Re: Thank you for responses
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.
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 ?????
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.
Re: Thank You for responses
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.
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
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 55°F 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.