# Thread: Understanding Electric costs with Rheem 2-stage

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Originally Posted by beenthere
His math is ok, except, he's using 1600 CFM on a 1400 CFM unit. And he's missing another part of the Manual S, that would regain 5,550 BTUs sensible. Meanig the 3.5 ton would be working more like a 3 ton.
Hey Beenthere, I see your post count is moving right along. Thanks for clearing that up as I was a little puzzled.

I went back to the original post where the OP indicated the unit as RARL-JEZ with no tonnage (or else I missed it).

Since this was all theoretical, and not knowing his latent requirements, I didn't want to add any confusion by considering excess latent capacity and used a 4 ton unit as an example.

AM

2. In your post you used an 042, which would be 3.5 ton.

I pulled up the extended performace of a 14 SEER unit.
At 95*F OD, 70*F ID DB 57*F ID WB it has a 42,000 total, and 30,000 sensible, at 1400 CFM.
At 1600 CFM same temp conditions, it has a total of 43,200, with a sensible of 32,200.

If the humidity is lower, with either of those CFMs, the sensible would increase anywhere from a few hundred BTU, up to 6,000 BTU. Depending on humidity.

On a tight house, that can make a big difference in unit selection.

I just picked a 4 ton single stage unit, it works the same with a 2 stage.

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Originally Posted by beenthere
In your post you used an 042, which would be 3.5 ton.

I pulled up the extended performace of a 14 SEER unit.
At 95*F OD, 70*F ID DB 57*F ID WB it has a 42,000 total, and 30,000 sensible, at 1400 CFM.
At 1600 CFM same temp conditions, it has a total of 43,200, with a sensible of 32,200.

If the humidity is lower, with either of those CFMs, the sensible would increase anywhere from a few hundred BTU, up to 6,000 BTU. Depending on humidity.

On a tight house, that can make a big difference in unit selection.

I just picked a 4 ton single stage unit, it works the same with a 2 stage.
My mistake. I am in the process of having an 048 installed and that was on my mind. That's where the 1600 CFM came from.

Mea culpa.

AM

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Hello everyone. I did a little more "investigating" in regards to how much electric this thing is using.

You may find my troubleshooting on this a little stupid, but I will eventually figure out how much I am using exactly.

Last night before I went to bed I decided to go check out my meter outside. I have a 5 digit analog clock type meter, along with a dial that spins horizontally (i guess this is some type of decibal reading).

I said to myself, lets see how much faster the dial moves when I have the AC unit on and off. So I decided to measure how many seconds it took for five complete revolutions on the dial.

With the AC completely off... it took 96 seconds for 5 complete revolutions on the dial. (I guess this would be my baseline).

I turned the AC on stage 1... dial started spinning much faster. 5 complete revolutions now only take 36 seconds.

I then adjusted my thermostat so the AC would kick in stage 2... dial started spinning a little faster. 5 revolutions now only take 28 seconds.

Now the biggest thing I found from doing this test, is there really isn't much of a difference in electrical consumption between stage 1 and stage 2. I really thought running in basic stage 1 would not consume as much electric as it is. Going from 96 seconds to 36 is more than doubling my total house electrical output.

Now my current meter read is 94622. If I can figure out how many revolutions it takes to go from 94622 to 94633, I can figure out how much KW/H (approximately) my CAC uses by subtracting the baseline from the amount used in stage 1. I can repeat for stage 2.

Yes I know this is a dumb way of looking at it since it will never be a perfect reading. But I think I will learn quite a bit as to how much this CAC really uses without getting an electrician in to hook a meter up.

Thoughts?

5. Well this may sound too easy.

But if you want to know how many revolutions. Turn your A/C on so it goes to second stage. And then count how many revolutions it takes to make turn the 1 KWH dial.

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Originally Posted by beenthere
Well this may sound too easy.

But if you want to know how many revolutions. Turn your A/C on so it goes to second stage. And then count how many revolutions it takes to make turn the 1 KWH dial.

Well exactly. That's what I was getting at. I guess I didn't explain myself that well. That's the next step I am going to do when I have a chance to get out there in daytime and it not be 96 degrees out.

But here is the million dollar question. Is there really not that much of a difference in electrical output between a stage1 and stage2 mode? Like I said 5 revolutions went from 96 seconds to 36 seconds going from off to stage1. Then it only jumped 8 more seconds to 28 seconds when stage 2 kicked on.

I can see stage 2 sucking up all the juice, but I was hoping running in stage1 would use much less electric. I guess I was mistaken. This is a 16seer unit too.

Today when I get home I am going to plug in an old 8000 BTU window AC and see how much electric that uses when its compressor kicks on (so I can compare between a window AC and this CAC system).

I also have a 110V KW/H meter so I could also use that to test the window AC, but that meter obviously won't work on my CAC system.

7. A 2 stage scroll compressor in first stage is rated at 67&#37; capacity. Your dial times reflect that percentage fairly close.

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Originally Posted by beenthere
A 2 stage scroll compressor in first stage is rated at 67% capacity. Your dial times reflect that percentage fairly close.

Ok. That makes sense then.

Ultimately the reason why I am doing this is to figure out the most efficient way for me to use my CAC to save on electric.

Both me and my wife work a normal 9-5 job. So I am trying to figure out of I should turn off the unit at 8am when I leave for work and then remotely turn it back on at say an hour before I anticipate coming home (I have a Proliphix thermostat with remote access).

I have a feeling with these scorching hot 95 degree days it might just be best to leave the unit on all day since the house would get VERY hot with the unit off (indoor temps will easily go past 80-85 degrees). Would probably take the unit a while to cool the house down from that temp.

But maybe on not so hot days where it only takes the system an hour or two to get its desired temperature its worth turning off during the day.

9. On 90 plus days, don't set it more then 2* higher then the temp you feel comfortable at.

Mid 80* days you might be able to set it hihger, maybe 4*

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Originally Posted by DjPiLL
... But here is the million dollar question. Is there really not that much of a difference in electrical output between a stage1 and stage2 mode? ...

I can see stage 2 sucking up all the juice, but I was hoping running in stage1 would use much less electric. I guess I was mistaken. ...
Here's an example to provide illustration to your question.

My WaterFurnace 3 ton heat pump has a 2 stage scroll compressor.

On an 'apples-to-apples' basis (80&#176; water @ 8 gpm), power consumption at compressor low speed (with blower motor at 1050 cfm) is spec'd at 1.30 KW. At compressor high speed (at 1250 cfm), it's spec'd at 2.28 KW. That's about a 75&#37; power increase for a 30% cooling capacity increase. Thus my observation is that you should see a similar scenario.

Further, here's a chart to affirm the low speed power value is in actuality about right - it shows my actual HVAC power usage (blue line). When only my 3 ton unit is running, it's the lowest of the 3 plateau levels, at about 1.5 KW (1500 W). This is close to the 1.30 KW spec, taking into account that I'm not running right at the water temp or water/air flow specs, and my 1.5 KW number includes power used by a water pump and the idle power for my other (5 ton) unit.

In my case, my 3 ton unit always runs in low speed - my ambient house conditions combined with my tstat setpoints never get to the point of needing 2nd stage cooling - so I don't have an actual power measurement to show for high speed.

In summary, it's hard to quantify, for your specific home, if it's more economical to force your unit to always run in 1st stage. It looks to me, though, on an actual power usage point-of-view, it costs you 75% more power for 30% more cooling capacity (given you have a 2-speed scroll compressor).

Best regards,

Bill

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Well for what it's worth I have a 4 ton RARL-JEZ and RBHM matched system.

I have to figure out how much KW/h this thing uses. Ill probably crunch some more numbers later this week and try to get an estimate.

12. ## Energy usage

The question has been asked about a different brand, but the Trane XL16i heat pump is what I have some documentation for. Being in the "16 SEER" category its energy usage ought to be comparable to other brands claiming 16 SEER. It too has the 67&#37; capacity in 1st stage, have to wonder if it might even use the same compressor as other brands.

Here is one 4-ton model's output and energy draw at 95F outdoor dry bulb, 75 indoor dry bulb and 63 indoor wet bulb conditions:
1st stage: 31.8 thousand btuh total capacity, 25.4 sensible, 2.79 KW draw.
2nd stage: 44.3 total, 33.5 sensible, 4.18 KW draw.

For those not too accustomed to the units of electricity, that will be 2.8 KWH if 1st stage runs a full hour, and so on. It looks to me like this model is 7.5% more efficient in 1st stage than in 2nd.

The one I have installed is similar to the XL19i, if I read the tables right it is 26% more efficient in 1st stage than in 2nd. It however uses an entirely different type of compressor (and is R-22 only).

Hope this helps -- Pstu

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