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  1. #1
    Join Date
    Nov 2018
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    2 Stage or Modulating Compressor with Variable Speed Furnace/Air Handler?

    I am building a new house, and have determined that I want the benefit of a variable speed blower in my furnace primarily for comfort purposes. What confuses me somewhat is how much benefit there is for going from a two stage compressor to a fully modulating (variable) speed compressor when paired with a variable speed furnace blower. I am not asking about the cost/benefit of energy savings - it is clearly not worth it other than any improvement in comfort. The two stage compressors cost significantly less than the new modulating compressors, and if the air handler is variable speed, how much benefit in temperature consistency, limited on/off switching, etc. is there for going from using a two stage compressor to a fully modulating one? Any thoughts would be appreciated. Even if you have thoughts on reliability, however, I would also like thoughts on the comfort question as well. I live in the southwest, so it is warm and dry most of the time.

    Thanks!

  2. #2
    Join Date
    Nov 2011
    Location
    St. Louis
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    I personally can never tell the difference when I walk into a home with high end equipment or basic stuff. I don't know it until I actually see the equipment; although sometimes the lack of noise gives it away.

    I can confidently state that any benefit in energy savings and comfort is instantly lost after the one year labor warranty is done. Generally speaking: higher end = more moving parts = decreased liability. The phrase "get what you paid for" has been turned upside down in this industry.

    In the southwest you don't have humidity to deal with like many of us - even less reason to have high end stuff for comfort.

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  3. #3
    Join Date
    Nov 2006
    Location
    Athens, Ohio
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    I suggest a single stage air conditioner and a ducted dehumidifier instead of a two-stage air conditioner. Keep the furnace with a variable speed blower. The dehumidifier will operate during low cooling load conditions when it is not hot enough to use the air conditioner.
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  4. #4
    Join Date
    Nov 2018
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    Thread Starter
    I appreciate the comments. To be clear, however, the thing that really bothers me about single stage HVAC is that I hate the hard on/hard off. In addition, I tend to keep the fan running all the time for air circulation. Accordingly, I have already decided that I will definitely have a variable speed blower in my furnace/air handler, and will have at least a two stage compressor for my AC. I would really appreciate any comments on the difference is between using a two stage compressor when the air will be blown by a variable speed furnace/air handler, and using the same variable speed furnace/air handler and pairing it with a variable speed compressor. Since the air is always going to be blown by a variable speed air handler, I am simply not clear as to exactly what to expect as the difference when using a two stage or variable compressor. Even if you don't recommend it - which I am happy to listen to - I would appreciate trying to figure this out to satisfy my knowledge.

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    39,268
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    I would only go with a modulating outdoor unit with zoning. Otherwise temps can be uneven when the thing throttled back to a lower setting. The duct system is deigned for full airflow, if you are running 50% airflow, air will go to the closest registers and distant rooms can be warm. But they are PERFECT for matching zoning - same brand of zoning as equipment.

    I'm not convinced that 2 stage A/Cs dehumidify that well on low. I'd also suggest along with Dean a single stage and whole house ventilating dehumidifier. Teddy Bear will be along to tell you more.

  6. #6
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
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    Quote Originally Posted by jrmpco View Post
    I appreciate the comments. To be clear, however, the thing that really bothers me about single stage HVAC is that I hate the hard on/hard off. In addition, I tend to keep the fan running all the time for air circulation. Accordingly, I have already decided that I will definitely have a variable speed blower in my furnace/air handler, and will have at least a two stage compressor for my AC. I would really appreciate any comments on the difference is between using a two stage compressor when the air will be blown by a variable speed furnace/air handler, and using the same variable speed furnace/air handler and pairing it with a variable speed compressor. Since the air is always going to be blown by a variable speed air handler, I am simply not clear as to exactly what to expect as the difference when using a two stage or variable compressor. Even if you don't recommend it - which I am happy to listen to - I would appreciate trying to figure this out to satisfy my knowledge.
    I like Dean's idea on the dehumidifier. They work great. I disagree that the "higher end" equipment is more problematic. I really like your idea of the 2-stage A/C and variable-speed furnace. It allows you to zone the system at some point. I had my "high-end" Bryant system in our home for 15 years with no problems. Then we moved.

  7. #7
    Join Date
    Jul 2006
    Location
    Prata di Pordenone
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    7,224
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    a properly sized 2 stage unit would be my choice . a properly sized and properly installed and setup correctly 2 stage unit should provide long term reliability and comfort . the flip side is a hacked in unit of any brand and configuiration will be nothing but problems .
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  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    8,803
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    Quote Originally Posted by jrmpco View Post
    I am building a new house, and have determined that I want the benefit of a variable speed blower in my furnace primarily for comfort purposes. What confuses me somewhat is how much benefit there is for going from a two stage compressor to a fully modulating (variable) speed compressor when paired with a variable speed furnace blower. I am not asking about the cost/benefit of energy savings - it is clearly not worth it other than any improvement in comfort. The two stage compressors cost significantly less than the new modulating compressors, and if the air handler is variable speed, how much benefit in temperature consistency, limited on/off switching, etc. is there for going from using a two stage compressor to a fully modulating one? Any thoughts would be appreciated. Even if you have thoughts on reliability, however, I would also like thoughts on the comfort question as well. I live in the southwest, so it is warm and dry most of the time.

    Thanks!
    A well built home any place needs a fresh air change every 4-5 hours when occupied and windows closed. Also include an air filter that is a minimum of Merv 11, mainly to keep the equipment clean.

    Fresh air change complicates the challenge of having real comfort. A week of outdoor dew points above 60^F with fresh filtered air change and moisture from the occupants can leave a home a mess with the start of mold growth.
    The dries home out quickly but leaves behind the millions of small invisible mold colonies. They go dormant and wait for the next high humidity opportunity to pickup where they left off. Depending on the repeated damp cycles and their duration, you may have a problem.
    All of us living green grass climates know that we have this cycle often enough that we should deal with dampness in our homes. But there are areas that in US that we can ignore the issue. High elevation and dry climates lessen the concern. But a wet spell of a week may warrant the need for humidity control to avoid the long term concern. Coastal humidity may also need to be considered.
    Monitor your indoor %RH, <50%RH is very comfortable.
    The current a/c equipment is unable to control humidity with out significant sensible cooling loads. A fresh air change like 100 cfm at +60^F dew point plust the moisture from the occupants is 2-3 lbs per hour moisture load. For an ac to remove that amount of moisture, you need a steady 1-2 ton steady sensible cooling load with an ideal setup a/c.
    A day or two of cycling low/no sensible cooling will leave the home +65%Rh. Cool surfaces will be much higher start mold growth.
    I feel that VS blowers are great for fine tuning a/c and low flow/energy circulation. Having lived with variable speed a/c/heating is more difficult. Heat tends to rise and cooling pours into the home like water. This makes
    uniform supplying VS heat/cool are varying flows needs management. Zoning Helps!
    Hope this helps. You decide. Using a small whole dehumidifier is a good way to add fresh air to home. A unit capable of adding 80 cfm of fresh air and removing 2-3 lbs of moisture to home with a well setup a/c will provide insurance to simple single speed a/c will give the accurate temp/%RH control during design high temperature/high dew point to the extreme of zero sensible cooling loads and design outdoor dew point.
    The cost is about the same as a VS a/c. Long term, you will be have much better control and reliability.
    Keep us posted. Monitor our home with a couple temp/%RH monitors loand term and tell us how everything works out.
    One of our sponsors manufactures the Ultra-Aire whole house dehumidifiers.
    Ultra-Aire.com
    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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