This section describes the building descriptors for water heaters. Typically, a building will have multiple water heating systems and each system can have multiple water heaters, so these building descriptors may need to be specified more than once.
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Water Heater Name |
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Applicability |
All water heaters |
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Definition |
A unique descriptor for each water heater in the system. Some systems will have multiple pieces of equipment, for instance a series of water heaters plumbed in parallel or a boiler with a separate storage tank. |
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Units |
Text, unique |
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Input Restrictions |
Where applicable, this should match the tags that are used on the plans such that a plan reviewer can make a connection |
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Baseline Building |
The naming convention for the baseline building system shall be similar to the proposed design |
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Water Heater Type and Size |
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Applicability |
All water heaters |
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Definition |
This building descriptor includes information needed to determine the criteria from baseline standards. The choices are listed below.
– Small (≤ 12 kW) – Large (> 12 kW) – Heat pump
– Small (≤ 75,000 Btu/h) – Large (> 75,000 Btu/h)
– Small (> 50,000 and < 200,000 Btu/h) – Large (≥ 200,000 Btu/h), <10 gal – Large (≥ 200,000 Btu/h), >= 10 gal
– Small (≤ 105,000 Btu/h) – Large (> 105,000 Btu/h)
– Small (≤ 210,000 Btu/h) – Large (> 210,000 Btu/h), <10 gal – Large (> 210,000 Btu/h), >= 10 gal
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Units |
List (see above) |
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Input Restrictions |
The water heater type shall agree with equipment specified in the construction documents. If no service hot water system exists or has been specified, but the building will have service hot water loads, a service water system shall be modeled that matches the system type in the baseline building design. For buildings that will have no service hot water load, no service water heating system shall be modeled. |
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Baseline Building |
Where a complete water heating system exists or a new service water heating system has been specified in the proposed design, the water heaters in the baseline system will be based on the building area type classification. See Table 96 below. For new service hot water systems, the system will be sized according to the provisions of Standard 90.1-2019, Section 7.4.1, and the equipment shall match the minimum efficiency requirements in Standard 90.1-2019, Section 7.4.2. Where the energy source is electricity, the heating method shall be electrical resistance. When the energy source is ‘Gas Storage’, the water heater shall be modeled using natural gas as their fuel. Where natural gas is not available or the proposed building site, as determined by the rating authority, gas storage water heaters shall be modeled using propane as their fuel. If no service hot water system exists or has been specified, but the building will have service hot water loads, a service water system shall be modeled for each building area type in the proposed design, in accordance to Table 96. and matching minimum efficiency requirements of Standard 90.1-2019, Section 7.4.2. Table 96. Baseline Building Water Heater Type (Standard 90.1-2019 Table G3.1.1-2)
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Rated Capacity |
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Applicability |
All water heaters |
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Definition |
The heating capacity of a water heater at the rated conditions specified in DOE 10 CFR Part 430 or ANSI Z21.10 |
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Units |
Thousands of British thermal units per hour (MBH) |
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Input Restrictions |
As designed. If the loads are not met, then the system needs to be autosized. |
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Baseline Building |
Autosize |
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Storage Volume |
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Applicability |
All water heaters |
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Definition |
The storage volume of a gas-fired water heater. This is used in the standby loss calculations and baseline calculations of energy factor (EF). |
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Units |
gallons |
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Input Restrictions |
As designed. If the loads are not met, then the system needs to be autosized. |
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Baseline Building |
Autosize |
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Energy Factor |
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Applicability |
Equipment covered by the National Appliance Energy Conservation Act (NAECA), which includes small storage and instantaneous water heaters |
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Definition |
The EF is the ratio of the energy delivered by the water heater divided by the energy used, in the same units. EF is calculated according to the DOE 10 CFR Part 430 test procedure, which specifies a 24-hour pattern of draws, a storage temperature, inlet water temperature, and other test conditions. These conditions result in the energy delivered for the test period. Energy inputs are measured for the same test period and the EF ratio is calculated. |
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Units |
Unitless ratio (between 0 and 1) |
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Input Restrictions |
Building descriptors for the proposed design should be consistent with equipment specified on the construction documents or observed in the candidate building |
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Baseline Building |
The EF for the baseline building system shall be determined from Table 7.8 of Standard 90.1-2019. Additional UEF efficiency requirements can be found in Table F-2 of Standard 90.1-2019. |
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Thermal Efficiency |
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Applicability |
Oil and gas-fired water heaters not covered by NAECA |
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Definition |
The full load efficiency of a water heater at rated conditions expressed as a dimensionless ratio of output over input |
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Units |
Unitless ratio (between 0 and 1) |
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Input Restrictions |
Building descriptors for the proposed design should be consistent with equipment specified on the construction documents or observed in the candidate building |
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Baseline Building |
From Table 7.8 of Standard 90.1-2019 |
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Tank Standby Loss |
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Applicability |
Water heaters not covered by NAECA |
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Definition |
The tank standby loss for storage tanks, which includes the effect of recovery efficiency |
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Units |
Btu/h for the entire tank |
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Input Restrictions |
As specified in manufacturer data and documented on the construction documents |
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Baseline Building |
As specified in Table 7.8 of Standard 90.1-2019 |
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Tank Off-Cycle Loss Coefficient |
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Applicability |
Water heaters |
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Definition |
The tank standby loss coefficient (UA) for the water heater. For small water heaters covered by NAECA, the loss coefficient is a derived parameter, a function of the energy factor and recovery efficiency. |
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Units |
Btu/h-°F |
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Input Restrictions |
For NAECA covered water heaters, the loss coefficient is calculated by the following: $$UA=\frac{1/EF - 1/RE}{67.5\times \left( \frac{24}{41094}-\frac{1}{RE\cdot Pon} \right)}$$ Where: EF = The energy factor of the rated water heater (unitless) RE = The recovery efficiency of the rated water heater. If this data is not available, the default shall be 0.78 for gas water heaters and 0.93 for electric water heaters. Pon = The input power to the water heater, in Btu/h |
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Baseline Building |
The baseline loss coefficient for NAECA water heaters shall be: 10 Btu/h-°F for gas-fired water heaters |
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Off Cycle Parasitic Losses |
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Applicability |
Water heater |
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Definition |
The rate of parasitic losses, such as a pilot light or controls, when the water heater is not heating. If modeled explicitly, pilot lights should contribute to off-cycle heating. |
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Units |
Watts |
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Input Restrictions |
As designed |
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Baseline Building |
0 |
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Off Cycle Fuel Type |
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Applicability |
Water heater |
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Definition |
The type of fuel that serves energy using parasitic equipment, such as a pilot light or controls, when the water heater is not heating |
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Units |
List: Electricity, Gas, Oil, Propane |
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Input Restrictions |
As designed |
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Baseline Building |
Not applicable |
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On Cycle Parasitic Losses |
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Applicability |
Water heater |
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Definition |
The rate of parasitic losses, such as a pilot light or draft fan controls, when the water heater is heating. This may be different than off cycle losses if the flue energy is considered. |
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Units |
Watts |
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Input Restrictions |
As designed |
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Baseline Building |
0 |
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On Cycle Fuel Type |
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Applicability |
Water heater |
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Definition |
The type of fuel that serves energy using parasitic equipment, such as a pilot light or controls, when the water heater is not heating |
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Units |
List: Electricity, Gas, Oil, Propane |
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Input Restrictions |
As designed |
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Baseline Building |
Not applicable |
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Water Heater Ambient Temperature Indicator |
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Applicability |
Water heater |
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Definition |
The location of the water heater for determining losses and energy interaction with the surroundings |
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Units |
List: Schedule, Zone, Outdoors |
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Input Restrictions |
As designed. When “Schedule” is used, a time of day schedule needs to be specified with temperature schedule for each hour. |
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Baseline Building |
Same as proposed |
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Fuel Water Heater Part Load Efficiency Curve |
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Applicability |
Water heating equipment for which a thermal efficiency as opposed to an EF is specified |
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Definition |
A set of factors that adjust the full-load thermal efficiency for part load conditions. The factor is set as a curve. |
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Units |
Percent (%) |
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Input Restrictions |
The following default curve shall be used unless detailed information is provided to justify alternative values. The default curve shall take the form of a quadratic equation as follows:
Where: FHeatPLC = The fuel heating part load efficiency curve Fuelpartload = The fuel consumption at part load conditions (Btu/h) Fuel design = The fuel consumption at design conditions (Btu/h) Qpartload = The water heater capacity at part load conditions (Btu/h) Qrated = The water heater capacity at design conditions (Btu/h) a = Constant, 0.021826 b = Constant, 0.977630 c = Constant, 0.000543 |
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Baseline Building |
The baseline shall use the default curve |
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