You have a relatively new Bryant Evolution furnace so it does make sense to mate it withe a Bryant AC condenser or heat pump. You can also buy the equivalent Carrier model since it is the same equipment. You will have to get a new coil since flushing out the R22 refrigerant is not practical.

The 2-stage AC does a better job of controlling humidity compared to a single stage AC. You will feel more comfortable on the hot humid days.

You could create a dual fuel system if you get the heat pump. Your gas and electric rates will determine whether it is cheaper to operate the heat pump or furnace. I know the Philadelphia area electric rates have gone up in the past few years. If you post your rates a calculation can be done.

You should have an Evolution thermostat to control your furnace. In opinion WiFI is not necessary. The latest version has a very nice display.

I think that it is humid enough in SE PA to want better humidity control.

Thanks so much for the input!

I'll dig up gas & electric rates to post, but in the mean time if there is a good site where I can learn to do the calculations myself, I would be very interested.

Calculators are easy to find. What you need is a crystal ball to predict what rates will be in the future. I'll be surprised if you don't find that gas is a lot less costly right now with drillers fracking the crap out of PA.

The real difference with variable-speed equipment is when it is humid and not so hot. Better humidity control will have a big impact on comfort.

Electric "price to compare" $0.0935 per kWh per PECO
Electric including all variable fees and taxes $0.16377 per kWh

Gas "price to compare" $0.5716 per Ccf, per PECO
Gas including all variable fees and taxes $1.04 per Ccf

I would forget the HP option and go straight AC.

Fuel comparison results below to be used as guide only.

Cost per 100,000 btu of useable heat

Electric baseboard: $4.22
Heat pump: $1.56
Natural gas: $1.06

I would look at the Evolution and Preferred high eff AC models with best matching ALA evap coil.

IMO

Thanks so much for taking the time to comment and to do the math to show efficiency of natural gas vs. a heat pump.

With your numbers as a guide, I was able to work my way through a calculator (linked) and get reasonably similar results.

Better yet, I was able to dig up detailed efficiency specs for the 286B heatpump and verify that even at very warm temperatures (47F external), high efficiency gas would be cheaper unless gas prices rise significantly (and no change to electric prices).

At this point, my decision is between a 3.5 ton 186B single stage Evolution system and a 3 ton 187B two stage Evolution system.

Unfortunately, I'm being quoted $1000 more for the two stage system which seems unreasonable, so may end up with the single stage.

Calculator Link:
www.eia.gov/neic/experts/heatcalc.xlsâÂÂ

You will be buying comfort with the extra coin.

ionized,

"You will be buying comfort with the extra coin"

Will she not also be saving electric costs?

There may be some costs savings if the 2-stage AC can lower the humidity level to a point where the thermostat could be set 2 or 3 degrees higher than you would with a single stage AC.

mike,

Even if the thermostat can not be lowered, the cost per BTU extracted from the home will be less with the first stage cooling versus the second. Same for heating.

If one had a situation where they could virtually never get by on just the first stage, then there would little or no efficiency improvement in paying more for two stage. I can not conceive of this situation in the real world.

"the cost per BTU extracted from the home will be less with the first stage cooling versus the second."

Can you explain why?

Do note that the 2 stage solution is smaller (3 ton) and slightly higher efficiency (17 SEER) than the single stage (3.5 ton, 16 SEER).

If a 3 ton 2-stage condenser is the correct size, then the 3.5 ton single stage is over sized. Conversely if you really need 3.5 tons of cooling, then you are going to be short on hot days with the 3 ton unit.

Thanks,

Existing A/C is 3.5 tons, and not used heavily due to shady location, cedar roof and added insulation & air tightening. My calculations show 3.5 tons is appropriate, but think likely OK downsizing, especially if only issue is slight discomfort on hot days.

My real concern is the $1000 pricing differential between the one stage and two stage. All the pricing I've seen shows difference in hardware cost should be about $300 with no difference in installation time / difficulty and I hate the idea that I'm being soaked for wanting a premium two stage product.

The general rule of thumb I have seen is you take the hardware cost and double it to come up with the installed customer price. Are you also getting a true 2-stage thermostat with the upgrade? If not, then I agree the extra $1000 is over priced.

Talk contractor and see if he can explain why it is an extra $1000.

I'm planning to get the Evolution Connex thermostat, but priced separately, so not part of the $1000.

I said,
"the cost per BTU extracted from the home will be less with the first stage cooling versus the second."

Mike asked, "Can you explain why?"

Sure. Work done by a HP compressor or pump is related DIRECTLY to the product of mass flow rate and change in temperature. Mass flow rate varies as the cube of the power required by the compressor.

Take a pump sitting out in the back yard. It's pumping down your swimming pool at 2 gallons a minute - you double the pump speed and the flow approximately doubles. The pump will now be drawing eight times the electric power that it was drawing at 2 gpm.

Generally a 2 stage A/C compressor does about 60% of full mass flow rate capacity when operating in single stage. The electric energy required in single stage would be approximately 22% of the electrical energy required at 2 stage (or full) capacity in order to do 60% of the work.

While there are other electrical loads associated with operating an A/C unit, the compressor is by far the major component electrical load.

EDIT: Mike, there are some Carrier performance curves that I had previously reviewed that demonstrate this performance - I can not currently put my finger on them.

This post was edited by saltidawg on Tue, Oct 29, 13 at 10:46

"Mass flow rate varies as the cube of the power required by the compressor."

Does this relationship depend on the other design and operating parameters of the system being constant? It seems to me that is must. If other variables are introduced, the cube relationship will change.

The other assumption is that as motor output is varied the motor efficiency is constant. I understand that hydrodynamics makes compressor/pump input increase faster than output, but as many motors are slowed, the conversion of electrical power to mechanical power drops off.

I don't know how all these dual speed systems work. Some of them, I understand, have two compressors so that could obviously boost motor efficiency at lower speeds

"Take a pump sitting out in the back yard. It's pumping down your swimming pool at 2 gallons a minute - you double the pump speed and the flow approximately doubles. The pump will now be drawing eight times the electric power that it was drawing at 2 gpm."

Saltidawg,

I know better to get into a discussion with you about water pumps. I also don't want to hijack this thread, but I want to understand how this cube law works in relationship to 2-stage compressors.

A water pump is a type of motor. The efficiency of the motor should be the work it does compared to the input energy it takes to drive it. Assume a water pump can move X gallons or water per minute at Y revolutions per minute, and consume Z amount of power. If I increase the speed to 2Y RPMs, I would expect to move 2X GPM and consume about 2Z Watts. But you are saying by doubling the rate flow, the power would increase to 8Z. This seems to be a big drop in efficiency.

If what you say it true, then I should see this big change in power consumption on the data sheets of 2-stage AC condenser between the low and high stages. Yet I don't see it.

What am I missing?

I apologize for not answering sooner. I had eye surgery yesterday for Entropion and had an eye bandage on until this aM and now with the nedications my vision is still not very good.

The cube relationship - double the flow for a compressor requiring eight times the input power has ABSOLUTELY NOTHING to do with moto efficiency. Overall heat pump efficiency IS affected by motor efficiencies and thus a lower compressor motor efficiency will have a effect on HP
Overall Efficiency. However, when comparing overall HP efficiency at two differing compressor loads, the motor efficiency differences are second order effects.

Let's imagine a 100% efficient power source for a compressor - be it a motor, a steam turbine, or a pneumatic motor. Double the mass flow rate requires eight times the input power. Nothing to do with driver efficiency.

In the real world of not 100% devices, to increase power by a factor of eight might require, say, nine times as much electical or steam or air input to the motor. (Just an example.)

An electrical analogy where a resistor is put acros a ideal source battery's output. The resisitor is by nature frequently regarded as 100% efficient in converting electrical energy to heat energy. In this case there is not a cubed relationship, but rather a squared relationship between current and power.

12 volt battery with a 12 ohm resistor across the terminals. One amp of current. The resistor dissipates 1X1 X12 - 12 Watts.(One amp squared times 12 ohms = 12 Watts.)

Add a second battery in series with the first. We now have 24 Volt battery source across 12 ohms. Now have 2 amps.

(Each battery is providing one amp of the two amps)

The resistor now dissipates 2 X 2 X12 = 48 Watts.

(2 amps squared times 12 ohms equals 48 Watts.)

NO change in the resistor's efficiency but doubling the voltage doubles the current and yields FOUR times the power.

To complete the analogy, you could add a small resistor in series with the battery that might represent internal resistence and/or other reall losses in battery performance. This would tweak thye result away from a actual squared law... similarly, differences in pump efficiencies can be modeled in compressor and pump model studies.

This is done all the time in ship design and other commercial applications.

Mike. re your example of motor power...

some humor (?)

If a hen and a half lays an egg and a half in a day and a half, how many eggs does nine hens lay in nine days.

Nope, not nie. :-).

Saltidawg,

I hope your eye surgery was successful. We need you to keep a close watch on this forum :).

I am struggling with with the cube law relationship. So here is my simple question.

If a 2 ton compressor draws 10 amps at 240 volts when operating in steady state condition, then how much current would I expect to see a 4 ton compressor use?

"If a 2 ton compressor draws 10 amps at 240 volts when operating in steady state condition, then how much current would I expect to see a 4 ton compressor use?"

Mike,

Sorry, my left eye is really messing with me... looks like I came out on the short end of a brawl. ;-)

Your question suggests to me that I have not been very articulate in presenting my fluid dynamics/mechanics explanation of power requirements versus mass flow rate thro a compressor or pump.

The answer to your question is, "I have no idea."

The issue is that we are talking about two different compressors in your question. One designed to an operating point supportive of a 2 ton A/C plant. The second compressor is optimized to a 4 ton plant.

The issue I have attempted to address concerns ONE compressor that may be operated at two different operating conditions.

Some compressors may be designed for only one operating point, others may be optimized over a range of operating points. The latter situation frequently leads to a multiple stage design.

PS I see that Carrier repeatedly makes reference to improved performance efficiency with 2 stage compressors. They do not advertise the second stage as being a better humidity remover with the nice benefit of possibly alllowing a lower set temperature. (I belive I am quoting their meaning correctly. lol)

By the way, is the answer you were looking for like an increase in current from 10 amps to like 10.5-11.0 Amps? Just curious.

This post was edited by saltidawg on Fri, Nov 1, 13 at 20:42

I am trying to understand the cube law you described in your earlier post. I think the the 4 ton compressor would use about 20 amps which is twice that of the 2 ton compressor. My logic is the SEER rating about the same so the relationship of cooling and power consumption should be about the same.

I have a 2-stage reciprocating compressor in my Carrier AC condenser. In high stage it is rated for 2 tons, in low stage it is about 1.2 tons. Are you saying if the compressor uses 10 amps in the low stage, it would use about 1000 amps in the high stage?

mike,

Did you not read my reply two posts up?

No disrespect, but I think you've also forgotten some of the earlier info I provided you.

I've added a link here to an early 2000's Carrier White Paper on Compressor design. It is guidance for A/C and HP designers in system design. It clearly shows that for a GIVEN compressor, acting at below design flow rates results in an increase in thermal efficiency... which makes the Thermodynamics guys and mechanics guys happy. :-)

Here is a link that might be useful: {{gwi:1595934}}

This post was edited by saltidawg on Mon, Nov 4, 13 at 19:52

saltidawg,

I read all your posts because I find them informative and they make me smile.

However I am still not connecting this statement:

"Mass flow rate varies as the cube of the power required by the compressor."

with the examples you have given.

Is there a paper associated with the link you provided? All I see is a chart and no document.