Honeywell DT90 User Manual

DT90

DIGITAL ROOM THERMOSTAT

PRODUCT SPECIFICATION SHEET

The new DT90 family of digital room thermostats is a

range of market leading products designed to provide
comfort with economy in modern heating systems.
Its large display and simple button interface make

DT90 extremely easy to use.

Energy efficiency is addressed by state-of-the-art TPI

control performance and an ECO button energy saving

feature.

Applications include control of gas or oil-fired boiler
systems, underfloor heating, electric heating and
zoning systems.

With a modern fresh look that complements any style
of décor, and a range of valued features for users and

installers alike, DT90 sets the standard for simple

environmentally-friendly room thermostats.

FEATURES

• Energy saving TPI control performance

• Advanced self-learning control adapts to the

environment and ensures close temperature
control with minimum energy usage

• Slim modern styling

• Large high contrast display with easy-to-read

characters

• Simple user interface

• Temperature setting procedure eliminates the

risk of accidental setpoint change

• Display shows room temperature, with option

to inquire about setpoint

• 5°C to 35°C setpoint range with 0.5°C

increments

• Temperature set using up or down buttons

• Off/standby button, allowing manual switch off,

with frost protection active

• Adjustable off/standby setpoint 5°C to 16°C or

DT90 can be set to off completely

• Battery powered by 2 x AA (LR6) alkaline cells

• Up to 4 years battery life (minimum 2 years),

with battery low warning

• Simple battery change by unclipping front

cover

• Installer Mode allows the thermostat to be

customised for the application and the needs of
the user

• NVRAM storage of setup parameters, ensuring

these are never lost

• Setpoint limits can be programmed in

• 24…230Vac SPDT potential-free contacts for

simple 2-wire installation

• 8 A resistive, 3 A inductive switch rating

• Surface or switch-box mounting options

• Sensor fault self-diagnostics

• Heat/cool changeover operation mode possible

FEATURES UNIQUE TO DT90E ECO MODEL

• Energy saving ECO button allows user to

change to a lower, energy saving setpoint for a
timed period of their choosing (1…24 hours)

• Display shows countdown of time remaining in

ECO energy saving mode

EN0H 8578 UK07 R1 12/08

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CONTROLS / DISPLAY LAYOUT

frost protect indicator heating demand indicator

DT90A

DT90E

DT90 DIGITAL ROOM THERMOSTAT

ECO countdown indicatorECO mode active

temperature display

setpoint indicator

fault indicator

battery low warning

off/standby indicator

SPECIFICATIONS

ELECTRICAL

Power supply : 2 x 1.5V IEC LR6 (AA) Alkaline cells

Battery life : Typically 4 years, minimum 2 years (with

correctly specified alkaline cells)

Battery low
warning

Switch type : SPDT (single pole double throw )

Electrical rating : 230 V, 50…60 Hz, 0.01 A to 8 A

Relay life : 100,000 operations minimum

Wiring : Terminal block capable of accepting

Wiring access : Rear, top and left side

ENVIRONMENTAL & STANDARDS

Operating
temperature
Shipping &
storage
temperature
Humidity : Humidity range 10% to 90% rh, non-

IP class : IP30

Approvals : CE mark, complying with standards

: Display will indicate when battery power

reserve is low. Unit will continue to
function for a minimum of 4 weeks after
the first indication is given

potential free

resistive, 0.1 A to 3 A inductive (0.6pf)

: 24 V, 0…60 Hz, 0.01 A to 8 A resistive,

0.1 A to 3 A inductive (0.6pf)

wires up to 2.5mm²

: 0°C to 40°C

: -20°C to 55°C

condensing

EN60730-1: 2001, EN60730-2-9: 2002
EN55014-1: 2001, EN55014-2: 1997

: WEEE & RoSH compliant

: C-tick

setpoint change buttons

ECO button

off/standby button

ECO mode countdown timer

TEMPERATURE CONTROL

Sensing element : 10K (@25°C ) NTC thermistor

Temperature
setting range

: 5°C to 35°C setpoint range in 0.5°C

increments

Control form : Self-learning TPI Fuzzy Logic algorithm

Proportional
band

Minimum on/off
time

: 1.5°C adjustable up to 3°C in 0.1°C

increments

: 1 minute, adjustable up to 5 min in 1 min

increments

Cycle rate : Adjustable to suit the application 3, 6, 9,

12 cycles per hour

Temperature
control accuracy

: ± 0.5°C (or better) at 20°C, 50% load and

3°C /hour temperature ramp

Frost protection : 5°C when thermostat switched to

off/standby, adjustable 5°C to 16°C

: Frost protection not available in cooling

mode

Positive off : Positive off possible (no frost protection)

by setting in Installer Mode

ECO energy
saving

Fail-safe
operation

: Setpoint default 18°C, adjustable 5°C to

35°C

: If temperature measurement system fails,

unit will continue to operate on the
assumption of a 10% load

METRICS

Dimensions
(unit)
Dimensions
(pack)
Weight (unit) : 165g

Weight (pack) : 192g

: 90 x 92 x 27mm

: 93 x 94 x 46mm

2

DT90 DIGITAL ROOM THERMOSTAT

ORDERING SPECIFICATION

Model Description Literature

DT90A1008 Digital room thermostat Multi-lingual

DT90E1012 Digital room thermostat with ECO energy saving feature Multi-lingual

MAIN FEATURES

Extra-Large Display

The DT90 display is more than double the size of its
predecessor, ensuring it is even easier to read and
allowing more information to be displayed, when
required. The large characters and high contrast
screen are especially important for those with impaired
vision.

Simple Interface

The user interface has been made as simple as
possible to make DT90 very easy to use. Buttons have
been labeled ﾒ and ﾔ to identify them as the means
of increasing and decreasing (respectively) the setpoint
temperature. The display normally shows the actual
room temperature. When one of the buttons is first
pressed, the setpoint temperature is shown flashing,
accompanied by the ﾒ and ﾔ symbols. Further
buttons presses will increase or decrease the setpoint
in increments of 0.5°C.

Off/Standby Button, with Frost Protection

The off/standby button allows DT90 to switch off the
heating (or cooling) system at the touch of a button. To
prevent accidental switch-off, the button must be held
for 2 seconds to activate the change. When off, DT90
will maintain control at a frost protection setpoint,
factory set to 5°C but adjustable between 5 and 16°C.
If required, frost protection can be switched off, to
provide a positive off function. These adjustments are

made by entering the DT90 Installer Mode.

Installer Mode

The Installer Mode is where DT90 can be configured
for different applications, and customized to meet the
needs of the user. The operating properties that can be
adjusted are called parameters, and these are
described in detail on page 6.
Parameters are as follows:

• Minimum on/off time

• Cycle rate

• Proportional band width

• Temperature measurement offset

• Upper setpoint limit

• Lower setpoint limit

• Energy saving ECO temperature (on DT90E only)

• Selection of heat/cool changeover operation

• Off/Standby setpoint

• Electric heat operation

• Reset all parameters to factory settings

Installer Mode is entered via a sequence of button
presses. The buttons are also used to scroll between

NVRAM Storage of Settings

All parameter settings are stored in a special kind of
memory called NVRAM so they will be retained
indefinitely even if the batteries are removed.

Advanced Self-learning TPI Control

DT90 uses a self-learning ‘fuzzy logic’ time-proportional
control algorithm. This form of control is better than
conventional PI control as it has a faster response and
better performance in steady state conditions. It
performs equally well in a wide range of different
installations, and ensures energy savings by controlling
closer to setpoint and minimising temperature
overshoots.

Additional Energy saving ECO feature

In a heating system, one of the best ways to save
energy is to reduce the setpoint temperature. The DT90
green ECO button provides a simple and convenient
way of doing this for a period of time of the user’s
choice. The ECO energy saving setpoint is pre-defined
in the Installer Mode. The factory setting is 18°C, but it
can be adjusted (between 5°C and 35°C) to give a
timed boost, if required. When the ECO button is
pressed, the user is given the chance to set the time
required at this new temperature, from 1 hour to 24
hours in 1 hour increments. The display indicates that
ECO mode has been set, and will count down the time
remaining in energy saving mode. Should they wish to
readjust the temperature setting, they can do that too,
using the ﾒ and ﾔ buttons.
ECO mode is cancelled simply by pressing the ECO
button again.

24 …230V 8(3)A SPDT Potential Free Contact
Rating

The DT90 switching relay has a high specification and
wide switching range, suitable for most domestic
applications. As the thermostat is battery powered, only
a 2 wire connection is required to operate the load.
Electric heating loads up to 8A (1.6kW) can be
switched directly, but please note that if more than 3A
is being switched, the electric heat parameter EH
should be set to value = 1 in the Installer Mode. This
ensures the relay is operated with a higher energy, to
switch higher currents reliably.

3 EN0H 8578 UK

INSTALLATION

Location

DT90 is for use in normal domestic environments, and
so should be located inside the house or building
where the ambient temperature is within the range 0 to
40°C. Avoid locations with high levels of condensing
moisture.
As the temperature control element in the
heating/cooling system, DT90 MUST be located in a
position with good air circulation at average room
temperature on an inside wall 1.2 to 1.5m from the
floor.
Do NOT position the thermostat near sources of heat
(radiators, hot air vents, TV or lights), near doors or
window, or in direct sunlight.

Mounting

DT90 can be mounted directly on the wall or onto an
electrical wall box.
The unit is supplied in 2 x halves, to enable quick and
easy installation of the wiring backplate.

DT90 DIGITAL ROOM THERMOSTAT

CAUTION –isolate power supply and make safe before
wiring the unit to prevent electric shock and equip men t
damage. Installation should be carried out by a qualified
Electrician or competent Heating Engineer.

Wiring

DT90 is for fixed wiring only and must be installed in
accordance with the latest Electrical Regulations.
Ensure the wiring connection to the supply is via a fuse
rated at no more than 8 amps and a switch having
contact separation of at least 3mm in all poles (formerly
Class “A”).

Completing the Installation

After wiring is complete, the front piece is attached to
the backplate with a hinging action, until it clips into

4

WIRING CONNECTIONS

a. Boiler
b. Boiler (potential free connection)
c. Thermal actuator
d. Zone valve
e. Electric actuator
f. Electric heat (maximum 1.6kW direct switching)

DT90 DIGITAL ROOM THERMOSTAT

PRODUCT DIMENSIONS

5

DT90 DIGITAL ROOM THERMOSTAT

y

p

INSTALLER MODE – APPLICATIONS SETTINGS

The DT90 has a special Installer Mode where some features can be adjusted to suit the application and the needs or
lifestyle of the user. Each adjustable feature is called a Parameter. These are described below.

Parameter Description and typical use

Minimum ON/OFF time DT90 is able to set a minimum limit for the on and off times of the controlled device or

appliance, to prevent damage or excessive wear. This minimum time can be set to 1,
2, 3, 4, or 5 minutes, and is particularly important for heating applications with oil
boilers, or air conditioning applications with compressors. Recommended settings are
shown in the next table.

Cycle rate Within the control proportional band, the output will be cycled several times per hour,

to give precise temperature control. This can be set to 3, 6, 9, or 12 cycles,
depending on the application. Recommended settings are shown in the next table.

Proportional band width The proportional band is the temperature band, adjacent to the set temperature point,

within which the thermostat will cycle the controlled output (to control the boiler, for
example). Above or below this band the output will either be fully on or fully off.
The band can be adjusted up to 3°C (default 1.5°C) to provide improved temperature
control. This is useful for (a) well-insulated homes with over-sized heating systems or

(b) air systems with a fast response
Temperature
measurement offset

If the thermostat is located in a particularly hot/cold location and cannot be moved

because of the wiring, then the measured/displayed temperature can be adjusted by

+/- 3°C. This is useful if the homeowner wants the reading to match the temperature

display on another appliance.
Upper / Lower
temperature limits

The normal upper temperature limit of 35°C can be reduced as low as 21°C to save

energy. The normal lower limit of 5°C can be raised as high as 21°C to protect

inhabitants from the cold.
Energy saving ECO
temperature

On the DT90E ECO model, pressing the ECO button immediately puts the thermostat

into energy saving mode, where it will control at this predefined temperature for as

long as the user wishes. This temperature can be set from 5°C to 35°C.
Heating or Cooling
operation

DT90 can be used for heating or cooling applications. If heating/cooling changeover is

selected, the thermostat can be manually switched from heating to cooling (and back

again) by pressing and holding the ﾒ and ﾔ buttons together for 3 seconds.
Off/standby setpoint When the off/standby button is activated, the DT90 will control to a special off/standby

setpoint. By default this is set to 5°C to provide frost protection, but it can be set up to

16°C if required. It can also be set off, by setting the parameter value to -- to provide

a positive off function. In cooling mode the default setting is off.
Electric heat operation If a direct electric heating application > 3A switching current is required, DT90 will use

more energy to operate its relay and so ensure the higher current is switched reliably.
Reset parameters This

rovides a means of resetting all the parameters to the factory supplied values.

Some parameters require to be set to match specific applications. Recommended settings are shown in the table

Specific Applications Setting What requires to be changed?

Cycle

s/

Minimum

on/off time

hour

Heating

Gas boilers
(<30kW)
Oil boiler 3 4 1. Set minimum ON/OFF time to 4 minutes

6 1 No changes required

2. Set cycle rate to 3 cycles/hour
Thermal actuator 12 1 Set cycle rate to 12 cycles/hour
Zone valve 6 1 No changes required

Air
conditionin
g

1. Configure the thermostat to allow switching between

heating and cooling modes (set parameter HC = 1)

2. Set the thermostat to the required mode of operation

(heating or cooling) by pressing the ﾒ and ﾔ
buttons together for 3 seconds when in normal run

mode (does not work in off/standby mode).
Heatpump / Air
conditioner

3 4 1. Set minimum ON/OFF time to 4 minutes

2. Set c

6

cle rate to 3 cycles/hour

DT90 DIGITAL ROOM THERMOSTAT

INSTALLER MODE – HOW TO SET PARAMETERS

Each parameter is identified by a 2-letter code, and has a range of values. These are shown in the table below,
followed by instructions on how to enter Installer Mode, how to select a parameter and change its value, and finally

Description Parameter Range of values Factory (default) value

Minimum ON/OFF time Ot 1, 2, 3, 4, 5 minutes 1 minute
Cycle rate Cr 3, 6, 9, 12 cycles/hour 6
Proportional band width Pb 1.5 to 3.0°C 1.5°C
Temperature measurement offset tO -3 to 3°C 0
Upper setpoint limit uL 21 to 35°C 35°C
Lower setpoint limit LL 5 to 21°C 5°C
Energy saving ECO temperature
(this parameter is only available on
the DT90E model)
Selection of heat/cool changeover HC 0 = disable

Off/standby setpoint * OS -- = off,

Electric heat operation EH 0 if < 3A

Reset parameters FS 0, 1 1 (factory settings)
* In cooling mode the off/standby setpoint defaults to OFF

ES 5 to 35°C 18°C

0

1 = enable

5°C

5 to 16°C

0

1 if > 3A

To enter Installer Mode:

a. Put DT90 into off/standby mode by pressing and

holding the off/standby button for 2 seconds.

b. Now press and hold both ﾒ and ﾔ temperature

adjustment buttons for 3 seconds, until the
display shows the word “Inst”.

c. Press the ﾒ button to get to the first parameter

Ot. The parameter code is shown on the display
separated by a colon from the parameter value.

Parameter

Value

To select and change a parameter:

d. Use the ﾒ and ﾔ buttons to move from one

parameter to another.

e. Press the off/standby button to select the

parameter value, ready to change

f. Use the ﾒ and ﾔ buttons to adjust the

parameter value. When the correct value is
flashing, confirm the selection by pressing the

off/standby button again, returning to the

parameter menu.

To exit Installer Mode:

g. Press and hold the off/standby button for 3

seconds

Note: Installer Mode will exit automatically after 10
minutes if no buttons are pressed.

7

INSTALLER MODE - FLOWCHART

DT90 DIGITAL ROOM THERMOSTAT

ES parameter
Only available

on DT90E

models

ENERGY EFFICIENCY AND THE ENVIRONMENT

Home energy use is currently responsible for more than ¼ of the total carbon emissions which contribute to climate
change. Heating and hot water systems based on boilers account for 2/3 of this figure, so it is important to
understand how controls can help to maximize energy efficiency while maintaining comfort.
DT90 temperature controls should be used in conjunction with appropriate time controls.
In order to save energy the following general points should be observed:

1. Ensure the system contains a room temperature thermostat and a hot water temperature thermostat (except for

Combi boiler systems), and that both are set to appropriate temperature levels.

2. Programme the heating and hot water to be off when the house is not occupied. If possible frost damage to any

exposed pipework is a concern, it is advisable to fit a frost protection thermostat or system. Honeywell can advise
on this.

3. It is normal to have the heating system switched off, or set-back at night when heating is not required.

4. Think about how domestic hot water is used. In a storage system, it is not necessary to have this switched on all

the time, even when the house is occupied.

5. Do not programme excessive heatup times for the central heating. If the occupier is out of the house, or still in

bed, much of this heating would be unnecessary.

6. In the evening, when the house is up to temperature, it is often possible to switch the heating off up to an hour

before going to bed, without any noticeable reduction in comfort.

The DT90 product family and its associated documentation and packaging are protected by various intellectual property rights belonging to
Honeywell Inc and its subsidiaries and existing under the laws of the UK and other countries. These intellectual and property rights may include
patent applications, registered designs, unregistered designs, registered trade marks, unregistered trade marks and copyrights.
Honeywell reserves the right to modify this document, product and functionality without notice. This document replaces any previously issued
instructions and is only applicable to the product(s) described.
This product has been designed for applications as described within this document. For use outside of the scope as described herein, refer to
Honeywell for guidance. Honeywell cannot be held responsible for misapplication of the product(s) described within this document.
Manufactured in the UK, for, and on behalf of the Environment and Combustion Controls Division of Honeywell Technologies Sàrl, ACS-ECC
EMEA, Z.A. La Pièce 16, 1180 Rolle, Switzerland by its Authorised Representative Honeywell Inc.

Honeywell Control Systems Ltd.

Newhouse Industrial Estate,
Motherwell ML1 5SB,
United Kingdom

http://europe.hbc.honeywell.com

EN0H 8578 UK07 R1 12/08