Digital Controllers for Dehumidifier and Dehumidistatic Control of Heat

The controllers share the following general features:
Off-the-shelf components.
Automatic switching between dehumidifier and heat.
Minimal human intervention once set and adjusted.
Remote RH and T sensors.
Ability to calibrate RH sensor.
Functionality at temperatures below 40EF.
Accuracy of RH controller to +/- 5%.
Ability to control set points.
Ability to adjust RH and T differentials.
Storage of settings in non-volatile EPROM memory.
Adaptability to different sites and systems.
Reasonable cost.
Components can be assembled for control of many different systems, including dehumidifier only, dehumidifier and dehumidistatic control of heat, dehumidifier and dehumidistatic control of heat plus thermostatic over-ride (low temperature or human comfort), or other configurations.
The control components are standard off-the-shelf items. However, how they are assembled is vastly different than in standard HVAC applications, which are most commonly concerned with temperature control. These control systems are designed primarily for historic buildings where there is a primary desire to control the RH within a given range, often selected as 40%-60%, allowing the winter and summer temperatures to fluctuate as necessary to achieve these RH levels. In most historic buildings, active humidification is detrimental to building preservation and is therefore to be avoided. Similar concerns exist for the use of air conditioning in historic buildings. However, the control components can be assembled to provide any type of temperature and relative humidity control desired.
The basic system logic that will control a dehumidifier and provide dehumidistatic control of heat begins with the humidity controller. A 24 Vac power supply and switching relay powers the controllers and provides a "hot" wire for system control. The "hot" wire inputs the humidity controller, which is set to the dehumidify mode. When the RH rises above the user-selected set point (generally around 50% RH), the output terminal is energized with 24 Vac. This terminal is wired in series to the input of the two-stage temperature controller. The temperature controller stages are set for the temperature at which RH control will switch from the dehumidifier to the heat, generally around 55 degrees for a commercial dehumidifier such as the DEC ThermaStor Santa Fe. This is a critical difference between this control system and how these controllers are generally used. If the temperature is above the set point, 24 Vac activates the switching relay, turning on the dehumidifier, and if the temperature is below the set point, 24 Vac will turn on the heat via the heating system thermostat input. Either of these pieces of equipment will run until they bring the RH down below the RH set point. Thus, switching between the heat and the dehumidifier will occur automatically as needed.
A museum can purchase the components separately and assemble them into a control system. However, experience has shown that licensed electricians often will not wire the controls correctly even when provided with a diagram, and many museum staff have difficulty properly setting the controls initially and calibrating the sensor. When this happens, the expense of a conservator site visit is added to the costs already paid for the electrician. These problems can be reduced or eliminated by having the control system pre-assembled and mounted in a metal hinged-lid circuit box by American Conservation Consortium. Wiring will be correct, settings will be pre-entered and calibration of the sensor will be performed. When the control system arrives, all that is required is to run the remote sensor wire, attach the thermostat wire to the heating system (if dehumidistatic control of heat is included), plug the dehumidifier into the grounded receptacle and insert the plug into a 120 Vac outlet.
Components for a dehumidifier and dehumidistatically-controlled heating system are:
Honeywell H775A electronic remote humidity controller;
Honeywell C7600B wall mount capacitance humidity sensor;
Ranco ETC-212000 2-stage remote temperature controller;
Taco 24 Vac power supply/ switching relay;
Metal circuit box;
50' 3-prong power cord;
18-5 x 100' thermostat wire (T and RH sensors);
18-2 x 50' thermostat wire (to heat);
Miscellaneous connectors and hardware.
Components for a dehumidifier-only control system include a single stage temperature controller, but the other components are essentially the same.
The performance of any environmental control system depends upon many factors, only one of which is the control system. Factors as simple as the placement location of the sensors can have a dramatic effect. Exterior walls will be colder or hotter than the interior depending upon the season, and sensors mounted in such locations will produce RH readings that are not typical of the room interiors. During certain times of day and at specific times of the year, sunlight may fall on the sensor, creating a spike in the RH reading that does not represent the room interior. Solar heating in western rooms can drop the RH levels rather sharply due to rapid heating of the air. A simple environmental control system based upon a dehumidifier and dehumidistatic heat can not raise the RH quickly to compensate for this. Similarly, a day with very dry outdoor air will cause a lowering of room RH, and the system can not compensate for this. It must be understood that each building and environmental control situation is different, and actual performance can only be determined by installing the controls, operating them through all of the seasons, evaluating the datalogged monitoring of the conditions produced, and making adjustments as required.
Simple environmental control systems can provide significantly enhanced preservation environments while minimizing potential damage to historic buildings. They can be successful in keeping the RH levels between 40% and 60% for the vast majority of the time throughout the year. Please refer to the environmental monitoring graphs attached to this document for comparison of the H775A controller with the conventional Honeywell H46C-1000 ribbon dehumidistatic control that is commonly used for inexpensive control functions and is the control factory installed in the DEC ThermaStor and many other dehumidifiers.
An extensive array of other factors influence the effectiveness of environmental control. Therefore, it is unlikely that simply purchasing controls and related HVAC equipment will have satisfactory results. It is highly recommended that a conservation consultant familiar with environmental control needs provide an initial site visit to verify that proper hardware is selected and that correct installation procedures and locations are determined. Additionally, there will be a need to evaluate monitoring results after system installation and make appropriate adjustments to guarantee that the system is providing the actual results desired. Use of appropriate consultant input during the planning and implementation phases will save untold headaches and unnecessary expenses.
Target RH levels are not the same in all parts of the United States or the World. While it is true that a RH range of 40%-60% is appropriate for much of the United States, the geographic location of the building is a primary factor in selecting appropriate system settings. Please feel free to contact us if you have questions.
These control systems were developed as part of a consulting project for the State of New Hampshire, Division of Resources and Economic Development. They are responsible for a number of historic houses throughout the state which hold collections. They wanted to develop a prototype control system that met the criteria listed on the first page of this document and could be easily adapted to most, if not all, of their sites. What on the surface appeared to be a rather straightforward investigative project turned out to be far more complicated. The manufacturers’ technical representatives had never envisioned their products being used in the manner desired. Their unanimous response to the question "will it work for this purpose" was "I don’t know, you will have to test it." So, the project morphed from investigation to testing. After the investigative work, prospective controllers were purchased, installed in the desired manner, and the environmental results were datalogged. This not only showed the strengths and weaknesses of each of the controls, but also taught valuable lessons in how to best adjust and tweak the controls for desired results. The flexible control system described herein is the result of this research. It has been installed and is operational in several historic houses utilizing Santa Fe dehumidifiers and forced air heating systems, and is providing vastly improved environmental control.
Due to liability issues, we are not able to provide assembled systems. However, we will be happy to consult with you about the appropriate equipment and the correct way to interconnect the parts. Due to the prevalence of internet vendors, you can probably purchase the equipment at a good discount, but if not, we may be able to obtain it for you at about 60% of the list price.


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American Conservation Consortium. Ltd.
4 Rockville Road, Broad Brook, CT 06016, 860-386-6058, acc@conservator.com
Copyright 2008, American Conservation Consortium, Ltd., all rights reserved


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