November 15, 2002
Hot-Humid

Abstract: 

It’s quite likely humid or hot or both in Houston, Texas. As a builder/buyer of high performance homes who naturally wants to deliver/buy a comfortable healthy home, just how do you most efficiently and cost-effectively cool and dehumidify inside air while maintaining sufficient introduction of outside air for ventilation? And perhaps most importantly, how do you condition for humidity when the season calls for neither heating nor cooling? These are two knotty, nasty questions with a bunch of potential answers, none of which have ever been systematically explored or compared….until now.

Background

Houston, Texas. It's quite likely humid...or hot...or both. You are a builder/buyer of high performance homes (http://www.buildingscience.com/buildingamerica/targets.htm), and naturally want to deliver/buy a comfortable healthy home. So, just how do you most efficiently and cost-effectively cool and dehumidify inside air while maintaining sufficient introduction of outside air for ventilation? And perhaps most importantly, how do you condition for humidity when the season calls for neither heating nor cooling? These are two knotty, nasty questions with a bunch of potential answers, none of which have ever been systematically explored or compared....until now.

One of the great things about the Building America program is hooking up with builders who, while they may not be as tickled with knotty nasty questions as building researchers, are willing and able to work with building scientists and see if a reasonable answer can create more value for them and their customers. Such a builder is Pulte Homes of Houston, Texas (http://www.pulte.com/market.asp?pro=0&ck=58|106). Pulte Homes arranged for twenty of their sold and occupied new homes in the greater Houston area to be in a year-long study of six different HVACintegrated dehumidification systems.

The homes were monitored hourly for indoor temperature and relative humidity (five locations), outdoor temperature and relative humidity, and energy consumption broken down by source (cooling, air handler unit, fan cycler, dehumidifier, ventilating fan). In this way, both the comfort conditions provided and energy consumed by each approach over time could be quantified and compared. Here is how each home was set up:

  • Control Homes Three of the Pulte homes monitored are pre-Building America—they are not high performance homes in terms of their building envelopes, their air tightness, and ventilation (they lacked mechanical central ventilation). These homes represent standard construction practice and performance in the Houston area.
  • Building America High Performance Homes Three homes were built to the BSC Building America performance targets (see: http://www.buildingscience.com/buildingamerica/targets.htm). These homes had no special dehumidification capability, just what the standard cooling system can provide (see the HVAC glossary and discussion of sensible heat ratio that follows). The results from these high performance homes provide a baseline in terms of comfort conditions and energy consumption.
  • Stand-alone conventional dehumidifiers Two homes have a conventional dehumidifier installed in a louvered closet within the living space and two have the units installed in the conditioned attic. The “dry” air supplied by these units is distributed throughout each home by the central-fan-integrated supply ventilation system (http://www.buildingscience.com/resources/presentations/practical_approac...), a system that is standard in BSC Building America homes.
  • Continuous filtration/ventilation Three homes have the DEC Filter Vent (http://www.thermastor.com/ventilation/ventilationcentral.html) and ducted dehumidifier located in the conditioned attic. These two units combine to provide air filtration, fresh air ventilation, and dehumidification.
  • Continuous filtration/ventilation/dehumidification Three homes have the DEC Ultra-Aire APD (Air Purification Dehumidifier) (http://www.thermastor.com/dehumid/dehumidresid.html), a single unit that provides air filtration, fresh air ventilation, and dehumidification.
  • Ventilation with energy recovery Three homes have Venmar ERVs (http://www.efi.org/products/ventilat/venterv.html) (Energy Recovery Ventilator) located in the attic. ERVs recover energy associated with both the heat and the moisture content of the exhausted indoor and supplied outdoor air. The Venmar system uses a dessicant wheel to accomplish the transfer and recovery.
  • High-efficiency, two-stage compressor and ECM AHU One home has a high efficiency Carrier cooling system with two-stage compressor ( http://www.residential.carrier.com/products/airconditioners/set_aircond.htm) and an ECM (electronically-commutated motor) air handler. The two-stage compressor provides a lower capacity run setting for more sustained operation and hence greater dehumidification. And the variable speed capability of the ECM air handler permits lower fan speed settings that increase the contact time of indoor air at the coils, also increasing dehumidification.

All of the test systems provide some level of ventilation, cooling, and dehumidification. They differ in:

As much as possible, there was more than one home with each system to reduce the effect of occupants (their number, their thermostat setpoints, and their activities that generate moisture such as cooking, cleaning, bathing). And the homes are concentrated in just a few Pulte developments to reduce the impact of micro-environmental differences.

  • the way in which they dehumidify (as part of cooling, as part of ventilation, or as a part of an individual conditioning activity);
  • what air they dehumidify (circulating interior air, incoming fresh air, or both);
  • when they dehumidify (at what stage in the conditioning sequence);
  • under what range of conditions they can dehumidify (only during cooling, only during ventilation, any time humid conditions exist), and finally;
  • the level of air filtration they provide.

This study set out to answer the following questions:

  • Which system(s) was the least expensive? We report the builder’s installed cost.
  • Which system(s) was the least expensive to run (operating cost)? We report both the total HVAC system daily energy cost as well as the cost of just the dehumidifying component, if available or applicable.
  • Which system(s) did the best overall job in providing humidity control? We report the % of hours each system kept the house average relative humidity below 60%.
  • What are other specific advantages/disadvantages of each system? We report builder and homeowner comments made to researchers over the course of the project, in a written short questionnaire, and in a face-to-face exit interview with the homeowner.

Basics

Humidity 101

Humidity is the water vapor in the air. There are two expressions of this vapor content—absolute and relative humidity. Absolute humidity refers to the total amount of water vapor in the air. Relative humidity refers to the amount of water vapor in air relative to the total amount of water vapor the air can actually hold. Air can hold different amounts of water vapor at different temperatures—warmer air can hold more than colder air. That’s why water can and often does condense on a cold glass—the air just around the glass gets cold enough that it can’t hold all of its water vapor and some condenses on the outside surface of the glass.

So, if you raise the temperature of the air, it’s relative humidity goes down; cool the air down and the relative humidity goes up. Cool the air down enough so that the relative humidity becomes 100% and you have reached that air’s dewpoint, or the point at which that air just can’t hold that amount of moisture anymore. Raising or lowering the temperature of a given volume of air changes the relative humidity but it does not change the absolute humidity. The dewpoint temperature of air is often used to describe or quantify absolute huimidity.

Relative humidity is critical to the performance of buildings for two reasons. One, all living things sense and respond to humidity based on the concentration of water in air--relative humidity--more than the absolute amount of water in the air--absolute humidity, within the temperature range of indoor environments. Two, in a surprising twist of science, materials adsorb and desorb water based on relative, not absolute humidity. . .

Download complete report here.