may have them install the soft start just to lighten the load a little.

 

[Frankman60]

That’s great. As long as the two condensers do not start at the same time, the individual LRAs are within the Tesla spec that was posted earlier in this thread (under 28 power PW). Because you have 3 PWs, the maximum LRA should be 86.

What brand and tonnage are your condensers? I need to find a 4 or 5 ton condenser with an LRA no more than 56 without a Soft Start. I’m a little concerned about relying on a Soft Start to control the LRA.

Got my 3 PWs installed this week. Was able to start up both of my ACs and even run my wall charger at 48A once the ACs were running. No soft starts were needed. My LRA is 75 on one unit and 67 on the other unit. I may have them install the soft start just to lighten the load a little.

 

[miimura]

That’s great. As long as the two condensers do not start at the same time, the individual LRAs are within the Tesla spec that was posted earlier in this thread (under 28 power PW). Because you have 3 PWs, the maximum LRA should be 86.

What brand and tonnage are your condensers? I need to find a 4 or 5 ton condenser with an LRA no more than 56 without a Soft Start. I’m a little concerned about relying on a Soft Start to control the LRA.

I am not an expert, but I think you will need to get a variable speed unit to get the LRA that low for that tonnage.

 

[DrSmile]

I have two A/C systems in the house that combined use about 8 KW (yikes!). The 2 Powerwalls have no problem running both of the systems (with soft starts), but obviously they won't power them for more than a few hours. A/C use is typically highest after the peak solar production, so when I run off grid I usually turn the larger A/C unit off at around 5pm and run on the smaller one which only uses about 3 KW. I have 13.2 KW of solar so I usually don't dip below 3KW production until around 6 pm in the Summer, at which point the Powerwall lasts well into the night.

 

[Frankman60]

I think you are right about that. I am searching for a variable speed condenser that will work with my existing heater/blower and coil because I don’t want to have to fully replace the entire HVAC system that isn’t that old.

I am not an expert, but I think you will need to get a variable speed unit to get the LRA that low for that tonnage.

 

[quitepossibly]

I think you are right about that. I am searching for a variable speed condenser that will work with my existing heater/blower and coil because I don’t want to have to fully replace the entire HVAC system that isn’t that old.

The variable speed condensers are typically require communicating equipment. So it is unlikely you'll find a variable speed condenser that works with your existing furnace.