1、OR-05-3-1 Evaluation of Hydronic Forced-Air and Radiant Slab Heating and Cooling Systems Evelyn Baskin, PhD Member ASHRAE ABSTRACT A contemporary house located in California has a unique HVAC system featuring radiant floor heating and cooling, forced-air hydmnic heating and cooling, and a system tha
2、t ventilates the house at night to reduce air conditioner energy use by pre-cooling building mass. The purpose of this study was to compare the energy consumption and electrical power demand characteristics of the various heating and cooling operating modes. Cooling season testing involved three dif
3、fer- ent operating modes during similar weather conditions and comparable indoor comfort conditions. Heating testing involved operation in two test modes: hydronic radiant jloor heatingand hydronic forced-air heating, both by heatingwater with natural gas. Energy performance results reveal that slab
4、 pre-cooling caused some of the energy demand to shijt from primarily in the afternoon. Coupling slab pre-cooling with nighttime ventilation air signijcantly shifted the energy demand load from primarily in the afternoon to late night with the load coming mainly from the fan. Also, during comparable
5、 outside conditions, there were appreciable energy differences among cooling modes and no identijable energy advantage was noticed during the heating mode. INTRODUCTION In this study, the energy consumption and electrical power demand characteristics of a house during the heating and cool- in season
6、s are assessed. The house is a custom 2484 fi2 (230.8 m ) two-story house completed in 2000 and located in Winters, California (see Figure i), approximately 30 miles (48.3 km) west of Sacramento. Cooling testing comprised operation in three different cooling modes during similar weather conditions w
7、hile maintaining equivalent comfort levels. Each of the three cooling modes was evaluated for a one-month period during the summer of 2002: B Figure 1 Test house: front view and back view. mode-C 1) mode-C2) mode-C3) Hydronic forced-air cooling in response to ther- mostat setting, Hydronic forced-ai
8、r cooling combined with hydronic slab pre-cooling, Hydronic forced-air cooling combined with night ventilation pre-cooling and supplemented as needed with hydronic slab pre-cooling. Heating testing was performed from November 2002 to Hydronic radiant slab heating (gas hot water heating) and Hydronic
9、 forced-air heating (gas hot water heat- ing). The energy and electrical power demand are compared among the three operational modes in cooling and between the two modes in heating, considering the indoor and outdoor environmental conditions. February 2003 and involved operation in two test modes: m
10、ode-H1) mode-H2) Evelyn Baskin is with Oak Ridge National Laboratory, Oak Ridge, Tenn. 02005 ASHRAE. 525 Figure 2a Condensing water heater und air handler/FAU (water coil and ventilator). Figure 26 Hydronic controls und water heater layouts. HVAC SYSTEM DESCRIPTION testing includes the following: Th
11、e HVAC indoor system used in heating and cooling Condensing gas water heater for heating water for com- bined domestic water and space (radiant and forced-air) heating, as seen in Figures 2a and 2b, along with the air handler and controls. A split-system HVAC system Condensing unit consists of a com
12、pressor, fan, and a refrigerant-to-water heat exchanger capable of delivering chilled water to the radiant floor tubing or the hydronic fan coil. Hotchilled water air-handler equipped with a hydronic coil and variable speed fan. Control system and damper that modulate ventila- tion air through the a
13、ir-handler fan. Controls allow the occupants to set desired ventilation cooling comfort range. Forced air distribution to all major rooms on the first floor and all rooms on the second floor. The system is shown in Figure 3a and is a standard flexible duct layout outside the conditioned space. 9 Rad
14、iant heating and cooling distribution to all first floor rooms-the main living area (great room, dining room, kitchen, entry, laundry, and baths) as shown on Figure 3b. The floor is a 3.5 in. (8.9 cm) thick concrete slab poured over -3 in. (7.6 cm) of rock and 3 in. (7.6 cm) of sand. There is a vapo
15、r barrier under the slab and the perimeter is insulated with 2 in. (5.1 cm) of extruded polystyrene to a depth of 16 in. (40.6 cm) below the top of the slab. The radiant tubing is tied to welded wire mesh reinforcement and is from 1 to 2 in. (2.5 to 5.1 cm) below the surface of the slab. The great r
16、oom, which is a two-story space with a cathe- dral style ceiling, connects the first and second floors. This results in a thermal connection between the two floors driven by air buoyancy effects. Although the HVAC system has multiple distribution systems, the entire house is controlled as a single z
17、one by a thermostat (set to 80F 26.7“C for cooling mode) located on the second floor hallway wall that is open to the open area extending from the great room to the second floor. The owner chose this location because the second floor is not usually occupied during the day, and the downstairs ther- m
18、ostat is not used during the cooling tests. The 80F (26.7“C) 526 ASHRAE Transactions: Symposia FIRST FLOOR 2- SECOND FLOOR Figure 3a House floor plan and forced-air distribution system (shown on second floor). Y! O1 -2003 Hydronic Forced-Air) Figure 8 Fijleen-minute averaged temperature profiles dur
19、ing 24 hours for outside air andJoor slab. Total (16-Min Avg) Energy Use During Heating (1 1-2002 01-2003 & 02-2003-Hydronic Forced-Air) Figure 9 Total energy use during test periods, mode-Hl and mode-H2. 7:OO and 9:00 a.m. and 3:OO and 6:OO p.m. during mode-H2. During mode-H1, the major energy use
20、occurs between 7:OO a.m. and 10 p.m. without any significant peaks, but during December, the energy consumption is much higher between 2:30 p.m. and 1O:OO p.m. due to the colder outside air temper- atures. Table 5 lists the energy performance results achieved during each testing mode for heating. Th
21、e gas consumption includes domestic hot water usage, which was assumed to be uniform through modes HI and H2. Durhg mode-Hl (December) the highest gas and electric energy consumption was recorded in December, while the lowest was recorded in ASHRAE Transactions: Symposia 533 Test Mode HI (Radiant) N
22、OV-02 Dec-02 Mode H2 (Forced-Air) Jan-O3 Feb-03 November, due to the milder outside air temperatures. During similar outside air temperatures, mode-Hl (December) and mode-H2 (January) total energy consumption is comparable. It should be noted that the measured duct leakage (using the Delta-Q method
23、Walker et al.) was found to be 8.1% of the total airflow. This is considerably lower than the 25-30% leak- age common to new construction. If the Table 5 total energy usage for January were increased by approximately 15-20%, mode-H I relative performance would be clearly superior. Total Power Gas T
24、herm (kWht) Auxiliaries (kWh) 25 (738) 8 53 (1554) 10 43 (1264) 49 40 (1 173) 49 CONCLUSIONS Cooling performance results reveal that slab pre-cooling caused some of the electric demand to shift from the on-peak periods (2 to 8 p.m.) to early morning off-peak periods (4 to 8 a.m.). This is due to the
25、 ability of the slab cooling to reduce on-peak cooling loads. Coupling slab pre-cooling with night- time ventilation significantly shifts the energy demand profile from primarily in the afternoon to the nighttime with most of the off-peak load coming from the fan. The combined benefits of slab pre-c
26、ooling and nighttime Ventilation significantly increase system overall performance and contribute to an “inverted“ demand profile relative to mode-Cl (forced-air) operation. Heating performance results indicate approximate equivalence for the hydronic forced-air heating mode relative to the radiant
27、floor heating mode during similar outside envi- ronmental conditions. The low duct leakage (8% of system airflow) affected the result because a more typical “leaky“ duct system would result in a performance advantage for the radiant floor heating. REFERENCE Walker, IS., M.H. Sherman, J. Wempen, D. Wang, J.A. McWilliams, and D.J. Dickerhoff. Development of a New Duct Leakage Test: Delta Q. LBNL 47308. Lawrence Berkeley National Laboratory. Berkeley, CA. 534 ASHRAE Transactions: Symposia
