Combi boiler cycles if flow temperature is below 60°

[jacoscar]

Hello, I have a Baxi DuoTec Combi 28 HE and 8 radiators.
I normally run it at 60-70 degrees flow temperature, but I wanted to see if I could run it at a lower temperature (can I get itnto condensate more? is my house heat pump ready?)

When I set it to 40 degrees the ‘gas’ light turns on while I hear the gas igniting inside, then the temperature rises rapidly reaching 50° in a matter of seconds, the fire and gas light turn off and the temperature drops to 30° in about a minute, then it ignites again and the temperature rises to 50° in a matter of seconds; then the cycle repeats.
Ok, the mean flow temperature may as well be 40° (oscillating between 30 and 50), but I don’t think this cycling is a good thing; does it even get a chance to condensate in those 5 seconds it stays on every 1-2 minutes?

The only way this doesn’t happen is if I set the flow temperature to 60°.

I have Tado valves and external sensors for each but 1 radiator and this test was done with all rooms calling for heat.

 

[muggles]

The minimum output of that boiler is 9.5kW, which likely exceeds the total output of your radiators and also the requirement of your home, so it's not all that surprising it's cycling like Bradley Wiggins. Not much to be done about that I'm afraid.

 

[Johntheo5]

Most gas boilers fire up at ~ 60% of max output before modulating down, if the flow rate isn't sufficiently high then the flow temperature will exceed its target value by 5C and trip the burner, if, realistically, the system temperature cant be much lower than say 28C after boiler overrun or recycle so you only have a dT of 42-28., say 14C so flowrate must be ~ 17.2LPM initially to ensure the boiler gets away before modulating down.
As above, it may also then cycle if heat demand is < 9.5kw, you could, as a test fire up the boiler at 60C and after a few minutes gradually reduce the target temperature to 40C to see if it continues to fire, but only reduce it in steps of less than 5C from its actual temperature otherwise the burner will trip and go into recycle.

 

[jacoscar]

Most gas boilers fire up at ~ 60% of max output before modulating down, if the flow rate isn't sufficiently high then the flow temperature will exceed its target value by 5C and trip the burner, if, realistically, the system temperature cant be much lower than say 28C after boiler overrun or recycle so you only have a dT of 42-28., say 14C so flowrate must be ~ 17.2LPM initially to ensure the boiler gets away before modulating down.
As above, it may also then cycle if heat demand is < 9.5kw, you could, as a test fire up the boiler at 60C and after a few minutes gradually reduce the target temperature to 40C to see if it continues to fire, but only reduce it in steps of less than 5C from its actual temperature otherwise the burner will trip and go into recycle.

Yes, I’ve tried lowering the temperature in small steps, but I cannot make it stay on with 40° flow temperature

 

[Johntheo5]

The burner will trip at target temp + 5C which means that its tripping at 45C, is this whats happening? or what target temp and actual temperature is it tripping at.
At a flowtemp of 45C the rads will output 36.7% of their nominal T50 rating so if 9.5kw is the minimum boiler output then the rad(s) output should be 25.9kw, if you have the rads zoned with say one zone on then not a hope IMO of running at 45C especially if the outside temperature is > say 5C.
I see you have 8 rads so maybe around a total of 12/15kw at full output.

 

[jacoscar]

The burner will trip at target temp + 5C which means that its tripping at 45C, is this whats happening? or what target temp and actual temperature is it tripping at.
At a flowtemp of 45C the rads will output 36.7% of their nominal T50 rating so if 9.5kw is the minimum boiler output then the rad(s) output should be 25.9kw, if you have the rads zoned with say one zone on then not a hope IMO of running at 45C especially if the outside temperature is > say 5C.

40 deg target temp
Turns on at 30
Turns off at 50
Time to get from 30 to 50 is about 10 seconds
Time to drop from 50 to 30 maybe 1-2 minutes

I also read the manual and put the boiler in calibration mode, where the CH knob regulates the gas flow (rather than regulating the gas flow to achieve a target temperature); by setting this to the minimum with all radiator valves open, the flow temperature settles to 60 deg

 

[Londoner2]

40c maybe too low, ideally 55c, also check where the thermostat is placed, do you get a quick drop in temp at that location?

I used to have a similar issue with a new combi boiler, i had a basic salus wireless thermosat T105RF, it was horrible with no real way to program it, after 4 months i replaced it with SALUS I500 internet/phone based one. Much simpler one to use, recycling stopped, and more control via phone app and internet.

 

[Johntheo5]

Thats a very interesting mode and the 60C is spot on as it will result in 15kw "worth of rads" emitting 10.01kw by my calcs so boiler runs continually.

 

[jacoscar]

40c maybe too low, ideally 55c, also check where the thermostat is placed, do you get a quick drop in temp at that location?

I used to have a similar issue with a new combi boiler, i had a basic salus wireless thermosat T105RF, it was horrible with no real way to program it, after 4 months i replaced it with SALUS I500 internet/phone based one. Much simpler one to use, recycling stopped, and more control via phone app and internet.

I have Tado thermostats and valves, the call for heat was permanently on during my tests (no cycling from the thermostat)

Thats a very interesting mode and the 60C is spot on as it will result in 15kw "worth of rads" emitting 10.01kw by my calcs so boiler runs continually.

But that means I cannot use weather compensation as it will try to lower the temperature in warmer days
I might as well keep the 60deg all year round

 

[Johntheo5]

Boiler cycling in itself doesn't result in any significent increase in fuel consumption as long as the run time is not less than 3 minutes, the problem with some boilers (like yours, it seems) is their very high ignition settings, Vaillants were notorious/famous for this ~ 65% gas flow on fire up and maintaining this for 60 secs before allowing modulation, you had to have very high flowrates + very long recycle times to get the flowtemperature as low as possible before refiring.
Range rating may help, can you range yours to say 15kw, it doesn't affect the DHW output. Also can the circ pump speed be increased?.

 

[jacoscar]

Boiler cycling in itself doesn't result in any significent increase in fuel consumption as long as the run time is not less than 3 minutes, the problem with some boilers (like yours, it seems) is their very high ignition settings, Vaillants were notorious/famous for this ~ 65% gas flow on fire up and maintaining this for 60 secs before allowing modulation, you had to have very high flowrates + very long recycle times to get the flowtemperature as low as possible before refiring.
Range rating may help, can you range yours to say 15kw, it doesn't affect the DHW output. Also can the circ pump speed be increased?.

Sorry, what do you mean by range rating?
The pump is fixed speed and was causing noise in the system so I had to install an external bypass to slow it down.
I have done these tests with the bypass set to a minimum to avoid too much hot water going back to the boiler

 

[Johntheo5]

Is this pump internal?, what type of bypass have you got is it a manual bypass or a ABV, if a ABV what make is it?.
Some boilers have a parameter where you can set the boiler output to (in kw) which only restricts the output in CH mode only.

Have you a link to your particular model installation/user manual.

 

[jacoscar]

Is this pump internal?, what type of bypass have you got is it a manual bypass or a ABV, if a ABV what make is it?.
Some boilers have a parameter where you can set the boiler output to (in kw) which only restricts the output in CH mode only.

Have you a link to your particular model installation/user manual.

The pump is internal

This is the bypass valve

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Here is a link to the manual; mine is the 28 version

https://www.freeboilermanuals.com/assets/pdf/baxi_disc/Duotec-Combi-HEA-NaturalGas.pdf