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  1. #1
    Join Date
    Nov 2018
    Location
    Quebec, Canada
    Posts
    19
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    Confused New system sizing question - Single-stage 2-ton vs 3-ton inverter (Lennox XP25)

    I need help deciding between a single-stage 2.5 HP vs a variable-speed 3-ton HP.

    I have a 20 years old electric furnace (15kw) with a 2 ton central AC that both need to be replaced. My current York air blower is rated 1000 CFM; it's a three speed PSC motor, so can't be sure of actual true speed (between ~850 to 1150 CFM based on installation manual). The speed is constant and I'm happy with the noise.

    I've already got three quotes and most of them suggest either a 2.5-ton single-stage HP (e.g. Lennox XP14) or a 3-ton variable-speed HP (e.g. Lennox XP25). In all cases, they also include a variable-speed air handler and 15kw heat strip (e.g. Lennox CBA38MV).

    I'm in Montreal (so cold winters) and my house is approx. 1700 sq-ft (on two floors) + basement (total 2500 sq-ft), but I usually keep the basement registers closed all year.

    If I go with a 2.5-ton XP14, I'm affraid the single-stage HP won't remove as much humidity than my current 2-ton AC does. So I thought the XP25 was a no-brainer since being variable-speed, it can work like a 1-ton system up to a 3-ton system depending on the needs.

    But the more I read on the XP25, I'm not so sure how "variable" and "automatic" the CFM control really is. I see jumper settings for fan speed in the air-handler documentation and this thread from 2-years ago (on this forum -- h_t_t_p_s://tinyurl.com/yaludlee) that scared me. I don't understand exactly why, but I'm afraid the CFM may end-up higher than my current setup and that I won't like the noise.

    My initial reasoning was to compare with my current York system. With its 15kw heat strip (~50k BTU), it works fine at around 1000 CFM airflow. The 3-ton XP25 at max heating performance would never output more than 36k BTU, so I thought the variable-speed air-handler should never need to go above 1000 CFM. But I am apparently wrong since the documentation allows for up to 1500 CFM with that air-handler / HP combination. And that other guy (in that older thread) was having similar issues as well.

    Does anyone has experience with XP25 systems and how airflow is managed?

    Any help is appreciated,

    Thanks,

    Larry.

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    8,803
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    You are right about the point of controlling air flow to remove moisture from the home while cooling. To get 50%RH in the space, you need a minimum of 30^F split from the return air temperature and the coil temperature.
    How do you deal with rainy weather, low/no sensible cooling loads, evenings, and the moisture from the occupants now. Have you monitored your indoor %RH before?
    Read my previous post if you are concern with solutions to this green grass climate problem.
    Adding a small whole house dehumidifier to your systems fixes the high humidity problem during rainy days and cool wet evenings. But being able to adjust the a/c air flow to remove enough moisture during significant cooling loads is critical even with a whole house dehumidifier
    Keep us posted on what you do and how it works.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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