WaterHeater:Mixed
WaterHeater:Sizing
Used in:
Supply side of DHW Loops
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WaterHeater:Mixed WaterHeater:Sizing
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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.
The name of the Water heater object.
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).
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.
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.
Select the Schedule that defines the hot water temperature setpoint in °C. Also known as the "cut-out" temperature.
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.
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.
The Ambient temperature indicator specifies how the temperature of the air surrounding the Water heater is to be defined. These options are available:
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.
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.
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.
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.
The control type can be one of:
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.
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.
The type of fuel used for heating. The fuel type can be one of:
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.
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 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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
The Water heater sizing data is entered on the Sizing tab.
This field describes the method to be used for sizing the water heater. There are six choices:
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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]
The average water tank temperature.
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.
The average heat loss rate due to the off- and on-cycle loss coefficients to the ambient temperature.
The heat loss energy due to the off- and on-cycle loss coefficients to the ambient temperature.
The use side mass flow rate. If stand-alone, this is the scheduled use flow rate.
The inlet temperature on the use side.
The outlet temperature on the use side.
The average heat transfer rate between the use side water and the tank water.
The heat transfer energy between the use side water and the tank water.
The source side mass flow rate. If in stand-alone operation, this is 0.
The inlet temperature on the source side.
The outlet temperature on the source side.
The average heat transfer rate between the source side water and the tank water.
The heat transfer energy between the source side water and the tank water.
The average heat gain rate to the tank water due to off-cycle parasitics.
The heat gain energy to the tank water due to off-cycle parasitics.
The average heat gain rate to the tank water due to on-cycle parasitics.
The heat gain energy to the tank water due to on-cycle parasitics.
The average heating rate demanded to maintain the setpoint temperature.
The heating energy demanded to maintain the setpoint temperature.
The average heating rate supplied by the heater element or burner.
The heating energy supplied by the heater element or burner.
The average heating rate unmet by the heater element or burner. The difference between the Total Demand Rate and the Heating Rate.
The heating energy unmet by the heater element or burner. The difference between the Total Demand Energy and the Heating Energy.
The average venting rate to keep the tank below the Maximum Temperature Limit.
The venting energy to keep the tank below the Maximum Temperature Limit.
The average net heat transfer rate when considering all losses and gains.
The net heat transfer energy when considering all losses and gains.
The number of times that the heater turned on in the time period.
The fraction of the time period that the heater was running.
The fraction of the Heater Maximum Capacity.
The average fuel consumption rate for the heater element or burner.
The fuel consumption energy for the heater element or burner.
The average fuel consumption rate for the off-cycle parasitics.
The fuel consumption energy for the off-cycle parasitics.
The average fuel consumption rate for the on-cycle parasitics.
The fuel consumption energy for the on-cycle parasitics.
The water consumption rate for the use side, if in stand-alone operation.
The water consumption for the use side, if in stand-alone operation.