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  • Paul Meleshkewich

Air Conditioning, Dehumidstats vs. Environment

Air Conditioning, Dehumidstats vs. Environment

We, as homeowners, living in Southwest Florida have many environmental issues we must contend with – one of which is a major problem, that four letter word MOLD. We would like to take this opportunity to introduce to you an easy-to-read, valuable resource we found; a book written by Walt Black, a Senior Consultant for American Management Resources Corporation, (AMRC) an environmental health and safety consulting firm located in Fort Myers. This book “A Practical Guide to Mold and The Indoor Environment in South Florida”, may be purchased by contacting Mr. Black at, or tel. no. (239)936-8266. We highly recommend this book to all of our clients.

Moving forward, Myhousetenders recommends that you set the dehumidistat to the “home” position and set the thermostat to 78 degrees to 82 degrees. This action will take the dehumidistat out of play.

If you want to proceed this way, you must put a note on your dehumidistat and notify Myhousetenders, in writing, by email — or mail – 3805 Jungle Plum Drive East, Naples, FL 34114


The Duct Stops Here, January, 2003: Destroy the Dehumidistat


By Andrew C. Ask, P.E. Consulting Engineer

“Living in Florida can be hazardous to your health, or at least to the health of your home if you have a Dehumidistat.

I’m talking about little beige boxes with a dial reading in “% Relative Humidity.” You’ll find them next to the thermostat, with a 3” x 5” card tape to the wall instructing the homeowner to “Set the Thermostat at x °F. and the Dehumidstat at y% when going away.” Some of you probably make good money selling them as upgrades.

We see a lot of them because they are popular in hot-humid climates where the owner is going to be gone for an extended period during the summer. We also see a lot of the damage they wreck.

People return to their home in Florida in late fall excepting that their dehumidistat will have kept it free of mildew. Instead, they find their walls and ceilings covered with mold. They also find inside temperatures in the sixties.

Here’s what seems to be happening. The dehumidistat senses high humidity and starts the air conditioning compressor. This indeed removes moisture from the air, lowering absolute humidity. But the air conditioning is being forced to run, and the house gets colder and colder. As the temperature falls, relative humidity goes up. The dehumidistat never satisfies. The compressor is now locked on, and runs straight through until someone does something. If this process is allowed to continue unabated for several months, mold will bloom on the cold, damp surfaces created by the cold air.

It costs from $25,000 to $50,000 to clean up one of these messes in an average home. More if you are cursed with wealthy and all of its trappings.

Much drywall and ductwork will have to be removed under containment conditions and replaced, and the homeowners will end up spending Christmas in a motel. Instead of decorations and carols, they will be observing that most American of traditions: finding someone to sue.

Much forensic effort goes into proving that the dehumidistat must have failed. What I think happens is that people leave for the summer, forget to turn the thermostat up and leave the dehumidistat at 50% relative humidity because that’s a “good” level. Upon discovering the catastrophe, they quickly return the settings on the wall to what the little white card says, then call their insurance company.

So what went wrong? The first thing to remember is that air conditioners aren’t dehumidifiers and you can’t turn them into one by the mere attachment of an additional control. A dehumidistat is like a Wonder bra® – it appears to do some good, but look at what’s inside and you will find nothing has really changed.

The dehumidistat in common use are notoriously inaccurate. They may be trying to maintain a relative humidity level 10% or so below what the arrow is indicating. Most of them are mechanical devices with cut-out points even lower than where they cut in. The homeowner could set one at 65% and it might not allow the compressor to stop until the rh falls below 50%, something that’s not going to happen.

The sequence that I described above is the so-called “parallel” hook-up, with the dehumidistat hard-wired in parallel with the thermostat so that either device can command the compressor to run. This is the worst possible way to set one up. You need some sort of Plan B to stop the compressor if the space gets too cold.

Here are some suggestions:

  • Wire the dehumidistat in series with the room thermostat so that both have to call before the compressor can run.

  • Add a remote bulb temperature controller located in the return air stream and wired in series with the dehumidistat to prevent over-cooling.

  • Limit compressor operation, as commanded by the dehumidistat, to about two hours per day. You should be able to do this with some of our modern time clocks.

  • Be certain that the AHU fan is cycling with the compressor and not running continuously.

I have one more idea to add, but first, let’s ask ourselves exactly what is it that dehumidistat do? They don’t dehumidify. The most they can do is to save electricity by stopping the compressor when it is merely hot outside, but not humid. They are, at best, an energy conservation device. We can normally prevent microbial growth just by setting the thermostat at 82°F., or so for the summer. All a dehumidistat can add to the equation is to stop the compressor any time the temperature rises above 82°F, but the humidity is low. Now how often does that happen in Florida?

SO LEAVE THE FOOL THINGS OUT. The risk outweighs the reward. Why risk $50,000 in damage to your home with a gadget that might save $50 a year in electricity? If you want to sell the homeowner something, sell them a dehumidifier. Until you do, The Duct Stops Here.

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