Water Heater

 

WaterHeater:Mixed

WaterHeater:Sizing

 

 

Used in:

 

Supply side of DHW Loops

 

The Water heater simulates a well-mixed, single-node water tank. It can be used for simulating many types of water heaters and storage tanks, including gas and electric residential water heaters, and a variety of large commercial water heaters. It models stand-alone operation, on- and off-cycle parasitic loads and thermal losses to the zone, instantaneous/tankless water heaters

 

The Water heater analytically solves the differential equation governing the energy balance of the water tank. Within a timestep, conditions are solved separately for when the heater element or burner is "on" (on-cycle) and when it is "off" (off-cycle). This approach allows ambient losses and parasitic loads to be divided into on-cycle and off-cycle effects and accounted for in detail.

 

For losses to the ambient environment, the ambient air temperature can be taken from a schedule, a zone, or the exterior. When used with a zone, a fraction of the skin losses can be added to the zone heat balance as internal heat gains.

 

Control options allow the heater to cycle or modulate to meet the load. When cycling, the heater element or burner is either on or off. The heater remains fully on while heating the tank up to the setpoint temperature. When the setpoint is reached, the heater turns off. The heater remains off until the tank temperature falls below the "cut-in" temperature, i.e., the setpoint temperature minus the deadband temperature difference. The heater continuously cycles on and off to maintain the tank temperature within the deadband. Most storage-tank water heaters cycle.

 

When modulating, the heater power varies between the maximum and minimum heater capacities. The heater stays on as long as the required total demand is above the minimum capacity. Below the minimum capacity, the heater will begin to cycle on and off based on the deadband temperature difference. Equipment is usually designed and rated to avoid this condition. Most tankless/instantaneous water heaters modulate.

General

Name

The name of the Water heater object.

Tank volume

The volume of the storage tank (in m3 or ft3). This field is autosizable based on the Sizing data. Although this field is allowed to go down to zero, even so-called "tankless" water heaters have some volume of water that is maintained around the heating elements or in the heat exchanger, typically around 0.00379 m3 (1 gallon).

External Heating Plant Connection

External heating plant connection

Check this option if the water heater is to be supplied with heat from a Hot water loop. In this case inlet and outlet connections are included on the bottom of the water heater allowing the connection to be made to the demand side of the Hot water plant loop.

Indirect water heating recovery time

This field is only required when the External heating plant option is checked, i.e. when the water heater is connected to the demand side of a plant loop. It is used to provide a design parameter for autosizing design flow rates The recovery time is expressed in hours. This is the time that the entire volume of the tank can be heated from 14.4ºC to 57.2ºC (58ºF to 135ºF) with an inlet temperature defined as the exit temperature in the associated Plant Sizing object. The default is 1.5 hours. The calculation is based on log-mean temperature difference (LMTD) and includes the heat transfer effectiveness factor entered above.

Temperature Settings

Setpoint temperature schedule

Select the Schedule that defines the hot water temperature setpoint in °C. Also known as the "cut-out" temperature.

Deadband temperature difference

The delta temperature difference (in Δ°C or Δ°F) between the setpoint and the "cut-in" temperature at which the heater will turn on. In other words, the "cut-in" temperature is Setpoint – Deadband.

Maximum temperature limit

The temperature (in °C or °F) at which the tank water becomes dangerously hot and is vented through boiling or an automatic safety. The tank temperature will never exceed the maximum. Any extra heat added to the tank is immediately vented. Note: The maximum temperature must be greater than the setpoint temperature at all times.

Ambient Temperature Settings

Ambient temperature indicator

The Ambient temperature indicator specifies how the temperature of the air surrounding the Water heater is to be defined. These options are available:

 

Ambient temperature schedule

Select the schedule specifying the ambient air temperature around the tank for skin losses. This field is only required if Ambient temperature indicator is 1-Schedule.

Ambient temperature zone

The building zone specifying the ambient air temperature around the tank for skin losses. This field is only required if Ambient temperature indicator is 2-Zone.

On-Cycle loss coefficient to ambient temperature

The loss coefficient (in W/K or Btu/h-F) to the ambient air temperature. Often this coefficient is identical to the "UA" for skin losses. If the loss effects of the flue are being modelled in the Off-Cycle Loss Coefficient, than this field would have a different value accounting only for the skin losses.

On-Cycle loss fraction to zone

If the Ambient temperature indicator is 2-Zone, this field adds the specified fraction of the on-cycle losses to the zone heat balance as an internal gain.

Heater Settings

Heater control type

The control type can be one of:

 

Heater maximum capacity

The maximum heat rate (in W or btu/h) that can be supplied to the water, probably the same as the "nominal" capacity. This field is autosizable.

Heater minimum capacity

The minimum heat rate (in W or btu/h) that can be supplied to the water. This field is only used when the Heater control type is 2-Modulate. If the total demand rate for heating is less than the minimum, even a modulating water heater will begin to cycle.

Heater fuel type

The type of fuel used for heating. The fuel type can be one of:

 

Heater thermal efficiency

The thermal conversion efficiency from fuel energy to heat energy for the heater element or burner. This is not the same as the overall efficiency of the water heater.

Heater part load factor curve

Select the Quadratic or Cubic curve that relates the overall efficiency of the water heater to the runtime fraction (if Control Type 1-Cycle) or part load ratio (if Control type is 2-Modulate). This is an additional multiplier applied to the Heater thermal efficiency above, to compute fuel energy use. The Part load factor curve should not have a value less than 0.1 in the domain from 0 to 1.

Note: If the curve accounts for ambient losses and/or parasitic fuel consumption, these effects should not also be input into the related fields in this object as that would result in double-counting.

On-Cycle Settings

On-Cycle parasitic fuel consumption rate

On-cycle parasitics include parts of the water heater that consume fuel when the heater is on, for example, an induction fan, or stand-by electronic control circuits. The fuel consumption rate (in W or Btu/h) is the total fuel that is consumed by all of the on-cycle parasitics.

On-Cycle parasitic fuel type

The type of fuel used by the on-cycle parasitics. The fuel type can be

 

 

The fuel type can be the same or different from the Heater Fuel Type.

On-Cycle parasitic heat fraction to tank

The fraction of on-cycle parasitic fuel energy that is converted to heat energy that ends up in the tank water. For example, an induction fan might (maybe) deliver a small fraction of its energy to the tank water for a value of 0.05. Electronic control circuits, on the other hand, do not add any heat to the tank and should be 0.

Off-Cycle Settings

Off-Cycle Parasitic Fuel Consumption Rate

Off-cycle parasitics include parts of the water heater that consume fuel when the heater is off, for example, a pilot light, or stand-by electronic control circuits. The fuel consumption rate (in W or Btu/h) is strictly the total fuel that is consumed by all of the off-cycle parasitics.

Off-Cycle Parasitic Fuel Type

The type of fuel used by the off-cycle parasitics. The fuel type can be one of

 

 

The fuel type can be the same or different from the Heater fuel type.

Off-Cycle parasitic heat fraction to tank

The fraction of off-cycle parasitic fuel energy that is converted to heat energy that ends up in the tank water. For example, a pilot light would deliver most of its heat to the tank water, as long as the thermal conversion efficiency must be taken into account, so perhaps 0.80 is reasonable. Electronic control circuits, on the other hand, do not add any heat to the tank and should be 0.

Off-Cycle loss coefficient to ambient temperature

The loss coefficient (in W/K or Btu/h-F) to the ambient air temperature. Often this coefficient is identical to the "UA" for skin losses. However, it can also be used to model the loss effects of the flue in a combustion water heater, in addition to the skin losses.

Off-Cycle loss fraction to zone

If the Ambient temperature indicator is 2-Zone, this field adds the specified fraction of the off-cycle losses to the zone heat balance as an internal gain.

Use-Side Settings

Use side effectiveness

This field specifies the heat transfer effectiveness between the use side water and the tank water. If the effectiveness is set to 1 then complete heat transfer occurs, simulating perfect mixing of the use side water and the tank water. If the effectiveness is lower, then the use side outlet water temperature will not be as hot as the tank water, simulating a heat exchanger.

Use side design flow rate

This field is optional and is used to specify the design flow rate through the Use Side of the water heater. The volumetric design flow rate is specified in m3/s or gal/min. The field is needed when the Use side is connected to a plant loop. The field can be autosized. Sizing results are reported in the EIO file.

Source-Side Settings

Source side effectiveness

This field specifies the heat transfer effectiveness between the source side water and the tank water. If the effectiveness is set to 1 then complete heat transfer occurs, simulating perfect mixing of the source side water and the tank water. If the effectiveness is lower, then the source side outlet water temperature will not be as hot as the tank water, simulating a heat exchanger.

Source side design flow rate

This field is optional and is used to specify the design flow rate through the Source Side of the water heater. The volumetric design flow rate is specified in m3/s or gal/min. The field is needed when the Source Side is connected to a plant loop. The field can be autosized. Sizing results are reported in the EIO file.

Sizing

The Water heater sizing data is entered on the Sizing tab.

Design mode

This field describes the method to be used for sizing the water heater. There are six choices:

Time storage can meet peak draw

This field provides the time, in hours, that the tank’s volume can sustain a peak draw. It is used to size the tank’s volume which is the simple product of peak draw volume flow rate and the draw time. There is no assurance that the water will be at the desired temperature for the entire draw. This field is only used if the Design mode is 1-Peak draw. The water heater must be connected to a full plant loop and be on the supply side.

Time for tank recovery

This field provides the time, in hours, that tank’s heater needs to recover the volume of the tank. The temperatures used to define recovery are a starting temperature of 14.4ºC (58ºF) and a final temperature of 57.2ºC (135ºF). Time for tank recovery data is only required if the Design mode is 1-Peak draw.

Nominal tank volume for autosizing plant connections

This field is used in case the water heater is indirectly heated by its source side connections and they are also autosized. The units are m3 or ft3. Because of the complexity of such a water heater and the timing for when sizing calculation happen inside EnergyPlus, the Source side connection flow rates need to be reported before the tank’s volume can be sized to meet Peak draw. This input field is used to provide a nominal tank volume to use temporarily while the flow connections are sized. This field is only required if the Design mode is 1-Peak draw and the water heater has autosized plant connections on the demand side.

Number of bedrooms

This field is used to enter the number of bedrooms in the model. It is only required if the Design mode is 2-Residential HUD-FHA minimum.

Number of bathrooms

This field is used to enter the number of bathrooms in the model. It is only required if the Design mode is 2-Residential HUD-FHA minimum.

Storage capacity per person

This field is used to enter the tank’s storage volume on per-person basis. The units are m3/person or ft3/person. The storage capacity per person is only required if the Design mode is 3-Per person.

Recovery capacity per person

This field is used to enter the recovery capacity per person in units of m3/person-hr or ft3/person-h. This is the volume of water the heater can recover in one hour per person. Recovery is heating water from a starting temperature of 14.4ºC (58ºF) to a final temperature of 57.2ºC (135ºF). The Recovery capacity per person is only required if the Design mode is 3-Per person.

Storage capacity per floor area

This field is used to enter the tank’s storage volume on a per-floor-area basis. The units are m3 /m2 or ft3/ft2. The Recovery capacity per person is only required if the Design mode is 3-Per person.

Recovery capacity per floor area

This field is used to enter the recovery capacity per floor area in units of m3/h-m2 or ft3/h-ft2. This is the volume water the heater can recover in an hour per floor area. Recovery is heating water from a starting temperature of 14.4ºC (58ºF) to a final temperature of 57.2ºC (135ºF). The Recovery capacity per floor area is only required if the Design mode is 4-Per floor area

Number of units

This field is used to enter the number of Units for use in sizing on per-Unit basis with the next two fields. The Number of units data only required if the Design mode is 5-Per unit. This can be used to account for any arbitrary item such as lodging rooms, desks, water fixtures, restrooms, etc.

Storage capacity per unit

This field is used to enter the tanks’ storage volume on per-Unit basis. The units are m3/h or ft3/h. The number of Units is entered in the previous field. The Storage capacity per unit is only required if the Design mode is 5-Per unit.

Recovery capacity per unit

This field is used to enter the recover capacity per Unit in units of m3/h or ft3/h. This is the volume of water the heater can recover in an hour per Unit. Recovery is heating water from a starting temperature of 14.4ºC (58ºF) to a final temperature of 57.2ºC (135ºF). This field is only required if the Design mode is 5-Per unit.

Storage capacity per collector area

This field is used to enter the tank’s storage volume on per-solar-collector-area basis. The units are m3/m2 or f3/m2. The Recovery capacity per unit is only required if the Design mode is 6-Per solar collector area.

Water Heater Outputs

The following Water heater output variables are available:

 

HVAC,Average,Water Heater Tank Temperature [C]

HVAC,Average,Water Heater Final Tank Temperature [C]

HVAC,Average,Water Heater Loss Rate [W]

HVAC,Sum,Water Heater Loss Energy [J]

HVAC,Average,Water Heater Use Flow Rate [kg/s]

HVAC,Average,Water Heater Use Inlet Temperature [C]

HVAC,Average,Water Heater Use Outlet Temperature [C]

HVAC,Average,Water Heater Use Rate [W]

HVAC,Sum,Water Heater Use Energy [J]

HVAC,Average,Water Heater Source Flow Rate [kg/s]

HVAC,Average,Water Heater Source Inlet Temperature [C]

HVAC,Average,Water Heater Source Outlet Temperature [C]

HVAC,Average,Water Heater Source Rate [W]

HVAC,Sum,Water Heater Source Energy [J]

HVAC,Average,Water Heater Off-Cycle Parasitic Heat Rate To Tank [W]

HVAC,Sum,Water Heater Off-Cycle Parasitic Heat Energy To Tank [J]

HVAC,Average,Water Heater On-Cycle Parasitic Heat Rate To Tank [W]

HVAC,Sum,Water Heater On-Cycle Parasitic Heat Energy To Tank [J]

HVAC,Average,Water Heater Total Demand Rate [W]

HVAC,Sum,Water Heater Total Demand Energy [J]

HVAC,Average,Water Heater Heating Rate [W]

HVAC,Sum,Water Heater Heating Energy [J]

HVAC,Average,Water Heater Unmet Demand Rate [W]

HVAC,Sum,Water Heater Unmet Demand Energy [J]

HVAC,Average,Water Heater Venting Rate [W]

HVAC,Sum,Water Heater Venting Energy [J]

HVAC,Average,Water Heater Net Heat Tranfer Rate [W]

HVAC,Sum,Water Heater Net Heat Tranfer Energy [J]

HVAC,Sum,Water Heater Cycle On Count []

HVAC,Average,Water Heater Runtime Fraction []

HVAC,Average,Water Heater Part Load Ratio []

HVAC,Average,Water Heater <Fuel Type> Consumption Rate[W]

HVAC,Sum,Water Heater <Fuel Type>Consumption [J]

HVAC,Average,Water Heater Off-Cycle Parasitic <Fuel Type> Consumption Rate[W]

HVAC,Sum,Water Heater Off-Cycle Parasitic <Fuel Type> Consumption [J]

HVAC,Average,Water Heater On-Cycle Parasitic <Fuel Type> Consumption Rate[W]

HVAC,Sum,Water Heater On-Cycle Parasitic <Fuel Type> Consumption [J]

HVAC,Average,Water Heater Water Consumption Rate [m3/s]

HVAC,Sum,Water Heater Water Consumption [m3]

Water Heater Tank Temperature [C]

The average water tank temperature.

Water Heater Final Tank Temperature [C]

The final water tank temperature at the end of the system timestep. If reported at the "Detailed" interval, this report variable can be used to verify an exact energy balance on the water heater. Also see the report variable: Water Heater Net Heat Transfer Energy.

Water Heater Loss Rate [W]

The average heat loss rate due to the off- and on-cycle loss coefficients to the ambient temperature.

Water Heater Loss Energy [J]

The heat loss energy due to the off- and on-cycle loss coefficients to the ambient temperature.

Water Heater Use Flow Rate [kg/s]

The use side mass flow rate. If stand-alone, this is the scheduled use flow rate.

Water Heater Use Inlet Temperature [C]

The inlet temperature on the use side.

Water Heater Use Outlet Temperature [C]

The outlet temperature on the use side.

Water Heater Use Rate [W]

The average heat transfer rate between the use side water and the tank water.

Water Heater Use Energy [J]

The heat transfer energy between the use side water and the tank water.

Water Heater Source Flow Rate [kg/s]

The source side mass flow rate. If in stand-alone operation, this is 0.

Water Heater Source Inlet Temperature [C]

The inlet temperature on the source side.

Water Heater Source Outlet Temperature [C]

The outlet temperature on the source side.

Water Heater Source Rate [W]

The average heat transfer rate between the source side water and the tank water.

Water Heater Source Energy [J]

The heat transfer energy between the source side water and the tank water.

Water Heater Off-Cycle Parasitic Heat Rate To Tank [W]

The average heat gain rate to the tank water due to off-cycle parasitics.

Water Heater Off-Cycle Parasitic Heat Energy To Tank [J]

The heat gain energy to the tank water due to off-cycle parasitics.

Water Heater On-Cycle Parasitic Heat Rate To Tank [W]

The average heat gain rate to the tank water due to on-cycle parasitics.

Water Heater On-Cycle Parasitic Heat Energy To Tank [J]

The heat gain energy to the tank water due to on-cycle parasitics.

Water Heater Total Demand Rate [W]

The average heating rate demanded to maintain the setpoint temperature.

Water Heater Total Demand Energy [J]

The heating energy demanded to maintain the setpoint temperature.

Water Heater Heating Rate [W]

The average heating rate supplied by the heater element or burner.

Water Heater Heating Energy [J]

The heating energy supplied by the heater element or burner.

Water Heater Unmet Demand Rate [W]

The average heating rate unmet by the heater element or burner. The difference between the Total Demand Rate and the Heating Rate.

Water Heater Unmet Demand Energy [J]

The heating energy unmet by the heater element or burner. The difference between the Total Demand Energy and the Heating Energy.

Water Heater Venting Rate [W]

The average venting rate to keep the tank below the Maximum Temperature Limit.

Water Heater Venting Energy [J]

The venting energy to keep the tank below the Maximum Temperature Limit.

Water Heater Net Heat Transfer Rate [W]

The average net heat transfer rate when considering all losses and gains.

Water Heater Net Heat Transfer Energy [J]

The net heat transfer energy when considering all losses and gains.

Water Heater Cycle On Count []

The number of times that the heater turned on in the time period.

Water Heater Runtime Fraction []

The fraction of the time period that the heater was running.

Water Heater Part Load Ratio []

The fraction of the Heater Maximum Capacity.

Water Heater <Fuel Type> Consumption Rate [W]

The average fuel consumption rate for the heater element or burner.

Water Heater <Fuel Type> Consumption Energy [J]

The fuel consumption energy for the heater element or burner.

Water Heater Off-Cycle <Fuel Type> Consumption Rate [W]

The average fuel consumption rate for the off-cycle parasitics.

Water Heater Off-Cycle <Fuel Type> Consumption Energy [J]

The fuel consumption energy for the off-cycle parasitics.

Water Heater On-Cycle <Fuel Type> Consumption Rate [W]

The average fuel consumption rate for the on-cycle parasitics.

Water Heater On-Cycle <Fuel Type> Consumption Energy [J]

The fuel consumption energy for the on-cycle parasitics.

Water Heater Water Consumption Rate [m3/s]

The water consumption rate for the use side, if in stand-alone operation.

Water Heater Water Consumption [m3]

The water consumption for the use side, if in stand-alone operation.