tekmar 374 User Manual

- Data Brochure

Universal Reset Control 374

D 374

03/09

The tekmar Universal Reset Control 374 is designed to maximize the comfort and efﬁ ciency provided by a hydronic heating system. The control automatically adjusts the boiler and mix water temperatures that are delivered to the heating system by using outdoor reset. The 374 can control two separate on / off stages (or one low / high ﬁ re) to provide outdoor reset while providing equal run time rotation of the boilers. The 374 can operate two mixing devices, which can be either two variable speed injection pumps or two ﬂ oating action valves. The mixing devices can be used to supply two different reset water temperatures or one reset and one setpoint water temperature to a space heating system. The 374 is capable of controlling an indirect Domestic Hot Water (DHW) storage tank and setpoint load. A large easy to read display provides current system temperatures and operating status. The control has an internal timer, which can have 2 events per day on a 24 hour, 5-1-1 day or 7 day schedule.

Additional functions include:

• Outdoor Reset

• Two Mixing Devices

• Two Boiler Stages

• Two separate mix demands for space heating loads

• Boiler demand for space heating loads

• DHW demand for domestic hot water loads

• Setpoint demand for setpoint loads

Menu Item

Universal Reset Control 374

Two Mixing, Two Stage Boiler, DHW & Setpoint

Output

Mixing

Valve &

Actuating

Motor

Output

Variable

Speed

driven

pump

Output

Mixing

Valve &

Actuating

Motor

Output

Variable

Speed

driven

pump

Input

032 Timer

Optional

Input

Universal

Sensor

Included

3 4

Opn

Cls1 Pwr Opn Cls2 UnO Com Boil Out Com Mix1 Mix2 Mix1 Power DHWPrim Mix2 Mix1 Com Mix2 Setp DHW Boil Com

Var1 Var21 Mix 2 Sw P2 L N P1 P3 N Pmp/Vlv Dem Dem Dem Dem Dem Dem Dem

Input

Outdoor

Sensor

Included

Universal

Included

Do not apply power

6789101112 15 161718

Input

Sensor

Input

Universal

Sensor

Included

Boiler Demand

Mix 1 Demand

Mix 2 Demand

DHW Demand

Setpoint Demand

13 14

Output

Mix 1

System

Pump

• Installer and Advanced access levels

• Primary pump and mixing system pump outputs

• Exercising

• Test sequence to ensure proper component operation

• Internal setback timer for energy savings

• Setback input for energy savings

• CSA C US certified

Rotate Floating Setback Advanced

Made in Canada by tekmar Control Systems Ltd.

Power: 115 V ±10% 50/60 Hz 1750 VA Relays: 230 V (ac) 5 A 1/3 hp, pilot duty 240 VA Variable Pump: 230 V (ac) 2.4 A 1/6 hp, fuse T2.5 A Demands: 20 to 260 V (ac) 2 VA

Signal wiring must be rated at least 300 V.

192021 222324 252627 2928

Stage 1/

Stage 2/

Boil Enbl.

Setp Enbl.

Input

115 V (ac)

Power

Supply

Output

Primary

Pump

Variable

Oﬀ

30% Enable

10% Enable

Output

Mix 2

System

Pump

Return

Boiler Sensor

SupplyInstaller

Test

oﬀ not testing red testing red testing paused

For maximum heat, press & hold Test button for 3 seconds.

Meets Class B: Canadian ICES FCC Part 15

Output

DHW

Pump

Note:

Boiler, DHW, setpoint, or mixed demand must be powered with 20 to 260 V (ac) before the control will operate pump/valve outputs or the boiler is able to fire.

Date Code

H2036B

Output

DHW Valve

Input

Boiler Demand

Input

DHW Demand

Input

Setpoint Demand

Input

Mix 2 Demand

Input

Mix 1 Demand

Output

Boiler

How To Use The Data Brochure

This brochure is organized into four main sections. They are: 1) Sequence of Operation, 2) Installation, 3) Control Settings, and

4) Testing and Troubleshooting. The Sequence of Operation section has five sub-sections. We recommend reading Section A: General of the Sequence of Operation, as this contains important information on the overall operation of the control. Then read the sub sections that apply to your installation.

The Control Settings section (starting at DIP Switch Settings) of this brochure describes the various items that are adjusted and displayed by the control. The control functions of each adjustable item are described in the Sequence of Operation.

Table of Contents

User Interface ..................................................Pg 2

Display ............................................................. Pg 3

Sequence of Operation .................................. Pg 4

Section A: General Operation .............. Pg 4

Section B: Boiler Operation.................. Pg 6

Section C: DHW Operation ................... Pg 10

Section D: Setpoint Operation ............. Pg 12

Section E: Mixing Operation .................Pg 13

Installa tion ....................................................... Pg 16

Control Settings .............................................. Pg 25

View Menu ..............................................Pg 25

Adjust Menu ...........................................Pg 26

Time Menu .............................................. Pg 30

Schedule Menu ......................................Pg 31

Testing the Control .........................................Pg 32

Error Messages ............................................... Pg 34

Technical Data .................................................Pg 36

Limited Warranty ............................................Pg 36

DIP Switch Settings ........................................Pg 23

User Interface

The 374 uses a Liquid Crystal Display (LCD) as the method of supplying information. You use the LCD in order to setup and monitor the operation of your system. The 374 has four push buttons (Menu, Item, ▲, ▼) for selecting and adjusting settings. As you program your control, record your settings in the ADJUST Menu table which is found in the second half of this brochure.

All of the items displayed by the control are organized into four menus: View, Adjust, Time, and Schedule. These menus are listed on the top left hand side of the display

(Menu Field). To select a menu, use the Menu button. By pressing and releasing the Menu button, the display will advance to the next available menu. Once a menu is

selected, there will be a group of items that can be viewed within the menu.

Note: The TIME and SCHEDULE menus are not available when there is no Setback selected.

Menu Item

Item

The abbreviated name of the selected item will be displayed in the item field of the display. To view the next available item, press and release the Item button. Once you have reached the last available item in a menu, pressing and releasing the Item button will return the display to the first item in the selected menu.

The items can be quickly scrolled through by holding the Item button and then pressing the ▼ button. To rapidly scroll through the items in the reverse order, hold the Item button and press the ▲ button.

Adjust

To make an adjustment to a setting in the control, begin by selecting the appropriate menu using the Menu button. Then select the desired item using the Item button. Finally, use the ▲ and / or ▼ button to make the adjustment.

Additional information can be gained by observing the Status field of the LCD. The status field will indicate which of the control’s outputs are currently active. Most symbols in the status field are only visible when the VIEW Menu is selected.

Menu Item

Display

Menu Field

Displays the current menu

Item Field

Displays an abbreviated name of the selected item

Status Field

Displays the current status of the control's inputs, outputs and operation

Symbol Description

Number Field

Displays the current value of the selected item

Boiler Demand Mix 1 Demand Mix 2 Demand DHW Demand Setpoint Demand

Time & Date Field

Displays days and AM or PM

Buttons

Selects Menus, Items and adjust settings

Open / Close

Displays when the actuator is opening or closing the mixing valve.

Mixing Device Output Scale

Shows output of injection pump or mixing valve.

Burner

Displays which stage relay is turned on.

Pump

Displays when the primary pump 1, mix 1 system pump and mix 2 system pump is operating.

DHW

Displays when the DHW pump or valve is on.

Warning

Displays when an error exists or when a limit has been reached.

Installer Access Level

Displays when the Installer / Advanced Dip switch is set to Installer

°F, °C, min

Units of measurement.

Pointer

Displays the control operation as indicated by the text.

UnOccupied Schedule

Displays when the control is in UnOccupied Mode.

Occupied Schedule

Displays when the control is in Occupied Mode.

Definitions

The following defined terms and symbols are used throughout this manual to bring attention to the presence of hazards of various risk levels, or to important information concerning the life of the product.

- Warning Symbol: Indicates presence of hazards which can cause severe personal injury, death or substantial property damage if ignored.

INSTALLATION

CATEGORY II

- Double insulated

- Local level, appliances

Sequence of Operation

Section A

General

Operation

Page 4 - 6

Section B

Boiler Reset

Page 6 - 9

Section C

DHW

Operation

Page 10 - 12

Section D

Setpoint

Operation

Page 12

Section E

Mixing

Operation

Page 13 - 15

Section A: General Operation

POWERING UP THE CONTROL

When the 374 control is powered up, all segments in the LCD are turned on for 2 seconds. Next, the control displays the control type number in the LCD for 2 seconds. Next, the software version is displayed for 2 seconds. Finally, the control enters into the normal operating mode.

TYPES OF DEMANDS

The control can control the supply water temperatures of two mix temperature systems and the boiler supply water temperature. The type of demand the control receives determines the operation of the control.

Boiler Demand

When a boiler demand signal from the heating system is present, the control operates the boiler(s) to maintain the boiler supply temperature based on the outdoor air temperature and the Boiler Characterized Heating Curve settings. Refer to section B.

DHW Demand

When a DHW demand is present, the control operates the boiler(s) to maintain the supply water temperature at least as hot as the DHW exchange setting. Refer to section C.

Setpoint Demand

When a setpoint demand signal is present, the control operates the boiler(s) to maintain the supply water temperature at least as hot as the Setpoint setting. Refer to section D.

Mix 1 Demand

When a mix 1 demand signal from the heating system is present, the control operates the mixing 1 device and the boiler(s) to maintain the mix 1 supply temperature based on the outdoor air temperature and the Mix 1 Characterized Heating Curve settings. Refer to section E.

Mix 2 Demand

When a mix 2 demand signal from the heating system is present, the control operates the mixing 2 device and boiler(s) to maintain the mix 2 supply temperature based on the outdoor air temperature and the Mix 2 Characterized Heating Curve settings or the control can provide a setpoint temperature. Refer to section E.

Design SupplyDesign Supply

CHARACTERIZED HEATING CURVE

The control varies the supply water temperature based on the outdoor air temperature. The control takes into account the type of terminal unit that the system is using. Since different types of terminal units transfer heat to a space using different proportions of radiation, natural convection and forced convection, the supply water temperature must be controlled differently. Once a terminal unit is selected, the control varies the supply water temperature according to the type of terminal unit. This improves the control of the air temperature in the building.

Terminal UnitTerminal Unit

Indoor DesignIndoor Design Outdoor DesignOutdoor Design

Decreasing Outdoor Temperature

TERMINAL UNITS

The control provides for a selection between six different terminal unit types: two types of radiant floor heat, fancoil, fin-tube convector, radiator and baseboard. When a terminal unit is selected, the control automatically loads the design supply temperature, maximum supply temperature, and minimum supply temperature. See section B for Boiler Terminal Units and section E for Mixing Termin a l U n i ts.

High Mass Radiant (1)

This type of a hydronic radiant floor is embedded in either a thick concrete or gypsum pour. This heating system has a large thermal mass and is slow acting.

Increasing Water Temperature

Low Mass Radiant (2)

This type of radiant heating system is either attached to the bottom of a wood sub-floor, suspended in the joist space, or sandwiched between the sub-floor and the surface. This type of radiant system has a relatively low thermal mass and responds faster than a high mass system.

Fancoil (3)

A fancoil terminal unit or Air Handling Unit (AHU) consists of a hydronic heating coil and either a fan or blower. Air is forced across the coil at a constant velocity by the fan or blower, and is then delivered into the building space.

Fin-Tube Convector (4)

A convector terminal unit is made up of a heating element with fins on it. This type of terminal unit relies on the natural convection of air across the heating element to deliver heated air into the space. The amount of natural convection to the space is dependant on the supply water temperature to the heating element and the room air temperature.

Radiator (5)

A radiator terminal unit has a large heated surface that is exposed to the room. A radiator provides heat to the room through radiant heat transfer and natural convection.

Baseboard (6)

A baseboard terminal unit is similar to a radiator, but has a low profile and is installed at the base of the wall. The proportion of heat transferred by radiation from a baseboard is greater than that from a fin-tube convector.

OUTDOOR DESIGN TEMPERATURE

The outdoor design temperature is the outdoor air temperature that is the typical coldest temperature of the year where the building is located. This temperature is used when doing the heat loss calculations for the building. If a cold outdoor design temperature is selected, the supply water temperature rises gradually as the outdoor temperature drops. If a warm outdoor design temperature is selected, the supply water temperature rises rapidly as the outdoor temperature drops.

WARM WEATHER SHUT DOWN (OCC AND UNOCC)

The Warm Weather Shut Down (WWSD) disables the space heating system during warm outdoor weather. There is a separate WWSD for both the occupied and the unoccupied periods. When the outdoor air temperature rises above the WWSD setting, the control turns on the WWSD pointer in the display and does not operate the space heating system to satisfy boiler, mix 1 or mix 2 demands. Mix 2 setpoint demands remain active. The control does respond to a DHW demand or a setpoint demand and operates as described in sections C and D.

SETBACK

To provide greater energy savings, the control has a setback feature. With setback, the supply water temperature in the system is reduced when the building is unoccupied. By reducing the supply water temperature, the air temperature in the space may be reduced even when thermostat(s) are not turned down.

The control has an internal setback timer with two events per day on a 24 hour, a 5-1-1 day or a 7 day schedule.

The control also has an external setback input. Any time the UnO Sw (6) and the Com (7) are shorted together, the control operates in the unoccupied mode.

The external setback overrides the internal setback timer schedule to place the control into the unoccupied period.

When in the unoccupied mode, the UNOCC segment is displayed in the LCD. The control adjusts the supply water temperature based on the UNOCC settings made in the control.

Com

Uno Sw

Timer Switch

FACTORY DEFAU LTS

The control comes preset with several factory defaults. These defaults are based on the terminal unit selection (see section B for Boilers and section E for Mixing Devices). To fine-tune building requirements, these defaults may be changed.

To reload the factory default, power down the control and wait for 10 seconds. Power up the control while simultaneously holding the Menu and ▼ buttons. An E01 error occurs forcing the installer to go through the ADJUST menu to ensure the settings are correct.

EXERCISING

The control has a built-in exercising feature, which ensures that each pump or valve is operated at least once every 3 days. If a pump has not been operated at least once every 3 days, the control turns on the output for 10 seconds. This minimizes the possibility of the pump seizing during a long period of inactivity. While the control is exercising, the Te st LED flashes quickly.

Note: The exercising function does not work if power to the control or pumps is disconnected.

BOILER PROTECTION

The control is capable of providing boiler protection from cold mixing system return temperatures. The control protects the boilers by either reducing the variable speed injection pump speeds or by closing the four way valves. This limits the amount of cool return water to the boiler and allows the boiler water temperature to recover.

PRIMARY PUMP OPERATION

The primary pump operates when the control receives boiler, mix 1 or mix 2 demands and is not in Warm Weather Shut Down (WWSD). The primary pump will operate in DHW MODES 3 and 4 when a DHW demand is present. The primary pump also operates in Setpoint MODE 3 during setpoint demands.

Primary Pump Purge

After any demand that operates the primary pump is removed, the control continues to operate the primary pump for an additional 20 seconds.

Section B: Boiler Operation

Section B1

Boiler Reset

Section B2

Boiler

Enable

Section B1: Boiler Reset

BOILER DEMAND

A boiler demand is required in order for the control to provide heat to the heating system. A boiler demand is generated by applying a voltage between 24 and 230 V (ac) across the Boil Dem and Com Dem terminals (30 and 31). Once voltage is applied, the Boiler Demand pointer is displayed in the LCD. If the control is not in Warm Weather Shut Down (WWSD), the control closes the primary pump contact. The control calculates a boiler target supply temperature based on the outdoor air temperature and the boiler characterized heating curve settings. The control then fires the boiler(s), if required, to maintain the boiler target supply temperature.

BOILER SUPPLY SENSOR

The boiler sensor can be located on the boiler supply if the 374 is the only control that is operating the boiler(s). When the Return / Supply DIP switch is set to Supply, the control determines the required operating temperature for the boiler supply and operates the Stage 1 and Stage 2 contacts in order to maintain the correct boiler supply water temperature.

24 to 230 V (ac)

Boiler

Sensor

Dem

Boil

Com

Dem

-20

(-29)

210 (99)

190 (88)

170 (77)

150 (66)

130 (54)

110 (43)

90 (32)

70 (21)

50°F (10°C)

BOILER CHARACTERIZED HEATING CURVE

The boiler(s) have a Boiler Characterized Heating Curve determined by the following settings:

Boiler Design Temperature

The boiler design supply temperature is the supply water temperature required to heat the building when the outdoor air temperature is as cold as the outdoor design temperature.

Boiler Indoor Design Temperature

The indoor design temperature is the room temperature that was used in the original heat loss calculations for the building. This setting establishes the beginning of the boiler characterized heating curve.

Boil Room

The Boil Room setting is the desired room temperature for the building and provides a parallel shift of the boiler characterized heating curve. The room temperature desired by the occupants is often different from the design indoor temperature. If the room temperature is not correct, adjusting the Boil Room setting increases or decreases the amount of heat available to the boiler zones in the building. A Boil Room setting is

Boiler Characterized

Heating Curve

BOIL MIN

BOIL INDR

(27)

(16)

Outdoor Air Temperature

OUT DSGN

WWSD OCC

WWSD UNOCC

ROOM OCC

ROOM UNOCC

40 (5)

BOIL DSGN

(-7)

BOIL MAX

(-18)

available for both the occupied (day) and unoccupied (night) periods.

BOILER TERMINAL UNITS

When a terminal unit is selected, the control automatically loads the boiler design temperature, boiler maximum supply temperature, and boiler minimum supply temperature. The factory defaults can be changed to better match the installed system. If a factory default has been changed, refer to section A to reload the factory defaults.

Supply Water Temperature

Terminal Unit High Mass

Radiant (1)

Low Mass

Radiant (2)

Fancoil

(3)

Fin-Tube

Convector (4)

Radiator

(5)

Baseboard

(6)

BOIL DSGN 120°F (49°C) 140°F (60°C) 190°F (88°C) 180°F (82°C) 160°F (71°C) 150°F (66°C)

BOIL MAX 140°F (60°C) 160°F (71°C) 210°F (99°C) 200°F (93°C) 180°F (82°C) 170°F (77°C)

BOIL MIN OFF OFF 140°F (60°C) 140°F (60°C) 140°F (60°C) 140°F (60°C)

Boiler Target Temperature

The boiler target temperature is determined from the boiler characterized heating curve settings and the outdoor air temperature. The control displays the temperature that it is currently trying to maintain as the boiler supply temperature. If the control does not presently have a requirement for heat, it does not show a boiler target temperature. Instead, “– – –” is displayed in the LCD.

Boiler Minimum

The boiler minimum is the lowest temperature that the control is allowed to use as a boiler target temperature. During mild conditions, if the control calculates a boiler target temperature that is below the boiler minimum setting, the boiler target temperature is adjusted to at least the boiler(s) minimum setting. During this condition, if the boiler(s) is operating, the minimum segment is turned on in the display when viewing either the boiler supply temperature or the boiler target temperature. Set the boiler minimum setting to the boiler

MIN Segment On

manufacturer’s recommended temperature.

Boiler Maximum

The boiler maximum is the highest temperature that the control is allowed to use as a boiler target temperature. If the control does target the boiler maximum setting, and the boiler temperature is near the boiler maximum temperature, the maximum segment will be displayed in the LCD while either the boiler target temperature or the boiler supply temperature is being viewed. At no time does the control operate the boiler(s) above 248°F (120°C).

MAX Segment

STAGING

The control operates up to two on / off boiler stages (or one low / high fire) in order to provide the required supply temperature. After a stage is turned on in the firing sequence, the control waits for a minimum time delay. The minimum time delay is adjustable using the Stage Delay setting. After the Stage Delay has expired, the control examines the control error to determine when the next stage is to fire. The control error is determined using Proportional, Integral and Derivative (PID) logic.

Proportional – compares the actual supply temperature to the boiler target temperature. The colder the supply water temperature, the sooner the next stage is turned on.

Integral – compares the actual supply temperature to the boiler target temperature over a period of time.

Derivative – compares how fast or slow the supply water temperature is changing. If the supply temperature is increasing slowly,

the next stage is turned on sooner. If the supply temperature is increasing quickly, the next stage is turned on later, if at all.

ROTATION

The control’s Equal Run Time Rotation function is fixed at 48 hours. The firing order of the boilers changes whenever one boiler accumulates 48 hours more running time than the other boiler. After each rotation, the boiler with the least running hours is the first to fire and the boiler with

the most running hours is the last to fire. This function ensures that both boilers receive equal amounts of use. When the Rotate / Off DIP switch is set to the Off position, the firing sequence always begins with boiler

720 hours 672 hours

one and then boiler two.

Note: When using a single two-stage boiler, ensure that the Rotate / Off DIP switch is set to Off.

Resetting the Rotation Sequence

To reset the rotation sequence, set the Rotate / Off DIP switch to the Off setting for 5 seconds and then return the DIP switch to the Rotate setting.

672 hours 720 hours

FIRE DELAY

The Fire Delay is the time delay that occurs between the time that the control closes a stage contact to fire a stage and the burner fires for that stage.

Stage Delay

The stage delay is the minimum time delay between the firing of stages. After this delay has expired the control can fire the next stage if it is required. This setting can be adjusted manually or set to an automatic setting. When the automatic setting is used, the control determines the best stage delay based on the operation of the system.

Boiler Contact Closes

Stage Delay

Boiler #1 Fires

Boiler #1

Fire

Delay

Time

Fire

Delay

Boiler #2 Fires

Boiler #2

Boiler Mass

The boiler mass setting allows the installer to adjust the control to the thermal mass of the type of heat sources used in the application. If the heating system is causing the boiler(s) to be staged on and off in rapid succession, a higher boiler mass setting will result in a decrease in the amount of cycling. Conversely, if the system is slow to respond to heat requirements, then decreasing the boiler mass setting will increase the response rate by staging the boilers at a faster rate.

Lo (1)

The Lo setting is selected if the boiler(s) that is used has a low thermal mass. This means that the boiler(s) has a very small water content and has very little metal in the heat exchanger. A boiler that has a low thermal mass comes up to temperature quite rapidly when fired. This is typical of many copper fin-tube boilers. The Lo mass setting provides a fast staging rate of on / off boiler stages.

Med (2)

The Med setting is selected if the boiler(s) that is used has a medium thermal mass. This means that the boiler(s) either has a large water content and a low metal content or a small water content and a high metal content. This is typical of many modern residential cast iron boilers or steel tube boilers. The Med mass setting provides a moderate staging rate of on / off boiler stages.

Hi (3)

The Hi setting is selected if the boiler(s) that is used has a high thermal mass. This means that the boiler(s) has both a large water content and a large metal content. A boiler that has a high thermal mass is relatively slow in coming up to temperature. This is typical of many commercial cast iron and steel tube boilers. The Hi mass setting provides a slow staging rate of on / off boiler stages.

DIFFERENTIAL

An on / off heat source must be operated with a differential in order to prevent short cycling. With the control, either a fixed or an auto differential may be selected. The boiler differential is divided around the boiler target temperature. The stage contact closes when the supply water temperature is ½ of the differential setting below the boiler target temperature. Additional staging occurs if the first stage is unable to raise the supply water temperature up to the boiler target temperature at a reasonable rate. As the supply temperature reaches ½ of the differential above the boiler target temperature, stages are staged off.

Fixed Differential

If the user desires to have a fixed differential, this is set using the boiler differential (BOIL DIFF) setting in the ADJUST menu.

Auto Differential

If the Auto Differential is selected, the control automatically determines the best differential as the load changes. This reduces potential short cycling during light load conditions.

Section B2: Boiler Enable

Desired temperature 160°F (71°C)

Boiler Oﬀ

Boiler On

Oﬀ

Diﬀerential

Time

165°F (74°C)

Time

155°F (68°C)

Diﬀerential 10°F (6°C)

Increasing Load

BOILER RETURN SENSOR

The boiler sensor should be located on the boiler return if the 374 is one of many controls that can call for boiler operation or the boiler has its own control with outdoor reset and setpoint operation. The boiler return sensor provides boiler return protection.

Boiler Enable Contact

When the Return / Supply DIP switch is set to Return, the 374 provides a boiler enable when

Boiler

Sensor

there is a requirement for heat. The 374 no longer tries to control the boiler supply water temperature directly but allows the boiler to operate at its operating aquastat setting. The boiler enable contact is closed at either 30% or 10% operation of the mixing device, which is set using the 30% Enable / 10% Enable DIP switch. The boiler enable contact remains closed until the mixing device(s) no longer requires heat.

Setpoint Enable Contact

If a DHW demand or a Setpoint demand is registered, the Setpoint Enable contact is closed to provide a setpoint demand to the boiler’s control if a setpoint input is available. When a return sensor is being used, the boiler should operate its own boiler pump.

NO BOILER SENSOR

The 374 is capable of operating without a boiler sensor if desired. In this case, there is no boiler protection provided by the 374.

To operate the 374 without a boiler sensor, the Return / Supply DIP switch is set to Return and the control must be powered up without the boiler sensor connected. This type of application is typical if the 374 is drawing heat from a heat source that already incorporates some form of boiler return protection.

Boiler Enable Contact

When the Return / Supply DIP switch is set to Return, the 374 provides a boiler enable when there is a requirement for heat. The 374 no longer tries to control the boiler supply water temperature directly but allows the boiler to operate at its operating aquastat setting. The boiler enable contact is closed at either 30% or 10% operation of the mixing device, which is set using the 30% Enable / 10% Enable DIP switch. The boiler enable contact remains closed until the mixing device(s) no longer requires heat.

Setpoint Enable Contact

If a DHW demand or a Setpoint demand is registered, the Setpoint Enable contact is closed to provide a setpoint demand to the boiler’s control if a setpoint input is available.

Section C: DHW Operation

Section C1

Domestic Hot Water (DHW)

Section C2

DHW with Low

Temp er a tur e

Boilers

Section C1: Domestic Hot Water (DHW)

DHW DEMAND

A DHW Demand is required in order for the control to provide heat to the DHW system. A DHW aquastat or setpoint control is used as a switch in the DHW demand circuit. Once the control detects a DHW demand, the DHW Demand pointer turns on in the LCD and the control operates the boiler to provide a sufficient boiler supply water temperature to the DHW tank. The control operates the pumps as described below.

24 to 230 V (ac)

DHW Dem

The control registers a DHW Demand when a voltage between 24 and 230 V (ac) is applied across the DHW Dem and Com Dem terminals (29 and 31).

Aquastat

BOILER TARGET DURING DHW GENERATION

The boiler target temperature is at least as hot as the DHW exchange setting (DHW XCHG). The DHW demand overrides the boiler reset target temperature, except when the boiler reset target is higher than the DHW exchange setting.

DHW MODE AND PRIORITY OPERATION

The control has four different settings available for DHW MODE. The required DHW MODE setting will depend on the piping arrangement of the DHW tank. It is often desirable to limit or even stop the flow of heat to the heating system when the DHW tank calls for heat. This allows for a faster recovery of the DHW tank.

Boil

Dem

Com

Dem

DHW MODE 1 - DHW in Parallel no Priority

When a DHW Demand is present, the DHW Pmp / Vlv contact closes. The primary pump (Prim P1) does not turn on, but may operate based on either a Boiler Demand, Mixing Demand or a Setpoint Demand. Refer to sections B and D.

It is assumed that the DHW pump will provide adequate flow through the heat exchanger and the boiler.

DHW MODE 2 - DHW in Parallel with Priority

When a DHW Demand is present, the DHW Pmp / Vlv contact closes and the primary pump (Prim P1) contact is opened.

It is assumed that the DHW pump will provide adequate flow through the heat exchanger and the boiler.

DHW MODE 3 - DHW in Primary / Secondary no Priority

When a DHW Demand is present, the DHW Pmp / Vlv contact is closed and the primary pump (Prim P1) is operated.

This mode can be used if a DHW tank is piped in direct return and a DHW valve is installed.

Primary

Pump

Primary

Pump

DHW Pump

Primary

Pump

DHW MODE 4 - DHW in Primary / Secondary with External Priority

When a DHW Demand is present, the DHW Pmp / Vlv contact is closed and the primary pump is operated. Priority can only be obtained using external wiring on boiler zones. The mix system pumps continue to operate but the variable speed pumps(s) is shut off or the mixing valve(s) is closed. This allows DHW priority over the mix zones. During a priority override, the DHW Pmp / Vlv contact is opened until the heating system has recovered before returning to DHW operation.

This mode can be used if a DHW tank is piped in direct return and a DHW valve is installed.

Power from

Boiler 3 / DHW

Contact

COIL

N.C.

Power to External

Boiler Zones

DHW MODE 4

N.O.

External Priority Interlock

DHW Pump

DHW PRIORITY OVERRIDE

The DHW Priority Override applies to DHW MODE 2 and 4. To prevent the building from cooling off too much or the possibility of a potential freeze up during DHW priority, the control limits the amount of time for DHW priority. As the outdoor air temperature becomes colder, the length of time that the control provides DHW priority is reduced. Once the allowed time for priority has elapsed, the control overrides the DHW priority and resumes space heating.

To provide external DHW priority, the boiler temperature space heating zones must be interlocked with the DHW Pmp / Vlv contact. During DHW demands, the DHW Pmp / Vlv contact must remove any power to all boiler temperature space heating zone valves or zone pumps.

Using External

Mix

System

Pump

DHW Priority OverrideDHW Priority Override

Increasing Air Temperature

Outdoor Air Temperature

Disable

Wiring

DHW Pump

Primary

Pump

Increasing Time

DHW priority demand time limit

CONDITIONAL DHW PRIORITY

If the boiler supply temperature is maintained at or above the required temperature during DHW generation, this indicates that the boiler(s) has enough capacity for DHW and possibly heating as well. As long as the boiler supply temperature is maintained near its target, DHW and heating occurs simultaneously.

DHW POST PURGE

After the DHW Demand is removed, the control performs a purge on the boiler(s). The control shuts off the boiler(s) and continues to operate either the DHW pump or the DHW valve and the primary pump if applicable. This purges the residual heat from the boiler(s) into the DHW tank. The control continues this purge for a maximum of four minutes or until the boiler supply water temperature drops 20°F (11°C) below the boiler target temperature during the DHW operation. The control also stops the purge if the boiler supply temperature drops below the current boiler target temperature.

DHW MIXING PURGE

After DHW operation, the boiler(s) is extremely hot. At the same time, the heating zones may have cooled off considerably after being off for a period of time. To avoid thermally shocking the boiler(s) after DHW priority, the control shuts off the boiler(s), but continues to operate the DHW pump while restarting the heating system. This allows some of the

DHW Pump

DHW return water to mix with the cool return water from the zones and temper the boiler return water.

Primary

Pump

DHW DURING UNOCCUPIED

If the control receives a DHW Demand during an unoccupied period, the control can either continue operation of the DHW system as it would during the occupied period or the control can ignore a DHW Demand for the duration of the unoccupied period.

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