Baseline Service Water-Heating Systems

This page defines the baseline service water heating systems required by ASHRAE 90.1 Performance Rating Method (PRM) Appendix G. The system selection is based on building area type per Table G3.1.1-2.

System Selection Table

ASHRAE 90.1 Baseline Service Water Heating Systems

Building Area Type	Baseline Heating Method
System A: Gas Storage Water Heater
Automotive facility	Gas storage water heater
Dining: Bar lounge/leisure	Gas storage water heater
Dining: Cafeteria/fast food	Gas storage water heater
Dining: Family	Gas storage water heater
Dormitory	Gas storage water heater
Exercise center	Gas storage water heater
Fire station	Gas storage water heater
Grocery store	Gas storage water heater
Gymnasium	Gas storage water heater
Hospital and outpatient surgery center	Gas storage water heater
Hotel	Gas storage water heater
Manufacturing facility	Gas storage water heater
Motel	Gas storage water heater
Multifamily	Gas storage water heater
Penitentiary	Gas storage water heater
Performing arts theater	Gas storage water heater
School/university	Gas storage water heater
Sports arena	Gas storage water heater
All others	Gas storage water heater
System B: Electric Resistance Storage Water Heater
Convenience store	Electric resistance storage water heater
Courthouse	Electric resistance storage water heater
Convention center	Electric resistance storage water heater
Health-care clinic	Electric resistance storage water heater
Library	Electric resistance storage water heater
Motion picture theater	Electric resistance storage water heater
Museum	Electric resistance storage water heater
Office	Electric resistance storage water heater
Parking garage	Electric resistance storage water heater
Police station	Electric resistance storage water heater
Post office	Electric resistance storage water heater
Religious facility	Electric resistance storage water heater
Retail	Electric resistance storage water heater
Town hall	Electric resistance storage water heater
Transportation	Electric resistance storage water heater
Warehouse	Electric resistance storage water heater
Workshop	Electric resistance storage water heater

Detailed System Descriptions

System A: Gas Storage Water Heater

Component Configuration

Primary Heating Component:

• Storage Tank: Insulated steel tank (50-100 gallons typical)

• Heating Element: Gas-fired burner with combustion chamber

• Thermostat: Dual setpoint control (upper and lower elements)

• Safety Relief: Pressure and temperature relief valve

Distribution System:

• Circulation Pump: Constant speed pump for hot water distribution

• Supply Piping: Insulated copper or PEX (minimum R-3.5 insulation)

• Return Piping: Optional circulation return or drain connection

• Expansion Tank: Accommodates thermal expansion (10% of tank volume)

• Check Valve: Prevents backflow from building to supply

Performance Characteristics:

• Thermal Efficiency: 0.78-0.84 (78-84%)

• Tank Insulation: Minimum R-12

• Recovery Capacity: 40-60 gallons per hour at 70°F temperature rise

• Standby Loss: 0.3-0.5% per hour of stored energy

• Setpoint Temperature: 48.9°C (120°F) baseline

EnergyPlus Objects

Primary heating component:

• WaterHeater:Mixed (Fuel Type: NaturalGas)

  • Thermal Efficiency: 0.80

  • Heater Capacity: 12,000 W (40,000 Btu/hr)

  • Tank Volume: 0.1893 m³ (50 gallons) typical

  • Off-cycle parasitic loss: 0.5 W/K

Supporting loop components:

• PlantLoop (Service Water Heating Loop)

• Pump:ConstantSpeed (circulation pump)

• SetpointManager:Scheduled (temperature control)

• Connector:Splitter and Connector:Mixer

• Pipe:Adiabatic (supply and return piping)

Demand-side components:

• WaterUse:Connections (fixture groupings)

• WaterUse:Equipment (individual fixtures)

Control and scheduling:

• Schedule:Constant (setpoint temperature)

• Schedule:Compact (demand profile)

• ScheduleTypeLimits (range definitions)

• PlantEquipmentOperationSchemes

• PlantEquipmentOperation:HeatingLoad

Application

Building Types (19 total):

• Automotive facilities

• Dining/food service (all types)

• Dormitories and residential facilities

• Exercise and recreation centers

• Fire stations

• Grocery stores

• Hospitals and medical centers

• Hotels and motels

• Manufacturing facilities

• Penitentiaries

• Performing arts facilities

• Schools and universities

• Sports arenas

• All other unlisted building types

Selection Criteria:

• High to moderate hot water demand

• Occupancy with sustained use patterns

• Food service operations

• 24/7 institutional operations

• Cost optimization with natural gas availability

Design Parameters

Tank Sizing:

Tank Volume = (Peak Hour Demand × Temperature Rise) / (Recovery Rate × Time)

Example: 200 gpm demand, 70°F rise, 50 gpm recovery, 1 hour target
Volume = (200 × 70) / (50 × 60) = 46 gallons → Select 50-75 gallon tank

Element Power:

Heater Capacity = (Tank Volume × Temperature Rise) / Recovery Time × 0.001163

Example: 50 gallon tank, 70°F rise, 1 hour recovery
Capacity = (50 × 70) / 1 × 0.001163 = 4.07 kW ≈ 12-15 kW gas input
(Gas heater has lower efficiency than electrical energy conversion)

Recovery Performance:

Recovery Time (hours) = (Tank Volume × Temperature Rise) / (Input Capacity × Efficiency)

Example: 50 gal, 70°F rise, 40,000 Btu/hr input, 0.80 efficiency
Recovery = (50 × 70) / (40,000 × 0.80 / 3,412) = ~90 minutes

Energy Consumption

Annual Energy = [Daily Volume (gal) × Days/Year × Temperature Rise (°F) 
                 × 8.33 lb/gal × 1 Btu/lb°F + Annual Standby Loss] / Efficiency

Standby Loss = Tank Surface Area × U-value × (Setpoint - Ambient) × Hours/Year

Example: 20 gal/day, 365 days, 70°F rise
Energy = [20 × 365 × 70 × 8.33 / 0.80] + Standby
       = 4.7 Therms energy use + ~2 Therms standby
       = 6.7 Therms per year (~$10 at $1.50/therm)

System B: Electric Resistance Storage Water Heater

Component Configuration

Primary Heating Component:

• Storage Tank: Insulated steel tank (40-50 gallons typical)

• Heating Element: One or two electric resistance elements

  • Upper element: 2-5 kW (primary heating)

  • Lower element: 2-5 kW (backup or simultaneous heating)

• Thermostat: Dual setpoint with element sequencing

• Safety Relief: Pressure and temperature relief valve

Distribution System:

• Circulation Pump: Optional (demand-based or timed circulation)

• Supply Piping: Insulated copper or PEX (minimum R-3.5 insulation)

• Return Piping: Optional circulation return or drain connection

• Expansion Tank: Accommodates thermal expansion (10% of tank volume)

• Check Valve: Prevents backflow from building to supply

Electrical System:

• Single Circuit: 240V, 20 A breaker, #12 AWG wire (single element)

• Dual Circuit: Two 20 A breakers or 40 A main (dual elements)

• Disconnect Switch: Manual disconnect within sight of heater

Performance Characteristics:

• Thermal Efficiency: 1.0 (100% - all electrical energy heats water)

• Tank Insulation: Minimum R-12

• Recovery Capacity: 20-40 gallons per hour at 70°F temperature rise

• Standby Loss: 0.3-0.5% per hour of stored energy

• Setpoint Temperature: 48.9°C (120°F) baseline

EnergyPlus Objects

Primary heating component:

• WaterHeater:Mixed (Fuel Type: Electricity)

  • Thermal Efficiency: 1.0

  • Heater Capacity: 4,500 W (single element) typical

  • Tank Volume: 0.1893 m³ (50 gallons) typical

  • Off-cycle parasitic loss: 0.5 W/K

Supporting loop components:

• PlantLoop (Service Water Heating Loop)

• Pump:ConstantSpeed (circulation pump)

• SetpointManager:Scheduled (temperature control)

• Connector:Splitter and Connector:Mixer

• Pipe:Adiabatic (supply and return piping)

Demand-side components:

• WaterUse:Connections (fixture groupings)

• WaterUse:Equipment (individual fixtures)

Control and scheduling:

• Schedule:Constant (setpoint temperature)

• Schedule:Compact (demand profile)

• ScheduleTypeLimits (range definitions)

• PlantEquipmentOperationSchemes

• PlantEquipmentOperation:HeatingLoad

Application

Building Types (17 total):

• Convenience stores

• Convention centers

• Courthouses

• Health-care clinics

• Libraries

• Motion picture theaters

• Museums

• Office buildings

• Parking garages

• Police stations

• Post offices

• Religious facilities

• Retail stores

• Town halls

• Transportation facilities

• Warehouses

• Workshops

Selection Criteria:

• Low to minimal hot water demand

• Public/government buildings with infrequent use

• Compact spaces with limited piping

• Buildings without commercial food service

• Adequate electrical service available

Design Parameters

Tank Sizing:

Tank Volume = (Peak Hour Demand × Temperature Rise) / (Recovery Rate × Time)

Example: 50 gpm demand, 70°F rise, 30 gpm recovery, 1 hour target
Volume = (50 × 70) / (30 × 60) = 82 gallons → Select 75-100 gallon tank

Element Power:

Element Power (kW) = (Tank Volume in gal × Temperature Rise in °F) 
                     / (Recovery Time in hr × 3.412 Btu/kWh)

Example: 50 gallon tank, 70°F rise, 1.5 hour recovery
Power = (50 × 70) / (1.5 × 3.412) = 6.8 kW
Select 4.5 kW for standard installation (90-120 min recovery)

Recovery Performance:

Recovery Time (hours) = (Tank Volume × Temperature Rise) / (Element Power × 3.412)

Example: 50 gal, 70°F rise, 4.5 kW element
Recovery = (50 × 70) / (4.5 × 3.412) = 2.27 hours ≈ 135 minutes

Energy Consumption

Annual Energy = [Daily Volume (gal) × Days/Year × Temperature Rise (°F) 
                 × 8.33 lb/gal × 1 Btu/lb°F / 3,412 Btu/kWh] + Annual Standby Loss

Standby Loss = Tank Surface Area × U-value × (Setpoint - Ambient) × Hours/Year

Example: 20 gal/day, 365 days, 70°F rise
Energy = [20 × 365 × 70 × 8.33 / 3,412] + Standby
       = 1.25 MWh energy use + ~0.3 MWh standby
       = 1.55 MWh per year (~$186 at $0.12/kWh)

System Selection Decision Process

Step 1: Identify Building Area Type

Match the proposed building to one of 36 listed building area types from Table G3.1.1-2.

Step 2: Determine Baseline Heating Method

If building area type matches one of 19 System A types: → Baseline: Gas Storage Water Heater

If building area type matches one of 17 System B types: → Baseline: Electric Resistance Storage Water Heater

If building area type is not listed: → Baseline: Gas Storage Water Heater (default "All others" category)

Step 3: Apply Baseline System Specifications

Use corresponding system's:

• Fuel type (Natural Gas or Electricity)

• Thermal efficiency (0.80 or 1.0)

• Heater capacity

• Tank volume based on building occupancy and use profile

• Temperature setpoint and deadband

Step 4: Model Baseline System

Build complete EnergyPlus model including:

• PlantLoop with pump and controls

• WaterHeater:Mixed with assigned fuel type and efficiency

• Demand-side equipment (fixtures) matching proposed building

• All piping, expansion, and relief components

• Schedules for hot water demand profile

Step 5: Compare Proposed vs. Baseline

• Calculate annual energy consumption for both systems

• Compare on: Fuel type, system efficiency, hot water availability

• Demonstrate proposed system performs at or better than baseline

System Selection Decision Tree

START: Building Area Type Classification

STEP 1: Match to Table G3.1.1-2 Categories

RESIDENTIAL/LODGING FACILITIES
├─ Dormitory, Hotel, Motel, Multifamily, Penitentiary
│  → System A (Gas Storage Water Heater)
│
└─ Other residential-type buildings not listed
   → System A (default: Gas Storage)

FOOD SERVICE OPERATIONS
├─ Dining (Bar lounge, Cafeteria, Family, Fast food)
├─ Grocery store
└─ School/university (with cafeteria)
   → System A (Gas Storage Water Heater)

RECREATIONAL FACILITIES
├─ Exercise center, Gymnasium, Sports arena
└─ Performing arts theater
   → System A (Gas Storage Water Heater)

INSTITUTIONAL/MEDICAL FACILITIES
├─ Hospital and outpatient surgery center
├─ Fire station, Manufacturing facility
│  → System A (Gas Storage Water Heater)
│
├─ Health-care clinic, Library, Museum
├─ Police station, Courthouse, Town hall
├─ Religious facility, Post office
│  → System B (Electric Resistance Water Heater)

COMMERCIAL/OFFICE BUILDINGS
├─ Office → System B (Electric Resistance)
├─ Retail → System B (Electric Resistance)
├─ Convenience store → System B (Electric Resistance)
└─ Automotive facility → System A (Gas Storage)

SPECIALIZED/STORAGE FACILITIES
├─ Parking garage → System B (Electric Resistance)
├─ Warehouse → System B (Electric Resistance)
├─ Workshop → System B (Electric Resistance)
├─ Convention center → System B (Electric Resistance)
├─ Motion picture theater → System B (Electric Resistance)
└─ Transportation facility → System B (Electric Resistance)

UNLISTED BUILDING TYPES
└─ All others → System A (Gas Storage Water Heater) [DEFAULT]

END: Apply selected system to baseline model

Component Specifications by System

Water Heater Tank Specifications

Parameter	Gas Storage	Electric Storage
Tank Material	Carbon steel with glass lining	Carbon steel with glass lining
Tank Volume	50-100 gallons (190-380 L)	40-50 gallons (150-190 L)
Insulation	Minimum R-12 wrap	Minimum R-12 wrap
Pressure Rating	150 psi (1,034 kPa)	150 psi (1,034 kPa)
Anode Rod	Sacrificial magnesium	Sacrificial magnesium
Typical Weight	100-150 lbs empty	80-120 lbs empty
Operating Pressure	75 psi relief valve	75 psi relief valve

Heating Element Specifications

Parameter	Gas Storage	Electric Storage
Primary Element	Gas burner, combustion chamber	Electric resistance element (2-5 kW)
Backup Element	Not applicable	Second electric element (2-5 kW) optional
Burner Efficiency	0.78-0.84 (78-84%)	1.0 (100%)
Capacity	40,000 Btu/hr (12 kW input)	4,500 W (15,000 Btu/hr) single
Burner Type	Natural gas pilot light ignition	Immersion electric element
Combustion Air	Required (flue vent)	Not required
Venting	Vent to exterior via flue	No venting required

Circulation Pump Specifications

Parameter	Specification
Pump Type	Constant speed centrifugal pump
Design Flow Rate	3-5 gpm typical
Design Head	100,000-300,000 Pa (1-3 psi)
Motor Efficiency	0.85 (85%) minimum
Control Type	Intermittent (on demand) or timed circulation
Power Consumption	0.25-0.5 kW continuous run

Distribution Piping Specifications

Parameter	Specification
Material	Copper (preferred), PEX, CPVC
Supply Line Diameter	1/2" to 1" (12-25 mm)
Return Line Diameter	1/2" to 3/4" (12-19 mm)
Insulation	Minimum R-3.5 (0.62 m²K/W)
Insulation Thickness	Minimum 1.5" (38 mm)
Max Line Length (no circulation)	30 feet (9 m)
Support Spacing	Every 4-6 feet horizontal; 8 feet vertical

Expansion Tank Specifications

Parameter	Specification
Volume	10% of water heater tank volume
Example (50 gal tank)	5-gallon expansion tank
Precharge Pressure	75 psi (75% of static pressure)
Type	Air bladder (replaceable preferred)
Installation	Saddle valve on cold water supply line

Relief Valve Specifications

Parameter	Specification
Pressure Setting	75 psi (517 kPa)
Temperature Setting	210°F (99°C)
Connection Size	3/4" NPT
Installation	Vertical on tank side
Discharge	To drain (visible termination)
Material	Brass body, stainless internals

EnergyPlus Simulation Inputs

Plant Loop Configuration

PlantLoop (Service Water Heating Loop)
├─ Loop Temperature Setpoint Node: SWH Supply Outlet Node
├─ Maximum Loop Temperature: 100°C
├─ Minimum Loop Temperature: 10°C
├─ Design Flow Rate: Autosize (based on demand)
├─ Plant Loop Volume: Autosize
│
├─ SUPPLY SIDE
│  ├─ Pump:ConstantSpeed
│  ├─ Connector:Splitter
│  ├─ Branch (WaterHeater:Mixed)
│  ├─ Connector:Mixer
│  └─ Pipe:Adiabatic (outlet)
│
└─ DEMAND SIDE
   ├─ Connector:Splitter
   ├─ Branch (WaterUse:Connections)
   ├─ Connector:Mixer
   └─ Pipe:Adiabatic (return)

Water Heater Object Configuration

For System A (Gas Storage):

WaterHeater:Mixed
├─ Name: Gas Storage Water Heater
├─ Tank Volume: 0.1893 m³ (50 gallons)
├─ Setpoint Temperature: 48.9°C (120°F)
├─ Deadband: 2.0°C (3.6°F)
├─ Maximum Temperature: 70°C (158°F)
├─ Heater Control Type: Cycle
├─ Heater Maximum Capacity: 12,000 W
├─ Heater Fuel Type: NaturalGas
├─ Heater Thermal Efficiency: 0.80
├─ Off Cycle Parasitic Capacity: 0.5 W/K
└─ Off Cycle Parasitic Energy Factor: 1.0

For System B (Electric Resistance):

WaterHeater:Mixed
├─ Name: Electric Storage Water Heater
├─ Tank Volume: 0.1893 m³ (50 gallons)
├─ Setpoint Temperature: 48.9°C (120°F)
├─ Deadband: 2.0°C (3.6°F)
├─ Maximum Temperature: 70°C (158°F)
├─ Heater Control Type: Cycle
├─ Heater Maximum Capacity: 4,500 W
├─ Heater Fuel Type: Electricity
├─ Heater Thermal Efficiency: 1.0
├─ Off Cycle Parasitic Capacity: 0.5 W/K
└─ Off Cycle Parasitic Energy Factor: 1.0

Demand-Side Configuration

WaterUse:Connections
├─ Name: Fixture Group (Bathrooms, Kitchen, etc.)
├─ Inlet Node: Hot Water Supply from Heater
├─ Outlet Node: Return to Drain
├─ Number of Equipment: 1 or more
│
└─ WaterUse:Equipment (per fixture)
   ├─ Name: Specific Fixture (Sink, Shower, etc.)
   ├─ End-Use Subcategory: Restroom, Kitchen, etc.
   ├─ Peak Flow Rate: m³/s (varies by fixture)
   ├─ Flow Rate Fraction Schedule: Time-dependent usage
   └─ Hot Water Supply Temperature Schedule: 48.9°C baseline

Typical Peak Flow Rates

Fixture Type	Peak Flow Rate	End-Use
Bathroom sink	0.000076 m³/s (0.12 gpm)	Restroom
Kitchen sink	0.000189 m³/s (0.30 gpm)	Kitchen
Shower	0.00315 m³/s (2.0 gpm)	Restroom
Mop sink	0.000632 m³/s (1.0 gpm)	Cleaning
Dishwasher	0.00315 m³/s (2.0 gpm)	Kitchen equipment

Key Controls and Setpoints

Baseline Temperature Setpoints

Hot Water Setpoint (Supply):

• Baseline: 48.9°C (120°F)

• Minimum (code): 43.3°C (110°F)

• Maximum (safety): 60°C (140°F)

• Deadband: 2.0°C minimum between on/off

Optional Setback:

• Reduced setpoint during unoccupied hours: 43.3°C (110°F)

• Reduces standby loss without scalding risk

Peak Hot Water Demand Schedule

Typical Office Building:

Midnight - 6 AM:     0% demand (closed)
6 AM - 8 AM:        10% demand (morning arrival)
8 AM - 12 PM:       20% demand (peak morning use)
12 PM - 1 PM:       30% demand (lunch period, if applicable)
1 PM - 5 PM:        10% demand (afternoon occasional use)
5 PM - 6 PM:        15% demand (afternoon departure)
6 PM - Midnight:     0% demand (closed)

Typical Residential Building:

Midnight - 6 AM:     5% demand (minimal)
6 AM - 8 AM:        40% demand (morning showers/baths)
8 AM - 12 PM:       15% demand (light use)
12 PM - 5 PM:       10% demand (occasional use)
5 PM - 8 PM:        25% demand (dinner, evening)
8 PM - 10 PM:       20% demand (evening showers/baths)
10 PM - Midnight:   10% demand (light use)

Typical Dining/Food Service:

Midnight - 6 AM:     5% demand (prep)
6 AM - 10 AM:       20% demand (breakfast)
10 AM - 12 PM:      10% demand (between meals)
12 PM - 2 PM:       50% demand (lunch peak)
2 PM - 5 PM:        15% demand (between service)
5 PM - 8 PM:        60% demand (dinner peak)
8 PM - Midnight:    10% demand (cleanup)

Circulation Pump Operation

Continuous Circulation:

• Operates continuously during occupied hours

• Maintains hot water availability at all fixtures

• Increases operating cost

• Reduces water wait time

Timed/Demand-Based Circulation:

• Operates on schedule (e.g., morning 6-9 AM, evening 4-7 PM)

• Timer-controlled during peak demand periods

• Reduces standby loss outside peak periods

• Recommended for energy efficiency

Outdoor Air Damper (if applicable)

Cold Climate Freeze Protection (CZ 3B-8):

• System 7+ with outdoor air intake must have freeze protection

• Damper minimum position to prevent freezing

• Not typically required for SWH systems (indoor location)

System Operation Availability

Schedule Status:

• Available: 1.0 (system always ready to operate)

• Unavailable: 0.0 (system disabled - rarely used for SWH)

Typical SWH System:

• Always available (Schedule:Constant = 1.0)

• Hot water demand varies by occupancy schedule

• System responds to thermostat and demand-side loads

Baseline Model Compliance Demonstration

Proposed vs. Baseline Comparison

Energy Consumption:

• Calculate annual fuel/electricity for proposed system

• Calculate annual fuel/electricity for baseline system

• Compare kWh (electric) or Therms (gas) per year

Efficiency:

• Proposed system efficiency ≥ Baseline system efficiency

• Or offset higher demand with improved HVAC efficiency

Hot Water Availability:

• Proposed system provides adequate hot water for building occupancy

• Baseline establishes minimum acceptable performance

Example Compliance Path:

A 10,000 sf office building proposes electric resistance water heater (System B).

Baseline (per Table G3.1.1-2): Electric resistance storage (Method B)
Proposed: Instantaneous electric water heater (higher efficiency)

Baseline Energy:
- 50-gal tank storage + standby loss
- Annual: 1,500 kWh estimated

Proposed Energy:
- Instantaneous (no storage, minimal standby loss)
- Annual: 1,000 kWh estimated

Compliance: Proposed meets or exceeds baseline requirement ✓