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  1. #1
    Join Date
    Jan 2008
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    91

    Trane Dual Fuel; Heat Pump Questions

    Looking for advice from people who know about this. I have a new xv95 gas furnace, which replaced a Carrier Weathermaker circa 1990. The AC unit (also Carrier) still works, but when the time comes to replace that, I am thinking of going with a heat pump.

    I live in Ohio. Average winter temperatures swing terribly -- we can have highs in the 20s for a week or two, then in the 30s or 40s, then be in the teens. Lows can be anywhere from 30s to single digits. So conditions seem right for a HP, as well as gas for when it's colder. Summers get kind of humid -- dew points in the high 60s and highs in the low 90s are about as extreme as we get, with the occasional high 90 day. Usually we are high 80s, dp mid 60s in summer.

    I've read up on the Trane XL HP models and my main question is: should I go single or multi stage? (I understand that the 16 is a multi-step and the 19 has two compressors). Either way, how much of a difference does this make on the heating side of things? I'm under the impression that when heating, the aux heat is the electrical strips. Does this essentially make every heat pump a two stage system (first stage = compressor, second stage = electric)? Does a multi-stage hp stage the compressor(s) when heating, before the electric strips are used? If so, what controls this?

    Sorry about the long post. Thanks for any advise.

  2. #2
    Join Date
    Mar 2006
    Location
    The South
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    2,142
    Cub

    First of all, you do not use electric heat strips in a dual fuel system. The gas furnace becomes your aux backup when the outside temps get cold and your HP can no longer maintain your home's inside temp comfort level.

    What size is your new furnace-post mdl number?
    What size is your existing AC condenser?

    If you go to a two stg XL16i heat pump, then you have 4 stgs of heat-two from the HP, two from the gas furnace.

    IMO

  3. #3
    Join Date
    Jun 2007
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    The Twilight Zone
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    The XL16i has a dual-stage compressor, the 19i has a dual compressor. Coupled with auxillary heat strips, these 2 units will provide at least 3 stages of heating - 1st stage heat pump, 2nd stage heat pump, 3rd stage heat pump plus aux gas.

    Note that these multi-stage compressors are more for comfort in the summer than for heating in the winter. Most heat pumps in use today use 1 single-stage compressor which works very well.

    With your winters, I would concentrate on getting as many btus out of the heat pump based on its btu rating. Anotherwords, if it is a 3-ton rated heat pump, you want a full 36,000 btu at the rating temperature of 47F, not 34,000 btu. Some systems are short of their rated btus.

    Note that a heat pump may have a lower life expectancy than straight a/c because it runs in all seasons.

    Best to you.

  4. #4
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    Look at the 4TWX50----- it is the 15I. Your dual fuel would be your gas furnace and heat pump combination.

  5. #5
    Join Date
    Jan 2008
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    91
    First of all, thanks for all your responses.

    Tiger, new furnace is a 80K. Old AC unit is a 3.5 ton.

    Still a little confused on the aux/backup heat. First, from reading this forum, I got the impression that you do not want the compressor and gas furnace on at the same time. So, for aux heat you have electric strips to "help" the HP until the temp is so cold that the HP is no longer worth running at all.

    Or, with a dual fuel, are you saying that the HP is "on its own" and when it gets too cold outside you lock out the HP and use gas ("Emergency" heat?). In this case, I would you still need the electric stips as backup at least when the hp defrosts?

  6. #6
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by Cub View Post
    First of all, thanks for all your responses.

    Tiger, new furnace is a 80K. Old AC unit is a 3.5 ton.

    Still a little confused on the aux/backup heat. First, from reading this forum, I got the impression that you do not want the compressor and gas furnace on at the same time. So, for aux heat you have electric strips to "help" the HP until the temp is so cold that the HP is no longer worth running at all.

    Or, with a dual fuel, are you saying that the HP is "on its own" and when it gets too cold outside you lock out the HP and use gas ("Emergency" heat?). In this case, I would you still need the electric stips as backup at least when the hp defrosts?
    Cub; a quick scenario of how your system would work with a two stage heat pump and your two stage furnace.

    Your hp would run in first stage, if it can not keep up with the heat loss of the house the Tstat calls for second stage compressor if that does not satisfy the load first stage of the furnace then second if needed.

    The type of Tstat you choose and the set up will dictate how the system operates.

    As the heat pump runs and defrost is determined the furnace will come on to suppliment while in defrost.

  7. #7
    Join Date
    Jan 2008
    Posts
    91
    OK, Second, that make sense.

    For a thermostat, they installed a TCONT803 with my current system. However, the Visionpro IAQ sounds like a much more versatile stat, esp if I go with a multistage HP. With a single stage HP and a stat like the IAQ, does this mean that once the IAQ decided the HP was not keeping up, it would stop the HP and run the furnace?

    Also, I thought it was a bad idea to run the gas furnace and HP at the same time -- something to do with the compressor's head temp? Except maybe this is OK during defrost?

  8. #8
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    You can use the 803 with a remote outdoor thermistor with two stage or one.
    I would look at the 15I it is a nice unit and is single stage.

    When the HP is in defrost it is in the A/C mode and running the gas furnace does not hurt anything. The HP running in the heat mode along with the furnace simultainiously would be bad. The IAQ is a good Tstat but I would not switch from the 803,you will not see a return on investment or comfort that would be noticed.

  9. #9
    Join Date
    Jul 2004
    Location
    Massachusetts
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    They're having a little trouble being clear, apparently. With the proper thermostat and outdoor temperature sensor, the T-stat should be programmed to turn the HP off when the OAT reaches the pre-determined lock-out temperature. At that temp, the t-stat will then call on the gas furnace, knowing the HP is locked out because of the OAT. When the OAT rises above the lockout temp, then the reverse happens; the gas furnace is locked out and the HP takes over. If at any time the cost of operation of the HP or for any other reason you desire, you can switch to "Emergency heat" and the HP will be locked out completely and just the gas furnace will be used. So the two components (HP/Gas) do not operate simultaneously.
    At and above 35F for example, just the HP. Below 35F, just the gas furnace.
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  10. #10
    Join Date
    Jan 2008
    Posts
    91
    Great board -- This is all very helpful!

    One other question I have involves the AC. The two main reasons it sounds like people get a multi-stage system are for dehumidification and energy savings. I never actually measured RH in our house, but with our old AC we seemed comfortable enough, from a cooling and humidity standpoint. And that was with no comfort-r thing. So it doesn't seem like humidity control by itself is really an issue for us. That leaves energy savings. I know there are a lot of other factors, but how much energy savings can you expect singe vs multi-stage? From eyeballing our electric bill, we use about 400 kwh a month when the ac is on. Under the same conditions, is it possible to figure that a single stage like the 15 would use a certain amount while the 16 would use x amount less?

  11. #11
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    Jun 2007
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    Quote Originally Posted by Cub View Post
    I know there are a lot of other factors, but how much energy savings can you expect singe vs multi-stage?
    Your cooling costs are based on the Energy Efficiency Ratio (EER) and not SEER, much to the misunderstanding of many people. Whichever SEER system you get, regardless of single-stage or dual stage, choose a system with a 12 EER minimum. You may find that some fancy 16+ SEER systems fail to meet 12 EER.

    Good luck.

  12. #12
    Join Date
    Jan 2008
    Posts
    91
    Back again with a few more questions...

    If I go with the single stage HP (say, xl15i), what blower speed will the furnace run in heat mode, as compared to the gas furnace stages? The reason I ask is because I really like the first stage gas heat of the xv95 -- you can barely hear it. Comfort R feature aside, I'm concerned that the HP will generally run the blower at a much higher speed -- closer to stage 2 gas heat. Is this usually the case?

    On the thermostat side, will my Tcont803 be able to run the two stages of gas heat as two stages, where the stat calls for the second stage of heat? Or with the 803, will I have to use the timer option on the furnace to get to second stage gas heat? The guys who installed my xv95 originally had it wired so that second stage came on after a time delay. I much prefer having the thermostat control this.

    Thanks again for all your help.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    With the 803, you will have to use the timer for second stage gas heat.
    If you opted for the IAQ, it would control it.

    A 3.5 ton HP, would run the fan at the same, if not higher speed then your second stage of gas heat.
    Should be about the same speed your A/C runs now.
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