andrews CWH 30/201 Instruction Manual

Andrews. Built to perform.
Please read and understand these instructions before commencing installation and leave this manual with the customer for future reference.
M8673
MAXXfl o EVO
High effi ciency condensing stainless steel storage water heater
CWH 30/201, CWH 30/301, CWH 60/201, CWH 60/301
CWH 90/302, CWH120/302
CONTROLS OPERATING INSTRUCTION MANUAL

2 MAXXfl o EVO
Blank Page

Introduction 3 MAXXfl o EVO
TABLE OF CONTENTS
Ref Subject Page
Warnings and important notices 4
1.0 Introduction to the controls
1.1 The heater controls 7
1.2 The Graphical User Interface (GUI) 7
1.3 Navigate and set using the control knob 8
1.4 Display symbol meanings 9
2.0 End user operation
2.1 The home page 10
2.2 Setting the time and date 10
2.3 Setting DHW heating time program 11
2.4 Setting DHW temperature 13
2.5 Setting DHW operating mode 14
2.6 Temporary DHW operation function 15
2.7 Information pages 16
2.8 Error messages 17
2.9 Maintenance messages 18
2.10 Error codes 19
2.11 Regional settings 23
3.0 Commissioning operation
3.1 Logging into Commissioning mode 24
3.2 Input / Output test 25
3.3 Communication 26
3.4 Source 27
3.5 Consumer 28
3.6 Versions 29
3.7 Complete parameter list 30
3.8 Commissioning wizard 30
4.0 Engineer operation
4.1 Logging into Engineer mode 31
4.2 Error history - diagnostic codes 32
4.3 Error history - Burner control phase 34
4.4 Complete parameter list 35
4.5 Complete parameter list table 36
4.6 Refresh operator unit 46

Introduction 4 MAXXfl o EVO
CAUTION HOT WATER
• Water temperature over 50OC can cause burns and in severe cases injury or death.
• Children, the elderly and the physically or mentally disabled are most at risk from scald injury.
• Feel water before bathing and showering.
• Temperature limiting devices such as mixing valves should be installed whenever possible to ensure safe
temperatures at outlets
SMELL OF GAS
IF YOU SMELL GAS - FOLLOW THESE SAFETY INSTRUCTIONS:
• Do NOT turn offor on any electrical switches (including light switches)
• Do NOT smoke
• Do NOT use the telephone
• DO evacuate persons away from the source of the gas smell
• DO close the main gas shutoffvalve
• DO open all the windows and doors where the gas leakage has occurred
• DO inform the gas authority or a competent specialist as soon as possible
FLAMMABLE SUBSTANCES
Do not store or use explosive or easily inflammable material (such as petrol, paint or paper) in the same room where
this heater has been installed. All other inflammable materials should be kept a minimum of 500mm away from the
heater and its flue system.
BEFORE START UP
• Ensure the area is clear and free from combustibles, flammable liquids and chemicals.
• Ensure that the primary system is filled with sufficient water and the gas and electric supply is turned on.
IMPROPER USE
This Andrews Water Heaters product has been designed and manufactured to comply with current European stand-
ards of safety. However, following an improper use, dangers could arise concerning the safety and life of the user or
of other people, or damage could be caused to the heater or other objects. This heater is designed to be used in a
pumped domestic hot water system and is intended exclusively to be installed on a gas supply with a meter and a
gas pressure regulator. Any other use of this heater will be considered improper. Andrews Water Heaters declines
any responsibility for any damage or injuries caused by an improper use. In order to use the heater according to its
designed scope, it is essential to carefully follow the instructions given in this guide.
USER COMPETENCY
This heater is not intended for use by persons with reduced physical, sensory or mental capabilities, or lack of expe-
rience and knowledge, unless they are given supervision or Instruction concerning the use of it by a person responsi-
ble for their safety. Children under the age of 12 years should not be permitted to use the heater.
ENGINEER COMPETENCY
The installation, adjustment & servicing of this heater must be carried out by a competent person and installed in ac-
cordance with current standards and regulations. Failure to correctly install or maintain this heater could cause injury
to persons or damage to property. The manufacturer shall not be held liable for any such injury and/or damage.

Introduction 5 MAXXfl o EVO
MAINTENANCE & CONDENSATION OUTLET
To ensure continued efficient operation of the heater it is recommended that the condensate outlet is checked and
serviced at regular intervals. The frequency of servicing will depend upon the particular installation and usage but in
every case a maximum of twelve months should be allowed between service inspections.
The condensate outlet shall not be modified or blocked. It is recommended that as part of the regular maintenance
the condensate systems is examined, cleaned and the condensate trap on the side of the heater is emptied and
cleaned. The trap can be examined by unscrewing by hand the bowl at the bottom of the trap. See illustration below:-
HEATER FAILURE
In the event of failure and/or suspected faulty functioning of this heater. Switch offthe heater and contact suitable
qualified technicians. Do not attempt to make any repairs unless you are suitably qualified and competent to do so.
This manual is intended for the Property owner, building users and service engineers.
A detailed guide to the Installation and maintenance of this heater is supplied separately (Installation
and Maintenance Manual) and this is intended for use by the specialist commercial installers and service
engineers.

Introduction 6 MAXXfl o EVO
NORMAL CAPACITY
The capacity in litres of the in-built DHW tank is given in the table below:-
Tank size 200 litres 300 litres
Models CWH 30/201
CWH 60/201
CWH 30/301
CWH 60/301
CWH 90/302
CWH 120/302
HEATER INSTALLATION AND MAINTENANCE
This heater has been designed for use with G20 and G31 gases and is manufactured to give an effi cient, safe and
long service life. To ensure continued trouble-free operation of this heater at maximum effi ciency, it is essential that
correct installation, commissioning, operation and service procedures are carried out strictly in accordance with the
instructions given in the “Installation & Maintenance Manual” supplied separately to this guide.
Only original parts and accessories from the manufacturer may be used on this heater. Using non-approved parts may
compromise the safety of the heater and invalidate any warranty.
REGULATIONS AND STANDARDS
It is the law that all gas heaters are installed by a competent person in accordance with the regulations. Failure to
install or maintain heaters correctly could lead to prosecution.
It is in your own interest, and that of safety, to ensure that the law is complied with. The installation of the Heater
MUST also be in accordance with the current Wiring Regulations, the Local Building Regulations, Building Standards,
the Bye Laws of the Local Water Undertaking, any relevant requirements of the Local Authority.
A detailed guide to the Installation and maintenance of this heater is supplied separately (Installation and
Maintenance Manual) and this is intended for use by the specialist commercial installers and service engi-
neers.
FROST PROTECTION
This heater is fi tted with an automatic Frost Protection system. Provided there is Mains power, Gas is connected and
the heater is not in a lockout condition, this system will be eff ective in preventing damage to the heater caused by
frozen water. This protection will be active even when the controls are in the standby operation mode.
If the temperature of the DHW tank should fall below 5OC heat engine(s) will operate to bring the DHW tank tempera-
ture up to 6OC before turning off again.
If the temperature of the internal fl ow pipework (heat engine) was to drop to 3OC the heat engine(s) will operate until
the temperature registered by the heat engine return sensor reaches 16OC. This will also heat up the tank temperature
to around 16OC

Introduction 7 MAXXfl o EVO
1.0 INTRODUCTION
1 Operation switch 2 Reset switch
3 Control knob (push to enter) 4 Graphical User Interface
1.2 THE GRAPHICAL USER INTERFACE
The control knob is used to operate the GUI. The display is organised into a navigation bar, status bar, and
work area.
Home page
Hot water page Work area
Information page
Setup page
The water heater is operated by rotating the control knob to move the selector over the screen then push-
ing the control knob to select the item. When selected the item will be displayed as inverse and can then be
changed by rotating the control knob and the change is confi rmed by pushing the control knob.
If the item was selected by mistake, take your hand off the control knob (do not push the control
knob) ,wait for 5 seconds and the item will de-select and revert to its previous value.
In order to return to the previous screen select the back “arrow” or “back” text in the bottom right corner of the
screen at any time
Cascade fl ow temp 60.1 oC
DHW temp 1 58.9 oC
Cascade fl ow temp 60.3OC
DHW temp 1 58.2OC
3
4
1
2
1.1 HEATER CONTROLS

Introduction 8 MAXXfl o EVO
Not selected: The operating object is displayed normally, black on white background.
Preselected: The operating object is framed.
Selected: The operating object is inverted with white on black background.
1.3 NAVIGATE AND SET USING THE CONTROL KNOB
Operating objects may have three display states:
To go to the navigation bar:
To set values in the work area:
Operating tips:
Turn control knob.
• The preselection is displayed with a frame around the symbol.
• The related topic page is displayed in the work area.
Press control knob.
• The symbol is selected on the navigation bar and is displayed inverted.
• The fi rst adjustable operating object of the work area is preselected.
Go back using the Back arrow on the navigation bar.
• The symbol in the navigation bar is once again preselected.
Turn control knob.
• The preselection is displayed with a frame around the operating object
Press control knob.
• The operating object is selected and is displayed inverted.
• The lower level is displayed if the operating object consists of multiple levels (e.g. Time
program).
Turn control knob. Set value.
Press control knob. Confi rm the set value.
• The set operating object is once again preselected.
Continue navigation
• To other pages, for selected and inverted displayed page titles.
Back “Back” goes a level higher within the work area.
Back arrow to return to navigation bar.
Editing time-out 5 seconds A changed setting reverts to the original state if not confi rmed
within this period.
Long button
pressure
> 3 seconds A long press of the knob on any expert view returns to the “Expert
view start page” (diagnostics page).
Locking time-out 1 minute Certain plant states are displayed in the foreground, e.g. Special
operations page. However, users are still able to go to any page
and set values. The foreground page returns after this period with-
out operator intervention.
Operating time-out 8 minutes The display switches automatically after this period without opera-
tion to the start page on the operator unit or display in standby on
the room unit.

Introduction 9 MAXXfl o EVO
1.4 DISPLAY SYMBOL MEANINGS
The following symbols are located on the navigation bar (left, vertical):
The following symbols can be displayed on the status bar (top, horizontal):
Accessible for end-user and experts:
Start page: Heater status. Access to system operation mode.
DHW page. Access to temperature and time clock settings
Info pages:
• Messages (errors, events)
• Heater information
Service/setting pages:
• Setting options for domestic hot water system
• Operate special operations (e.g. for maintenance work)
• Login in expert view (see note below)
Available in addition for experts:
Diagnostic pages: Analyse and test heater and system.
Adjust/repair pages:
• Adapt parameters in ‘Complete parameter list’
Symbol ‘Alarm’ indicates a system error.
Symbol ‘Maintenance/Special operations’ indicates the presence of a maintenance message
or special operation feedback.
Symbol ‘Event’ indicates an event message from the system.
‘Hand’ symbol The ‘Hand’ symbol is displayed if the heater operation mode is being held “off ”
due to the operation switch being in “standby” position or the hot water operation mode is set
to off in the Domestic hot water page.
12:00 The device clock is synchronized with the clock from the connected controller.
Symbol ‘User’ and the number to the right (access level 1 to 3) indicate which user level is
currently active.
• None: User
• 1: Commissioning engineer
• 2: Heating engineer
• 3: OEM
Symbol ‘Producer’ indicates the water heater burner is currently operating and a fl ame has
been successfully detected.

End user 10 MAXXfl o EVO
2.0 END USER OPERATION
2.1 THE HOME PAGE
Evaluation of information displayed:
Heater GUI Display
The start page for the heater GUI unit displays the most important information on the water heater and system
operation mode. There are no settings that can be applied whilst in this screen.
2.2 SETTING THE TIME AND DATE
To set the current time and date proceed with the following steps
To set the time:
1. Rotate the control knob until the settings tab is highlighted and press the control knob. “Regional settings (1/3)”
should already be preselected
2. Rotate the control knob to select the time fi eld value and press the control knob
3. Rotate the control knob and enter a new hour value by pressing the control knob
4. Rotate the control knob and enter a new minute value by pressing the control knob
To set the date:
5. Rotate the control knob to select the date fi eld and press the control knob
6. Rotate the control knob and enter a new date, month and year value by pressing the control knob after each item
set
7. Rotate the control knob for additional regional settings or exit regional settings with “Back”.
8. It is also possible to adjust the dates set for the start and end of summer time, for the purposes of automatic
clock adjustment
Cascade fl ow temp 60.1 oC
DHW temp 1 58.9 oC
Line only displayed for
90 or 120 kW models
Regional settings (1/3)
Time
14:42
Date
09.08.2016

End user 11 MAXXfl o EVO
2.3 SETTING DHW HEATING TIME PROGRAM
In order to set the DHW heating time program the Domestic Hot Water operation mode of the heater must be
set to “ON”.
This is achieved by turning the operation switch on the front facia of the heater from standby (0) to on (I)
Select the Domestic Hot Water (DHW) tab by rotating the control knob and pressing the control knob. The operating
mode should be displayed as “On”. See example below:
With the DHW tab selected rotate the control knob to highlight the time program fi eld,
Press the control knob to enter the time clock setting page. Once the fi eld is selected rotate the control knob to reach
the time programming screen.
OPERATION SWITCH
Domestic Hot Water
Operating mode On
Temporary ...
Nominal set point 28oC
Time program 00 12 24
Domestic hot water
Operating mode On
Temporary ...
Nominal set point 28oC
Time program 00 12 24
When operation switch is set to ON (I)
DHW operating mode should be ON
If the operating mode is not ON when the
operation switch is ON, change the mode to ON
When operation switch is set to STANDBY (O)
DHW operating mode will be OFF

End user 12 MAXXfl o EVO
Select the day fi eld to change the times of that day
Turn and push the control knob to set the start and end times of the program. Scroll to the beginning (00 hours) to
insert another time band (up to a total of 3 time bands per day can be set).
To delete a time band use the control knob to move the start and end times of the band to be the same then press the
control knob.
The time settings of any day can be copied and applied to other days in the weekly schedule
After setting up the times of the day required to be copied, rotate the control knob and select copy in order to move to
the “copy to screen”.
Domestic hot water
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
00 12 24
Back
Monday
Set time program
12:00 to 18:00
00 06 12 18 24
Back
Copy
Monday
Set time program
12:00 to 18:00
00 06 12 18 24
Back
Copy

Commissioning 13 MAXXfl o EVO
Using the control knob select the day to be copied to and press the control knob. Repeat this operation as required.
Once times have been copied select “Done “ to return to Domestic hot water page.
2.4 SETTING DHW TEMPERATURE
With the operation switch on the front facia turned ON (I) select DHW tab and rotate the control knob to highlight the
“Nominal set point” fi eld and push the knob to select.
Once the fi eld is selected rotate the control knob to change the Domestic Hot Water set point. Confi rm the selection
by pressing the control knob within 3 seconds.
Domestic hot water
Operating mode On
Temporary ...
Nominal set point 60oC
Time program 00 12 24
Copy Monday to
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
Done
Copy Monday to
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
Done

End user 14 MAXXfl o EVO
2.5 SETTING DHW OPERATING MODE
Changing the operating mode is achieved using the operation switch on the front of the facia (see below).
When operation switch is set to position O (STANDBY), DHW operating mode should be OFF (see example below)
When operation switch is set to position I (ON), DHW operating mode should be ON (see example below)
NB: If the operating mode is not ON when the operation switch is set to I (ON), change the operating mode to ON
using the control knob and push to select. Test that the operating mode changes when the position of the operation
switch changes from ON to STANDBY and back to ON.
Domestic Hot Water
Operating mode On
Temporary ...
Nominal set point 28oC
Time program 00 12 24
OPERATION SWITCH
Domestic hot water
Operating mode Off
Temporary ---
Nominal set point ---
Time program 00 12 24

End user 15 MAXXfl o EVO
2.6 TEMPORARY DHW OPERATION FUNCTION
Outside of the programmed operation times for water heating, the operation of the water heater can be turned on for
a single occasion where the domestic hot water will be heated to the nominal set point. The heater control returns to
normal operation using pre-settings as soon as the hot water reaches set point.
This function is possible to activate even if the operation switch on the front facia is in the
STANDBY (O) position
Once this function has been started it can only be stopped by changing the operation switch
position 3 times. e.g. If this function has been started with the operation switch in STANDBY (O)
then it will be necessary to switch to ON (I) then back to STANDBY (O) to cancel the function.
With the icon highlighted press the control knob to select the Domestic hot water page. Using the control knob
move the indicator over the “Temporary” fi eld and press the control knob to highlight the fi eld
Rotate the control knob to change the “Temporary” fi eld from “...” to “Recharging” and press the control knob to select
the value
The heater operation in now active and will recharge the DHW tank water to the nominal set point previously pro-
grammed
It is not possible to view or change the nominal set point of the hot water once this operation has been
started. If you have any doubts as to the current DHW set point, cancel the operation (as described
above) and start the heater using the operation mode to view and set the nominal set point.
Domestic Hot water
Operating mode Off
Temporary ---
Nominal set point ---
Time program 00 12 24
Domestic hot water
Operating mode Off
Temporary Recharging
Nominal set point ---
Time program 00 12 24

End user 16 MAXXfl o EVO
2.7 INFORMATION PAGES
Information regarding the operation state of the water heater can be viewed by selecting “ ” pages . See example
screen showing the status of DHW:-
To view further pages select the “ ” page and press the control knob. The fi eld at the top of the screen can now be
selected with the control knob and rotated and selected again for the info page required. See example: -
On a 90kW or 120kW model the “Cascade” screen will display the combined fl ow temperature of both operating heat
engines and this represents the temperature of the water being charged into the tank. On a 30kW or 60kW model
“Boiler temp” represents the temperature of the water being charged into the tank.
Domestic hot water
Charging active
DHW temp 1 58.6oC
Boiler
In operation for domestic hot water
Boiler temp 62.0oC
Water pressure 1.2bar Water pressure only
available on 90 & 120kW
models
Cascade
Cascade fl ow temp 61.9oC

End user 17 MAXXfl o EVO
2.8 ERROR MESSAGES
If the heater controls are unable to operate the gas burner or it detects a undesirable condition, the controls will put
the heater into a “lockout” mode and the heater may be prevented from operating again, until manually reset. If the er-
ror detected is a minor one, the controls will allow the heater to operate normally, but in either case an error message
symbol will be displayed on the top of the GUI display and a text explanation will be displayed on the home
screen in the Information menu. Any inportant messages aff ecting the operation of the heater should be displayed
instead of the normal “Home” screen.
The error may need to be cleared before the heater will function again. Viewing the info page menu will provide the
error detail (see section 2.10 in this manual for possible error codes). Press the RESET button (located to the left of
the GUI display) and provided the error condition has been corrected the heater will run through the ignition sequence
again.
A Lockout condition should not be repeatedly reset. If the condition persists then a qualified repair
engineer should be consulted.
Example :
In this example the heater has gone into lockout mode, because the primary system pressure has exceeded 9.0 bar.
The resolution would be to rectify the overpressure condition and reset the error to restart heater operation again (see
below)
Once the causes of an error have been rectifi ed, the lockout is cleared by accessing the reset switch on the front of
the heater. A press of the reset switch of between 1 and 3 seconds will be suffi cient to reset the heater controls.
Reset rocker switch
A full list of error codes is provided in Section 2.10
Error
322: Water Pressure 3 too high

End user 18 MAXXfl o EVO
Further diagnostic codes and the history of the last 20 error messages can be found by logging in as
“Engineer” and accessing the “Error” menu. For details on how to do this please refer to
section 4.2 of this
section 4.2 of this
manual.
manual.
2.9 MAINTENANCE MESSAGES
The display will show a symbol if the heater is reporting a maintenance issue. The heater will continue to operate
as normal, but it may in future stop if the maintenance message is ignored and an error develops as a result.
Example :
In this example ‘105: Maintenance message’ is used to notify the user, the heater has been in operation for more
than 12 months and requires a service inspection. When servicing has been completed, the service counter should be
reset using parameter 7045, in the Service / special operation menu
(see section 4.4 for details)
(see section 4.4 for details)
Maintenance
105: Maintenance message

End user 19 MAXXfl o EVO
2.10 ERROR CODES
Error
Code Error Code Description Diag. Code Notes
20 Boiler temperature 1, sensor
error
Other Boiler fl ow sensor (B2) is outside normal limits
439 Boiler fl ow sensor (B2) is short-circuit
440 Boiler fl ow sensor (B2) is open-circuit
26 Common fl ow temperature
sensor error
Any Check connections or replace faulty sensor located on com-
mon fl ow pipe work up stand section (B10)
28 Flue gas temperature, sensor
error
539
543
Flue gas sensor (B8) is short-circuit
540
544
Flue gas sensor (B8) is open-circuit
40 Return temperature 1, sensor
error
Other Boiler fl ow sensor (B7) is outside normal limits
441 Boiler fl ow sensor (B7) is short-circuit
442 Boiler fl ow sensor (B7) is open-circuit
50 DHW temperature 1, sensor
error
Any Check connections or replace faulty DHW tank sensor (B3)
81 LPB short-circuit or no power
supply
Any LPB Short circuit or no bus power supply. Check operation of
OCI345 modules and connecting cables
82 LPB address collision Any LPB address duplicated on another Control PCB - Check ad-
dress of all control PCBs via parameter 6600
83 BSB short circuit Any No BSB communication or wires shorted - Check operation of
GUI or OZW672 (if fi tted)
84 BSB address collision Any Two appliances connected through BSB (e.g. OZW672) have
the same allocation address. Change one of the appliances
address via parameter 6600
98 Extension module 1, error Any AGU2.5 confi guration error - Check parameters
99 Extension module 2, error Any AGU2.5 confi guration error - Check parameters
100 More than one clock time
master
Any Only one device should be set as the time master, check on
the GUI (parameter 6640) and any OZW672 connected (in
case of connection to an OZW672, this device should be the
master)
102 Clock master without power
reserve
Any The control PCB battery backup for the time clock has been
depleted following the restoration of power to the heater. If this
message has not cleared within 10 minutes of power being
turned on, turn the power off , wait 10 seconds and turn the
power on again. This message does not prevent the operation
of the heater in any way.
105 Maintenance message Any Item requiring attention but not preventing appliance operation
(e.g. 12 Month service due). Check details of message on the
appliance Front GUI screen.

End user 20 MAXXfl o EVO
2.10 ERROR CODES (CONTINUED)
Error
Code Error Code Description Diag. Code Notes
110 SLT Lockout Others General overheating issue. Check for pump operation, trapped
air and heat exchanger blockages. Monitor temperatures of
system to establish problem area. Check that the connector
X18 (yellow looped wire) is in place on the control PCB
428
438
817
Maximum Delta T of primary system exceeded. Check that the
fl ow and return sensors are operating correctly. Check pump
operation and check for trapped air in the heat exchanger.
426
437
815
Flow temperature heat up gradient exceeded. Check pump
operation and check for trapped air in the heat exchanger.
Flow through heat exchanger may be restricted.
420
421
819
820
Return temperature greater than fl ow temperature. Check that
the fl ow sensor is operating correctly and the pump direction is
correct.
430
419
422
423
809
810
813
814
Flow temperature exceeded. Check pump operation and
check for trapped air in the heat exchanger. Flow through heat
exchanger may be restricted.
424
425
811
812
Return temperature outside plausible limits. Check return sen-
sor (B7) and replace if necessary .
422
423
809
810
Flow sensor readings are outside plausible limits. Check fl ow
sensor (B2) and replace if necessary
111 Shutdown limit thermostat Any Heat exchanger temperatures have been exceeded. Investi-
gate fl ow rates and controls to determine causes of temporary
fault code.
125 Maximum boiler temperature
exceeded
Any Flow temperatures have been exceeded. Appliance will
continue once temperatures have dropped below maximum
allowed limits.
126 DHW charging temperature not
reached
Any Charging temperature never achieved. Check operation and
heat up times for DHW
127 DHW legionella temperature
not reached
Any Check operation of legionella function and DHW system.
Legionella setpoint has not been achieved within 48 hours of
the legionella function operating. If using a secondary return
system and sensor, do not set the legionella function setpoint
too high, as the heat losses around the hot water pipework
may not permit this setpoint to be achieved
128 Loss of fl ame during operation 394 Flame lost in operation within the fi rst 7 seconds of the start
of the modulation phase. Check the CO2 is set correctly and
check operation and condition of ionisation electrode.
854 Flame lost in operation after the fi rst 7 seconds of the start
of the modulation phase. Check the CO2 is set correctly and
check operation and condition of ionisation electrode.
625 Flame lost in operation more than 24 times in a 24 hour pe-
riod. Check the preceding error codes for details on when the
ionisation current was lost. Check the operation of the ionisa-
tion electrode, gas supply, and fl ue system. Check that there is
suffi cient gas available to the heater at all times.