Baxi Duo Tec 40 HE A Fluctuating Hot Water Temperature

[bernardgreen]

Just give up Bernie.

with answers like this there probably is no point in asking questions.......

= the square root of fu¢k knows!

One does wonder if the reason for such answers is that the person does not know the answer.

 

[picasso]

Baxi don't publish minimum gas rates for DHW but they give a figure of 6kws for central heating so would assume the same minimum would apply for DHW, they also give this piece of advice...

 

[Master Mech]

Well I am more confused now than when we started.

Are we still saying the boiler is at fault?

 

[DP]

Well I am more confused now than when we started.

Are we still saying the boiler is at fault?

I would be looking at the boiler to see 2hy boiler not behaving as it should. Ultimately burner size and output are determined by the temperature sensors.

It is like driving a car, your foot and eyesight decide are used to propel the car to a a target speed. You boiler keeps shutting down before analogical car reaching 70 on the motorway

 

[bernardgreen]

Baxi don't publish minimum gas rates for DHW but they give a figure of 6kws for central heating so would assume the same minimum would apply for DHW, they also give this piece of advice...

That 2.5ltr/min flow rate does fit with the calculations for water cooling of electronic equipment.

I altered the parameters T(in) T(out) and heating load to match the application of a boiler needing to be kept stable.


To calculate minimum water flow rate required to ensure thermal stability

For heating water the formula is

Heat required = volume in litres x delta T in degC / k
where factor k = 853 for the result in kWhr

transposing the formula
volume in litres = heat produced x k / delta T

Given maximum temperature rise is 40 deg C
T (in) = 20 deg C
T (out) = 60 deg C

Assume 6 kW for 20 minutes ( one third of an hour )
hence heat produced = 6 / 3 = 2 kWhr
volume = 2 x 853 / 40 = 42.6 litres
42.6 litres in 20 minutes is a flow rate of 2.13 litres per minute.

Assume 6 kW for one hour
hence heat produced = 6 kWhr
volume = 6 x 853 / 40 = 127 litres
127 litres in one hour is a flow rate of 2.1 litres per minute.


Adapting this to the intentional heating of flowing water in a water heating system

To heat 100 litres of water from 20ºC to 60ºC, delta T = 40
100 x 40 / 853 = 4.7

Hence 100 litres of water require 4.7 kWhr if the temperature of the water is to be raised by 40 deg C

Assume flow rate of 5 litres per minute through the heater,
hence 100 litres passes in 20 minutes
20 minutes is one third of an hour
to create 4.7 kWhr in one third of an hour requires a heating rate of 4.7 x 3 = 14.1 kW

Assume flow rate 10 litres per minute through the heater,
hence 100 litres passes in 10 minutes
10 minutes is one sixth of an hour
to create 4.7 kWhr in one sixth of an hour requires a heating rate of 4.7 x 6 = 28.2 kW

 

[nwgs]

That 2.5ltr/min flow rate does fit with the calculations for water cooling of electronic equipment.

I altered the parameters T(in) T(out) and heating load to match the application of a boiler needing to be kept stable.


To calculate minimum water flow rate required to ensure thermal stability

For heating water the formula is

Heat required = volume in litres x delta T in degC / k
where factor k = 853 for the result in kWhr

transposing the formula
volume in litres = heat produced x k / delta T

Given maximum temperature rise is 40 deg C
T (in) = 20 deg C
T (out) = 60 deg C

Assume 6 kW for 20 minutes ( one third of an hour )
hence heat produced = 6 / 3 = 2 kWhr
volume = 2 x 853 / 40 = 42.6 litres
42.6 litres in 20 minutes is a flow rate of 2.13 litres per minute.

Assume 6 kW for one hour
hence heat produced = 6 kWhr
volume = 6 x 853 / 40 = 127 litres
127 litres in one hour is a flow rate of 2.1 litres per minute.


Adapting this to the intentional heating of flowing water in a water heating system

To heat 100 litres of water from 20ºC to 60ºC, delta T = 40
100 x 40 / 853 = 4.7

Hence 100 litres of water require 4.7 kWhr if the temperature of the water is to be raised by 40 deg C

Assume flow rate of 5 litres per minute through the heater,
hence 100 litres passes in 20 minutes
20 minutes is one third of an hour
to create 4.7 kWhr in one third of an hour requires a heating rate of 4.7 x 3 = 14.1 kW

Assume flow rate 10 litres per minute through the heater,
hence 100 litres passes in 10 minutes
10 minutes is one sixth of an hour
to create 4.7 kWhr in one sixth of an hour requires a heating rate of 4.7 x 6 = 28.2 kW

What a load of rubbish. I go to loads of houses where the water pressure is poor and the boiler just modulates down to the flow provided

I’ve never read such crap

 

[bernardgreen]

I’ve never read such crap

If you consider it to be crap then you should be able to explain why you believe it is crap, can you explain ? An explanation would be useful for people asking questions about their boiler switching ON and OFF

and the boiler just modulates down to the flow provided

No domestic boiler can modulate down to 10% or less ( 10:1 turn down ). As people are mentioning on the forum when the flow provided is too low the boiler starts to cycle ON and OFF.

Disclaimer. boilers with variable geometry combustion chambers can modulate to less than 10% but are too complicated for domestic service

 

[nwgs]

If you consider it to be crap then you should be able to explain why you believe it is crap, can you explain ? An explanation would be useful for people asking questions about their boiler switching ON and OFF



No domestic boiler can modulate down to 10% or less ( 10:1 turn down ). As people are mentioning on the forum when the flow provided is too low the boiler starts to cycle ON and OFF.

Disclaimer. boilers with variable geometry combustion chambers can modulate to less than 10% but are too complicated for domestic service

Modern boilers need a “minimum” flow rate to activate. So the OP’s boiler has met the minimum requirement for flow. Or it wouldn’t fire up

 

[Blooflame]

Jeez.
Too much theory, not enough practical!

I'd love to help but am amazed that the responses just go on..and on...and on. Lime you say...too much theory.

 

[ianmcd]

No domestic boiler can modulate down to 10% or less ( 10:1 turn down )

wrong again bennyboy

 

[bernardgreen]

wrong again bennyboy

Prove that. List those that can.

EDIT OK I accept there may be some that can modulate down to 10%

"" Our Linea One 38kW combi boiler boasts a class-leading 10:1 modulation ratio, enabling it to modulate its output to 10 per cent of its maximum – 38kW down to 3.8kW. ""

I do still wonder how the efficiency changes between 100% and 10%

Do manufacturers provide any data about modulation versus efficiency ?

 

[ianmcd]

Prove that. List those that can.

Go google, it is what you do anyway, start with Navien who actually claim ven better turn down ratios

https://www.navieninc.com/residential/combi-boilers

 

[ianmcd]

well @bernardgreen has your computer broken down ?

 

[ianmcd]

Prove that. List those that can.

EDIT OK I accept there may be some that can modulate down to 10%

"" Our Linea One 38kW combi boiler boasts a class-leading 10:1 modulation ratio, enabling it to modulate its output to 10 per cent of its maximum – 38kW down to 3.8kW. ""

I do still wonder how the efficiency changes between 100% and 10%

Do manufacturers provide any data about modulation versus efficiency ?

Thought you had a J&S not a Vokera ?

 

[bernardgreen]

Go google, it is what you do anyway,

Other sources are available ( some have restricted access ) as are other search engines

start with Navien who actually claim ven better turn down ratios

High turndown ratios up to 11:1

well @bernardgreen has your computer broken down ?

No. I was doing other things

Thought you had a J&S not a Vokera ?

Yes you are right I have a Johnson and Starley, does that matter ?

I do still wonder how the efficiency changes between 100% and 10%

I did briefly google for information that is in the public domain but nothing found.