Baxi Duo Tec 40 HE A Fluctuating Hot Water Temperature

[bernardgreen]

I have had reason to look up the range of temperature for mains water supply in the London area.

"Mains water is distributed across London through a network of pipes, and varies in temperature between 5 and 20°C during the year."

That led to some thoughts about boilers.

With primary flow set for 60°C the delta T varies from 55°C ( in winter ) down to 40° ( in summer )

domestic hot water thermostat set to 43 °C

Delta T varies from 38°C ( winter ) and 23°C ( summer )

It would seem sensible that the flow restrictor in the boiler should allow enough flow for the 23°C delta to be achieved before the modulation reaches its bottom limit and rapid cycling has to happen.

 

[DP]

With primary flow set for 60°C the delta T varies from 55°C ( in winter ) down to 40° ( in summer )

Primary flow is not set to 60. When boiler runs full tilt to deliver hot water, flow temperature hovers around 70

 

[DP]

Delta T varies from 38°C ( winter ) and 23°C ( summer )

It would seem sensible that the flow restrictor in the boiler should allow enough flow for the 23°C delta to be achieved before the modulation reaches its bottom limit and rapid cycling has to happen.

Combi is manufactured To raise the cold water temperature by 35 degrees by restricting flow through the boiler. This temperature can then be adjusted by the end user by adjusting the control on the boiler ( if one is fitted)

 

[bernardgreen]

When boiler runs full tilt to deliver hot water, flow temperature hovers around 70

Not talking about "full tilt"...... It is about minimum "tilt" being too high for the DHW flow rate.

 

[DP]

Not talking about "full tilt"...... It is about minimum "tilt" being too high for the DHW flow rate.

Bernard, end user does not control the primary temperature. OP is getting 60 degrees due to a fault condition. This fault is causing the hot water temperature to swing about instead of steady temperature because boiler is shutting down when 60 degrees is reached for primary temperature As has been stated by OP, fan starts to slow down and then stop ( which then causes delivered hot water temperature to fall). Then boiler starts and hot water temperature rises to primary temperature reaches 60

For correct operation, burner should not shut down.

 

[bernardgreen]

For correct operation, burner should not shut down.

What option has the control system got if the amount of heat being produced at minimum burn rate cannot be taken away from the boiler ?.

 

[DP]

What option has the control system got if the amount of heat being produced at minimum burn rate cannot be taken away from the boiler ?.

Fan runs slower to match heat generated to heat used

 

[Deleted member 221031]

What option has the control system got if the amount of heat being produced at minimum burn rate cannot be taken away from the boiler ?.

How would that happen ? Everything increase And decreases with the flow of water. You know a boiler that can supply 16 pm will manage to adjust its self if only 8lpm is available. As DP said, fan speed, which reduces gas demand, which reduces heat used, which reduces the load.

40kw would be capable of causing CH issues, but given you live in the dark ages, it’s really not an issue for you

The mind boggles

 

[Master Mech]

Sorry guys this is all way over my head so what is my next move?

Today I removed the strainers from the mains supply cock & the one in the hydraulic inlet assembly where the flow restrictor is. Both were immaculately clean.

 

[bernardgreen]

Fan runs slower to match heat generated to heat used

So the fan could in the most extreme case could slow down to. ? You tell me what is the lowest percentage of maximum fan speed that the boiler can safely operate at.

I assume you are aware that almost all combustion chambers have a minimum gas rate, below this gas rate the combustion becomes unstable and the boiler has to be shut down

 

[DP]

So the fan could in the most extreme case could slow down to. ? You tell me what is the lowest percentage of maximum fan speed that the boiler can safely operate at.

I assume you are aware that almost all combustion chambers have a minimum gas rate, below this gas rate the combustion becomes unstable and the boiler has to be shut down

Bernard, let us leave this discussion where it is as we are not making any progress here
You are talking as a layman, me as a service person who works with these type of boilers.

Combustion is set for exact values at low and high fire, anything out of spec causes boiler to shut down

 

[Deleted member 221031]

So the fan could in the most extreme case could slow down to

Why extreme cases ? Fan speeds change constantly

You’re obviously thinking of a 240v standard speed fan, not a premix boiler.

 

[bernardgreen]

You’re obviously thinking of a 240v standard speed fan, not a premix boiler.

No I am thinking of a variable speed fan.

Combustion is set for exact values at low and high fire, anything out of spec causes boiler to shut down

What is the "exact value" for low fire compared to the amount of heat energy required for the minimum specified water flow rate and mains water at the high end of temperatures to be expected in summer.

It does seem that in some boilers the low fire produces more heat than can be removed from the boiler when delta is low and there is a restricted flow of water. The restriction being due to the boiler having a flow restrictor as part of it's design.

 

[Deleted member 221031]

No I am thinking of a variable speed fan.



What is the "exact value" for low fire compared to the amount of heat energy required for the minimum specified water flow rate and mains water at the high end of temperatures to be expected in summer.

It does seem that in some boilers the low fire produces more heat than can be removed from the boiler when delta is low and there is a restricted flow of water. The restriction being due to the boiler having a flow restrictor as part of it's design.

Just give up Bernie.

 

[dilalio]

What is the "exact value" for low fire

= the square root of fu¢k knows!