Two Stage or Single Stage Pump?

[pieatro]

I've got a heat pump quote with one unit with a two-stage pump and one with single-stage, both units have variable speed fan on air handler. Contractor included estimated energy use/cost. There is so little difference that I'd have to keep the system for 20 years to justify cost of two-stage pump. What is the advantage of a two-stage pump and when would you consider using it?

[SouthTex]

Most of the time your comfort level is better, air flow quieter and more energy savings than what most contractors can guarantee. Due to the lower humidity we are able to run our temperature higher in the cooling season and still feel comfortable.

[pieatro]

SouthTex, maybe I need more knowledge about what the two-stage pump does and how it operates. The compressor pump moves the refrigerant throught the system and produces a high and low side. Are both stages/pumps the same size? In a two-stage system under what conditions does the second stage run and when does the first stage run?

[gary_g]

SouthTex, maybe I need more knowledge about what the two-stage pump does and how it operates. The compressor pump moves the refrigerant throught the system and produces a high and low side. Are both stages/pumps the same size? In a two-stage system under what conditions does the second stage run and when does the first stage run?

What part of the country?

I would consider a 2-stage condenser if I lived in a region that had long summers.

Depending on the mfr and model number, a 2 stage condenser can be 50% capacity on first stage, or 66%, or 80% of capacity (or some other percentage).

In cooling mode, a 2-stage condenser allows for better humidity removal when it runs on 1st stage (less cycling on/off, more run time at lower capacity). Also in cooling mode, EER (Energy Efficiency Ratio) is higher on 1st stage than second stage.

The thermostat usually determines when to kick the system into 2nd stage.

2 stage condensers are more for comfort than energy savings.

If you can afford it, go for the 2-stage. If not, a single stage condenser (properly sized and installed) should work just fine.

Take care.

[SouthTex]

Some mfg use 2 seperate compressors and some use single compressor that unloads. Each mfg has a different split on the capacity of each stage. On a call for cooling the first stage comes on if the temp continues to go up the second stage comes on. My varible speed fan changes with the stage change, thus at first stage my air flow is less and quieter. 100 degrees here for 37 days and mine has stayed in first stage most of the time. House is of average construction, would do even better with sealing the house better and sealing the ducts.

[pieatro]

Yes gary_g I'm beginning to understand that location matters. I live in North Carolina. We certainely get our share of mid 90s in summer but usually our severe summer is only for about 2 months. It appears a 2 stage pump has more value/pay-back in places like Florida, Louisianna, etc. but I'm doubting energy savings here in NC. I can see advantage of long run times with lower humidity with 2 stage but can't seem to justify the extra cost. Does 2 stage pump factor into heat pump heating cycle?

[beenthere]

Common misconception, that 2 stage is better in hotter/longer summer areas.

A 2 stage in an area with a mild summer, canstill provide better comfort then a single stage.
And in some cases, save money on operating cost.
Many people find they can set their thermostat up a degree or 2 because of thelower humidity. And save money on operating cost.

In heating. The longer run time in first stage helps provide a more even temp through out the house. And reduce the feeling of a cool draft that you can sometimes get from a short on cycle.

[danglerb]

Its more about comfort and quiet than dollars and sense. Comfort is temperature and humidity together, and with two stage cooling the lower stage keeps you in the comfort zone better. Single stage on the less than hottest days will cycle off too long, so you have much larger changes in humidity and associated comfort.

[Alden_Sloe]

Are you comparing apples to apples? The two stage units tend to be higher end units that carry higher SEER/HSPF numbers. They typically use higher quality parts and often have a better warranty.

I'm not sure when they rate the 2 stage units if they are "averaged" or just assuming conditions that require total capacity. If the later then I can see where real world numbers would be much better with the 2 stage.

Even here in Washington State the the heating and cooling system has a huge range over which it must operate. While a single stage might be "just right" 80% of the time that other 20% it's really nice to be able to shift gears. :D

I'm amazed at how much water the AC pulls out of the air. Our coil drains to a sump in the basement. The sump will run even in the summer because of the ground water so I hadn't really thought too much about it. I was down in the basement the other day after it had emptied and was shocked at how much water was draining into the sump from the AC. We don't have heat spells all that often but that makes it all the worse when we do because nobody is accustomed to it. It's made even worse by the fact that it's almost always a muggy heat. A very nice bonus to have the dehumidifier :)

[pieatro]

Are you comparing apples to apples? The two stage units tend to be higher end units that carry higher SEER/HSPF numbers. They typically use higher quality parts and often have a better warranty.

Carrier 25HPA524A30 - 15.5 SEER, 8.5 HSPF Total annual cost = $709
Single-stage pump

Carrier 25HNA924A30 -17.9 SEER, 9.0 HSPF, Total annual cost = $738
Two-stage pump

What am I getting from 2 extra SEER, .5 HSPF and almost 30% more $$? - If the contractor's calculated cost is correct. He stated he got his estimates of cost from Carrier. Warranty is the same.

[beenthere]

No prices, we don't talk prices here.

I removed it.

[Alden_Sloe]

I think he's quoting estimated operating cost which should be OK but the numbers make no sense. The higher efficiency unit will cost less not more. Look up the numbers on the AHRI website. Note that the coil used and blower/furnace selection makes a big difference (that's why the advertising always says "up to" 17 SEER - your mileage may vary). Also get the contractor to write down the actual model numbers of the furnace and coil on the contract; and then make sure that's what they actually install. Not everyone's as upright as the Pros on this forum ;)

[beenthere]

No, he had a price, which I edited out. That you can't see.

[pieatro]

No, he had a price, which I edited out. That you can't see.

I actually had a price difference not a price. Are price differences not allowed? How about a % difference?

[beenthere]

Nope, no price differences.

Percentages, slide by.

They really don't mean anything since no dollar amount is applied.

[pieatro]

Percentages, slide by.

I made the edit.

[pieatro]

but the numbers make no sense. The higher efficiency unit will cost less not more.

Do you mean cost less to operate? If so then it does, just not enough if you accept the annual cost given by the contractor, or at least Carrier.

[Alden_Sloe]

Carrier 25HPA524A30 - 15.5 SEER, 8.5 HSPF Total annual cost = $709
Single-stage pump

Carrier 25HNA924A30 -17.9 SEER, 9.0 HSPF, Total annual cost = $738
Two-stage pump



I'm looking at this and see the two stage annual cost being $19 more per year. That can't be right.

[pstu]

I think an important point is that in a mild summer climate, the "payback period" due to higher SEER/EER may be very long or nonexistent -- if you measure everything in dollars and cents. The best of the 1-stage units come awfully close to the 2-stage rated efficiencies. The comfort argument is valid in both hot and mild climates. I'm a homeowner, not trained in the technical parts but can understand things like payback period and runtimes. My 2-stage Trane has about 55% of its capacity on 1st stage and that allows for some long runtimes, a 1-stage of the same capacity would surely cycle lots more.

Hope this helps -- Pstu

[pieatro]

I'm looking at this and see the two stage annual cost being $19 more per year. That can't be right.

Well, if one of the pros here has access to Carrier's cost software then maybe I can determine if this IS a mistake.... or not. I was surprized as well. Maybe the contractor has a unit on the floor he wants to move so he fudges the cost payback???

[Alden_Sloe]

Not sure what climate you're dealing with. On heating the two stage is about 6% more efficient. If you're only spending $700 a year on heating that's only $42 a year in saving. The difference on the cooling side is a bit more but still probably less than $100 a year total.

One reason I went with the two stage is I felt it was OK to oversize it. I know, not supposed to do that. But, pretty much every contractor that bid the job said they'd put in a bigger unit to match the total load of the house but the ductwork wouldn't support it. Planning on doing major remodeling over the next 2-3 years and figured I'd be better off with the extra capacity now and add ducting when we have the walls torn apart. As a back-up plan I figured there would be a way to lock it in single stage if it was a problem. It only runs on second stage when there's extreme temperatures; the 20F winter cold snap and the 104F heat wave. During our "week of summer" here in Seattle I was sure glad we had the bigger unit. During the day it was running full bore at about 70-80% duty cycle. Same thing in the cold. I figure we probably saved $100-150 in gas bills by virtue of rarely ever having to rely on the furnace. That paid for my AC this summer :cool:

[pieatro]

Not sure what climate you're dealing with.

Central North Carolina


Planning on doing major remodeling over the next 2-3 years and figured I'd be better off with the extra capacity now and add ducting when we have the walls torn apart.

I added 450 sq ft to my main floor which had/has a 1 1/2 ton A/C system with propane furnace. I spent a lot of time sealing and insulating the addition. I haven't noticed any change in the loading on my system. I would do as much as you can to seal out drafts/leaks/infiltration and put in good insulation.