Page 3 of 5 FirstFirst 12345 LastLast
Results 27 to 39 of 63
  1. #27
    Join Date
    Apr 2004
    Posts
    7,680
    1. no
    2. no

    Assuming a proper match, the system if properly installed will give you optimum efficiency and performance, any alterations will reduce both.

    Reducing airflow within limits can (and I say this carefully) add to comfort by increasing the latent capacity of the machine but the equipment needs to be sized correctly. It will however not aid in the efficiency.

    Your best bet is to follow all the mfg's instructions and attend as much factory training as you can, if you do that, your customers will be happy and you'll be rich.

  2. #28
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,088

    Re: Thank you for responses

    Originally posted by bornriding
    When considering the entire hvac system, I feel that the purpose of central heat & air is to supply an area ( say a home ) with the right volume of air ( air changes/hr ) at the right temperature. Is this correct ??

    And, I ass u me that i can be more efficeient if I either:
    1) With a certain movement of air ( cfms ), If I can make the air cooler, then my system is more efficient.
    2) If I can increase my air movement ( cfm ), and maintain the same supply air temperature, then I have increased my efficiency.

    How far off base am I ?????
    CFM per room is based upon a proper heat load calculation done for each room. The purpose of a central heating and cooling system is to provide year round indoor climate control, including humidity control and ventilation (air changes), for human comfort and longivity of interior contents.

    1) As doc said, colder air works in your favor...to a point. A colder coil will dehumidify the space better during high humidity conditions, but will be slower to lower the sensible heat of the space if the airflow is reduced to accomplish a colder coil.

    2) To maintain the same coil temperature with an increase in air flow over the coil would require an increase in system capacity. Look at any commercial system with a variable frequency drive air handler and variable air volume units in the ductwork. The system capacity varies in direct accordance with the load via changes in airflow over the coil and total heat load. An increase in airflow typically is brought on by an increase in airflow and heat load, thereby compressor unloaders and other staging devices are cycled to meet system capacity.

    In my opinion, the most efficienct system would be one that would exactly match load with capacity. Most residential systems are sized for extreme conditions, both for heating and cooling, so unless the system is running under design conditions, it's typically running oversized. We all know that oversized systems are inefficient.
    Residential gas heating systems can gain efficiency by being multi-staged (lo/hi fire) and cooling system gains can be had via variable speed blowers by static pressure or discharge plenum temperature, multi-stage evaps, two speed compressors, TXV's, head pressure control for low ambient operation, etc.

    Most residential cooling efficiency gains could EASILY be had by a properly sized, properly charged system with proper ductwork/airflow. Even a 10 SEER unit would run much better than most of them do with the typical installations they receive.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #29
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    Thank You for responses

    Ok lets see if I got this right.

    What ya'll are saying is, if I have more volume of air into an area at a certain ( say 55 'f' ) temperature, it won't cool faster.

    Or if I have a smaller volume of air but at a colder temperature, it still won't cool the area faster.

    I'm not sure i agree ! But I'll keep thinking on it

  4. #30
    Join Date
    Apr 2004
    Posts
    7,680
    Strictly depends on the latent/sensible load at the time. There is too much ambiguity to the question.

  5. #31
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,088

    Re: Thank You for responses

    Originally posted by bornriding
    Ok lets see if I got this right.

    What ya'll are saying is, if I have more volume of air into an area at a certain ( say 55 'f' ) temperature, it won't cool faster.

    Or if I have a smaller volume of air but at a colder temperature, it still won't cool the area faster.

    I'm not sure i agree ! But I'll keep thinking on it
    Think of one room that is served by a residential HVAC system. Common thought is that you cool a room by pumping cold air into it. That's really not the case. You cool a room by removing heat at a faster rate than what is being added to the room through infiltration, lighting, people, solar load, etc. On forced air cooling this is accomplished by removing the air that is at an undesirable temperature and humidity level and refreshing it with air that replaces the air removed to be conditioned.
    You cannot have effective cooling of a room without both sufficient supply and return air.

    When a proper heat load calculation for a room is done, the amount of CFM necessary to maintain design indoor temperature at outdoor design temperatures (typically an extreme condition) is set. Using design conditions indoors and out, if you were to increase air volume into a room and yet maintain a consistent supply plenum temp of say, 55 degrees, the room would tend to overcool, IMO. The air exchange would be more than what is necessary to maintain design conditions within the room.
    Reducing air flow yet maintaining the same 55F supply would undercool the room, as air is not being exchanged at a rate consistent to meet the heat load on the room.

    So, yes, to address your question, a room will cool faster if it has a greater volume of air exchange under these conditions.
    However, as doc said, there are many variables, the most basic being the sensible and latent heat loads, moving on to what temperature the room is when the equipment is started vs. what it is to be brought to so the thermostat satisfies.

    If you have a hot room and start the equipment with dry conditions, it will pull the temperature down faster as the equipment is doing less latent heat removal than sensible. However, the objects in the room will give up heat at a slower rate than the air itself. Therefore if you were to juice the system so it pulled down the room temp quickly, you'd end up with temperature swings until all the objects in the room equilized in temperature. After that you'd have too much capacity at the same juiced up levels.

    Under humid conditions the room will pull down more slowly, because there's not only room content load and sensible load, but also an increased latent load.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #32
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    Let me try again

    Shop, Doc, Thank ya'll very much. I've enjoyed reading and discussing your replies.

    Here is my second try at explaining what I mean.

    I've always thought that the reason for conditioning the air in an area ( say a home ) was to mixed the existing air in the home with cooler air from tha a/c system, at a proper level ( cfm's ), which determines how fast ( or slow ) the air is 'mixed' ( for lack of better word ) and then returned to the unit.
    To me, there were two factors in determining how long it would take a 'home' to cool down from the time the thermostat turned the unit on until the thermostat was satisfied. And they were, the amount of cool air delivered to the home ( and recirculated within ) and #2 - the temperature of that air that was delivered.
    I'm talking the same home, whatever the heat gain.
    So, I've got a certain home, with a certain heat gain, and say my unit is sized correctly.
    I kinda, at this moment, don't consider humidity or latent heat because by the time the air is delivered to the home from the system, only sensible heat is being sent.
    I'm also not talking about the effects on a system from raising or lowering the volume of air or the temp. of the air.
    But, to me, the amount of air and the temperature of that air are the two factors that actually govern the time that the a/c system will have to run to 'satisfy the thermostat'. And if I can get more air from a system, and maintain the same temperature of the air then the effect will cool the area ( home) quicker at less expense ( electric bill )
    Or, if I cannot get more air, but if I lower the sensible temperature of the same air that is now being delivered to the home, then the effect will cool the home quicker at less expense. ( realizing that too much of either one is bad )

    One of my points, I guess, is that all I here about how to set up a system deals with 'superheat' and yet, everyone gives a range of values, such as 10 to 15 degrees superheat ( + - 5 ).To me, the difference between 10 & 15 degrees superheat is a difference in the supply air temperature ( I have done tests for this ). If I set a system at 15 degrees superheat, the supply air will be (say) 59 degrees. But by setting the superheat at 10 degrees, the supply air reduces to (say) 55 degrees. So I prefer to set a system at 10 degrees s/h so that my supply air temp will be lower.... and I actually think that I am doing right.
    All this, knowing of course that I must be careful to avoid liquid getting to the compressor.

    What's ya'lls thoughts??










  7. #33
    Join Date
    Dec 2004
    Posts
    200
    I think you need to measure the indoor wet bulb and the temp out side,then use a slide chart to find out what the required superheat is,,,the + or - 5 is the vapor line temp not superheat

  8. #34
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579

    http://hvac-talk.com/vbb/showthread.php?threadid=49522


    Proper charging and troubleshooting the refrigerant system go hand in hand.




  9. #35
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,818
    Since your going to continue charging the way you are to get the lowest coil temp you can, next time you go from 15 to 10 degrees super heat, check your amp draw on both the indoor blower, and the compressor at 15, and then at 10, you'll find that your amps went up, so your not more eff.

    A vs blower will raise the eff. of a system not because it delivers more air, or colder air, but because it uses less electric to move the same amount, and temp air.

    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  10. #36
    Join Date
    Apr 2004
    Posts
    7,680
    Also if you're charging by superheat you are using a fixed oriface. If the unit calls for 15 and you give it 10, thats great until the load changes and it calls for 5, now you just bought a compressor. Over 95% of the compressors replaced are killed, they rarely fail on their own when they have proper motr cooling and lubrication. Overcharging, undercharging, low loads, even too high loads prevent one, the other or both.

    My Advice? Dont futz with the basic fundimentals, its really not in your best interest, especially when your using a customers equipment as an experment.

    Next there is no such thing as cold, only heat or a lack there of. I suspect a course in basic fundimentals of thermodynamics and psycromentrics. Then a class in a/c troublshooting and system diagnostics and you should have the tools to troublshoot and even make fine adjustments to get the efficiency and performance you want and maintain a long lasting, efficeint and effective peice of equipment and a happy customer.

  11. #37
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,818
    Originally posted by docholiday


    Next there is no such thing as cold, only heat or a lack there of. I suspect a course in basic fundimentals of thermodynamics and psycromentrics. Then a class in a/c troublshooting and system diagnostics
    Doc, he teaches Hvac.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  12. #38
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579
    Originally posted by beenthere
    Originally posted by docholiday


    Next there is no such thing as cold, only heat or a lack there of. I suspect a course in basic fundimentals of thermodynamics and psycromentrics. Then a class in a/c troublshooting and system diagnostics
    Doc, he teaches Hvac.


    When they should be students, they are instructors!


  13. #39
    Join Date
    Mar 2002
    Posts
    516
    The difference between 10 and 15F SH in cooling mode is just the length of suction line containing liquid. The lower the SH the longer the liquid run out of the evap. before it picks up enough heat to vaporize. In a fixed orifice piston system lower ID load and higher ODA both reduce SH. So if you run the SH lower than the spec calls for with clean filters on a moderate day no big deal. When the filter gets dirty and reduces the ID load and the ODA goes to 95+F the SH goes to zero and the comp. might get slugged. If it's a HP the accumulator might save you. A scroll comp. might tolerate some liquid for a while but the efficiency will drop. The highest eff. will result when NO liquid leaves the evap. A TXV maintains a set SH which doesn't allow any liquid into the suction line. IOW you want the evap. completely flooded but you want no liquid spilling over into the suction line as cooling in the suction line is wasted. You can't get this with a fixed orifice. All you can do is set the proper SH according to what the chart calls for your conditions which on an 85F ODA day might be 15F. Come back on a 70F day and you might get see 25F SH or on a 95F day 10F. Gotta go eat dinner now....Hope this helps!

Page 3 of 5 FirstFirst 12345 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event