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Thread: Options for new HVAC system with low Amp draw

  1. #1
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    Question Options for new HVAC system with low Amp draw

    My situation:
    House 2 story, total ~4,500 sq ft.
    Installed solar system that produces plenty of power
    Limit for items running on solar during grid outage 40AMP breaker
    Two systems (one per floor)
    Houston, TX area

    Looking for a HVAC system that could run on solar only (during power outages), to heat and cool house (if possible).
    Alternatively a system that can cool on solar through one feed and heating on grid only (no 40A limit) through a second feed.

  2. #2
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    Runnings not your problem starting is. At 40 amps 1-Ton mini maybe 1 1/2-Tons.

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    One of those Bosch units? I'd imagine that the peak inrush on those is minimal compared to a straight scroll. That and an indoor unit that varies the blower speed based on temp differential and you might have a good chance at full solar.
    “A dying culture invariably exhibits personal rudeness. Bad manners. Lack of consideration for others in minor matters. A loss of politeness, of gentle manners, is more significant than is a riot.”
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  4. #4
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    Unless I am not mistaken,a standard solar install will shut down and will not allow production while the grid is down (called anti-islanding). Maybe your system is different but if you are net metered i would be surprised if it would allow it.

    Just because you have a 40amp breaker doesn't mean you have 40amps available. What size pv system do you have? When there is no sunlight, shade, rain or higher temperatures your output will change and the last thing you would want to do is brownout your equipment or inverters with low voltage

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  5. #5
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    Thread Starter
    Thanks for the feedback,
    my current ~15 year old 5 ton unit can start up and run just fine (limit is 40A per breaker, not total).
    The issue is the heating coils in the air handler require 60A breaker(s).
    Hence with the old system we have 2 feeds, one for heating coils and another for the compressor, blower, and transformer

    The solar system is backed up by battery with an automatic disconnect from the grid in order to allow operation during grid outages.
    The size of the system is 64 panels at (I think 350W) per panel -> 22.4kw with 32kw/h of battery capacity

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  7. #6
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    Your starting a 5-Ton unit on utility power. The inrush current is in the 100 - 300 amp range. That breaker has a time delay for 3-5 seconds before tripping.

  8. #7
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    Quote Originally Posted by pecmsg View Post
    Your starting a 5-Ton unit on utility power. The inrush current is in the 100 - 300 amp range. That breaker has a time delay for 3-5 seconds before tripping.
    This is correct...
    Starting an elec motor (including the compressor motor in your AC), requires 300-400% of the run amps, for about 1-2 seconds...
    As noted above, this is called 'inrush current'.

    This can be lowered with a hard start kit, however not to 40A.

    You need a variable capacity unit, that has a variable capacity compressor...
    Note these are the premium units available, and they are NOT cheap.

    Another option would be to get a 10-15KW storage battery for your solar system... the battery will provide the inrush current.
    If you plan to do this... get a sparky that understands motors and inrush current to help you with choosing the battery.

    Just because a solar storage battery says it holds 15KW... does NOT mean it will throw 125A inrush current to start a 5T AC unit.

    There is a LOT more to this than is apparent...

    Here is a place to start:

    NorthernTool.com
    Go to the generator section and read the articles on choosing generator sizing...
    That will get you started
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *The value of comfort, over the long term; leave economic choices behind!
    Choose your contractor wisely!

  9. #8
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    ....and the Bosch Bova is a heat pump with capability to -7F
    “A dying culture invariably exhibits personal rudeness. Bad manners. Lack of consideration for others in minor matters. A loss of politeness, of gentle manners, is more significant than is a riot.”
    ― Robert A. Heinlein

  10. #9
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    A soft start kit will lower your inrush considerably. I have only used them twice but it was night and day


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  11. #10
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    Quote Originally Posted by Juan Madera View Post
    ....and the Bosch Bova is a heat pump with capability to -7F
    WHEN will Houston TX have -7'F.?..??...???
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  12. #11
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    Quote Originally Posted by Timrf79 View Post
    Thanks for the feedback,
    my current ~15 year old 5 ton unit can start up and run just fine (limit is 40A per breaker, not total).
    The issue is the heating coils in the air handler require 60A breaker(s).
    Hence with the old system we have 2 feeds, one for heating coils and another for the compressor, blower, and transformer

    The solar system is backed up by battery with an automatic disconnect from the grid in order to allow operation during grid outages.
    The size of the system is 64 panels at (I think 350W) per panel -> 22.4kw with 32kw/h of battery capacity
    Not sure I'm getting a clear picture here.

    Are both of the AC systems (forget about the heat for now) connected to the solar system?

    Have you run both of the systems on cooling on just the solar system (by shutting off the grid power) with good results or what are the limits you found?
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  13. #12
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    Quote Originally Posted by Timrf79 View Post
    My situation:
    House 2 story, total ~4,500 sq ft.
    Installed solar system that produces plenty of power
    Limit for items running on solar during grid outage 40AMP breaker
    Two systems (one per floor)
    Houston, TX area

    Looking for a HVAC system that could run on solar only (during power outages), to heat and cool house (if possible).
    Alternatively a system that can cool on solar through one feed and heating on grid only (no 40A limit) through a second feed.

    I would presume a 4,500 SF residence
    < 20 years old in TX
    would only require 4 or 5 tons Total.

    3_ Ton and 2_ ton equipment, 2 Stage with soft start
    will _ SIGNIFICANTLY _ decrease INRUSH Current.

    + Post # 5 __
    " ...
    The size of the system is 64 panels at (I think 350W) per panel -> 22.4kw
    with 32kw-hr of battery capacity. ..."

    ATTACHMENT
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    Attached Images Attached Images
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

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  15. #13
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    A Trane XV-20i will start. Supposedly they start lower than running amps and will give full heat value to around 0 with the heat pump. As far as the electric elements 40 amps will get you close to 10 kW. Some elements are not rated for a full 20A so with the right kit you could get 19kW but you will not be running anything else.

  16. #14
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    Even a 2-Ton system will draw in excess of 150 amps during start up.

    Best chance is an inverter driven unit and even thats questionable.

  17. #15
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    Most of the inverters I work on... will ramp up slowly... thus minimal amp draw.

    Note that a soft start kit is NOT the same thing as an inverter that ramps up slowly...
    HUGE technical difference!
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *The value of comfort, over the long term; leave economic choices behind!
    Choose your contractor wisely!

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  19. #16
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    Quote Originally Posted by ga-hvac-tech View Post
    Most of the inverters I work on... will ramp up slowly... thus minimal amp draw.

    Note that a soft start kit is NOT the same thing as an inverter that ramps up slowly...
    HUGE technical difference!

    Does this device actually operate close to its CLAIMED Performance?

    http://www.micro-air.com/

    http://www.micro-air.com/support-doc...tion_Sheet.pdf
    Attached Images Attached Images  
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  20. #17
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    Quote Originally Posted by dan sw fl View Post

    Does this device actually operate close to its CLAIMED Performance?

    http://www.micro-air.com/

    http://www.micro-air.com/support-doc...tion_Sheet.pdf

    Anyone take one of these apart and determine what is in it???
    Kinda an expensive cap/relay in a box thingy...

    OTOH... does it have electronics in it???
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *The value of comfort, over the long term; leave economic choices behind!
    Choose your contractor wisely!

  21. #18
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    Your starting a 5-Ton unit on utility power. The inrush current is in the 100 - 300 amp range. That breaker has a time delay for 3-5 seconds before tripping.
    While you may be correct, this is not the question.
    The breaker that secures the unit it a 40A breaker, which means the inrush current is irrelevant.
    As a mater of fact my entire panel is secured by a 150A breaker....

  22. #19
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    Breakers for AC units are 'slow-blow'... in the sense that they allow inrush current for a short time.

    As noted earlier... there is a LOT more to this, than meets the eye!

    Personal thought...
    Wisdom... is knowing when to let a PRO handle something...
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *The value of comfort, over the long term; leave economic choices behind!
    Choose your contractor wisely!

  23. #20
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    Thread Starter
    Quote Originally Posted by firecontrol View Post
    Not sure I'm getting a clear picture here.

    Are both of the AC systems (forget about the heat for now) connected to the solar system?

    Have you run both of the systems on cooling on just the solar system (by shutting off the grid power) with good results or what are the limits you found?
    Essentially there are 3 breaker panels in my house, there is a disconnect that can isolate one of the panels from the grid.
    In this case this one panel is only solar/battery fed.

    During system stress test we:
    - disconnected the grid
    - started and run AC, two ovens, several hair dryers, PC, internet, stove, space heater (maybe a few other things)) until the system breaker finally tripped
    - hence the solar system can easy run the one connected AC

    Limitation is 40A per breaker

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