Tag: dehumidification


I’ve been reading a book called “Cool, How Air Conditioning Changed Everything” and it got me interested once again in the history of air conditioning and refrigeration. Like many things the people who are credited with “inventing” are the ones dogged enough to make an idea commercially successful, not the idealists forever tucked away in the lab.

I bought a 1921 version of the periodical “Ice and Refrigeration” and mixed in with the ads for absorption ice machines and “mineral wool” insulation was the advertisement shown above. Willis Carrier understood how to connect ideas and make sense of emerging technology, first to keep paper dry in a factory and later to cool the world with “Manufactured Weather”. Look carefully at the ad, you will notice it mentions many things… but not cooling, the ad is in ICE AND REFRIGERATION and the ad doesn’t mention COOLING.

Many of you know that in 1906 Willis Carrier patented what is now referred to as the “First Air Conditioning System” but do you know what it was that he actually invented?

You may be led to believe that Willis Carrier invented compression refrigeration? Nope, the first commercial attempts at compression refrigeration began in the 1830’s and the patent above actually has no compression refrigeration in it whatsoever. Many will say that he was the first to dehumidify the air, this is also false, there had been compression refrigerated cooling coils in use that dehumidified the air before Willis came along they just didn’t do it on purpose.

What Willis Carrier understood better than anyone else in his day was the RELATIONSHIP between humidity, temperature and saturated air or “dew point” and how to manipulate water temperature, water volume and air volume to produce a CONTROLLED humidity environment first and later a controlled temperature, humidity, and ventilation environment.

The Carrier “Air Washer” was nothing more than water pumped through nozzles that produced a mist of water. The air would blow through the water mist and it would clean the air, drop it to dew point (100% RH) and then continue to sensibly cool the air. Willis worked in northern states with cold groundwater at a time before water use restrictions so the cold water would serve to cool AND dehumidify the air. At the time it seemed like black magic that running air over water could REMOVE water from the air, but so long as the water temperature was below the dew point temperature of the air that is exactly what would happen. All Willis had to do to change the dehumidifier to humidifier was to increase the water temperature or change the dehumidifier to a sensible cooling machine was to use cold water and give the air more dwell time or passes through the water to decrease the sensible temperature.

In the process Carrier and his team made many discoveries about air and in 1911 Carrier presented possibly his greatest work which he called the “psychrometric formulae” which is the founding document on which all of current understanding of psychrometrics is built. Carrier took a VERY SIMPLE idea and pursued it and understood better than the others around him and because of that, we remember him today. He thought about cooling, heating, ventilation, humidity and air cleanliness and combined them together into one machine that controlled it all.

Later on, Carrier would begin actively “cooling” the air with compression refrigeration and replaced water sprays with refrigerant evaporator coils to leverage the latent capacity of refrigerants, but it all started with a mist of water an understanding of dewpoint, some dogged determination and some clever marketing for his “manufactured weather”.

— Bryan

To find the catalog where I found some of this information I created a link to the national archives at hvacrschool.com/willis

 

 

 

 

Sometimes you find yourself in a position where you are going to replace a fancy thermostat with a simple one. It may be because the customer got fed up with all the options or because you are there on a weekend service call and all you have is a basic stat.

No matter the reason you need to make sure the new thermostat can do the job the old one did before you quote, an option that gets overlooked in matching up is dehumidification.

Most manufacturers of residential variable speed air handlers have a terminal that will drop the blower speed when de-energized. It may be marked DH or D or dehum or something else. From the factory, they generally have that terminal connected to R using a pin or jumper so that the blower will run up to full speed. When one of these special thermostats get installed the tech is supposed to remove that jumper or pin and connect a wire from that terminal to the thermostat dehumidification terminal so that the thermostat can energize the terminal for full blower speed or drop 24v to the terminal to go into dehumidification.

If we install a new thermostat and forget to reconnect that pin or jumper then the system will ALWAYS run in dehumidification mode because there will never be any power on that terminal at the air handler board.

The lesson is to pay attention to whether or not a system is wired for dehumidification. If you do need to replace it with a basic thermostat make sure to replace the pin or jumper (J1 on the board example above).

If you forget to do so the system will run less efficiently with lower airflow, suction pressure and coil temperature.

— Bryan

 

 

Dehumidification features are common on residential systems ever since the introduction of variable speed blower motors. The system is set up so that the blower can produce less CFM per ton when latent (humidity) load in the space is higher than the setpoint relative humidity. Slowing the blower increases moisture removal by reducing the sensible load on the evaporator coil and therefore dropping the coil temperature and surface dewpoint.

Most variable speed fan coils and furnaces have a terminal designated for Dehumidification and it can be called D, dehum, DH or something else depending on the manufacturer.  In all cases I am aware of, this dehumidification terminal must be energized for the blower to go to full speed and when that terminal is de-energized the blower speed (usually) drops to 80% of full speed.

For years we have seen thermostats with designated dehumidification terminals to match up with the fan coil/furnace terminal, so it was just a matter of disconnecting a jumper from the dehumidification terminal to the R terminal in the unit and connecting a wire from the designated thermostat terminal to that dehumidification terminal in the unit. The diagram below is an example of this on an old Carrier Thermidistat with a variable speed Carrier fan coil.

We now have 24v control smart thermostats like Ecobee, Cor, Nest and Lyric with a lot more flexibility in how they can be set up rather than having a single, designated dehumidification terminal.

I am a big fan of EcoBee for many reasons including their Alexa integration, remote wireless sensors and application flexibility… but you need to be really careful with how you set them up, ESPECIALLY when setting up dehumidification.

The image above is a GIF and should show you the first part of the dehumidification setup. I am setting it up for a single speed compressor heat pump with a variable speed fan coil. EcoBee has contacts labeled acc+ and acc- that can be set up to do a wide variety of functions. For this typical dehumidification function using the system you would select Menu>Installation Settings>Dehumidifer >1 Wire ACC+>Open contact state to activate dehumidifier.

This setup uses 24V power from the R terminal to energize the acc+ terminal and therefore the dehumidification terminal in the fan coil/furnace when there is NO call for dehumidification.

Now for a controversial part. Go to the equipment menu and select Dehumidifier to “dehumidify with fan”= no. We have seen several occasions where the blower continues running with no cooling call if this setting is set to yes when there is a dehumidification demand and no cooling demand.. According to the EcoBee website HERE it appears to say the opposite, but we have confirmed on a few occasions that this occurs and there appears to be no adverse effects from setting it to off becasue the blower is still controlled by the thermostat for cooling operation and dehumidification without cooling is not possible without an external dehumidifier.

In order for the system to over cool below the temperature setpoint to dehumidify you need to go into the thresholds menu and set up AC over cool Max to the maximum temperature below setpoint that would be allowed during dehumidification by the equipment.

— Bryan

 

 

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