Field Evaluation: Habitat for Humanity
Project Summary
Final Report (PDF) August 2003
This Schenectady Habitat for Humanity Home was a pilot demonstration project partially supported by the Schenectady Builders & Remodelers Association Green Building program. Construction of the two-story 1344 sq. ft. home began in May 2001 and was completed in October of the same year. The design and construction of the house includes insulating concrete form (ICF) basement walls, vinyl siding with integral insulation, and whole house mechanical ventilation. PATH technologies under evaluation at this site include a radiant floor heating system and air admittance valves (AAV’s).
The primary evaluation for this project is a side-by-side comparision of two different types of heating systems installed in the same home. One heating system is a radiant floor design with the tubing installed below the floor, using a sealed combustion boiler. The other system is a forced air ducted design with a condensing furnace. The evaluation will compare energy consumption and operating costs of both systems during occupancy. Monitoring includes energy consumption, both electricity and natural gas, and measurement of floor temperatures and room air stratification in specific rooms.
System operation alternated in two-week periods throughout the 2001 – 2002 heating season. In other words, the radiant system was operated or in use for one period and the forced air system was operated and in use the following period. A datalogger monitored and recorded energy consumption and environmental conditions.
A secondary evaluation focused on the design, installation and acceptance of air admittance valves or AAV’s. Air admittance valves are plumbing vents that allow air intake into the drain-waste-vent system, but prevent gases from escaping. They can reduce the need for long runs of vent pipe and multiple penetrations through the roof. Installation costs, code and permitting issues, and contractor feedback regarding AAV’s were documented throughout the construction process.
Blower Door and Duct Blaster Testing
Blower Door and Duct Blaster testing was conducted on the home to characterize the tightness of the building envelope and ductwork. The Blower Door results showed the building envelope to be very tight at 2.2 ACH50. Using the LBL method to calculate natural air changes per hour, the results were .31 ACHnat. No significant leakage sites could be found when typical areas were investigated during a “manual” investigation, e.g., around windows and doors, receptacles, attic hatches, plumbing and electrical penetrations to the exterior.
Duct Blaster test results were not reliable because a built-in bookcase had been installed in front of the heat register in one bedroom. Although the base of the bookcase had been neatly routed to allow for airflow, we could not access the register in order to seal it off during testing. Therefore, total duct leakage cannot be accurately reported. However, since duct leakage to unconditioned space is a relative measurement, the test for leakage to outside of the building envelope is more reliable. All the ductwork is located within conditioned space and duct leakage to unconditioned space was very low at 25 CFM at 25 Pascals.
Air Admittance Valves (AAVs)
Air admittance valves were also evaluated at this site. Air admittance valves are plumbing vents that allow air intake into the drain-waste-vent system, but prevent gases from escaping. They reduce the need for long runs of vent pipe and multiple penetrations through the roof. Resultant benefits include reduction in both material and labor costs, and enhanced durability given fewer penetrations of the building envelope. Studor air admittance valves were used for this project, but other brands are available.
The field evaluation of air admittance valves included the following:
Documentation of relevant building code requirements and associated permitting procedures
Feedback from local code officials and trade contractors regarding the installation and performance of the valves
Monitoring during the construction phase to provide information regarding impact upon plumbing labor or other trade contractors
Documentation of labor and material costs associated with installation of AAVs, and comparison to conventional venting systems
Periodic inspections and energy use monitoring once the home is occupied, to provide assessment of performance of the heating systems
COMPARISON OF RADIANT FLOOR HEATING AND FORCED AIR SYSTEM
Radiant Floor Heating System
A Slant/Fin® Model CB-45 boiler provided the heat for the radiant heating system. The boiler was a sealed combustion unit with adjustable output. The advertised efficiency rating of this boiler is approximately 86 percent Annual Fuel Utilization Efficiency (AFUE). Since the calculated heat loss of this home was very low (22,000 btuh), a boiler was selected that could come close to that output and still remain within the same cost and efficiency range as the furnace. The CB-45 boiler also allows for variable Btuh output when combined with direct vent applications; this is accomplished by a field adjustment of the gas input rate on the boiler.
The radiant floor system that was installed is a “dry system.” The radiant industry uses that term for tubing that is not imbedded in a slab. Both “wet” and “dry” systems circulate liquid inside the tubing. “Wet” system refers to tubing imbedded in a slab above the floor and “dry” system refers to tubing installed below the floor. Cross-linked polyethylene (PEX) tubing was installed beneath the floor and secured into aluminum plates designed to improve heat transfer. Tubing was spaced 8 inches apart in each joist space. Volunteers working for Habitat for Humanity helped install most of the radiant floor tubing and aluminum plates below the floor sheathing of the first and second levels.
Forced Air System
The forced air system installed in this house was a conventional ducted system with the d uctwork located within conditioned space. Supply and return trunk lines were made of sheet metal. Branch lines to each supply register were flexible. Floor registers are used throughout the home, and separate return air inlets were located on the first and second levels.
A Carrier® Model 58MCA040-08 sealed combustion condensing furnace provided the heat. The advertised efficiency of this furnace is approximately 90 percent AFUE. This furnace has the lowest output within its product line (37,000 btuh). It utilized sealed combustion to meet our parameters and was the closest match to the design load without going to a more expensive higher efficiency, dual stage, variable speed unit.
Monitoring
The datalogger installed in the home continuously monitored different sensors and equipment. Data was compiled and then periodically retrieved electronically by Research Center engineers. The data is currently being analyzed to compare fuel use under each heating system versus outdoor conditions, e.g., temperature and wind speed. The occupants also kept a record of their activities as well as their perceptions of comfort during the monitoring period. Other qualitative information that will be presented is differences in temperature between the first and second floor, temperatures at different heights off the floor, floor temperature, and indoor humidity levels.
The following parameters were monitored on a continuous basis:
Indoor and outdoor temperature and relative humidity
Wind speed and direction, solar radiation
Sub-floor temperatures in four interior locations
Stratification temperature measurements in two interior locations
Warm air furnace: gas valve status, blower operation, supply and return plenum temperatures
Boiler and radiant floor piping: gas valve status, flow rate, supply and return temperatures of primary, loop and second floor loop, off/on status of first and second floor pumps
Energy Recovery Ventilator: outdoor air supply flow rate, temperature at one interior supply location
