The joy of being a Heating Engineer

 

GC No. 47 313 11 (with clock) 
GC No. 47 313 12 (without clock)

Made in France and marketed in the UK under the Hepworth Group flag, this combi is similar to the earlier SD623 boiler but has cosmetic changes to the control panel door and a printed circuit board (PCB) that features more low-voltage controls. The Thelia 623 has a fan-assisted flue that can be arranged rearward.

Using a simple gas control with a permanent pilot and thermoelectric flame supervision, the boiler has a two in one form of heat 
exchanger and is range-rated from 8.8kW to 23.4kW. Domestic hot water flow rate is 11.0 litres/minute at a temperature rise of 30°C.

GENERAL

Chassis

The chassis consists mainly of a brown enamelled air casing with framework extending downward to enclose the gas and water section components and support the control panel. Two rearward-facing stand-off ‘legs’ at the upper rear corner of the chassis hook onto a wall-mounted plate so that, together with a wall mounted services connecting plate at the lower edge of the chassis, an insulating space is formed in which is fitted a six litre expansion vessel.

Removable side panels hook over tabs at the top of the chassis and are each secured at the bottom by one screw. The front panel also 
hooks over tabs but doubles as an air-casing cover by fitting down over the rubber sealed edge of the case and then being secured at 
the bottom over two spring clips. Plastic side trims are attached to each side panel during installation. A safety strap attached to the top of the front panel is located around a flue gas test point fitted on the upper face of the boiler (see Figure 11).

The air casing is divided by a fixed shelf with an integral flue hood on which the fan is secured by two tabs and screws. Perforations in the shelf allow intake air to the combustion section. Although supplied as a back outlet for a rear or side flue, the fan can be repositioned to exit from the upper face of the boiler for a vertical side or rear flue (see Installation’).

Fan

When there is no call for heat the 240V fan operates at low speed via a large ceramic resistor clipped to the inside of the chassis.

Air-pressure switch

Also clipped to the inside wall of the chassis, the air pressure switch senses positive pressure from the air intake duct via a silicone tube but reacts to the negative pressure (no tube) created by the fan operating at full speed. A blocked intake duct or lighting themain burner without the air casing fitted will prevent switch change-over.

Combustion section

The front and side walls of this ceramic-fibre lined chamber form part of a ‘U’-shaped cover that is retained by two wing nut-secured side rods and two toggle fasteners. The rear lining is located between the main burner and the heat exchanger. The fourteen-blade burner has a die-cast manifold with a centrally
located pilot, electrode and flame supervision device (FSD) thermocouple. A burner 
pressure test point projects downward through the air chamber.

The pilot burner tube, which is located and spring clipped into a slot on the manifold, produces a two-pronged aerated flame that straddles the thermocouple. An overheat thermostat on the heat exchanger outlet pipe is connected onto the FSD thermocouple to act as an interrupter in the event of a temperature in excess of 102°C. A heat shield plate located in front of the burner assembly protects the front air case from localised heating.

WATER SECTION

Heat exchanger

The finned-copper exchanger consists of a two-pass, large bore core’ for the primary system, with each pass containing a two-pass length of small bore tubing for the domestic hot water (DHW) system. A capped air bleed pipe incorporating an otherwise loose ball valve is fitted on the primary section. The flow pipe features a DHW thermistor, an overheat limit switch (see below) and an overheat thermostat for the FSD system (see ‘Gas Control’).

Reversing (diverter) valve

Fitted in the lower left-hand corner of the chassis, this is a control panel lever actuated valve that controls the primary system flow and return. Selecting the lever to the right-hand (Winter) setting allows full primary system flow and return for central heating (CH) and allows the valve position to be changed by the action of a ‘water valve’ to gain DHW. On demand for DHW, the water valve closes’ the reversing valve to convert the primary system into a closed circuit within the appliance, temperature then being monitored by a 
DHW thermistor fitted in the primary outlet from the heat exchanger. Selecting the Summer position closes the reversing valve as a semi-permanent feature so that only DHW is available.

Water valve

This component is in reality only a mains flow-sensing diaphragm that pushes the reversing valve into the DHW mode when a draw-off tap is opened. It is fitted between the appliance connecting plate and the reversing valve and secured by a single screw accessed by removing the microswitch assembly from the reversing valve. The assembly features a restrictor valve that allows any pressure to be relieved back to the DHW expansion vessel (see Figure while the draw-off tap(s) are closed. The diaphragm is renewable.

Microswitch assembly

Operation of the mode selector lever or the water valve controls the position of two numbered microswitches fitted in a block on top of the reversing valve.

No. 1 - Closed in the Winter (CH + DHW) position of the lever to ensure that the pump and main burner will operate when a call for either CH or DHW is received. Open in the Summer setting until closed by action of the water valve when a draw-off tap is opened.

No. 3 - Remains open in either setting of the selector lever until a draw-off tap is opened This starts the pump, increases fan speed and changes the temperature controlling circuitry from the default (CH) mode to the DHW mode.

Pump

The pump has a single-speed 240V AC Grundfos UP 15-40/SD motor and a plastic scroll assembly incorporating an automatic air vent. The pump is bracketed to the reversing valve, a guide on the rear end of the pump locating into a rubber grommet in the chassis. Inlet and outlet connections are each sealed with an ‘O’-ring and secured with a retaining plate and wire clip respectively.

A spring-loaded pump bypass valve is fitted on the bottom of the reversing valve body that allows a partial bypass within the primary circuit to control the pump output. Adjustment can he made by turning the hexagon-headed screw clockwise to decrease the bypass.

Loss of water switch

Fitted in the left-hand side of the reversing valve, this pressure sensor-operated microswitch isolates all electrical power to the control 
circuits if the primary system pressure falls below the safe operating minimum.

Pressure-relief valve

Located on the connecting plate at the rear of the boiler, the valve is set to relieve system pressure in excess of 3 bar. To speed up system draining, the valve can be opened by first prising off the red cover and then levering the valve spindle up with the blade of a suitable screwdriver.

DHW thermistor

This is a PTC-type (positive temperature coefficient) resistor fitted in the outlet pipe of the heat exchanger. Resistance at 25°C should 
be 8 -15 Ohms rising to 40 Ohms at 75°C.

Limit thermostat

Fitted on a plate clamped to the heat exchanger outlet pipe, this plate-type switch will open at a temperature of 92°C, automatically remaking on cooling at 60°C (see Operation’).

GAS CONTROL

This comprises a Saunier Duval multifunctional control with a thermoelectric FSD ana 24V DC solenoid-operated main valve. The clear plastic push-button box’ attached to the gas control features a black and green ’start’ button and a red ‘off button. Pushing the start button opens a spring-tensioned safety valve in the neck of the inlet connection which in turn opens a spring-tensioned check valve in the connecting plate fitting. FSD action will hold the valves open when the pilot is lit. Pushing the off button will trip the holding mechanism to close the inlet valves. In addition to opening the gas inlet valves, operating the start button makes/breaks electrical contacts in the push-button box (see ‘Operation’).

The gas control is essentially a spring closed valve that can be biased toward fully open by a variable current applied to the solenoid coil. When initially energised in response to a call for heat, the solenoid only partly opens the main valve to produce a low burner flame light-up. After two seconds the gas rate will either continue at a low rate or go to full rate – all depending on the sensed temperature of the boiler control thermostat or the DHW thermistor (see ‘Operation’). The control is factory-set and adjustment is not recommended.

Ignition generator

Fitted on the rear inside face of the chassis, this is a 240V AC-powered generator feeding a conventional spark electrode that operates for approximately seven secondwhile the start button is depressed.

USER CONTROLS

Control panel

Fronted by a protective cover, the control panel is secured to the chassis by two quarter-turn fasteners, but it can be lowered to give full access to all gas and water components. The single PCB is housed in a hinged cover on the inside face of the panel, together with a 
transformer and the boiler thermostat.

Timer

This 24-hour digital clock/timer can be operated manually and/or programmed with up to six on/off settings. A ‘hold’ facility allows the 
set programme to be held on or off for up to 27 days, after which the programme will continue normally. The clock is powered by a low voltage from the PCB but has a backup battery on its rear face to retain any programmed memory while mains power is isolated. 
Instructions for normal use are given on the control panel cover, but to override any set programme for servicing, set the vertical slide switch to Timer’. The readout will show an ‘h’ (meaning hours), then press the on/off button to show ‘ON’.

Selector lever

Selects either a Winter (CH+DHW)ora Summer (DHW only) mode – see ‘Reversing (diverter) valve’.

Temperature/pressure gauge

This is a dual gauge indicating primary system readings. The very short capillary of the pressure gauge locates directly into the reversing valve body (requiring system drainage for removal) while the temperature gauge sensor locates into a dry pocket on the flow pipe.

Boiler thermostat

This is a dual, two-stage thermostat with a control range of approximately 30°C to 90°C. The control has separate but overlapping 
operating ranges that can be identified by a lower and an upper range switch that can be heard to click while rotating the control knob. With the control in the lower range the thermostat acts initially as a burner on/off switch and controls at a low gas rate. This is particularly useful in reducing the efficiency drop that can result in a low range-rated boiler. The upper range provides control at full 
gas rate.

On/off push buttons

For lighting or extinguishing the pilot burner see ‘Gas Control’.

OPERATION

Pilot

When the ‘Start’ button is pressed to light the pilot, the FSD safety inlet valve and the check valve in the connecting plate fitting are pushed open. At the same time, the upper set of contacts on the green push-rod make and hold mains power (via the loss of pressure switch) to the fuse on the PCB and on to energise the transformer. This then powers the low- voltage circuits on the PCB which in turn cause the fan to run at full speed and start a ten-second lighting delay period while the fan purges the combustion section. One contact (on the black push-rod) is held open at this time to prevent the gas control valve solenoid being energised, while another contact closes to provide a neutral for the fan relay and to kick-start the pump to ensure future operation. When the air-pressure switch contacts have changed over and the purging delay is complete, the ignition generator will energise for about seven seconds. If the pilot fails to light it will then be necessary to abort the start procedure and wait three minutes before re-attempting the start.

With the FSD satisfactorily energised and the start button released, both inlet gas valves remain open and the upper set of contacts remain closed. However, the lower set shunts the fan to operate via its resistor at low speed, stops the pump and reinstates the gas control circuit ready for a call for heat.

Heating

With the mode selector lever in the Winter setting and the room thermostat calling for heat, the pump will start and the fan will switch to full speed. When the air-pressure switch contacts change over, the gas control solenoid valve is energised open to light the main burner – the opening current being limited for two seconds to produce a low flame output. Thereafter the current is dictated by the setting of the boiler control thermostat as follows:

• a) control knob set between minimum and first ‘click’ – low flame output

• b) control knob set between maximum and second ‘click’ – high flame output.

The thermostat will extinguish the burner when the set temperature is reached, the fan reducing to low speed, but the pump continuing to run until cancelled by the room thermostat. If set in the upper range of control, the boiler thermostat will retire the boiler when the temperature falls below its operating tolerance, but if the thermostat is operating in the lower range of control, relighting will be delayed by an anti-cycling device. 

Note: Since this delay cannot be overridden to save time, servicing should preferably be carried out with the thermostat set in the upper range of control.

DHW

Whenever a draw-off tap is opened – whether operating in the Summer or Winter mode – the water valve will respond by pushing the 
reversing valve to close the heating return line. As previously described, this action also closes No. 3 microswitch on the reversing valve which results in starting the pump, switching the fan to full speed and bringing the DHW thermistor into the control circuit. Temperature is then controlled by gas valve modulation until the tap is closed. Maximum hot water temperature is 60°C.

General notes

If the air-pressure switch does not change over when the fan goes to full speed, the main burner will not light. If the switch reverses during normal operation, the burner will extinguish but will then automatically relight if the pressure switch changes over again.

Loss of primary system water pressure will shut down the boiler and prevent all further operation until the pressure has been re-established and the pilot relit.

An overheat condition in excess of 92°C will open the limit switch on the heat exchanger flow pipe. This will cause a reduction in fan 
speed and consequent burner outage. On cooling, the switch will remake at 60°C to relight the burner if a call for heat still exists.

If the flow pipe temperature exceeds 102°C the overheat thermostat will cause the thermocouple FSD circuit to be interrupted. This will cause total shut-down of both gas and electrical supplies, necessitating re-lighting the pilot.

INSTALLATION

Using the template, mark out and cut the flue hole and fit the boiler mounting and services connecting plates. 

Note: The flue must have a downward fall of about 35mm/m from the boiler to the terminal.

Saunier Duval recommend installation of both a bypass valve and a flow control valve as shown. Connect electrical power to the terminal block, flush the heating system and purge the gas supply.

Flue

The boiler is supplied ready with the fan to exit from the rear wall – to go directly rearward or via an elbow to either side. Alternatively, 
the fan can be removed, repositioned on its backplate and refitted to exit through the top of the boiler. Via an elbow, the flue can then project rearward or sideways, or with a concentric flue kit is suitable to exit vertically through a flat or pitched roof.

Prior to installing the boiler, the two W clamps must be removed from the rear face of the chassis and then refitted as follows:

• a) Rear flue from rear face -fit short concentric connector onto fan and secure with W clamps.
• b) All other configurations – fit elbow (or vertical connector on top face only) and secure with W clamps.

Rear or side flues can be extended up to 2.9m and are suitable for high-rise flat fitment.

Permitted vertical flue arrangements and lengths are as shown. 

Note: Although this is not a condensing boiler, the vertical flue is fitted with a drain trap to cater for rare occurrences of condensation. Any water in the flue drains into an internal gutter formed in the boiler to flue socket and then via a stub pipe to the drain trap. This should then be led to an internal/external drain or an external soakaway.

Fitting

Remove the upper front and side panels before lifting and hanging the appliance onto the two hooks of the mounting plate.

Before lowering toward the connecting plate, insert the metal filter (supplied) into the CH return fitting on the mounting plate. Also insert the white plastic filter into the mains fitting on the connecting plate and the flow restrictor into the mating coupling on the boiler (this coupling is telescopic to aid fitment and allow in-situ removal during servicing). Lower the boiler and couple the gas connection first as it has no flexibility.

 

When fitting the terminal take care to ensure correct fitment of the flue pipe within the air duct as shown.

COMMISSIONING

Filling

With the electrical supply isolated and the mode selector lever set to Winter, fill and vent the primary system to 1.5 bar. Hold a container under the heat exchanger air bleed and slacken, but do not completely remove the cap or the small ball may be lost. Tighten the cap when all air has been bled. Open the pump automatic air-vent cap two or three turns and leave open.

• Fill the DHW system by opening draw-off taps until an air-free flow is obtained.
• Purge the pilot burner supply pipe by disconnecting the burner end of the pipe and holding the black Start button in.
• Check for gas tightness after refitting the pipe (ensure the ‘O’-ring is fitted).
• Check the pump for free rotation by removing the end cap and turning the shaft with a suitable screwdriver.
• Remove the two clear plastic transportation pieces from the fan assembly.
• Fit the front casing.

Lighting the pilot

Conventional thermoelectric action

Press and hold in the black button – the fan will start at full speed and after approximately ten seconds’ purging, the ignition will light the pilot. Continue holding in the button for a further ten seconds, then release and check that the fan reduces to low speed operation and that the pilot remains alight. If not, wait three minutes before repeating the procedure.

Checking burner pressures

Remove the burner pressure test point screw and connect a suitable gas pressure gauge. With the room thermostat (if fitted) and the boiler thermostat set to maximum, set the clock/timer sliding switch to Timer’ and press the ON/OFF button to show ON. The main burner should light.

• From the maximum setting, turn the boiler thermostat control knob slowly back to the first click, then clockwise again to maximum. This ensures the control is operating in the upper, full gas rate range of the control. Check the gas pressure against the data plate on the inside face of the control panel. If incorrect, shut the boiler down by pressing the red stop button and transfer the pressure gauge to the inlet gas pressure test point. Check the inlet gas pressure and/or use the fault-finding charts. Adjustment of any part of the gas control is not recommended.

Range rating (where required)

With a gas pressure gauge connected to the burner pressure test point and the CH potentiometer on the back of the control panel turned fully anticlockwise (about 3/4 of a turn), start the boiler in the Winter mode. From maximum, turn the boiler thermostat control knob back to the first click, then clockwise again to maximum. Now slowly adjust the CH potentiometer clockwise to attain the 
required burner pressure and refit the adjustment screw cover. Check that the adjustment has not affected the DHW circuitry by fully 
opening a tap – the burner pressure should increase to the maximum shown on the data-plate for maximum heat output.

General

Check for gas and water tightness and for correct operation of any external controls.

SERVICING NOTES

Screws

The majority of screw heads on this appliance require a Torx 20 screwdriver (see Figure 2).

Pipe couplings

A number of pipes are of the ‘O’-ring sealed, push-in type that are retained by a forked plate. When refitting, make sure the ‘O’-ring is fitted (preferably new and silicone greased) and that the plate is correctly located.

Combustion chamber

Take extra care to avoid damage to the insulation linings when removing and refitting the chamber cover. The upper edge of the front 
lining fits between projecting lugs on the front of the heat exchanger and can be easily broken if misaligned.

Removing the reversing microswitch

Removal of the microswitch assembly (one screw) may allow a link to become disengaged from the internal mechanism of the reversing valve – particularly if the mode selector lever is moved while the microswitch assembly is removed. If this occurs, ensure the link is refitted as shown. The right hand end has an integral pin that locates into a hole in the reversing valve casing – the left-hand end is recessed to locate over a projecting pin. The adjacent spring-tensionedlevers need to be pushed aside by hand to refit the link.

SEQUENCE OF OPERATION

No demand

1. Permanent pilot alight.
2. Low-water pressure switch giving continuity.
3. Fan running at low speed to maintain pilot (approximately 80V AC).

Hot water demand

1. Demand for hot water.
2. Water flow through boiler operates the diaphragm- operated water valve.
3. The movement of the water valve spindle moves the diverter internal mechanism into the DHW position, closing the heating port. This movement also closes microswitch 3, giving continuity (24V DC).
4. Pump operates (230V AC).
5. DHW thermistor is checked to see if in range (thermistor readings depend on the PCB fitted: see table opposite).
6. Air-pressure switch (APS) is checked for rest position (Common to Normally Closed). Note: If the APS is not at rest, the sequence does not continue.
7. Fan speed increases to full speed (230V AC).
8. APS operates (Common to Normally Open). Note: If APS does not operate the fan will run continuously but no power is supplied to the gas valve.
9. PCB supplies 24V DC to the gas valve, which opens to allow full gas at the main burner (igniting off the permanent pilot). Note: Gas valve = 9V DC low gas and 24V DC high gas.
10. DHW outlet temperature is maintained at approx. 65°C. This is controlled by the DHW thermistor in conjunction with the PCB and the DC gas valve.Note: A high-limit thermostat shuts down the burner if the primary water temperature exceeds 92°C. Note: If the high-limit thermostat fails to operate, then a 95″C overheat thermostat will interrupt the thermocouple. This will extinguish the pilot burner.
11. Demand ends.
12. Water valve retracts to rest position, microswitch 3, going to open circuit.
13. Burner extinguishes.
14. Pump stops.
15. Fan reverts to low speed (no demand) to maintain the permanent pilot.

Heating demand

1. Demand by clock/external controls (out at terminal 2,back at terminal 3 at the appliance wiring connections).
2. Reversing valve lever in winter position, giving continuity at microswitch 1 (24V DC).
3. Pump operates (230V AC).
4. Air-pressure switch (APS) is checked for rest position (Common to Normally Closed). Note: If the APS is not at rest the sequence does not continue.
5. Fan speed increases to full speed (230V AC).
6. APS operates (Common to Normally Open). Note: If APS does not operate the fan will run continuously but no power is supplied to the gas valve.
7. PCB supplies 24V DC to the gas valve which opens to allow full gas at the main burner (igniting off the permanent pilot). Note: Gas valve – 9V DC low gas and 24V DC high gas.
8. Boiler temperature is monitored by the customer-operated boiler thermostat located on the facia. This is a high/low type thermostat: minimum approximately 30°C, maximum approximately 87°C. Note: A high-limit thermostat will shut down the burner if the primary water temperature exceeds 92°C. Note: If the high-limit thermostat fails to operate then a 95°C overheat thermostat will interrupt the thermocouple. This will extinguish the pilot and burner.
9. End of demand.
10. Pump stops.
11. Fan reverts to low speed (no demand) to maintain the permanent pilot.

DATA

Pump	Single speed	Grundfos 40/50
Fan	Two speed (one winding)	Low speed 80V AC (1200 Ohm inline resister fitted) High speed 230V AC
			Thermistor (original PCB)	Hot water thermistor (orange spot) located on the left-hand side of heat exchanger in a dry pocket	With the original PCB this is a PTC thermistor, 8-15 Ohms at cold 40 Ohms at 75°C
Thermistor (new PCB)	Modified to clip on thermistor, clipped to domestic hot water flow pipe from heat exchanger	NTC thermistor  10-12k Ohms at cold  2.5k Ohms at 60°C
						New PCB dip switches	Switch 1	Set to ON
			Switch 2	Set to OFF
Switch 3	A set to OFF
(incorporates three smaller switches)	B set to OFF
	C set to ON
Gas valve	One (DC coil)	9V DC low gas
		24V DC high gas

Hints and tips

Fault	Comment	Checks
Boiler inoperative/no low fan		Check the low water pressure switch, located bottom left,
		near the reversing valve. This will require continuity to
		power the PCB/components.
Insufficient hot water after	Modified PCB should come with	Check a new thermistor has been fitted. This will be
new PCB	modified thermistor, because the	clipped onto the DHW flow pipe. (The original was
	appliance will now require a NTC	located in a pocket below the high limit thermostat.)
	thermistor.
Poor heating performance		Filter fitted into the heating return isolation valve. If
	restricted or blocked it will give circulation/temperature
	problems when in heating demand.
	NB: Because this filter was fitted by the installer it may
	have been fitted in the flow isolation valve.
No spark from spark generator		After voltage checks at the spark generator (230V AC)
on ignition sequence		try reversing the live and neutral wires at the generator.
		The reason being is that the spark generator could be
		polarity sensitive.
On heating demand the	This can cause radiators to	1. Locate the connections on the PCB marked HT
stops when the boiler	cool dramatically before the	2. Follow the grey wires to the right-hand side of
thermostat is satisfied	boiler re-fires	the boiler.
(usually after new PCB)		3. Locate the one grey wire with a black plastic
		blanking cap
		4. Locate the two grey wires connected together with
		black plastic blanking cap.
		5. Connect these three wires together with a recognised
		electrical connector.
		The pump will operate continually whilst there is a heating
		demand from the time switch or room thermostat.
Water OK, no heating		Internal clock does not turn boiler ON, it holds boiler OFF!
(internal clock)		If the clock is suspect, unplug it from the PCB. Boiler
		should now operate with the selector arm set to Winte
		and any external thermostats calling (out at terminal 2.
		back at terminal 3).