Combi boiler - HOT WATER Firing Sequence

[harryfox121]

Hello all, I have a question and have searched by cannot find an answer.

I have a combi boiler in my house, I was wondering how it works.

When you open a hot tap, the boiler fires up and hot water is shortly deliered at the hot tap.

My boiler has no intermittent or perm pilot btw.

My question is how does the boiler operate to firing, is it like this, or am I way off.

I'd be grateful to anyone who can help me.

Hot water tap opened
Water flows through diverter valve (if combi boiler has 1)
Diaphragm lifts (if this type of diverter valve)
Port closes to main heat ex and allows water to flow through DHW heat exchanger (plate heat ex etc, if this type of combi)
Spindle raises and makes contact with micro switch
Pump runs
Pump flow switch is activated
Fan runs
Air pressure switch proves fan is running ok, flueways are clear, air pressure hoses/ports are clean/not blocked
HW thermistor/stat is calling (within set range)
Ignition/gas valve opens
Gas flows through to burner
Gas ignites
FSD probe senses gas and proves back to board
Gas valve opens at full rate until stat/sensor up to temp/demand satisfied

and the PCB is the electronic part which receives signals (voltages) and allows firing sequence to move onto next part if all Ok

All this happens in a split second or so

I know some combi's don't use the pump with HW but a lot do.

 

[twgas]

Port closes to main heat ex and allows water to flow through DHW heat exchanger (plate heat ex etc, if this type of combi)

Port to CH (not main hex) closes and the port to the plate hex opens. the rest is generally about right.

 

[harryfox121]

sorry that's what I meant so the water only circulates through the main heat ex and dhw heat ex, not the ch circuit/rads etc

 

[harryfox121]

One last point: At what point does the thermistor/stat usually come into play, ie is it before pump/fan running/proving or after, as if the stat/sensor was stuck open (infinity reading) then the boiler wuldn't fire.....

 

[twgas]

is it before pump/fan running/proving or after

before.

 

[harryfox121]

thanks for your help.

 

[APHSLTD]

Hello all, I have a question and have searched by cannot find an answer.

I have a combi boiler in my house, I was wondering how it works.

When you open a hot tap, the boiler fires up and hot water is shortly deliered at the hot tap.

My boiler has no intermittent or perm pilot btw.

My question is how does the boiler operate to firing, is it like this, or am I way off.

I'd be grateful to anyone who can help me.

Hot water tap opened
Water flows through diverter valve (if combi boiler has 1)
Diaphragm lifts (if this type of diverter valve)
Port closes to main heat ex and allows water to flow through DHW heat exchanger (plate heat ex etc, if this type of combi)
Spindle raises and makes contact with micro switch
Pump runs
Pump flow switch is activated
Fan runs
Air pressure switch proves fan is running ok, flueways are clear, air pressure hoses/ports are clean/not blocked
HW thermistor/stat is calling (within set range)
Ignition/gas valve opens
Gas flows through to burner
Gas ignites
FSD probe senses gas and proves back to board
Gas valve opens at full rate until stat/sensor up to temp/demand satisfied

and the PCB is the electronic part which receives signals (voltages) and allows firing sequence to move onto next part if all Ok


All this happens in a split second or so

I know some combi's don't use the pump with HW but a lot do.

you could add

thermistor senses hw temp and modulates burner pressure to keep temp stable, unless hw temp exceeds max temp setting by 5 degrees then burner off until hw temp is 5 degrees below set temp.

Depends on what combi you have though as older ones tended to have just high and low gas rate and nothing in between