Immergas EOLO 21 Maior Guide

Technical documentation Technical documentation
STEM@ ed 06/01
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EOLO MAIOR @
EOLO Maior @
Instantaneous wall-hung boilers
with sealed combustion chamber
fan assisted
EOLO 21 Maior @
EOLO 24 Maior @
EOLO 27 Maior @
- GENERAL FEATURES
The EOLO Maior @ series comprises sealed-chamber boilers with forced draught (fan assisted) for
central heating and the production of domestic hot water.
The range includes electronic-ignition versions with working outputs of 21,000 kcal/h (24.4 kW), 24,000
kcal/h (27.9 kW) and 27,000 kcal/h (31.4 kW).
Adjustment and control of the boiler are entrusted to a microprocessor-controlled electronic board
thanks to which the appliance can be connected to the Remote Friend Control (optional).
The hydraulic circuit features a copper water-gas exchanger, an automatic system by-pass and a new
brass monobloc to which is fitted the hydraulic 3-way valve which ensures, according to need,
circulation of boiler water in the heating system or in a stainless-steel plated exchanger for the
instantaneous production of d.h.w.
Combustion air flow into the sealed chamber and flue extraction are ensured by a fan, the correct
operation of which is controlled by a differential pressure switch.
All the models feature as standard an antifreeze safety device that protects down to temperatures
as low as -5°C.
- MAIN DIMENSIONS AND CONNECTIONS
DIMENSIONS (mm)
HEIGHT WIDTH DEPTH
EOLO 21/24/27 850 450 350
CONNECTIONS
GAS D.H.W. SYSTEM
G U ERM
EOLO 21/24/27 1/2 “ 1/2 “ 1/2 “ 3/4 “ 3/4 “
V - power connection
G - gas supply
U – domestic hot water outlet
E – domestic hot water inlet
R - system return
M - system flow

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EOLO MAIOR @
MAIN COMPONENT PARTS
1 - Intake points(A= air) (F=flues)
2 - Fan safety pressure switch
3 - Flue extraction fan
4 - NTC probe for regulating the d.h.w.
5 - Gas valve
6 - System emptying stopcock
7 - System filling stopcock
8 - Automatic air vent
9 - Sealed chamber
10 - Expansion vessel
11- NTC limit and adjustment probe
12- Overheat. safety thermostat
13- Combustion chamber
14- Circulator
15- System water pressure switch
16- Plated exchanger
17- 3-bar safety valve
18- Automatic bypass disengage. screw
19- 3-way hydraulic valve
20- Positive signal pressure point
21- Negative signal pressure point
22- Draught diverter
23- Primary heat exchanger
24- Ignition and detection electrodes
25- Burner
CONTROL PANEL
1 - Heating temperature switch
2 - D.h.w. temperature switch
3 - Burner operating yellow indicator
4 - Green power indicator
5 - Switch 0 / Summer and CAR / Winter / Reset
6 - Boiler thermo-pressure gauge
7 – Red ignition stop indicator

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EOLO MAIOR @
EOLO Maior S @
Sealed combustion chamber fan
assisted boilers for central heating
only
EOLO 21 Maior S @
EOLO 24 Maior S @
- GENERAL FEATURES
The EOLO Maior "S" @ is a sealed combustion chamber boiler with forced draught (fan assisted)
for central heating only.
The range features two versions with electronic ignition with working outputs of 21,000 kcal/h (24.4
kW) and 24,000 kcal/h (27.9 kW).
Adjustment and control of the boiler are entrusted to a microprocessor-controlled electronic board
thanks to which the appliance can be connected to the Remote Friend Control (optional).
The appliances feature a copper water-gas exchanger and a standard antifreeze safety device that
protects down to temperatures as low as -5°C.
- MAIN DIMENSIONS AND CONNECTIONS
DIMENSIONS (mm)
HEIGHT WIDTH DEPTH
EOLO 21/24 S 850 450 350
CONNECTIONS
GAS D.H.W. SYSTEM
GERM
EOLO 21/24 S 1/2 “ 1/2 “ 3/4 “ 3/4 “
V - power connection
G - gas supply
E - d.h.w. inlet
R - system return
M - system supply

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EOLO MAIOR @
MAIN COMPONENT PARTS
1 - Intake points(A= air) (F=flues)
2 - Fan safety pressure switch
3 - Flue extraction fan
4 - Gas valve
5 - System emptying stopcock
6 - System filling stopcock
7 – Automatic air vent
8 - Sealed chamber
9 - Expansion vessel
10- NTC limit and adjustment probe
11- Overheating safety thermostat
12- Combustion chamber
13- Circulator
14- System water pressure switch
15- 3-bar safety valve
16- Positive signal pressure point
17- Negative signal pressure point
18- Draught diverter
19- Primary heat exchanger
20- Ignition and detection electrodes
21- Burner
CONTROL PANEL
1 - Heating temperature switch
2 - Red ignition stop indicator
3 - Burner operating yellow indicator
4 - Green power indicator
5 - Switch 0 / Summer and CAR / Winter / Reset
6 - Boiler thermo pressure gauge

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EOLO MAIOR @
HYDRAULIC CIRCUIT
EOLO Maior 21-24-27 @ EOLO Maior 21–24 S @
1- Minimum flow shutter (*)
2- Flow limiting device (*)
3- 3-way hydraulic valve (*)
4- Gas valve
5- D.h.w. exchanger (*)
6- NTC d.h.w. probe (*)
7- Burner
8- Primary exchanger
9 - Sealed chamber
10 - Flue hood
11 - Fan
12 - Fan safety pressure switch
13- Overheat. safety thermostat
14- NTC adjustment and limit
probe
15- Automatic air vent
16- Expansion vessel
17- Circulator
18- One way valve (*)
19- Automatic bypass (*)
20- Bypass disengag. screw (*)
21- 3-bar safety valve
22- System water pressure
switch
23- D.h.w. flow microswitch (*)
24- Filling stop cock
25- One way valve (**)
G - Gas supply
U - Hot water outlet (*)
E - D.h.w. inlet
R - System return
M - System delivery
(*) only on versions with d.h.w.
production
(**) only on version 27
PRIMARY CIRCUIT (BOILER CIRCUIT)
The primary circuit with relevant control and safety devices is started every time there is a central heating or
domestic hot water request.
- OPERATION
The heat contained in the combustion products is absorbed by the copper blades of the water-gas exchanger (8),
which in turn transfers this heat to the water circulating inside thanks to the boiler pump (17).
In the case of the models for central heating only (S), the water flows directly into the system, while in the case of
boilers with d.h.w. production, this can be deviated inside an instantaneous plate exchanger (5) made of
STAINLESS steel.
This depends on the position of the 3-way hydraulic valve (3), which when idle permits flow through the system
delivery pipe (M) and return pipe (R) while, following running of the d.h.w., flow is deviated towards the d.h.w.
exchanger (5).
- CIRCULATOR
This operates on the primary circuit return and is located on
the brass header assembly.
It is connected to the enbloc by means of a threaded
connecting piece, while the primary exchanger and expansion
vessel are connected by means of coupling pipes (2) with O.R.
seal (3), locked by means of special forks (1).
The pumps used differ according to boiler output (see head -
flow graph).

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EOLO MAIOR @
- HEAD FLOW GRAPH
The curve that represents the relation between the flow
rate and the available head depends on the pump
operating speed and on whether the system bypass is
engaged or not (optional on "S" versions)
The curves shown on the graph alongside refer to:
- A = Circulator at top speed with bypass disengaged
- B = Circulator at top speed with bypass engaged
- C = Circulator at second speed with bypass disengaged
- D = Circulator at second speed with bypass engaged
EOLO 21 Maior @
EOLO 21 Maior S @
EOLO 24 Maior @
EOLO 24 Maior S @
EOLO 27 Maior @
- PRIMARY EXCHANGER
This is a water-gas blade exchanger with pipes and fins in
copper at the outlet (delivery) of which are positioned the
NTC heating sensor (6) and the overheating safety
thermostat (2).
The size and layout of the pipes differ according to boiler
output.
In the 21 and 24 versions the four pipes making it up are
connected in series (1) while on the EOLO 27 they are
connected in parallel (7) two by two to reduce flow
resistance to the utmost and provide the greater head
required by boiler output.
It is connected to pump delivery and to primary circuit
delivery by means of coupling pipes (3) with OR seal (5)
fastened with special forks (4).

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EOLO MAIOR @
- SAFETY DEVICES AND CONTROLS
Automatic system by-pass (1)
(combined versions only)
This ensures circulation in the primary circuit even
when this is prevented by the high resistance of the
system.
It operates between the supply and return of the
primary circuit and is accessible after the three-way
enbloc in brass into the two sections of which it is
made.
It can be disengaged (closed) by turning the bypass
disengagement screw (2) clockwise.
In the case of models for central heating only (S), the
bypass is optional and is fitted between the system
delivery and return connection pipes.
System filling unit (5)
This stopcock is positioned between the boiler circuit
and the cold water inlet and makes it possible to
pressurise the heating system.
It is housed in the lower part of the enbloc to which it
is screwed up.
In the case of version 27 only, downstream of the
stopcock, an one way valve is fitted (6)
(disconnector) that prevents accidental flow of water
from the primary circuit to the water network (d.h.w.
circuit).
System water pressure switch (3)
This reads the pressure inside the primary circuit.
It is housed in the enbloc and is coupled to a
microswitch that prevents boiler operation when the
read pressure is below 0.3 bar.
This way overheating of the main exchanger is
prevented.
3-bar safety valve (4)
This prevents safety pressure being exceeded in the
circuit (3 bar).
It is fastened in the front part of the enbloc by means
of a socket-head screw.
When this engages, water exits from the delivery pipe.
Automatic air vent (7)
For automatically expelling any gaseous substances in
the circuit.
Fitted on the water side of the expansion vessel.
Expansion vessel (8)
Compensates changes in volume following heating of
water and permits reducing pressure changes.
This is positioned in the rear of the appliance and is
connected to the pump by means of a coupling pipe (9).
It has a pre-load pressure of 0.8 bar and a capacity of 8
l in versions 21-24 and 10 l in version 27.

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EOLO MAIOR @
HYDRAULIC CIRCUIT
SECONDARY CIRCUIT (D.H.W. CIRCUIT)
The models with instantaneous production feature a secondary hydraulic circuit that starts up only when
domestic hot water is run.
- OPERATION
Running the hot water results in transit of cold water inside the 3-way valve (1) and its consequent shift
to operating position (see 3-way hydraulic valve operation).
As a result, the return pipe (R) closes and the transit way is opened to the d.h.w. exchanger (2)
This way, circulation in the central heating system is prevented while this becomes possible in the plate
exchanger (2), inside which the cold water absorbs the heat in the primary circuit (see d.h.w. exchanger).
In this phase, the central heating function is disengaged and priority is given to the production of d.h.w.
- DOMESTIC HOT WATER EXCHANGER
This is a water-water exchanger consisting of
STAINLESS-steel plates placed one on top of the other
(15 in version 21 and 19 in version 24, 21 in version 27)
on the surface of which runs in counterflow the water of
the boiler circuit and the cold water for the d.h.w., and
through which the heat exchange occurs between the
two liquids.
To reduce mineral deposits to the utmost at the hot water
inlet, a single-acting valve (2) is fitted that prevents the
exchanger temperature from rising during operation in
central heating phase.
Hydraulic coupling is achieved by means of four O.R.
rings (1) for direct coupling to the 3-way enbloc to
which the exchanger is fastened.

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EOLO MAIOR @
- HYDRAULIC 3-WAY VALVE
Whenever there is a d.h.w. request, the valve deviates primary circuit flow from the heating system to
the plate exchanger.
The component part is an integral part of the 3-way brass enbloc and comprises an area where the transit
of d.h.w. is detected and another where, following this, deviation occurs to the boiler circuit.
The core of the valve consists of a noryl assembled sealing unit (10), which can be removed from the
enbloc after removing the cover (11) and diaphragm (1).
In the case of the Maior @series, both the assembled cartridge used during production and the pre-
assembled cartridge used as a spare are interchangeable with those fitted on previous MAIOR versions,
despite rods (8 - 9) being used with 3 mm diameter and O.R. rings in EPDM peroxide that give these
greater resistance to mechanical and chemical stress.
At the cold water inlet is a flow limiting device (5) the value of which differs according to appliance
output (8 l/min in 21e versions, 10 l/min in 24 versions and 12 l/min in 27 versions).
Idle position (central heating)
The thrust of the spring (4) keeps the shutter (3) tight
so the opening towards the d.h.w. exhanger is closed
and, at the same time, the system return pipe (R) is
opened.
Following a central heating request, pump operation
enables circulation in the system through the delivery
pipe (M) and return pipe (R).

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EOLO MAIOR @
- Operating position (domestic hot water)
Running the domestic hot water generates a flow
inside the valve that invests the d.h.w. valve (6).
This represents a constriction (venturi pipe)
downstream of which the pressure of the water is
lower than upstream.
With a flow of at least 1.85 l/min and a dynamic water
pressure of 0.4 bar, the pressure difference between
the two points is such as to cause the movement of the
diaphragm (1).
The movement is conveyed to the rods (8-9) and plate
(2) which first of all cause shutter (3) to move and
then the d.h.w. microswitch (7) to close with
consequent powering of the pump and modulation
board.
In this condition, the return pipe (R) is closed, while
the opening is open towards the exchanger inside
which the water of the primary circuit circulates.
When the d.h.w. supply has terminated, the pressure
difference upstream and downstream of the d.h.w.
valve (6) is cancelled out, enabling the valve to return
to idle position.

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EOLO MAIOR @
GAS CIRCUIT
The circuit consists of an atmospheric burner and of a modulation type valve which permit gas
combustion and flow adjustment respectively.
- OPERATION
When the main coils (3) are energised, both inner valve shutters open, allowing the gas to flow towards
the burner.
The flow rate/outlet pressure is regulated by means of the gas valve stabiliser and the modulation coil
(1).
By means of the burner nozzles (7), the fuel is injected into the venturi pipes (ramps) inside which the
air-gas mix is obtained that is ignited by the spark from the ignition electrodes (5).
- MODULATING GAS VALVE
The gas valve (Honeywell VK 4105 M) features two main coils controlled by the electrical circuit
(ignition unit) and a third controlled by the modulation board.
The maximum and minimum outlet pressure settings can be made on this valve (see gas adjustments).
Main electrical coils (3)
Two coils of the ON-OFF type are energised (230V
AC) by the ignition unit when the burner has to be
ignited.
These are electrically connected in series and
energised by rectified mains power through a diode
bridge inside the special connector (2).
Modulation coil (1)
This is a low-voltage coil controlled by the modulation
board.
It controls the gas valve stabiliser and permits
changing the outlet pressure in a way proportionate to
the DC current that runs through it.
Honeywell VK 4105 M
- BURNER
The burner consists of horizontal venturi pipes (6) in which the gas is injected by an equal number of
nozzles (7) mounted on the special header (8).
The number of nozzles, the diameter of which changes according to the gas used (see technical details)
is 13 in the 21,000 kcal/h version, 15 in the 24,000 kcal/h version and 16 in the 27,000 kcal/h version .
Ignition occurs by means of an electronic board (unit) which controls the ignition electrode (5) and
detection electrode (4).
Ignition electrodes (5)
These are controlled by the ignition unit which
produces an electrical charge between the two
electrodes.
They are positioned on the front of the burner in line
with the centre ramp.
Detection electrode (4)
This is controlled by the unit and detects burner
ignition.
It is positioned on the front of the burner on the ramp
alongside the ignition electrodes.

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EOLO MAIOR @
GAS ADJUSTMENTS
Max and minimum gas pressure adjustments
can be made by means of the gas valve
respecting the values shown on the tables
relating to each boiler for the type of
corresponding gas.
Measurement is made using a pressure gauge
connected to the outlet of the gas valve (6)
and to the positive pressure point located on
the upper part of the sealed chamber.
- VALVE VK 4105 M (with yellow protection cap)
Minimum pressure adjustment
- After interrupting the power supply to the
modulation coil (*), turn screw "3"
clockwise to increase pressure to the burner
and anticlockwise to decrease it.
Maximum pressure adjustment
(operation to be performed after minimum
pressure adjustment)
- Draw d.h.w. after setting the temperature
switch at maximum.
Turn nut ”4” clockwise to increase pressure
to the burner and anticlockwise to decrease
pressure.
1) Modulation coil 5) Yellow protection cap
2) Coil pin 6) Gas valve outlet press. point
3) Min. output adjust. screw 7) Valve inlet press. point
4) Max. output adjust. screw 8) Air press. point hose connector
- - VALVE VK 4105 M-M with integrated flanges (with transparent protection cap)
Maximum pressure adjustment
- Draw d.h.w. after setting the temperature
switch at maximum.
Turn nut ”3” clockwise to increase pressure
to the burner and anticlockwise to decrease
pressure
Minimum pressure adjustment
(operation to be performed after maximum
pressure adjustment)
- After interrupting the power supply to the
modulation coil (*), turn screw "4"
clockwise to increase pressure to the burner
and anticlockwise to decrease it.
1) Modulation coil 5) Transparent protection cap
2) Coil pin 6) Gas valve outlet press. point
3) Min. output adjust. screw 7) Valve inlet press. point
4) Max output adjust screw 8) Air press. point hose connector
(*) only on boilers featuring B&P modulation boards,when the modulation coil is detached, LEDs H1
(yellow) and H2 (red) flash alternately

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EOLO MAIOR @
- CHANGING TYPE OF GAS
Adaptation to a type of gas different to that for which the boiler is intended are standard and can be done
by using the special kits (natural gas or LPG).
The changeover consists in replacing the burner nozzles and in switching to the modulation board of the
“NATURAL GAS- LPG” (S1) bridge.
The minimum and maximum pressures are then set on the gas valve in the way described above.
Adjustment of the minimum and maximum output (version 27 only) during the heating phase and of
burner ignition pressure (see table below) can be done by means of the respective trimmers mounted on
the modulation board (see modulation board operation).
Slow ignition values (mm H2O)
Natural gas LPG
EOLO 21 Maior @ / 21 Maior S @ 25 60
EOLO 24 Maior @ / 24 Maior S @ 25 60
EOLO 27 Maior @ 23 70

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EOLO MAIOR @
FLUE CIRCUIT
- OPERATION
The combustion residues, after investing the water-gas exchanger (1), are conveyed to a hood (3) at the
top of which is a draught diverter (4).
Operation of the fan ensures forced extraction of the flues and at the same time creates a vacuum in the
sealed chamber (2) that permits extraction of the combustion air from outside.
Correct flue extraction is controlled by a differential pressure switch (5), operation of which enables or
does not enable burner ignition.
- AIR/FLUE SAMPLING SUMPS (6-7)
The top outer part of the chamber features two
sumps with screw closing accessible from the front
and through which combustion air (6) and flues (7)
can be extracted.
- FLUE PRESSURE SWITCH SIGNAL
PRESSURE POINT (8-9)
The top outer part of the sealed chamber features
two pressure points with screw closing for
measuring the signal at the ends of the flue pressure
switch (5).
The negative pressure point (8) is linked through a
Y-pipe to the top of the draught diverter and to the
negative pressure point of the flue pressure switch.
The positive pressure point (9) is directly linked to
the inside of the sealed chamber.

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EOLO MAIOR @
- FLUE PRESSURE SWITCH (5)
This is positioned in the top inner part of the sealed chamber and, by means of the points provided, reads
the difference in pressure between the top of the draught diverter (the point where the extractor is
positioned and operates) and the inside of the sealed chamber.
The signal measured from the pressure switch varies according to the length of the extraction/exhaust
terminals and is measured by the special pressure points fitted to the top part of the sealed chamber (8-
9).
Its operation causes a deviation of a switch contact that acts on the unit (IGN. BOARD) and enables or
fails to enable burner ignition.
- FAN (4)
The extractor operates downstream of the combustion chamber and is physically "rested" on the top part
of the hood (3) from which it extracts fumes into the exhaust pipes to which the boiler is linked.
It is controlled from the ignition unit and its operation coincides with that of the boiler.
The fans used differ according to the output of the appliance (21,000, 24,000 and 27,000 kcal/h).

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EOLO MAIOR @
EXTRACTION AND EXHAUST SYSTEMS
(see extraction and exhaust end instructions )
The EOLO Maior @ series of boiler are ready for connection to the special coupling type extraction and
exhaust pipes.
- EXHAUST
Connection to the exhaust pipes is done by means of a flange
(1) or a flanged curve to be fastened to the union (4) on the top
part of the sealed chamber after placing in between a special
shaped seal (6).
The flange differs according to whether the doubled or
concentric system is used. In the former case, the opening for
extraction of combustion air (5) is closed, while in the latter
case, this is used.
For correct boiler operation, a diaphragm (7) must be fitted on
the exhaust (4) in between the flange used (1).
Diaphragms of different diameter are available which must be
fitted according to the type of pipe and its length (see boiler
instruction booklet).
- EXTRACTION
Using the doubled system, connection to the extraction
pipes is done in the same way as the exhaust pipes, by
connecting up to one of the two 80 mm diameter holes (2)
on the top part of the sealed chamber. The hole which is not
used is closed by means of one of the special caps (3)
equipping the boilers.
If coaxial pipes are used, extraction is done through the concentric hole outside the exhaust pipe (5).
- EXTRACTION EXHAUST KIT
The kits and relevant accessories make it possible to use four concentric systems and two doubled systems.
As regards pressure drop relating to each accessory, the various possible combinations and the use of the
diaphragms to be fitted according to the length of the pipes, see the instructions concerning the extraction
and exhaust ends (boiler instruction booklet). The accessories (curves, extensions and end pieces) are of the
coupling type and seal is provided by special lip seals.
Coupling type horizontal concentric kit 60 /100
The exhaust pipe (φ60 mm) is fitted inside the extraction
pipe (φ100 mm).
Connection to the boiler is made using a 90° curve (1)
that can be positioned in any direction and which, by
means of the necessary extensions, must be connected to
the special extraction and exhaust end (2).
Max possible overall length beyond the first curve (1) is 3
straight and horizontal metres.
Coupling type horizontal concentric kit 80 /125
The exhaust pipe (φ80 mm) is fitted inside the extraction
pipe (φ125 mm).
Connection to the boiler is made using a 90° curve
diameter 60/100 (1) that can be positioned in any
direction and which, by means of the adapter 60/100-
80/125 (2) and necessary extensions, must be connected
to the special extraction and exhaust end (3).
Max possible overall length beyond the first curve (1) is
7.3 straight and horizontal metres.

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EOLO MAIOR @
Coupling type vertical concentric kit 60 /100
The exhaust pipe (φ60 mm) is fitted inside the
extraction pipe (φ100 mm).
Connection to the boiler is made using a flange (1)
that, by means of the necessary extensions, must be
connected to the special 60/100 extraction and exhaust
end with aluminium tile.
Max possible overall length is 4.7 straight and
vertical metres.
Coupling type vertical concentric kit 80 /125
The exhaust pipe (φ80 mm) is fitted inside the
extraction pipe (φ125 mm).
Connection to the boiler is made using a flange (1)
that, by means of the adapter 60/100-80/125 (3) and
necessary extensions, must be connected to the special
80/125 extraction and exhaust end with aluminium tile
(2).
Max possible overall length is 12.2 straight and
vertical metres.
80/80 coupling type separator kit
Both pipes have a diameter of 80 mm.
Connections to the boiler are made using two special
flanges for exhaust (1) from the centre pipe and
extraction (2) from one of the two side holes.
Max possible length (extraction + exhaust) is 33
straight horizontal metres and 41 straight vertical
metres.
To prevent condensation problems, the exhaust pipe
should not be more than 5 m long
Insulated 80/80 coupling type separator kit
Both pipes have a working diameter of 80 mm.
Connections to the boiler are made using two special
flanges for exhaust (1) from the centre pipe and
extraction (2) from one of the two side holes.
Insulation is obtained thanks to special seals (3)
whereby an air space can be created by means of a φ
125 mm external concentric pipe.
Max possible length (extraction + exhaust) is 33
straight metres.
To prevent condensation problems, the exhaust pipe
should not be more than 12 m long

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EOLO MAIOR @
ELECTRICAL CIRCUIT
The electrical circuit of the EOLO MAIOR @ boilers is controlled by an electronic board that controls
all the boiler functions and to which the electrical devices on the machine are connected..
Some of the control and safety devices operate at mains voltage and others at low voltage.
230 V AC CIRCUIT
SAFETY DEVICES AND CONTROLS
Detection electrode
(E3) This detects ignition of the burner by whose flame it is invested.
It is connected to the detection circuit of the ignition unit
Fuse This interrupts power to the circuit when power input is over 3.15 A.
It is fitted on the modulation board. Fuse
3.15 A 250 V
Flue pressure switch (S4) This controls the ignition unit and enables burner operation when fume
extraction occurs correctly.
If this is closed with the fan off, the ignition cycle will not start.
Two-position
switch
Overheat. safety device
(E4)
When the safety threshold is exceeded (115 ºC) this interrupts power
to the main coils of the gas valve (Y1).
It is located at the outlet of the primary exchanger.
Double-contact
clicson thermostat
LOADS
Ignition unit
(IGN. CARD)
This is controlled by the modulation board when burner ignition is required.
It controls the devices needed for ignition (fan, main gas valve coils, ignition electrodes)
and flame detection (detection electrode).
The power cables of the boiler envisage use of the B&P unit.
The use of a Brahma unit will require switchover of the two connection leads of the reset
button of the master switch S5.
NOTE. the instructions for performing this operation are shown on the EOLO Maior @
wiring diagrams in the “Wiring diagram” folder.

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EOLO MAIOR @
Circulator (M1) This is powered by the modulation card whenever there is a demand for d.h.w., heating
or antifreeze.
Permits circulation of water in the primary circuit.
Ignition electrodes
(E1 - E2) These provoke an electric spark that ignites the air-gas mix.
They are controlled from the ignition unit (IGN. BOARD).
Modulation card This is powered when the main switch is positioned on (summer) or
(winter) (see modulation board operation).
Gas valve (Y1)
(main coils) This is powered by the ignition circuit when the burner has to be ignited.
Permits flow of gas to the burner.
Fan
(M2)
Ensures air flow inside the sealed chamber and exit of combustion flue.
Powered by the ignition card.
LOW-VOLTAGE CIRCUIT
SAFETY DEVICES AND CONTROLS
Zone control unit (CZ)
(external optional)
Permits boiler operation with 3 area valves or external pumps
controlled by the respective room thermostats.
Area 1 controls (times and temperatures) through the CAR.
See control zone
unit operation
Remote control (CAR)
(external optional)
Permits remote control of boiler (SUM/WINT switch, temperature
adjustment and indication, alarm display, reset, etc.) and acts as a
weekly chronothermostat.
See Remote
Friendly Control
Operation
(CAR)
Depending on position, permits :
Main switch
(S5)
0
RESET
-low-voltage circuit not powered
-d.h.w. operation (combined models only) and CAR
(optional)
- d.h.w. and central heating operation
- release after tripping of ignition failure, temperature and
stack safety device (the RESET position works at the mains
voltage by closing a contact that controls the ignition unit)
3 position double
switch + reset
button
D.h.w. switch (S6)
(combined models only)
Following an hot water request, this affects the adjustment board,
causing the pump (M1) to operate and enabling the burner pump.
When idle, it enables operation in central heating mode.
Two-contact
switch
System water pressure
switch
(S7)
This affects the modulation board and interrupts power to the circulator
(M1) and to the ignition unit when the pressure in the boiler circuit is
below 0.3 bar.
Two-contact
switch
Central heating sensor
(B1)
Permits the modulation card to read the temperature of the delivery
water in the primary circuit.
If this breaks, burner operation will be stopped in both central heating
and d.h.w. mode.
This is located at the exit of the main exchanger.
NTC sensor
10 kohm 25 ºC
D.h.w. sensor (B2)
(combined models only)
Permits the modulation board to read the outlet temperature of the
d.h.w.
If this breaks, burner operation will be stopped in d.h.w. mode.
This is located in the d.h.w. pipe at the plate exchanger exit.
NTC sensor
10 kohm 25 ºC
LOADS
Modulation coil
(Y2) This is powered by the modulation board with variable DC current.
Permits changing the gas pressure to the burner.

Technical documentation Technical documentation
STEM@ ed 06/01
20
EOLO MAIOR @
ELECTRICAL CIRCUIT
CENTRAL HEATING PHASE
(*) the main coils of the gas valve run at mains voltage rectified by means of a diode bridge (U1) only on the
Honeywell VK 4105 gas valve (diode bridge fitted on the valve connection connector)
Operation with room thermostat
When the main switch (S5) is in WINTER position, it powers the modulation board and enables
operation in central heating mode.
When the room thermostat (TA) contact is closed, the low-voltage circuit starts the pump (M1)
through relay contact K1.
If the system water pressure switch (S7) reads a pressure in the primary circuit above minimum
value (S7 switch closed) and if the temperature read by the NTC heating sensor (B1) is below the
setting made on the control panel by means of the heating potentiometer, the board closes the
contact of the requested relay (K2).
This way, the ignition unit (IGNITION CARD) starts the ignition cycle by first checking the correct
position of the flue pressure switch (S4) and then powering fan (M2).
After the contact of flue pressure switch (S4) has been deviated by fan operation (M2), the unit
(IGN. CARD) powers the ignition electrodes (E1-E2) and, with the consensus of the overheat. safety
thermostat (E4), the main coils of gas valve (Y1).
Burner ignition is detected by the unit by means of the ionisation electrode (E3).
Operation with Remote Friendly Control
The min switch (S5) in "SUMMER and CAR" position powers the modulation board and remote
control (CAR).
If the conditions found by the remote control (CAR) require ignition in central heating phase
(SUM/WINT switch in WINTER position, central-heating temperature adjustment above that read
by the central-heating sensor (B1), room temperature adjustment above that read), the board
powers pump (M1) through relay K1 and ignition occurs as described above.
NOTE: In both cases, at each switch-off due to temperature having been reached, the modulation
board stops burner operation during central-heating phase for 180 sec., which can be reduced to 30
seconds by intervening on the special bridge “S2” (see modulation card operation).