Central City Air

Mold, Mildew and Humidity - Causes and Remedies

An article by David Debien

A controversial subject too often misunderstood in the air conditioning business is the control of humidity in a home environment in the cooling season. Humidity removal is usually a by-product of air conditioning, not it's main goal. While this may be appropriate in some cases, it overlooks the facts that sometimes cause serious comfort problems in our homes. The result is sick building syndrome. This is a term used to describe a house that tends to make the people ill just by living in its environment. It is no joke to know that in many areas of the country, serious conditions exist that are causing people to visit allergy clinics. A more sealed environment that most homes are providing today can exacerbate these conditions. In a recent survey, a leading consumer magazine found that the number one compliant by 41% of the people was that they wanted better humidity control.

Infiltration is a condition where the house is built to poor standards so that air passes through it or into it very easily. This is more common in years past, especially areas like Houston, Texas where the goal is to ventilate the heat or attempt to minimize air infiltration from hot, humid sources. As buildings became more energy efficient, the comfort system followed this trend. Not only were electric bills lower, air infiltration was minimized, while equipment was improved in efficiency. These are just of few of the factors that influence humidity build-up in any home.

Since Manual J books have been written and used, they often take into account the conditions that exist in a particular home in a specific part of the country. These guidelines, while excellent, should be used only for what they are intended, namely sizing equipment under one condition. However, they are too often used in defense as to why equipment will perform. The equipment analysis show that under Manual J conditions this equipment will perform to provide comfort. In most homes, Manual J conditions may exist only 5% of the time or less. So why do we design to these factors as the main focus of equipment selection? We HAVE to, we MUST do this. There is no choice! Granted, then we must use Manual J.

This means that when Manual J conditions occur, our customer will not call us and complain that the system we sold them does not give them the comfort under the worst conditions. So we are now safe, correct? Yes and No. We are safe from a judgment proving that we cannot provide comfort. But what about the other times when comfort is THE issue and Manual J misses the mark?

Do we not have the same obligation to provide the customer with the closest to absolute comfort that is known to man at this point in our History? No question about it --- this is our most important goal. So why do we miss the mark so often? The answer lies in the approach to the physics going on in the house. Granted, it varies from place to place to some degree, but the problems we are encountering are relevant to most of the United States and the World, as a matter of fact.

Let's look at one specific example of home design that has educated me beyond normal given absolute. This is where I operate, Houston, Texas, the highest grains of moisture per pound on a yearly basis anywhere in the United States. A customer was building a custom home and asked for our proposal. This house was to be surrounded by a Styrofoam 1 foot thick wall with stucco on the outside. The roof was made likewise. The first thing I did, was try to talk the customer out of using this material. Vapor barriers don't well work in this area, and can actually cause a house to fail due to a build-up of moisture in the walls. But the customer was determined to have it his way. I tackled the job as a challenge, Horror stories of green walls in similar house designs in this area made me wonder if it was even possible to accomplish the goal. The customer added thermo Payne glass to reduce the heat load even more. They were well off DINKS (double income, no kids) that were going to have the home of their dreams and this was it. I was sure it would teach me something, but would it cost me more to install a system and deal with the problems due to the house design? There was only one way to find out. I took the job.

The sensible heat load was low. The latent heat load was very high. Even with the moisture vapor barrier, humidity would find it's way in into this house, especially in Houston. The true sensible heat ratio looked closer to .55 or .60 but no higher. Making phone calls and asking for help got me nowhere fast. Some advised strongly to not deviate from an ARI match. Manufacturers just gave the standard line. Install a reheat system of hot gas bypass or electric heat and control it with a humidistat. This approach was unacceptable to my way of thinking, running the heat and cooling cycle at the same time. Build a super tight house and then add heat to make the unit run time longer and remove more humidity? No way, was my response.

The local utility company had run a load analysis at the time. They came up with 7.5 tons as the total cooling requirements for this 5000 square foot home. Boldly, I rejected their claim as this being the proper size. Their theory was to slightly undersize the load to allow it to run longer, thus removing more humidity. Makes sense so far, but I decided to go the opposite direction. My equipment selection was 10 tons.

How many times have you heard the statement "If you oversize the equipment, run time will be insufficient to remove the humidity"? This is a very common argument, even today. It is simply not true as a general statement to say that over sizing equipment will cause a humidity problem. If that were the case, with Manual J conditions existing only 5% of the time, the equipment will be oversized 95% of the time resulting in insufficient run time to provide comfortable humidity levels. This would be especially true in Houston, Texas if it were true anywhere. I am here to tell you it is not a given anywhere. On a daily basis, 90% of the contractors in this area will agree with the approach that over sizing the equipment will cause high levels of humidity in the house. Now that I have your attention, just what is it that is being said here?

Was 10 tons too much for a house that someone else showed to be a 7.5-ton load? Would this house have serious problems with humidity or comfort when the equipment ran shorter cycles? Would a shorter cooling cycle be a by-product of an oversized equipment design? Would the customer see a lower electric bill as a result of over sizing the system? What about mildew? Would it grow inside the system or the house? The answer to all of the above questions is a simple NO! Impossible you say, I can hear it nationwide.

What determines the equipment selection we make? We look in the manufacturer's engineering catalog and select a system that meets the goals that we want our system to achieve. These catalogs do not always distinguish between a glass house or a Styrofoam house. The equipment is usually listed in capacities and factors that concern the average home in the entire United States. The job above is not the average home. Since the sensible heat ratio is so low, the off the shelf selection would not do the job. At best, a .75 SHR was attainable. This is where the challenge becomes how to best address a sensible heat ratio that was not in the books. I knew it was nearly impossible to attain a .5 sensible heat ratio, but an aggressive approach could get the SHR much lower.

The full size 5 ton coils with an expansion valve gave about a .78 SHR. Sizing down the coil lowered the SHR to about .7 as best could be determined. Since I needed more humidity removal, why not try a nominal rated three-ton coil? Impossible some would say, but we did it anyway. We asked for help from our national distributor and were discouraged from this approach, but it was determined that it would work. We even received an engineer's stamp with an ARI rating. So, we wound up putting two 5 ton furnaces with 350 CFM per ton with two five ton condensers and yes, 2 three ton expansion valve coils. This along with selected fan speed controls gave us exactly what we wanted, extremely high humidity removal with normal run time in the cooling cycle.

The run cycle was not shortened to any great degree. Remember, we are now taking out more latent heat than normal. Since the thermostat relies on dry heat to cycle on and of. The lower the dry heat removal, the higher the wet heat removal. The result is a cycle that allows humidity levels to be aggressive removed by oversized equipment that was redirected to increase humidity removal. This approach lowers efficiency because it is more efficient to remove sensible heat than it is to remove latent heat. For the same tonnage equipment, if you remove mostly dry heat, the run time will be lower, humidity removal will be lower, the home will have high humidity levels and great discomfort. The thermostat could need to be lower by as much as 8 degrees to obtain necessary run time in this tight house to get the comfort if you do not design for the conditions that exist in this application.

Now does this system perform? Last year the customer called me up on a 100 degree August day to complain about his humidity. I'll never forget the call. "Dave you have do something about my humidity." was his statement. "What does your hygrometer say?" I asked. "78 degrees with 39% relative humidity." he said. "Can you raise the humidity a little"? "Yes", I said knowing that I had mastered this customer's environment. We simply changed the fan speed a notch increasing the sensible heat ratio of the system and humidity levels returned to 50% at 76 to 78 degrees. Maybe it was slight overkill, but I now knew that SHR was THE dominant factor in the humidity level and comfort needs of this and every other customer. This system has been operating for about three years now and works very well. The customer, one of the former Houston Rockets, gives me his front row seats when business interferes with his game attendance. He is very happy, but what about my company employees? The praise they received drives them to do quality work on every job. We all win, big time!

This became a starting point for specific designs and continued experimentation. We lost rebate after rebate from the utility company because we violated the ARI rating that said we would get a particular condition. But the ARI rating did not address the more than double humidity levels that were encountered on the Gulf Coast. They were just averages, nothing more, they didn't apply. I even personally protested the local utility's rebates as wrong in q protest appearance before the Texas Public Utility Commission in Austin, to no real avail. Criticism reigns where ignorance abounds. It has been an uphill battle, customer by customer, one at a time. Finally, after 7 years of this approach becoming a solution to most who nearly gave up on comfort, it is now being applied more aggressively, by the competition. I say, welcome!

A common criticism of downsizing coils is that you will shorten the life of the compressor. In addition, the manufacturer will void the warranty. You cannot control the superheat. Running a colder coil will cause the system to generate mold and mildew. The system capacity will severely be reduced if you down size a coil. These statements that are made by competitors in the business really are saying, "I don't understand what you are doing, therefore I can only criticize the approach". They may be incorrect, but customer confusion reins because more support is available on the oblivious side of the table.

As designers build more efficient homes, with tighter wraps on them, humidity removal becomes a problem in 90% of the U.S. In extremely low humidity areas, mold is almost non-existent, no matter what you do. In these areas, humidity needs to be raised not lowered. This too, is attainable by equipment selection and application.

Next: Six effective ways to increase humidity removal on existing equipment answering the customer cry for improved comfort in the cooling cycle.

Approaches to solving existing problems:

One of my personal quirks is that I am a driving perfectionist. Nothing makes my day more than a call from a frustrated consumer with a problem that 5 companies have attempted to solve and have failed. It sends me into a different mode and I can hardly wait to see the potential job. When a house has a cooling coil that is oversized or operating with a cap tube or flow rater we replace the coil or modify it. There are several approaches to change the dynamics of the present operation and achieve dramatic results.

The first indicator of a humidity or moisture problem is customer complaints of sticky environment especially in the late night hours. As the sun goes down, sensible heat gain is reduced and run time is also reduced. This could be solved by several methods. As the sensible requirements are reduced, often the latent load is increased more than enough to be a problem. The customer will attempt to lower the thermostat to accommodate this condition. If that's all it takes, then do it. But there are customers that don't want to go through this daily ritual. They want to set it and forget it. Since an enthalpy thermostat is not economically available, simple approaches can achieve the results that separate you from your competitor.

Here they are:

Strategy #1

Expansion valve coils
Don't even debate this fact. Every system installed in the world under all conditions operates better with an expansion valve. It is a given. If you don't have an expansion valve, install one immediately. Selection of the valve requires careful analysis of the conditions. Did you know that a 3-ton expansion valve, nominally rated, would produce 5 tons of cooling in hotter ambience? Be careful, an improperly selected expansion valve, improperly installed, operates poorly and will cause the very problem you are trying to eliminate, namely better latent removal.

The best place to install an expansion valve is outside the coil. Install a sight glass before the expansion valve and an access valve on the suction line coming out of the coil. This will allow you to charge, analyze and check superheat and performance right in the attic. What a concept! Try it first before proceeding to any other approach. If it solves the problem, you can walk away with a happy customer and future referrals.

Strategy #2

Lower fan speeds
In a defined psychometric analysis, a major manufacturer wrote a document called "Equipment Selection in Humid Applications". This document was based on Houston, Texas climate conditions. The direction of the report was to slow the fan down from 450 CFM to 300 to 350 CFM. The first item to address is that you must use an expansion valve for this application. It would be an asset to use an adjustable expansion so that super heat can be set. It would be advisable to install a sight glass and an access port to allow for adjustment at the coil, if necessary. A balanced port expansion valve does the best job of controlling superheat as you push the refrigeration cycle to precise performance. With a full sight glass and normal summer ambient temperatures for your area, you will be able to make quantitative analysis. Accuracy is critical if you are to succeed. Once you have the expansion valve installed correctly and have balanced the superheat, you are ready to take your first temperatures.

Measure temperature drop across the cooling coil, not across the system. Record the superheat at the expansion bulb. If the temperature drop across the coil is below 20 degrees, slow the blower down one notch. If it is above 25 degrees, either raise the speed one notch or correct the ductwork restriction causing restricted airflow. We are assuming a super clean coil. After you slow the blower down, observe the suction pressure and the superheat to make sure the superheat is in the 12 to 15 degree range at the coil. If after 10 minutes of running, the superheat is too low, adjust the superheat of the valve to correct the problem. Ironically, this is rarely required since most factory built expansion valves are present at the factory for normal superheat conditions. The balanced port expansion valve adjustments must be approached with caution as with any expansion valve changes. Improper adjustments or a wrongly selected valve will encounter liquid flood back. This is true not matter what you install, because you are always pushing the limits of the expansion valve. Many people operate the expansion valve as a flow rater and not as an expansion valve. This failed effort is caused by improper refrigerant balance from day one. Proper operating expansion valves alone can change the performance of any system.

Now that you have experimented with a slower blower and seen the results, you are ready to control the fan correctly to tune the performance of the system to the customer's demands. This is where you must decide the best way to control the customer's comfort. Since humidity control is the direction we are taking, several methods will solve you unique problems. You must choose the one that you feel will satisfy the customer best.

Strategy #3

Electric Eye
When the Sun is out, the sensible heat load is higher. By installing an electric eye on the house, you can slow the blower down one notch when the Sun is no longer a factor. With a simple SPDT relay the fan changes speed, no one touches anything. This method works best when the customer complains that the system works poorly at night but great during the day. This also works best when the equipment is not sized for the required latent load.

Strategy #4

Two Speed Thermostat
Install a two-speed thermostat that starts out in low speed. If the system is satisfied without a temperature increase, the unit will cycle in a lower speed. If the thermostat rises a degree or two, the fan will change speeds and allow the sensible heat ratio to change and remove more dry heat. As it cools down, the fan will again slow down, and go after more humidity. This can be series wired in with an electric eye, allowing only the fan to operate in lower speed at night. The only problem with the 2-speed thermostat is it works best when the equipment is sized aggressive to satisfy the thermostat and not when the equipment is undersized.

Strategy #5

Humidistat
A simple humidistat can control the fan speed. When humidity is high, the fan runs at a lower speed. When the humidity is low, the fan will run at a higher speed. This, like the two-speed will compensate best when installed with an aggressively sized unit and not one that is undersized. It maximizes performance, allowing higher efficiencies when conditions permit. This control also gives the customer another dial to adjust to zero in on their comfort.

Strategy #6

Time Delay
This is a very simple and effective way to control the fan speed. A simple "delay on make" time delay is wired into the "Y" of the thermostat. When the thermostat calls for cooling, the timer is energized with the same "Y" signal. Then the adjustment is set from 1 to 10 minutes. The fan starts out in a lower speed and when the set time is reached, it shifts to a higher speed. Unlike the two-stage thermostat, the time delay holds the fan in the higher speed until the thermostat reaches set point. This works well with all systems and the customer can even adjust the control if you set it up that way. This type of control works with oversized equipment as well as undersized equipment. The adjustment determines the rate of cycle change you want. If the cycle is longer than 8 minutes, then it can be assumed that the sensible load may be higher. This would be especially true in the heat of the day. In the middle of the night, you may never have an 8-minute cycle, thus staying in the lower speed and increasing the humidity removal.

Top ten comfort solutions for improved comfort and performance.

Use a balanced port expansion valve, externally equalized, and with adjustable stem. Size the expansion valve to the flow of the system, and the condenser capacity.
Install a sight glass at the coil right before the expansion valve.
Select the cooling coil for the load not the SEER rating.
Install balanced ductwork for proper airflow requirements.
Adjust the fan speed to make the system perform to the house and the customer.
Select the furnace that provides the proper airflow for all the conditions that exist.
Install digital thermostats to control the system operation.
Measure and record all start up and running conditions information. Keep these records for long-term reference.
Make sure your superheat is correct and avoid adjusting it once you have set the system up initially. The reason most expansion valve are not adjustable is because most manufacturers do not trust the serviceman to mess with the superheat setting.
Don't assume anything. Analyze the facts and make sure you know what the consumer is unhappy about before you attempt any of the approaches mentioned in the article. Don't walk away from any problem until you eliminate it.

The Ten Air Flow Rules of Thumb:

Higher airflow increases the sensible heat ratio.
Mold will not grow if indoor air is kept at 55% or below.
Lower airflow increases humidity removal.
The tighter the house, the higher the quality of supply air required.
Improperly sized ductwork will never allow you to deliver quality air from any equipment.
Return air grills in the ceiling help balance out airflow conditions especially in areas of the house where the load varies.
The lower the humidity in a given space, the higher the set point can be and still maintain comfort.
Temperature drops across the cooling coil should be 20 to 22 degrees for maximum humidity removal in the cooling cycle.
Lower airflows will not decrease efficiency enough to measure, but will improve comfort enough to notice.
The two factors in airflow are quantity and quality. You must provide both correctly since comfort is directly related.

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