An Assessment of Energy Efficient Methods of Indoor RH Control in the Florida Housing for Florida Building Commission Research

Oscar L. Calleja and Jim Cummings

August 13, 2013

Definition of the Problem: The thermal efficiency of homes has improved in recent decades. Some of those improvements have reduced the amount of sensible heat entering the house (sensible heat is associated with temperature rise while latent heat is associated with water vapor in the air). Wall and attic insulation, improved duct insulation, improved windows, and better shading of windows and houses have reduced sensible heat entry into the home, which can cause the A/C system to operate less. Reduced A/C operation can lead to a reduction in water vapor removal, and the potential for an increase in indoor RH.

On the other hand, there are two other factors that tend to counter the reduced envelope load. The first is that more electronic devices (computers, TVs, video games, etc.) are operating in our homes producing considerable internal sensible heat and extended A/C system operation. The second is that air infiltration has declined dramatically in recent decades. Typical new homes have declined in air leakage from about 22 ACH50 (air changes per hour at 50 pascals of pressure; a blower door test result) to about 5 ACH50 in recent years. Since about 85% of the cooling load associated with air entering from outdoors during hot and humid weather is latent heat (water vapor) and only about 15% is sensible heat, the tightening of homes has greatly reduced the amount of water vapor that must be removed by the air conditioning system. Tightening of duct systems has also substantially reduced the amount of water vapor entering the house.

On the other hand, there is a trend for various codes, standards, and programs to require mechanical ventilation (typically in compliance with ASHRAE Standard 62.2), which then re-introduces a considerable portion of the outdoors-to-indoors air exchange that was eliminated by house tightening. The net result of the reduction in envelope sensible loads, increased internal sensible loads, reduction in envelope and duct leakage latent loads, and increase in ventilation-induced latent loads is that in some cases the conventional A/C system does not adequately control indoor RH.

There is a need, therefore, to identify methods of improved humidity control in homes. It is, of course, essential that humidity control be achieved in ways that are energy efficient and cost-effective. For Code development purposes in our Florida climate there is also a need to take into account the role of humidity control when developing code for envelopes and mechanical systems.

Our Approach to This Problem: Two tasks are proposed. Task 1: Our research team proposes to perform a review of literature, examination of experimental data, and an assessment of the energy efficiency and cost-effectiveness of various approaches to managing the latent load in homes. Humidity control is not just a question of which appliance or technology to choose.

In existing homes it is often an issue of eliminating various types of failures which lead to increased latent load. It makes sense, for example, to eliminate various failures prior to installing expensive and energy-intensive technologies.

· If large duct leaks are drawing humid air into a house, the solution would be to repair the duct leak instead of installing a high-efficiency dehumidifier.

· If a house has large pressure differentials and air infiltration due to unbalanced return air, then installation of return pathways would be the energy efficient and cost effective approach.

· If an A/C has the air flows that are too high, incorrect refrigerant charge, or long air handler fan shut-off delay, the fixing those problems would be the correct solution.

It is often an issue of adopting design approaches which improve the chances of effective latent cooling.

· If an A/C system is oversized so that it cools primarily with short cycles, then replacement with a right-sized system would be better than installing a dehumidifier.

· If a thermostat has characteristics which produce short-cycles, and therefore poor humidity control, the thermostat should be replaced.

· If ventilation air is introduced into the home, deliver it so that it passes primarily through the A/C system prior to mixing into the room air.

Once system failures have been eliminated and advantageous approaches to system operation have been adopted, and acceptable humidity control has not yet been achieved, then enhanced space conditioning systems should be contemplated and those that are most energy and cost-effective should be considered first. This study will examine a variety of technologies and approaches, and provide recommendations of those that are most energy and cost-effective.

In the recent versions of the Florida Building Code many of the “failures” listed above have been addressed, such as building air leakage, duct leakage and proper AC equipment sizing. Nevertheless additional humidity control measures might be needed and recommendations for potential inclusions to Code language will be part of this study.

Task 2. Experiments will be implemented in a highly instrumented lab facility to assess indoor RH and energy consumption (with measurement of temperature and humidity, and system energy consumption) that results from various approaches and equipment types (including a high-efficiency dehumidifier) are implemented when various levels of ventilation air are introduced to the space.

Expected Outcome and Impact on The Code: The outcome of this research will be a report that assesses the pros and cons of various approaches to indoor humidity control, with a focus on the solutions that are most energy and cost-efficient. It will address the issue of ventilation and optimized humidity control. Recommendations will be presented on fixing problems that cause elevated indoor humidity, approaches to system design, and selection of systems/technologies that can achieve energy efficient humidity control. From the recommendations of this report, it is expected that the Florida codes will be modified or adjusted to take into account the best approaches to energy- and-cost effective control of humidity in the Florida climate.

Budget: Budget total is $54,600. Task 1: $27,000 Task 2: $27,600 for the lab experiments and data analysis.