Btu Buddy 112: Too Much Load for Air Conditioner
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim were on their way to a residence where the occupants said their unit runs all day long, but the space temperature rises to 82°F while the thermostat is set at 75°. The unit shuts off during the night and the temperature inside is 75° in the morning, but during the day it rises.
Tim said, “Sounds like the system may be low on refrigerant.”
Bob said, “We can’t really draw conclusions until we get there, but that sounds like it may be the case.”
They arrived and talked to the owner. It is a patio home community in a Southern city. There are hundreds of these types of homes that older people downsize into from their larger family type homes that were needed to raise children. The owner explained, “The home is about five years old and really very well designed, except the air conditioning system does not keep the place cool. I have talked to several of my neighbors and they all are saying the same thing. It is too warm inside during the day.”
Bob said that they would give the system a good check up and see what they could find.
Bob and Tim went to the outdoor unit, did a visual check, and found:
1. The suction line was cold all the way to the compressor.
2. The condenser was clean.
3. The outdoor fan was running and really hot air was coming out of the condenser.
4. The liquid line was really warm, not hot.
5. The outdoor ambient was 101°.
6. The air filters were clean.
Bob asked Tim, “What do you think?”
Tim said, “Boy, that unit really seems to be working. I think we should check the amperage to see if it is really loaded up and working to capacity.”
Bob said, “That is a great next step.”
They checked the amperage and the compressor was drawing nearly full load amps. Bob said, “This system is doing all it can do and it is just not man enough for the job.”
Tim asked, “What do you mean by that?”
Bob explained, “The system is not carrying the load. The load is greater than this system can handle. We can look for heat leaks into the structure, but I doubt if we can find enough to make up the difference.”
They looked over the structure and found:
1. One window that was opened slightly in the laundry room.
2. They started the dryer and went to the outside vent and the dryer was venting correctly.
3. They looked at the roof ventilation system and it was a ridge vent with vents cut into the soffet plates.
4. The house was built on a slab.
5. There were no exhaust fans running all of the time.
Tim said to Bob, “I think you are right. The load is more than the air conditioner can stand. More heat is getting in than the unit can remove.”
Bob agreed, “There is not much we can do. We either have to get more capacity out of the system or reduce the heat transferring into the structure. We can do a little of both and maybe we can get these people some relief.”
“How in the world are we going to do that?” asked Tim.
Bob explained, “The only way that I know of involves water. Let’s go to the big box hardware store down the street and look around.”
They informed the customer where they were going and that they would be back in a few minutes.
Tim asked, “What are we going to do?”
Bob said, “We are going to look for some soaker hoses that have small pin holes that will give off a fine mist. Then we will stretch it over the roof at the top and hang the tail end of the hose off of the end of the house where the condenser is located. The mist will cool the roof and spray a fine mist towards the condenser that will evaporate and cool the air entering the condenser. Cooling the roof will take some of the heat transfer load off of the whole structure and lowering the entering air temperature to the condenser will give it just a little more capacity. With those two things working, it may be enough to get some relief.”
Tim said, “Wow, what an idea. That just may work.”
They went to the store and found just the hose they needed. It was 75 feet long. They figured that should be about right.
They installed the hose and went to the owner and described what they had done. Bob said, “You should turn the water off at dark and turn it back on about 9 a.m. in the morning. This is only a temporary measure. This system is undersized for these weather conditions. You really need to have a new system installed to take care of these really hot summers. You could also install a window unit as an assist in this kind of abnormal weather. Fans can be used to circulate air in the areas where you stay the most. That will also help you to be comfortable. Moving air cools the body.”
The owner said, “This is happening to many of these homes in this development. How can the systems all be undersized? They were supposed to be engineered.”
Bob responded, “The common design practice in this area is to use national weather data that has been collected over decades of time. The data shows that if the designers use 95° for the local outside design temperature that we may spend no more than 1 percent of the summertime hours over that 95° temperature (Figure 1 - click on the PDF link at the end of this article). That should only happen a couple of afternoons a year. The rest of the time the system is oversized and shuts off and then turns back on when the space temperature rises above the thermostat set point. A system is most efficient if it runs all of the time. On and off operation is not the most efficient. Most of the season the system is operating below the full capacity of the system.
“Systems that were designed for 95° will not maintain 75° indoors when there are long-term hot spells like we are having now.
“The temporary methods that we are using should only be used in extremes like we’re experiencing now. It is likely that once we get through this hot spell that there will not be another one, but no one can guarantee that.
“The city water department is likely to frown on using the soaker hose idea if the city is in drought conditions, but we will see if we can get you some relief until the weather breaks.”
Tim asked, “What about capacity control for better efficiency for this type of situation?”
Bob said, “That is probably the ideal way to solve a problem with big fluctuations. The system can be sized for 100° design temperatures and the system capacity can automatically adjust to different capacity loads.”
The owner then asked, “Why aren’t all systems equipped with capacity control?”
Bob said, “More control for a system costs more. It gets into variable speed compressors and fan motors, so system initial costs go up. Most people want the least expensive equipment they can buy. This is particularly true in the speculative home market. The builder wants to choose appliances that are as economical as possible and will sell as low as possible.”
The owner asked, “What is next?”
Bob said, “We are going to leave it with you to operate the water and I will call you tomorrow afternoon and you can tell me if it was worth the effort we put in.”
Bob and Tim then left the job. When Bob called back the next day, the owner reported that the home was operating well within the temperature set point of the thermostat. He said that the unit was still running all of the time, but it was comfortable in the house.
Bob told Tim, “Remember, that is a temporary repair. Water can sometimes be used when a condenser fan fails and a replacement can’t be readily found. Realize that it takes a lot of water and you have to be prepared for the runoff water when it is used, because it will not all evaporate.”
Publication date: 7/23/2012
Figure 1 PDF - This table shows the climatic conditions for various cities across the country. The complete table is published by ASHRAE. (From Practical Cooling Technology, by William Johnson, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)