Page 2 of 2 FirstFirst 12
Results 21 to 25 of 25

Thread: Question on upcoming HVAC installation .

  1. #21
    Join Date
    May 2021
    Location
    Phoenix, AZ
    Posts
    22
    Post Likes
    Thread Starter
    Quote Originally Posted by BNME8EZ View Post
    The only true test for short cycling is at design conditions it runs constant and maintains temp your golden. If it runs less than 75% of the time you are way over sized, if you loose a degree then makes it up again as the sun goes down your just a bit under.
    Where can I know more about these "design conditions" of my system ?

  2. #22
    Join Date
    May 2021
    Location
    Phoenix, AZ
    Posts
    22
    Post Likes
    Thread Starter
    I'm thankful to everyone who have replied to my queries. I'm getting to learn a lot. Hope I had done this sooner, but better late than never.

    Now I have three outstanding queries on this topic.

    1). Can the blower speed be controlled independently of the compressor size?
    Meaning given an oversized system, can one control the rate of supply air/CFM (via blower motor), so that it takes longer to cool the house, hence preventing short-cycling? I'm attaching the Airflow table for my system. Not sure, what to make of it though.

    2) If humidity is not an issue, is it true that the problem with oversizing/short-cycling will be only wear on the system and lower-efficiency ?

    3) I have Nest E thermostat. Is there a way I can run the compressor based on a time limit and not the target room temp (say, run for 45 minutes and then "sleep" for 1 hour) ? Is that even recommended ?

    AIR FLOW TABLE
    Name:  Airlfow.png
Views: 57
Size:  107.4 KB

  3. #23
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    14,634
    Post Likes
    Quote Originally Posted by Froodo View Post
    I'm thankful to everyone who have replied to my queries. I'm getting to learn a lot. Hope I had done this sooner, but better late than never.

    Now I have three outstanding queries on this topic.

    1). Can the blower speed be controlled independently of the compressor size?
    Meaning given an oversized system, can one control the rate of supply air/CFM (via blower motor), so that it takes longer to cool the house, hence preventing short-cycling? I'm attaching the Airflow table for my system. Not sure, what to make of it though.

    2) If humidity is not an issue, is it true that the problem with oversizing/short-cycling will be only wear on the system and lower-efficiency ?

    3) I have Nest E thermostat. Is there a way I can run the compressor based on a time limit and not the target room temp (say, run for 45 minutes and then "sleep" for 1 hour) ? Is that even recommended ?

    AIR FLOW TABLE
    Name:  Airlfow.png
Views: 57
Size:  107.4 KB
    1 yes to a point. You can run it slower but the coil will freeze and the heat exchanger will over heat, or you can run it faster but get less dehumidification and not feel as cool, then there is the noise issue from moving to much air.

    2 yes an oversized system will run a shorter cycle and all the stops and starts will wear out the equipment faster.

    3 I don't know if the Nest has that capability or not and I suppose you could run the system on a timer but it will overshoot on run time and then over shoot the other way when off. This is also a very inefficient way to run a system. What kind of gas mileage would you get if you ran your car full throttle for 30 minutes then let it coast for 15? What would your average speed look like doing that? Part of the 15 you would be stopped by the side of the road not getting where you want to go right. Consider a thermostat like the cruise control on your car. Much better trip and better fuel economy with the cruise than the timer, right? The think you have to remember is the the car has a throttle the HVAC system does not. So it is better to have the system sized so it can run most of the time than be too big and only run part of the time.

    There is one exception, kinda. A true Variable Speed system will work to match the output of the system to the needs of the house. Even at that though operation is much better when properly sized.

  4. Likes Froodo liked this post.
  5. #24
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    14,634
    Post Likes
    Quote Originally Posted by Froodo View Post
    Where can I know more about these "design conditions" of my system ?
    Design conditions are not for the system they are for the area you live in. They are based on historical averages and how many hours a year you are at a specific temperature range in your area. For example my area is -2 outdoor 70 indoor for heating and 93 outdoor 75 indoor for cooling. There are times when you will be outside these design temps but on average it is only a few times a year for just a few hours. When it goes outside of design the house will slowly lose ground and the system will run all the time. One thing to note is that when a load is done on your house it will say X#BTUh at design conditions. It is very seldom that a unit can be sized exactly to the load as equipment only comes in increments of 6k for most cooling and 15-20k for most furnaces. Because of this the equipment is sized as close as possible but will likely still be bigger than needed. This is why the "if a little's good a lot is better" philosophy is bad news with HVAC equipment.

  6. Likes Froodo liked this post.
  7. #25
    Join Date
    Feb 2013
    Location
    Sonora, California, United States
    Posts
    6,147
    Post Likes
    Quote Originally Posted by Froodo View Post
    Where can I know more about these "design conditions" of my system ?
    design conditions are not of the system. Design conditions are 30 years a wether data averaged. Been a while since I needed to run a Load Calculation and even when I did I just memorized my local design conditions. I might have got them from the load calculation software but honestly I cant remember.

    Im sure someone here will know and be able to link you or advise you of what your design conditions are....

    The system is than sized based on a whole lot of stuff: Window size, Window kind, Blinds/Drapes/Color, Roof type and color, Wall type, door type, insulation in walls/attic/floor, how the house faces the sun, over hangs, is house surrounded by grass/concrete, is ductwork in attic/crawlspace/between floors, etc. All this data and more is collected and input into software along with those design conditions to determine what size equipment your home needs and how much airflow each room needs to heat and cool properly.(or a block load calc can be done to just determine the entire house as a whole)

  8. Likes Froodo liked this post.
Page 2 of 2 FirstFirst 12

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •