July 30, 2013
Mixed-Humid

Abstract: 

This document covers a description of the need and applied solutions for supplemental dehumidification in warm-humid climates, especially for energy efficient homes where the sensible cooling load where the sensible cooling load has been dramatically reduced. Available supplemental humidity control options are described and discussed, with application guidance. Some options are less expensive but may not control indoor humidity as well as more expensive and comprehensive options. The best performing option is one that avoids overcooling (cooling below the requested set point) and avoids adding unnecessary heat to the space by using waste heat from the cooling system to reheat the cooled and dehumidified air to room-neutral temperature.

Introduction

This document is focused on indoor humidity control in warm-humid climates. In the last decade, building codes and market demands have been quickly pushing up the energy efficiency requirements of residential buildings. Overall, this is good, and produces a significant net energy and cost savings. However, this situation has forced us to rethink the way we have traditionally thought about conventional residential space conditioning system design in warm-humid climates. Most building efficiency improvements brought about by code requirements and above code incentive programs, such as more insulation, better windows, low-power lighting and more efficient appliances, are directed at lowering sensible gains while latent (moisture) gains remain mostly unchanged. Latent gains are mostly related to internal moisture generation by occupants and their activities, and ventilation requirements. Because conventional cooling systems are directed to control to a temperature set point, cooling systems in these more efficient, low sensible gain houses, have longer off-times. During those longer off-times, indoor moisture can build up and cause elevated levels of indoor relative humidity (RH). Elevated RH impacts comfort, indoor air quality, and sometimes material durability if mold or fungi growth occurs. Therefore, at some times when there is no need to lower the space air temperature, supplemental dehumidification will still be needed to maintain the relative humidity below acceptable levels. Maximum indoor RH thresholds vary depending on the criteria. For example, to control for dust mite allergen, a maximum of 50% RH is recommended. To control for comfort, at typical indoor cooling season temperatures, a maximum of 60% RH is recommended. To avoid wintertime condensation on metal window frames and single-glazed windows, often resulting in mold on sills, the threshold would be 50% RH or lower.

The information provided in this Measure Guideline applies whether the home was constructed new or retrofitted to be high performance/low sensible heat gain.

Extensive field testing was done with BSC builder partners in Texas and Florida in 2001 to 2007 (Rudd et al. 2003, Rudd 2004, Rudd et al. 2005, Rudd 2006, Rudd and Henderson 2007, Rudd 2007(b)). In part, that testing revealed that supplemental dehumidification was needed in high performance, low sensible heat gain homes in order to maintain indoor relative humidity below 60% year- round. Detailed simulations later confirmed that and expanded on those findings (Rudd et al. 2013).

Off-the-shelf stand-alone supplemental dehumidification systems and central system integrated supplemental dehumidification solutions that allow year-round indoor relative humidity control between 50% and 60% were employed to address this problem.  Supplemental dehumidification provides an opportunity to market year-round comfort in warm-humid climates, and is intended to enable further reduction in sensible cooling loads, through further efficiency improvements, without the risk of elevated indoor humidity.

While these advancements have been important and needed in the residential space conditioning industry, supplemental dehumidification technology continues to improve and evolve, and the market for these products is still in its infancy.  Design capacity prediction is subject to many unknowns, the most important being sensitivity to internal moisture generation by occupants.

Decision Making Criteria

Generally, the decision to employ supplemental dehumidification is coupled with efficiency improvements resulting in low sensible cooling loads in warm-humid climates (Figure 1 below the red line).  However, there are some situations where supplemental dehumidification is needed that are not related to efficiency improvements.  Those situations are generally found in multi- family buildings, especially first floor units with little outside wall or roof exposure, or where homes are very shaded by trees or buildings. High outdoor humidity in coastal areas can make supplemental dehumidification important further north than the IECC warm-humid line extending generally from Wilmington, NC to Dallas, TX (Figure 1). The type of construction, and time of year of construction and occupancy, can also have an important impact on indoor humidity control.  For example, a slab- on-grade, concrete or masonry unit wall home constructed in late summer and occupied in the fall, or constructed in winter and occupied in spring has a lot of interior moisture to remove due to construction materials drying but little chance of consistent moisture removal until the main cooling season begins.  While in some cases, this may require only temporary supplemental dehumidification, builders of high performance homes in warm-humid climates will likely find it more efficient and acceptable to their overall customer base to treat all homes (at least in a given community) alike by employing the more robust permanent supplemental dehumidification solution. 

Figure 1. IECC Climate Zone map showing warm-humid line generally extending from Wilminton, NC to Dallas, TX

Cost and Performance

Supplemental dehumidification, in and of itself, does not save energy, rather, it is justified by enabling the energy savings from dramatically reduced sensible cooling loads in warm-humid climates. The supplemental dehumidification solution is intended to enable further reduction in sensible cooling load, through further efficiency improvements, without the risk of elevated indoor humidity.

The estimated equipment cost of supplemental dehumidification, including labor, can range from $400 to $2,000 depending on the system solution chosen. A stand-alone dehumidifier will cost the least and a desiccant dehumidifier integrated with the central space conditioning system will cost the most. The PROGRESSION SUMMARY chart at the beginning of the document follows this order and Table 1 provides more cost estimate detail. 

Table 1. First-Cost Estimates for Supplemental Dehumidification Systems

Supplemental
Dehumidification System
First-Cost
Estimate
Stand-alone dehumidifier
with remote dehumidistat
$400
Integrated ducted
dehumidifier
$1,000
Sub-cooling reheat$1,600
Full-condensing reheat$1,750
Desiccant dehumidifier$2,000

The most effective solutions, having relatively low operating cost and essentially eliminating indoor humidity above 60% RH, are:

  • full condensing and subcooling reheat integrated with the central cooling system;
  • ducted dehumidifier;
  • stand-alone dehumidifier with central system mixing; and
  • DX condenser-regenerated desiccant dehumidifier.

Supplemental dehumidification operating energy of about 170 kWh/yr can be expected for a HERS Index 50 house (having ducts inside conditioned space) with a 60% RH set point. About five times that can be expected with a 50% RH set point.

A second tier performer is the subcooling reheat system but that system allows more elevated RH hours.  A third tier (not considered as supplemental dehumidification) is the enhanced cooling option which uses controls for 2oF of overcooling and lower airflow (200 cfm/ton) activated at 50% RH.  Two-speed and variable speed systems do little to reduce hours of elevated relative humidity in warm-humid climates unless coupled with the enhanced cooling option listed above.

Risk Identification

Elevated RH impacts comfort, indoor air quality, and sometimes material durability if mold or fungi growth occurs.  Counting hours above a given RH threshold is a reasonable metric to determine a system’s effectiveness in reducing or eliminating elevated indoor RH.  60% RH is a reasonable and commonly used threshold.

Technical Description

Table 2 gives a listing of recommended minimum supplemental dehumidification capacities for several different conditions1. Those conditions depend on the desired RH set point, and on houses size and occupancy, resulting in expected internal moisture generation. The capacities listed reasonably well match the capacities of available dehumidification equipment, and are given at the AHAM rating condition of 80oF and 60% RH. Moisture removal capacity in typical homes at lower temperatures, or lower RH, or both, will be lower than the AHAM rated capacity, but that was considered in making Table 2.

Table 2. Minimum Supplemental Dehumidification Capacity Guideline

 Minimum Supplemental Dehumidification Capacity1
 Small to Medium House Size
roughly <2500 ft2
Larger House Size
roughly >2500 ft2
 60% RH
set point
50% RH
set point
60% RH
set point
50% RH
set point
Average internal
moisture generation,
for 4 people
(12 lb/day)
40 pint/day
(1.7 lb/h)
60 pint/day
(2.5 lb/h)
60 pint/day
(2.5 lb/h)
80 pint/day
(3.3 lb/h)
Higher internal
moisture
generation, or
more than 4 people
(24 lb/day)
50 pint/day
(2.1 lb/h)
70 pint/day
(2.9 lb/h)
70 pint/day
(2.9 lb/h)
90 pint/day
(3.8 lb/h)

1 Capacities listed are based on AHAM rating at 80°F and 60% RH

System Interaction

Designers or builders sometimes want to know if the cooling system capacity can be reduced because of installed supplemental dehumidification capacity.   The answer is, no.  The reason for that is that operation of supplemental dehumidification is not coincident with the peak design conditions of a cooling system.  A cooling system is designed to meet the summer peak sensible cooling load, while understanding that at that condition it will also have some latent (moisture removal) capacity (generally being 20 - 25% of total capacity).  At summer cooling design conditions, supplemental dehumidification is not needed.  Supplemental dehumidification is needed during spring/fall seasons, summer shoulder months, rainy periods, some summer nights, and even some winter days in warm-humid climates.

Measure Implementation

Scope of Work

A.  Determine the minimum supplemental dehumidification capacity needed. See Table 2.
B.  Determine whether a less expensive stand-alone dehumidifier approach or a more expensive and comprehensive central system integrated approach will be used. See Figure 2 to Figure 17.
C.  Determine how whole-house distribution of dehumidified air will be accomplished (i.e. timed periodic operation of the central system blower or make sure interior doors stay open).
D.  Determine a good representative location for installing the dehumidifier controller (dehumidistat) with relative humidity display.
E.  Install the system.
F.  For dehumidifiers integrated with the central system, install a backflow preventer damper in the dehumidifier outlet duct. Use a “Wye” fitting to connect to the dehumidifier supply duct to the central system supply duct/plenum to help move the dehumidified air downstream for better distribution. See Figure 6 and Figure 7.
G.  Verify proper operation of the dehumidifier and control system.

Install Procedure

Enhanced Cooling Option (Lower Evaporator Airflow and Overcooling)

For comparison, the Conventional System for the HERS Index 50 house referred to in this guideline was a SEER 17.7, 2-speed compressor system with a BPM motor variable speed indoor fan (Rudd et al. 2013; Rudd 2013).  The Enhanced Cooling humidity control option is the same as the Conventional System except with controls to provide lower airflow and space overcooling when space humidity is high. These are usually programmable settings on higher-end cooling equipment and thermostat/dehumidistat products.  HVAC contractors are typically familiar with this and would implement it for the builder.  Operation in Enhanced Cooling mode is limited to 50% runtime (10 minutes ON, 10 minutes OFF) and the operating fan power for the brushless permanent magnet (BPM) motor drops from 0.35 to 0.1 W/cfm at low airflow.

Conventional direct expansion (DX) cooling systems are typically AHRI rated with airflow of between 350 – 400 cfm/ton of total cooling capacity.  Lowering the cfm/ton improves latent cooling performance (moisture removal) but increases energy consumption because of the work it takes to condense additional water vapor. For the results shown in this guideline, the airflow was lowered to 200 cfm/ton. There is a practical limit to lowering the cfm/ton because if parts of the evaporator coil drop to 32°F, frosting and eventual icing of the coil will occur.  Ice further blocks airflow, creating more ice, and eventually makes it impossible to deliver the conditioned air. As a protection against icing, the low airflow operation is typically limited in time, and for further protection, temperature switches or sensors can be strategically placed on coil to stop the compressor if the coil temperature drops too low.

Overcooling means, when space humidity increases above the RH set point, the cooling temperature set point is reduced 2°F below the requested temperature set point to continue the cooling operation in hopes of meeting the RH set point.  This operation also carries with it a risk of occupant discomfort complaints due to the wide range of temperature control.   This humidity control option is “enhanced cooling” rather than “supplemental dehumidification” because when the minimum cooling set point is satisfied (2oF below the requested set point in this case) then there will no longer be any call for cooling or the moisture removal that typically occurs with that cooling.  In other words, moisture removal is locked out at that point.

The information in all of the humidity control performance tables in this document come from two much more detailed references: Rudd et al. 2013 and Rudd 2013. . .

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Footnotes:

  1. These recommendations are based on the author’s best estimates accumulated over a decade of experience with applying and analyzing supplemental dehumidification systems.  Also see references Rudd 2013 and CMHC 2009 for additional information.