Technical Info - Combi Boilers
Introduction
Design & Construction
Operation
Pump Overrun
Fault Finding
Failure
Electrical Supply (Faulty Capacitor)
Worn Out Bearings

Rotor Seized
Burnt Out Motor Windings
Pump Speed Low
Pump Noise
Impellor
Air

Venting

Pump/Pump Head Replacement
Flushing and Protecting the System
Fitting the New Pump
Fault Finding
Service & Repair
Circulating Pump Overhaul
Re-Assembly

Extract from 'Central Heating Combination Boilers - Fault Finding & Repair'

Introduction
When a combi boiler fires, the water in the heat exchanger will be quickly heated. With low water content heat exchangers the system water within the exchanger is rapidly heated to temperature. This heated water is needed to meet a demand from the central heating or a domestic hot water requirement. The most efficient way of moving this heated water to either the central heating system or to the secondary heat exchanger is by using a machine. This machine will circulate the heated water to where it is needed so it can give up its heat to meet the demand. This machine is commonly referred to as a pump and less commonly as a circulator or an accelerator. To cover most situations the phrase ‘domestic circulating pump’ has been popularised.

Design & Construction
Modern quality pump construction consists of a cast iron housing, stainless steel rotor can and rotor cladding with ceramic upper and lower bearings, a carbon based thrust bearing and rubber ‘O’ rings. The terminal box is constructed of plastics. The electric motor rotates the shaft at speeds of between 700 – 2000 rpm.

Pre-installed pumps can operate within a pressure range of 0.1 – 10 bar (being factory tested to 15 bar) although most combi boilers have a pre-set maximum working pressure of 3 bar. The pumps are designed to operate at temperatures of typically 10 – 110°C so they can easily accommodate most conditions. In most combi boilers the pump is installed on the central heating return pipe.

The water contained within the pump both lubricates the bearing and keeps the pump motor temperature within its designed operating range. These pumps are robust and corrosion resistant.

Many older combi boilers are fitted with a fixed head circulating pump. This operates at one speed and has no adjustment. Its operating tolerances against pressure (head) is wide so suits many and varied systems. Unfortunately it is not especially efficient and can cause system noise. These pumps provide a 5 metre head of water with some providing a 6 metre head.


In many modern condensing combi boilers an automatic variable speed circulating pump is fitted. This automatically adjusts its speed to meet the pressure fluctuations caused by differing heating demands. As thermostatic radiator valves close in response to set room temperatures the pump speed automatically adjusts, reducing the differential pressure as the resistance of the system rises. This saves energy (electricity) and reduces the possibility of system noise such as radiator valve noise and pipe flow noise. When they are fitted within the combi boiler the manufacturer may also fit an automatic By-pass, therefore DO NOT fit an automatic By-pass valve. Always follow the manufacturer’s instructions.

Circulating pumps installed by the manufacturer within the combi boiler casing require a 230V mains supply. These pumps have a low power consumption ranging between 35 – 100 watts, (similar to a standard light bulb). Pre-installed pumps are electrically protected by the combi boiler fuse normally rated at 3 amps. The pumps are always earthed by the combi boiler manufacturer. DO NOT remove the terminal box cover without first disconnecting the electric power supply and carrying out electrical safety procedures.

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Operation
As the motor rotates the shaft a circular veined wheel, known as an impellor, draws water in through its centre (eye) and throws it out at its edges by centrifugal force. As the water is thrown outward it creates a pressure drop causing more water to be drawn in at the eye. The faster the impellor spins the greater the centrifugal force created, which increases the water pressure in the system.

Pumps are controlled by the time and temperature controls of the central heating system or by flow switches (micro switches) in the secondary hot water circuit. Circulating pumps will pump in one direction only. The flow direction is usually indicated with an arrow molded to the base. If a pump is installed the wrong way around the combi boiler will overheat and often go to lockout. ALWAYS ensure the pump is fitted correctly.

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Pump Overrun
This is a time OR temperature device which enables the pump to continue running for a short time after the boiler has shut down. This facility allows the pump to continue circulating the water through the pipe work removing the heated water from the combi boiler heat exchanger. If the water was not circulated the residual heat left in the exchanger would overheat the water, possibly causing it to boil. This could cause a noise (kettling) or cause the overheat thermostat to operate and send the boiler to lockout. Further, by preventing the water from over heating it reduces the potential for scale deposition.

By providing 230V to the pump when the water is over 60°C and after the combi boiler has shut down, the pump continues to run until the water drops below 60°C. The thermostat/sensor is normally closed when the water is above 60°C and opens when the water is below 60°C.

Note: If the pump continues to run indefinitely the thermostat/sensor or microswitch is faulty. Check the water temperature and also check the voltage back to the PCB.

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Fault Finding
On every occasion the combi boiler fires the circulating pump should run and when the boiler turns off the pump will either stop immediately or will run for a short time and then stop. When the pump runs on for a short while it will remove latent heat from the boiler. This is called pump ‘over-run.’ Circulating pump faults will either be:

1. Pump failure
2. Pump speed low
3. Pump noise

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Failure
There are four ways a circulating pump can fail to start:

1. No electrical supply
1a. Faulty Capacitor
2. Worn out bearing(s)
3. Rotor seized
4. Burnt out motor windings

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Electrical Supply (Faulty Capacitor)
If the pump does not start it may be there is no electrical supply going to the pump.

1. Remove terminal cover
2. CAUTION – using a multimeter check the voltage at the pump terminals. It should read 230V across the     brown or red (live) and blue or black (neutral)
3. If there is no voltage the problem is at the combi boiler (check terminals for voltage), or if the fault is when     the central heating is calling for heat the programmer or room thermostat is faulty.

Check setting

You can check the circulating pump has failed:

a) Turn off and electrically isolate the combi boiler
b) Remove the three wires from the pump terminals making a note of positions
c) Using a test lead fitted with a plug and 3 Amp fuse, connect the three wires to the pump terminals    (Brown-Live, Blue-Neutral, Green/Green and Yellow-Earth)
d) Plug the test lead into a socket
e) Switch the socket on and if the pump runs then the fault is not with the pump but with another component
f)  Remove the vent screw using the pump tool and using a 4mm screwdriver or the pump tool, quickly spin the    rotor shaft. If the pump starts the capacitor is leaking/faulty. Replace pumpor pump head

Note: If the pump does not start immediately turn off the electrical supply or the pump motor (windings) may burn out.

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Worn Out Bearings
If the bearings are worn out then the pump or pump head will need to be replaced. When checking for this be prepared for water spillage. Protect all electrics and electronics.

1. Turn off and electrically isolate the combi
2. Remove the vent plug in the face of the circulating pump using a pump tool or large blade screwdriver
3. If water pours out then the bearings are worn out. Mop up the spillage
4. Replace vent plug using pump tool or large blade screwdriver
5. Fit new pump or pump head

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Rotor Seized
The rotor may have seized due to particles of rust/sludge or scale preventing it from rotating. When checking for this be prepared for water spillage. Protect all electrics and electronics.

1. Turn off and electrically isolate the combi boiler
2. Remove the vent plug in the face of the circulating pump     using pump tool. Mop up any water spillage
3. If just a few drops of water seep out fit a 4mm screwdriver     (use pump tool) through the vent plug opening and gently     turn the shaft anti-clockwise to dislodge particles
4. If the rotor shaft feels ‘rough’ as it spins or it will not move     a new circulating pump or pump head is required
5. If the rotor shaft turns freely (anti-clockwise) replace the     vent plug using pump tool, reconnect the power, turn the     boiler on and check the pump is working correctly

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Burnt Out Motor Windings
Having checked the electrical supply and there is 230V at the pump terminals, the rotor has been freed and now turns freely (anti-clockwise), but the pump does not work, then the motor has burnt out due to the rotor seizing and causing overheating in the motor windings. Often, when the motor windings do burn out the 3 amp fuse blows. Always check the fuse in the plug or fused connection unit AND any fuse within the combi boiler.

To test and prove the motor windings have burnt out:

1. Turn off and electrically isolate the combi boiler
2. Remove pump terminal cover and remove wires to pump terminals making a note of the connections
3. Use multimeter set on Ohms(Ω) and test the resistance across live and neutral terminals:

Less than 1 Ohm = Motor windings burnt out – Replace pump or pump head

Mega Ohms/Infinity/Overload = Motor windings burnt out – Replace pump or pump head

120 – 200 Ohms = Motor windings O.K.

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Pump Speed Low
To prevent kettling and overheating, the pump must run at speeds appropriate to the burner output level. If the pump does slow down due to sludge/rust/debris fouling the impellor or the pump is reaching the end of its useful life the following may be evident:

1. Lack of heat output at radiators/domestic hot water
2. Boiler kettling
3. Frequent boiler shut down
4. Boiler cycling on limit thermostat

The debris fouling the impellor must be dislodged so the central heating system should be drained, flushed and protected (see Flushing & Protecting – 2.5). If the pump is simply coming to the end of its working life then it must be replaced.

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Pump Noise
The major cause of system noise is incorrect impellor speed. When the circulating pump speed is too slow the following may occur:

Boiler ‘kettling’ due to water not passing over the heat exchanger
fast enough causing it to become overheated.

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Impellor
As with any central heating system component there is wear and tear. The circulating pump will give year after year of trouble free service but the bearings and rotor will gradually wear and lose their precision running ability. This ‘out of true’ state causes the impeller to run out of balance and create noise which is evident by the pump slightly moving when operating. This is a sign the pump is nearing the end of its working life and the only solution is to fit a new pump or pump head.

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Air
When air enters the pump the noise can be very loud. It is relatively simple to vent the pump but the reason for the air entering the system must be established and corrected as dry running/cavitation of a circulating pump will damage it very quickly.

If the combi boiler is installed wrongly or at the wrong angle the top bearing may not receive system water to both lubricate and cool it. The pump will fail quickly if this is not corrected immediately. Use a spirit level to check the boiler is installed correctly.

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Venting
It is essential that when a new circulating pump has been installed, the combi boiler or the central system system has been drained the circulating pump is vented. It must be vented before it is allowed to start. This will remove air and allow system water to enter. The system water will cool the pump, preventing overheating and will also lubricate the shaft and bearings. Permanent damage may result from the pump being run dry even for a short time. ALWAYS vent the pump.

Expect some water spillage so have old towels/rags available to protect floors etc and to mop up.

1. Turn off and electrically isolate the combi boiler
2. Remove circulating pump terminal cover
3. Caution – using a multimeter check the voltage at the pump terminals. There should be a ZERO reading
4. Replace terminal cover
5. Remove the vent plug using a pump tool or wide blade screwdriver. Mop up water spillage
6. Fit a 4mm screwdriver (or use pump tool) through the vent plug opening and gently turn the shaft anti     clockwise
7. Rotate the shaft several times to remove any air
8. Replace vent plug. Do not over tighten
9. Replace any fuses and turn on the electrical supply
10. Set the room thermostat to maximum
11. Set the programmer to central heating ‘on’
12. Turn on the combi boiler and check correct operation of pump and combi boiler
13. Reset room thermostat to original setting

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Pump/Pump Head Replacement
When replacing a combi boiler pump there will be water spillage so be prepared to protect all electrics and electronics including flooring etc and have a bowl and old towels/rags to mop up.

1. Turn off and electrically isolate the combi boiler
2. Remove the circulating pump terminal cover
3. Caution – using a multimeter check the voltage at the pump terminals. There should be a ZERO reading
4. Note (write it down!) the colour of each wire and its corresponding terminal letter or number.
    Brown (or red) – Live to Terminal L or 1
    Blue (or black) – Neutral to Terminal N or 2
    Green/Yellow (or green) – earth to Terminal E or 3
5. Note (write it down!) the way the pump is installed
6. Note (write it down!) the water flow direction. This is indicated by an arrow on the pump often cast into the     base of the pump
7. Remove each electric wire from its terminal
8. Drain down the central heating system or use a pipe freezer
9. Using a pump tool remove the vent plug and deal with any water spillage. Note the colour of the water. If it is     clear with no debris the system water does not need to be changed. If it is dirty (a reddish or blackish     colour) or contains debris the system MUST be flushed and protected
10.Hold the circulating pump in one hand to steady it, loosen the union nut between the pipe work and pump      with a pipe wrench, pliers or large spanner
10a.Alternatively, remove the pump head by using a pump tool or a 4mm hexagonal ‘T’ bar
11. Holding the pump, completely undo both union nuts
12. Remove the circulating pump

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Flushing and Protecting the System
It is essential to minimise the possibility of future damage or inefficient operation by ensuring the whole system is clean and protected against corrosion.

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Fitting the New Pump
Circulating pumps should be replaced either on a like for like basis, if available, or leading manufacturers provide replacement guides and fitting kits/recommendations for using their pumps.

1. Do not apply sealants or jointing compounds to any surface including the threads of the isolating valves or     the pump port threads. Assemble dry
2. With the flow arrow in the correct direction (check your notes) place the new circulating pump into position     between the union nuts
3. Slide the new gaskets into place
4. Hand tighten both union nuts ensuring the gaskets do not move out of position
5. Steady the new circulating pump with one hand and tighten both union nuts. Do not over tighten. Ensure the     pump is at the correct angles
6. Important – vent the circulating pump
7. Remove the terminal cover and using your notes rewire the new circulating pump
8. Replace the terminal cover
9. Replace any fuses and turn on the electrical supply
10. Set the room thermostat to maximum
11. Set the programmer to central heating ‘on’
12. Turn on the combi boiler

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Fault Finding

No Central Heating/No Hot Water

Reason
Pump failure – no electrical supply.

Pump failure – burnt out motor windings.

Pump failure – rotor seized due to
rust/sludge sediment

Pump failure – worn out bearing(s)
due to wear and tear

Pump failed – dry running


Pump failed - impellor broken from shaft

Pump failure – burnt out motor
windings due to seized rotor

Pump capacitor faulty


No Central Heating/Hot Water O.K.

Reason
Room thermostat faulty so no signal to
combi boiler

Diverter valve faulty

No Hot Water/Central Heating O.K.

Reason
Diverter valve faulty

Insufficient Heat To Radiators

Reason
Pump speed slow due to
sediment/sludge etc

Frequent Combi Boiler Shut Down

Reason
Pump speed slow due to sediment
causing combi limit thermostat to activate

Pump Noise

Reason

Air in system/pump

Dry running of pump

Impellor out of balance due to wear and tear

Pump buzzing due to selector switch set midway
between settings

Pump buzzing due to jammed rotor

Pump Overrun

Reason

Faulty water temperature thermostat/ sensor or faulty microswitch

Repair
Check electrical connections at pump.

Replace pump head.

Repair impellor movement or replace pump or pump head.

Replace pump head.


Replace pump head and vent before turning on combi boiler

Replace pump/pump head

Replace pump/pump head


Test capacitor and replace if faulty or replace pump/pump head



Repair
Check call for heat at room thermostat


Check diverter valve



Repair
Check diverter valve



Repair
Drain, flush and protect system and overhaul pump




Repair
Flush and protect system and overhaul pump




Repair

Vent system and pump

Position combi boiler correctly

Replace pump head

Reset to the higher speed

Free the rotor or replace pump/pump head




Repair

Measure water temperature. If less than 60°C and voltage from thermostat/sensor or microswitch to PCB then check thermostat or microswitch

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Service & Repair
Circulating pumps should give many years of quiet efficient hot water circulation. They are precision manufactured to a high quality standard and are robust and resistant to corrosion. They may seize if the system water contains corrosive particles of rust and sludge.
After long periods of non-operation, rust and sludge sediment may settle within the body which cause the pump to seize when first started. Therefore, before the pump is started it is good practice to remove the vent plug and rotate the shaft using a small screwdriver or pump tool. Continue to spin the shaft until it runs freely, replace the vent plug and turn on the combi boiler.

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Circulating Pump Overhaul
Most modern pumps can be overhauled for cleaning etc. Note that the following procedure DOES NOT form part of an annual service:

1. Turn off the boiler
2. Turn off the electrical supply to the boiler and pump, remove     plug(s), fuse(s) and put them in your pocket,etc.
3. Remove circulating pump terminal cover
4. Caution – using a multimeter check the voltage at the pump     terminals. There should be a ZERO reading
5. Drain down the central heating system or use a pipe freezer
6. Unscrew the four bolts holding the pump head to its base     using a pump tool or a 4mm hexagonal ‘T’ bar
7. Carefully pull the circulating pump head forward. Mop up     water spills
8. If the water is red or black or contains debris the whole      central heating system must be flushed and     protected
9. Carefully place a flat-head screwdriver between the bearing      plate and rubber gasket. Do not try to lever off     the impellor
10.Carefully lever around the circumference between the bearing plate and the rubber gasket. This will      withdraw the rotor can and the rotor/impellor assembly
11.Tap the end of the rotor shaft to remove the rotor/impellor assembly from the rotor can
12.Completely remove the rotor/impellor assembly
13.Using a detergent mixture clean the rotor and shaft and the inside of the rotor can
14.Rinse thoroughly. Protect electrical components from water

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Re-Assembly
1. Refit the rotor can into the motor head. Ensure the rubber upper sealing ring is in place at the top of the can
2. Fit the lower rubber gasket onto the rotor can and lightly position the rotor assembly into the rotor can
3. Re-fit the head to the pump base and hand tighten the four bolts
4. Fully tighten the four bolts in turn diagonally a little at a time, this will pull the assembly into position
5. Remove pipe freezer and allow ice plugs to melt
6. Check the rotor movement, using a pump tool or 3mm screwdriver place the tip in the end of the slot of the     rotor. Make sure it moves freely
7. Replace the vent plug
8. Replace terminal cover
9. Replace any fuses and turn on the electrical supply
10. Set the programmer to central heating ‘on’
11. Turn on the combi boiler
12. Reset room thermostat to original setting

Extract from 'Central Heating Combination Boilers - Fault Finding & Repair'

 


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