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  1. #1

    Single Stage vs. Dual Stage Heat Pump?

    I am in the process of selecting a contractor for a replacement furnace and air conditioner in Minnesota.

    One contractor stood out as trying to meet our needs and sizing things properly (including looking at all supplies and returns). They are recommending a 3-stage Carrier 95% furnace and a 3.5 ton, single stage Carrier heat pump (which matches the size of the existing AC). The heat pump recommendation surprised me, because of all of the benefits touted by the other contractors about the humidity control that a 2-stage HP could offer.

    We have two stories and a finished basement. The bedrooms in the upper story are usually too warm at night with the existing setup, even with the supply dampers closed in the basement. We have returns in each of the bedrooms. However, we have a large two-story great room, and there are no returns covering that space, as well as a similar entry-way space (about the size of two bedrooms in total).

    The contractor's arguments for recommending a single stage instead of a dual stage heat pump are:

    1) The single stage has a higher SEER and COP for most temperature ranges

    2) The single stage is available in 3.5 ton, while the dual stage is only available in 4 ton, and our need is 3.5 ton

    3) More importantly, he believes that if we went with a 2-stage and operated in low, the cold air would not adequately reach upstairs. On the heating side, they would rarely use the low setting on the heat pump. And, with the variable speed capabilities of the furnace blower, combined with the smart controller, we would get sufficient humidity control with a single stage.

    Given this application, are there opinions about the logic and recommendation to go single stage instead of two stage for the heat pump?

  2. #2
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    1.Savings from running in low stage may trump that.

    2.Not a big issue,unless the ducts are undersized now for the 3.5 ton.

    3.If that's true,ask how the hot air will get there,in the low stage of heating.

    Two stage for humidity control while cooling is great.
    Last edited by dash; 06-30-2009 at 07:04 AM.

  3. #3
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    Seems to me the higher efficiency is more important.

    Heat rises so not as hard to get warm air upstairs.

    While they say 2 stage dehumidifies better, that's not much of an issue in MN vs Florida.

    Just a few things to consider on the other side of the fence!

  4. #4
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    Quote Originally Posted by BaldLoonie View Post
    Seems to me the higher efficiency is more important.

    Heat rises so not as hard to get warm air upstairs.

    While they say 2 stage dehumidifies better, that's not much of an issue in MN vs Florida.

    Just a few things to consider on the other side of the fence!
    How is it not an issue in MN? It should be more of an issue in MN because it is can be humid and not as hot. There are 10,000 lakes in the state. I'd think 2-stage would be far more beneficial in MN than it would be in Florida where there would be much more heat to force the run times up.

  5. #5
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    Except on 1 Carrier model and 1 Trane model, low cool is 75-80% of high cool. Not much difference, run times won't be that much longer. One popular model of 2 stage has some high sensible heat ratios so on low it doesn't dehumidify well anyway. The TS equipped Carrier could do better with more like 60% on low. But any unit on low is using an awfully big coil compared to capacity, that's why the moisture sucking can be reduced.

    While I never had trouble feeling comfy in MN with a conventional A/C system, if someone really wanted dry air they could go with a control that reduces blower speed to really suck out the moisture or since much of the time it is very humid, the cooling may be on so little that no matter what you do, the A/C doesn't dry the air enough, go with a whole house dehumidifier.

  6. #6
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    Quote Originally Posted by badtlc View Post
    How is it not an issue in MN? It should be more of an issue in MN because it is can be humid and not as hot. There are 10,000 lakes in the state. I'd think 2-stage would be far more beneficial in MN than it would be in Florida where there would be much more heat to force the run times up.
    While you do have 10,000 lakes.... Florida has the Gulf of Mexico. Not much of a comparison.

    While MN does get humid days, the actual dewpoint rarely gets much above 65F thanks to cool overnight temperatures. You outdoor design tmperature will be lower so the equipment, if sized correctly, should have enough load to keep the humidity well controlled without needing 2 stage. With such a short cooling season, I think it's somewhat of a waste. Getting a VS blwoer and using dehumidify on demand to slow the blower for more latent capacity, should do just fine.

  7. #7
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    Quote Originally Posted by BaldLoonie View Post
    Except on 1 Carrier model and 1 Trane model, low cool is 75-80% of high cool. Not much difference, run times won't be that much longer. One popular model of 2 stage has some high sensible heat ratios so on low it doesn't dehumidify well anyway. The TS equipped Carrier could do better with more like 60% on low. But any unit on low is using an awfully big coil compared to capacity, that's why the moisture sucking can be reduced.

    While I never had trouble feeling comfy in MN with a conventional A/C system, if someone really wanted dry air they could go with a control that reduces blower speed to really suck out the moisture or since much of the time it is very humid, the cooling may be on so little that no matter what you do, the A/C doesn't dry the air enough, go with a whole house dehumidifier.
    Yup.... and worse, to get higher SEER one might foolishly be inclined to oversize the coil on a 2 stage unit. Guess how cold a 4 ton coil gets with 2.2 tons of capacity of a 2 stage 3 ton unit??? Not very cold. I'm struggling with this problem. I'm comming to the conclusion that 2 stage units are somewhat overrated for dehumidification.

  8. #8
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    *

    Quote Originally Posted by motoguy128 View Post
    Guess how cold a 4 ton coil gets with 2.2 tons of capacity of a 2 stage 3 ton unit??? Not very cold. I'm struggling with this problem. I'm comming to the conclusion that 2 stage units are somewhat overrated for dehumidification.
    talking about coil temps;

    the fact is, the blower speed is lowered as the compressor kicks into low speed

    which in turn keeps the coil as cold as in high speed

    low speed compressor and indoor blower is going to run longer than if in high speed

    which will remove more humidity than a single stage system, when set-up properly



    .

  9. #9
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    Nobody has mentioned quietness yet. Things are much quieter in low speed. I have a modulating furnace. I hardly ever hear the furnace run. Unfortunatley, my AC is single-speed and I do hear the system in cooling mode. A home is so much more enjoyable when the hvac equipment is quiet.

    Larry

  10. #10
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    [QUOTE=larryb;3749842]Nobody has mentioned quietness yet. Things are much quieter in low speed. I have a modulating furnace. I hardly ever hear the furnace run. Unfortunately, my AC is single-speed and I do hear the system in cooling mode. A home is so much more enjoyable when the hvac equipment is quiet.

    If your system is that loud in cooling mode your duct work could be undersized?

    The 3 stage Carrier furnace is a nice system, I have had mine for 1-1/2 yrs now and I love it. I'll be upgrading my 10yr old Payne 10 seer AC sometime this summer and I to am contemplating going with a 2 stage AC or HP but am still unsure if it's worth the extra money to do the HP so the jury's still out as far as that goes.

    If your farther north than I am here in Wisconsin about a 100 miles south of Greenbay than I'm not sure your going to benefit from a 2 stage AC or HP as your payback will be somewhat non-existent. I would have someone run some numbers for you in your area to give you something to compare side by side with a single stage AC/HP and a 2 stage AC/HP so you can see just how they compare with that of area temps for your area.

    Call your local utility company and see what they have to offer as far as a home accessment goes which would include a blower door test and other test in and around the home and see if you can't improve your house first then get a Load calc done and see where your at. I would bet a single stage AC/HP would suffice for you in your area ?

  11. #11
    Thanks to all for the thoughts on single vs. dual stage. While there is no clear answer, the debate has helped me lean toward going with a single stage. The same contractor also recommended a 14 SEER unit instead of the 16-17 quoted by the others, because it would not yield enough savings in our climate to warrant the extra cost. So, they are giving me the impression of selling me only what I need, and no more. They also have a great deal of experience in our area.

    On the heat pump vs. AC question:

    I downloaded 10 years worth of hourly climate data for the Twin Cities and found that we spend about 85% in heating season (< 70 degrees), and 15% in cooling season (> 70 degrees). In the heating season, we spend 30% of the time below 30 degrees (gas furnace range) and 70% between 30 and 70 (heat pump range). Given our energy costs and our heating gas usage (adjusted for a new 95% efficient furnace), the heat pump would save about an additional $300/year, or a 5.5 year payback given the increased cost for the heat pump. Not great, but probably enough to tilt me to the heat pump vs. the AC.
    Last edited by chrysalishead; 07-01-2009 at 01:33 PM.

  12. #12
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    Quote Originally Posted by chrysalishead View Post
    Thanks to all for the thoughts on single vs. dual stage. While there is no clear answer, the debate has helped me lean toward going with a single stage. The same contractor also recommended a 14 SEER unit instead of the 16-17 quoted by the others, because it would not yield enough savings in our climate to warrant the extra cost. So, they are giving me the impression of selling me only what I need, and no more. They also have a great deal of experience in our area.

    On the heat pump vs. AC question:

    I downloaded 10 years worth of hourly climate data for the Twin Cities and found that we spend about 85% in heating season (< 70 degrees), and 15% in cooling season (> 70 degrees). In the heating season, we spend 30% of the time below 30 degrees (gas furnace range) and 70% between 30 and 70 (heat pump range). Given our energy costs and our heating gas usage (adjusted for a new 95% efficient furnace), the heat pump would save about an additional $300/year, or a 5.5 year payback given the $1600 increased cost for the heat pump. Not great, but probably enough to tilt me to the heat pump vs. the AC.
    Good analysis.

    Not sure if I would use 70F as the cut-off between heating season and cooling season, but I don't think that would change your end result.

    Best of luck.

  13. #13
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    Quote Originally Posted by chrysalishead View Post
    Thanks to all for the thoughts on single vs. dual stage. While there is no clear answer, the debate has helped me lean toward going with a single stage. The same contractor also recommended a 14 SEER unit instead of the 16-17 quoted by the others, because it would not yield enough savings in our climate to warrant the extra cost. So, they are giving me the impression of selling me only what I need, and no more. They also have a great deal of experience in our area.

    On the heat pump vs. AC question:

    I downloaded 10 years worth of hourly climate data for the Twin Cities and found that we spend about 85% in heating season (< 70 degrees), and 15% in cooling season (> 70 degrees). In the heating season, we spend 30% of the time below 30 degrees (gas furnace range) and 70% between 30 and 70 (heat pump range). Given our energy costs and our heating gas usage (adjusted for a new 95% efficient furnace), the heat pump would save about an additional $300/year, or a 5.5 year payback given the increased cost for the heat pump. Not great, but probably enough to tilt me to the heat pump vs. the AC.
    Careful with that temperature data. You may spend 70% of the TIME above 30F... but you only consume maybe 30% of the total energy at those termpatures. Remeber you burn at least twice the energy maintaining 70F indoor temp at 10F than you do at 40F.

    So 1 hour at 10F uses hte same energy as 2 hours at 40F.

    I initially made that mistake when running calculations for a dual fuel system. The savings were about half what I originally estimated.

    So you will get longer run times on the heat pump, and save energy. But it only outputs about 1/2 - 1/4 the heat of the furnace.

    I'm still a fan of dual fuel for flexibility and cost savings, but the furnace still does the "heavy lifting".


    I'd put it this way. In the north, it's a gas furnace with a heat pump. In the south, it's more like a heat pump system with supplemental gas heat instead of all electric.

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