deciding between 2.5 ton 14 SEER or 3 ton 16 SEER/2 stage
I am having a hard time deciding on which system to use to replace my existing 2.5 ton HP that is 14 years old. Average electric bill over the last 123 months is $70.66. The minimum is $48.67 and the maximum is $116.00. Note: the few times that my electric bill is over $100.00 is during the heating season and not the cooling season.
Both of the new upgrades will replace the existing Air Handler with a new Variable Speed Air handler model.
Location is Rock Hill, SC. Single story house (1333 sq ft). Design conditions for the load calculation are: Winter Outdoor Temp: 10, Summer Outdoor Temp: 100, Winter Indoor Design Temp: 72, and Summer Indoor Design Temp: 76.
Loss/Gain Summary. Loss: 35791. Gain: 25479. using a Capacity Multiplier of 1.05 the recommendation was for a 2.5 ton single stage HP system with a variable speed air handler. SEER : 15.0 and HSPF 9.0.
A second recommendation was made for a 3.0 ton 2 stage/single compressor HP system also with a variable speed Air Handler. Note: different Air Handler for the 3 ton system. This 2 stage system runs at 70% capacity in stage 1. SEER 17.0 and HSPF 9.20. Note: a 2 stage system at 2.5 tons is not available.
The 3.0 ton system costs $$$ more than the 2.5 ton system. Now the contractor admits that the 3 ton system will not save me that much more money than the 2 ton system. The contractor does say that the 3 ton system will provide a better, overall comfort level than the 2.5 ton system.
Two stage HP seem to fairly new so my question is: Are there really any advantages to 2 stage systems vs. 1 stage system. Or is this nothing but marketing hype. They key point for me is will the 3 ton running at stage 1 be able to do the the job - say 85% of the time. If I end up running at stage 2 the majority of the time I would be better off with the 2 ton system because then the 3 ton system will be over sized. Am I correct in my thinking?
Last edited by scJohn; 04-03-2008 at 07:53 AM.
Reason: correct typo andremove $ difference
First remove the pricing, its not allowed on this forum.
Next. According to the load calc you posted, a 2 ton can't maintain the design temp set point.
So I wouldn't consider a 2 ton. A 3 ton 2 stage in first stage will be 2.1 tons. Or enough to do the entire load without going to second stage. What was the cooling sensible load.
This can be a problem in high humidity areas.
How well did the old 2.5 ton keep up? Might want to stay with a 2.5 ton.
Those outdoor design temps should be revisited. Rock Hill may see those temps, but you do not design systems based on the short term extremes. This is but one of the areas where load calcs can me skewed and cause the systems to be over sized.
Originally Posted by scJohn
Properly sized with two staged compressor, on demand dehumidification from the t-stat, and you will have better control over the humidity and probably allow for you to keep your stat set a degree or two higher and be just as comfortable. This could lower the equipment size (and cost) and reduce your operating cost.
Personally I'd go the single stage 2.5 ton with variable speed blower and use a Vision Pro IAQ (if not a Bryant or Carrier) for dehumidification. Chances are the HSPF for the single stage won't be much less than the 9.00 of the expensive one so your only savings will be in the summer and probably not add up enough to justify the premium.
If you go to a bigger unit, chances are you'll need an increase in supply & return duct size. Did the contractor suggesting the 3 ton for your little house bring that up?
Your design temps seem very extreme for that area. HVAC-Calc list outdoor temps for your area at summer 94 and winter 23. Your 3 ton at 70% might still be oversized. The ductwork also needs to be evaluated for the increased size.
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BeenThere. My general thoughts are that the cycles are short in the summer. I set the t-stat (non-digital) at around 78 and forget it for the summer season. For winter, the cycles are much longer with the t-stat set a 68 for night time and 70 during the day. Ten minutes or longer cycles are very common in the winter (mostly during the evening and night) whereas during the summer 10 minute cycles or longer would only be on the hottest days.
BadLoonie. No mention was made of increasing the overall duct size. There was no mention of that in the proposal. The only thing mentioned was swapping out the 14" return with a 16" return. Air handler is located underneath the house in the crawl space.
If those are your current run times with the 2.5 ton, you don't need or want a 3 ton.
2 stage units in the summer time, need long run times to remove enough moisture, the same as a single stage.
Some 2 stage don't remove a lot of moisture in first stage, so if they have short run times, you end up with high humidity.
PS: I believe you will find that Trane has a 2.5 ton 2 stage. I'm not a Trane fan.
So I can't tell you about its total performace.
Very true, Been.
The 19i starts at 2.5 tons. Being 2 compressor, it has 50/100% staging so low is really low. Also on low, it's moisture suckin' ability is better than the 16i on low if you really want 2 stage.
With baldies info.
If I was going with Trane, and the previous 2.5 ton was doing ok, I'd go with the 19i 2.5 ton then.
The load calc that has been run used rather extreme outdoor temps and said the home needed 2.5 tons. The real world experience shows 2.5 tons is significantly short cycling in both heat and cooling. Sounds like he needs to have a hard look at the assumptions input into the calc and adjust accordingly (currently garbage in - garbage out).
May be able to get it down to 2 tons with some improvements on the building envelop saving him money every day. Lots of 2 ton two stage units to choose from.