Combi Boiler Explanation

[Monkfish]

I have just had a Combi Gas WB 30CDi boiler installed. I would like to understand how it works; e.g.

1. How does it heat both domestic and central heating.
2. Does it have two boilers.
3. How much water does it store and keep heated internally.
4. Does it turn off the central heating when the domestic is run.

Just generally how it works so I can understand what's going on inside my own boiler.

If someone could explain or point me to an article pertinent to my boiler I'd be grateful. Thanks

 

[REDSAW]

Sequence of Operation (Domestic hot water)
1. Main switch on – Green standby neon illuminates.
2. Turn on hot water tap and the combined flow switch and diverter valve will make circuit (continuity) at
microswitch and move to DHW position.
3. Hot water demand Red neon illuminates.
4. PCB sends 240 Vac to pump which runs.
5. PCB checks primary and hot water sensors (thermistors) are in range (1 kohms–30 kohms) and the
HW potentiometer is calling for heat.
6. PCB starts ignition sequence.
7. PCB checks the air pressure switch for NO air.
8. PCB sends 240 Vac to fan which then runs and air pressure switch makes.
9. Power is sent to the solenoids (20v DC) and 6v DC to the modureg to give low flame.
10. Flame rectification confirms flame presence within 10 seconds – Red burner neon illuminates. (If
unsuccessful lockout occurs.)
11. The 6 Vdc at the modureg raises to 14 Vdc to give full gas rate (13.8 mbar) within 4 seconds.
12. Temperature is monitored by the thermistors in conjunction with the HW potentiometer.
13. Demand ends. Flame neon and burner goes off – cooling overrun function occurs.
Sequence of Operation (Central Heating)
1. Main switch on – Green standby neon illuminates.
2. External controls calling for heat (clock and roomstat).
3. Central heating demand – Red neon illuminates.
4. PCB sends 240 Vac to pump which runs.
5. PCB checks primary sensor is in range (1 kohms–30 kohms) and the CH potentiometer is calling for heat.
6. PCB starts ignition sequence.
7. PCB checks the air pressure switch for NO air.
8. PCB sends 240 Vac to fan which then runs and air pressure switch makes.
9. Power is sent to the solenoids (20 Vdc) and 6 Vdc to the modureg to give low flame.
10. Flame rectification confirms flame presence within 10 seconds – Red burner neon illuminates. (If
unsuccessful lockout occurs.)
11. Temperature is monitored by the primary heating thermistor in conjunction with the CH potentiometer.
12. Demand ends. Flame neon and burner goes off – cooling overrun function occurs.
Sequence of Operation (Hot water interrupting heating)
1. Heating running – Burner and central heating demand – Red neon illuminated.
2. Turn hot water tap ON – combined flow switch and diverter valve will make circuit (continuity) at
microswitch and move to HW position.
3. PCB maintains 240 Vac at pump.
4. Central heating Red neon remains ON and the hot water demand Red neon illuminates.
5. PCB checks hot water sensor in range (1 kohms–30 kohms).
6. Burner to maximum within 4 seconds (14 Vdc to modureg).
7. Burner modulates under control of the domestic HW sensor and the HW potentiometer.​

8. When the hot water is turned off the combined flow switch

and diverter valve will break circuit (open​

circuit) and the central heating operation resumes.


roughly.

 

[Monkfish]

Thanks for the detailed description. So what happens if the domestic hot water is set to a lower temperature than the central heating? How is the pre-heated water in the boiler reduced down to tap temperature? How much water is pre-heated in an average boiler?

 

[dougs.cowan]

Hi Monkfish,
The temp of the heating circuit will not be the same as the temp of the hot water. The water is heated by passing through a small heat exchanger. This exchanger has water from the main heat exchanger passing in close proximity to the incoming cold water mains supply. A standard combi boiler such as yours does not store any hot water, the water is heated as it passes through. If you turn off the incoming cold main supply to the boiler, you also turn off the hot water. Hope I have explained this ok for you. Regards Doug

 

[Blackcatgas]

Congrats on your new combi, the Installation/servicing instructions have detailed descriptions of how your new combi works, usualy in the first few pages.

 

[Monkfish]

Hi, well the user manual just deals with knobs & buttons, whilst the instruction manual covers the component parts, installation and commissioning. There's nothing that really describes what goes on inside.

There should be a diagram that shows how the thing works. I'm still unclear.

1. So the heat exchanger heats the water in the sealed central heating system?
2. The large expansion vessel is in series with this flow to accommodate the expansion of the water as it is heated?
3. The domestic hot water is heated by a separate heat exchanger that in turn is heated by the water in the central heating loop?

Is that all correct?

I have two settings for domestic hot water: Normal and Economy. The Normal mode gives me hot water almost immediately, the Economy mode takes a little longer. Am I right in thinking that in Normal mode hot water is passed through the DHW exchanger all the time ensuring that I have hot water ASAP. Under Economy mode the hot water is diverted to the DHW exchanger only after turning a tap on? Not sure how this would work when the central heating is turned off though. I guess just by the principal heat exchanger making sure the DHW is up to temperature.

A diagram would make it much easier to understand.

 

[Blackcatgas]

Hi, well the user manual just deals with knobs & buttons, whilst the instruction manual covers the component parts, installation and commissioning. There's nothing that really describes what goes on inside.

There should be a diagram that shows how the thing works. I'm still unclear.

1. So the heat exchanger heats the water in the sealed central heating system?
2. The large expansion vessel is in series with this flow to accommodate the expansion of the water as it is heated?
3. The domestic hot water is heated by a separate heat exchanger that in turn is heated by the water in the central heating loop?

Is that all correct?

I have two settings for domestic hot water: Normal and Economy. The Normal mode gives me hot water almost immediately, the Economy mode takes a little longer. Am I right in thinking that in Normal mode hot water is passed through the DHW exchanger all the time ensuring that I have hot water ASAP. Under Economy mode the hot water is diverted to the DHW exchanger only after turning a tap on? Not sure how this would work when the central heating is turned off though. I guess just by the principal heat exchanger making sure the DHW is up to temperature.

A diagram would make it much easier to understand.

Greenstar 30CDi literature - Worcester, Bosch Group UK installer site

My appologys, WB used to write a brief sequence of operations in thier MI instructions.download the technichal information on boilers pdf file, from the link above.

 

[Monkfish]

Thanks for the pointer to that info, I'm sure that will come in handy. Unfortunately none of the manuals show a block diagram of the internal workings. Surely there must be such a thing for training purposes?

 

[stevetheplumber]

basicly water is heated in the main heat exchanger and is pumped through a threeway valve. In heating mode this goes out the boiler and through rads. when you turn a tap on the threeway valve is moved by a diaphram and directs the flow thro a water to water plate heat exchanger which heats mains water running through it enroute to your taps
Temperature is controlled by various thermostats which alter the voltage to the gas valve which raises and lowers the flame to suit demand
there are also various saftey devices which cut boiler out if water preasure is low, water is to hot, theres a fault on the gas supply or no flame
before lighting boiler also checks there is circulation of water and that the fan is working
oh for the days of stat, gas valve and thermocouple

 

[Monkfish]

Thanks Steve, that's the sort of info I was after. I have a couple more questions if it's not too much trouble:

1. Is the expansion vessel simply in-line with the central heating circuit? Is that just a metal box that can expand?
2. How does the condensing part work in conjunction with the principal heat exchanger? Does the fan have something to do with this?

Thanks

 

[stevetheplumber]

Thanks Steve, that's the sort of info I was after. I have a couple more questions if it's not too much trouble:

1. Is the expansion vessel simply in-line with the central heating circuit? Is that just a metal box that can expand?
2. How does the condensing part work in conjunction with the principal heat exchanger? Does the fan have something to do with this?

Thanks

exspansion vessel is a metal box with a ballon full of water in side and the space around the ballon is charged with gas into which the water can expand
condensing isnt a seperat part its just the way the whole burner heat exchanger is designed to extract maximum heat from the flue gases causing the gas to cool to the point the water vapour condenses and runs down through the condense pipe the change of state from vapour to liquid releases extra heat
the fan is there to draw air in and push fumes out ,on these boilers it actually draws the gas in from a nozzle as well(much the same way as a carburetta works on a car)
just realised weve gone from injecting fuel under preasure to save the planet and cars have dropped the carb and started injecting fuel under preasure to save the planet

 

[Monkfish]

Thanks again