Low air flow

[Pjskinner]

Hello all.... I have a couple questions regaurding air flow on a Amana Air Command 80 SSE in my house . Okay first off when the air conditoner is on you barely feel the air blowing from the vents in all bedrooms, kitchen and living room. But it blows kinda good in the bathroom. I keep all the doors open in the house but the bedrooms do not feel to cool "but the living room, bathroom and kitchen does". I have no clue as to the tonnage of the unit but the house is roughly 1100 sq'. Is there any thing I can do to make more air flow. The air filter is new and always changed when needed, The condensor unit is a Amana High efficiancy 14 Model RCE36AZA. Also I cannot find the evap. coil on this one. Also How can I tell what blower settings this thing is on.


Please help,
Pj


Attached is a drawing of vents in the house.

http://smg.photobucket.com/albums/v497/MrWrong/hvac/?start=#imgAnch1

[LazyPup]

What you have just described is a very well designed duct system that is functioning correctly.

A properly designed HVAC system should provide two or perhaps three complete air exchanges per hour. To determine the size of the blower unit we then compute the total cubic footage of the living space to be conditioned. Once we have the total we mulitply that by the desired number of air exchanges per hour to determine the total volume of air to be moved per hour.

Example, let us assume you have 1800sq.ft with 8' ceilings. Your total cubic footage is then 1800 x 8 = 14,400cu.

We then multiply the total cubic footage by the desired number of air exchanges per hour so for our example let us use 2 air exchanges per hour.

14,400cu.ft x 2 air exchanges per hour equals = 28,800cu.ft per hour.

Blowers are rated in cu.ft per minute so we then divide the total cu.ft per hour by 60 to determine CFM.

28,800 / 60 = 480CFM

From this we can determine the blower and main duct must be rated at 480CFM but now we must go back room by room to determine the size of the drops off the main duct and size of the grilles.

Example: Let us assume we have a living room that is 18' x 24' and again we have 8' ceilings thus the room volume is:

18 x 24 x 8 =3456cu.ft. Here again, we desire 2 air exchanges per hour so we must move 3,456cu.ft x 2 = 6852cu.ft per hour or 6852 /60 =114.2cu.ft per min.

We could use one vent but an 18 x 24 room is a fairly large area and if we were to only use one grille it would result in a hard blast of air causing striation and uneven air currents throughout the room. (This problem is common with large window AC units).

The solution here is to use multiple vents to get a more even air flow. The estimators books show tables to help design how many vents to use but for the sake of illustration let us assume that we want 4 outlets in this room. Each outlet must now provide 1/4 the amount of air required for the room so the outlets would be sized at 114.2CFM room total / 4 vents = 28.55CFM per vent thus when properly designed you should feel little more than a faint gentle breeze at the vent usually just enough to lightly flutter a ribbon tied to the vent.

Understanding that air conditioning not only cools the air but it also dehumidifies, it then stands that we would plan on a greater amount of air exchange in a bathroom to help remove the excess humidity which is present in bathrooms.

Properly balancing air flow in a duct system is a very complicated task that should only be done by a trained professional with the proper test equipment. Any effort on the part of a homeowner to adjust fan speeds or duct balancing dampers can result in totally improper air flow and could result in serious system damage from coils icing.

[Pjskinner]

I appreceiate your replies they are always so informative. Ill take your advice about the tinkering with things I will call some one as soon as I do some investigating.. How can I tell If the condensor unit is the right one for the house. Reason asking is that if only you could see the way things were installed you would feel the way I do about all this mess.


Thanks,

Pj

[LazyPup]

Before you call someone there are a few things you can check on your own..

1. The leading cause of reduced air flow or complete system failure is a dirty air filter. While discussing air filters there is another point I would like to strongly emphasize. Far too often the homeowner opts for the high Merv rating filters as opposed to the conventional fiberglass mesh type. That in itself is great because the higher the MERV rating the finer the filter medium material and consequently the more dirt and debris it will trap, but High Merv filters cost two or three times the price of the conventional filters and people are then lulled into the false economy of expecting it to last much longer. Such is not the case. The actual amount of air that can pass through a square inch of high filtration medium is much less. To compensate for the reduction in air flow through the medium high filtration filters are made in a pleated form that greatly increases the actual amount of surface area. While the filter has a much greater surface area it is also trapping much smaller particulates, therefore the air flowing through the filter will obviously be cleaner but then end effect on the filter medium is that due to the increased amount of dirt trapped by the filter the filter will actually clog faster.

For peak efficiency, especially in a hot and dusty enviroment like Texas I would suggest you get in the habit of changing the filters monthly. This may seem a like overkill but consider the actual cost involved. Typically you can buy a case of 12 filters for about $12 to $24 at any HVAC supply house and in most Home Supply Centers. Changing the filter monthly will then cost you an average of $1 to $2 a month. Operating a system with a dirty filter can nearly double your run time and consequently double the cost of the electrical energy to operate it. In addition, if a system is operated for an extended time with a dirty filter you run a serious risk of the coils icing up and potentially icing all the way back to the compressor, which is one of the leading causes of compressor failure. With all things considered, even changing a $5 high merv filter monthly seems like a bargain to me.

2. Check all return air vents to make sure they are not obstructed by furishings or personal belongings. Even a partial obstruction on a return air vent can have a dramatic effect on overall air flow. My rule of thumb is there should be no furnishing or personal possessions within a space eqaul to the diagonal measurement of the grille.

3. Do not run ceiling fans when the AC is on. Ceiling fans create very strong air currents that often interfere with the designed air flow of the AC system. I once worked on an AC in a large one story on slab house in Florida that had three bedrooms on one end of the house, a very large living room in the center and the AC evaporator in a utility closet on the opposite end of the living room. The house was very well insulated new construction and the system had been engineered specifically to the heat load needs of the structure yet the living room, family room and kitchen were quite comfortable while the bedrooms remained an untollerable 90degF. For four consequetive days we checked the condening unit, evaporator unit, all fans and refrigerant pressure and all was perfect, but the bedrooms remained 90degF. There we were, three service techs, the Rheem Field rep and our company president all standing outside by the pickup discussing the problem when the lady of the house came out an announced, "Whatever you did, its working great now, the bedroom is 72degF. We all stood there dumbfounded, what had we done? We went back inside to see what had changed and as we entered the living room it was darker than usual. Someone snapped the overhead light on while we were looking in the utility closet. A few minutes passed and the lady came out of the bedroom area to announce it was getting hot in there again. We all agreed that the only change was turning the light on and surely a light could not effect the AC but when we looked up at the vaulted ceiling there was the answer. The ceiling light was a fan/light. A follow up smoke test revealed that when the ceiling fan was running it caused a down draft to the center of the room, which then spread across the floor and caused a backdraft pressure in the hallway preventing the return air from coming through the hall, across the living room and back to the unit. We were then confronted with two options. !. We could have installed a dedicated return air duct at a considerable cost or 2. Instruct the homeowner not to run the ceiling fan when the AC was on. (Can you guess which solution the homeowner opted for?). Having learned a valuable lesson there, I now make it a practice to check for ceiling fans any time there are erratic temperature variations and far too often that proves to be the problem.

There is a very simple diagnostic test that you can perform to get a fairly accurate indiction of how well your system is performing. Using a pocket probe type thermometer measure the temperature of the air entering the return air duct closest to the unit, then measure the temperature of the air discharging out of a supply grille closest to the unit. The air coming out of the supply grille should be approximately 12 to 15degF cooler than the return air entering the unit. If it is less than 10degF or greater than 20degF you should first change the filter and repeat the test. Repeat the test again in 20 or 30 minutes. If the temperature differential remains less than 10degF or greater than 20degF you should have the unit checked by an HVAC technician.

[Pjskinner]

Thanks pup I will check the ducts and return air temps. About the filters I change those every month like you advised. But I do not know how often the previous owners changed them.. Also in the bedrooms there is no ceiling fans just lights.

Thanks again,
Pj