Gas Rate/ Heat input....why so important?

[Savva]

Hi all I'm being asked to do more & more GSR's and want to develop my understanding of WHY heat inputs/ gas rates are so important. With a flueless appliance like a HOB, where the POC's fill the room, it's obvious why the app needs to burn the correct amount of gas. But for a room sealed app such as a boiler, it's still safe even though the heat input is higher/ lower than the MI's...isn't it?

 

[ShaunCorbs]

Depends if it's ranged rated then you can see what it's set at

And with a new condensing boiler your allowed 10% either way no more tho

 

[Pssst]

If its a natural draught burner then it may be that the burner cant be range rated and is specifically designed to work within tight parameters eg a gas fire. The injector is designed to pas a certain volume of gas at a certain pressure and the burner is designed to entrain a certain volume of air to ensure good combustion. If you run it at higher pressure then you up the volume of gas delivered but you cant really greatly control to amount of air drawn in.

Forced draught high efficiency boilers are similar. Amount of air and therefore gas is controlled by fan speed/gas control train etc. Combustion is kept a lot closure to stoichiometric than a natural drive burner to drive up efficiency .

 

[Savva]

If its a natural draught burner then it may be that the burner cant be range rated and is specifically designed to work within tight parameters eg a gas fire. The injector is designed to pas a certain volume of gas at a certain pressure and the burner is designed to entrain a certain volume of air to ensure good combustion. If you run it at higher pressure then you up the volume of gas delivered but you cant really greatly control to amount of air drawn in.

Forced draught high efficiency boilers are similar. Amount of air and therefore gas is controlled by fan speed/gas control train etc. Combustion is kept a lot closure to stoichiometric than a natural drive burner to drive up efficiency .

Thanks for the reply Shaun I guess I'm approaching this from a safety point of view. If a modern boiler, RS with a fan, is giving safe gas analysis readings BUT the gas rate is out then what is the correct procedure....is it an advisory only? or AR? what do you think?

 

[London Gas]

Depends if it's ranged rated then you can see what it's set at

And with a new condensing boiler your allowed 10% either way no more tho

Its +5% and -10% will state this usually in the instructions most have a table stating norm and +5% and -10%.

Lets say for arguments sake you had a Vaillant 837 and you did a reading which was high but within and that's okay but you didn't gas rate it and due to the power of this boiler and you taking this reading at peak time, the boiler could be modulating down perhaps due to lower flow rate at that time of the day, your readings are safe but the heat input is out, you tick all the boxes and leave, the next day the customer is using the boiler and now its operating at the correct HI but is burning with a higher now unsafe ratio you've left an unsafe situation but you didn't realise because you didn't check if it was burning correctly either by BP or HI.

I had this exact situation with a GSI on my check when I got my own license and his view was if the HI is out its AR as its not operating as per manufacturers instructions regardless if it was the boiler or a third party issue such as in the situation being a block of flats with poor water pressure and you can't therefore test the boiler at max rate.

 

[GrahamM]

The 'Gas Rate' indicates the appliance is operating within the Manufacturer limits. ie. the design of the appliance.
Unless MI's state otherwise as per TB141 you are permitted +5% & -10% of the heat input.
If heat input is outside these parameters you may be looking at some installation defect or fault with appliance that may require an AR notification until rectified.

 

[RoryD]

Hi all I'm being asked to do more & more GSR's and want to develop my understanding of WHY heat inputs/ gas rates are so important. With a flueless appliance like a HOB, where the POC's fill the room, it's obvious why the app needs to burn the correct amount of gas. But for a room sealed app such as a boiler, it's still safe even though the heat input is higher/ lower than the MI's...isn't it?

For a multitude of possibilities, safety long term cannot be guaranteed. We can only ensure at that snapshot in time that we have done our best to ensure the correct amount of gas is being consumed as per MI's. What I find baffling is that a GSR Engineer has to ask such a question.

 

[Savva]

For a multitude of possibilities, safety long term cannot be guaranteed. We can only ensure at that snapshot in time that we have done our best to ensure the correct amount of gas is being consumed as per MI's. What I find baffling is that a GSR Engineer has to ask such a question.

longterm safety can't be "guarenteed" under any circumstances....plonka.

 

[Savva]

Its +5% and -10% will state this usually in the instructions most have a table stating norm and +5% and -10%.

Lets say for arguments sake you had a Vaillant 837 and you did a reading which was high but within and that's okay but you didn't gas rate it and due to the power of this boiler and you taking this reading at peak time, the boiler could be modulating down perhaps due to lower flow rate at that time of the day, your readings are safe but the heat input is out, you tick all the boxes and leave, the next day the customer is using the boiler and now its operating at the correct HI but is burning with a higher now unsafe ratio you've left an unsafe situation but you didn't realise because you didn't check if it was burning correctly either by BP or HI.

I had this exact situation with a GSI on my check when I got my own license and his view was if the HI is out its AR as its not operating as per manufacturers instructions regardless if it was the boiler or a third party issue such as in the situation being a block of flats with poor water pressure and you can't therefore test the boiler at max rate.

Interesting scenario...thanks for the response

 

[Lambed84]

Manufacturers engineer their boilers to operate as close to the Stoichiometric requirments of gas as possible outside of laboratory conditions with increasing success.
If an appliance was to operate outside of this (heat input too high or low) then you get incomplete combustion and therefore carbon. The longer this goes unnoticed the more soot builds up. The more soot, the greater the UNSAFE SITUATION.
Remember the combustion triangle, burner pressure - heat input - CPA.
If you get 2 correct it's a fact for the third to be. With the introduction of smart meters and loosing the accuracy gas rating once had, it's imperative to confirm atleast 2 of the combustion triangle. Making it essential to not only know how to gas rate but cary out the test.
Just my humble opinion.

 

[bacon_sandwich]

I look at it also from a slightly different perspective. Modern boilers modulate at least down to 1/4 full power. I want to ensure that the design can ensure the boiler reaches its maximum power. So a good cold start gives me a good indication that say a 65kw boiler is actually delivering that amount of power.

 

[RoryD]

longterm safety can't be "guarenteed" under any circumstances....plonka.

Which is what I said you imbecile. Now we're even in the name calling sequence I'll suggest you spend more time swotting up and less time stating the bleeding obvious

 

[Savva]

Manufacturers engineer their boilers to operate as close to the Stoichiometric requirments of gas as possible outside of laboratory conditions with increasing success.
If an appliance was to operate outside of this (heat input too high or low) then you get incomplete combustion and therefore carbon. The longer this goes unnoticed the more soot builds up. The more soot, the greater the UNSAFE SITUATION.
Remember the combustion triangle, burner pressure - heat input - CPA.
If you get 2 correct it's a fact for the third to be. With the introduction of smart meters and loosing the accuracy gas rating once had, it's imperative to confirm atleast 2 of the combustion triangle. Making it essential to not only know how to gas rate but cary out the test.
Just my humble opinion.

Thanks for the reply...never knew incomplete combustion produces soot. Useful to know

 

[Lambed84]

Thanks for the reply...never knew incomplete combustion produces soot. Useful to know

Ch4 + 2o2 =
co2 (carbon dioxide) 2h20 (water)
Being complete combustion.
If gas rate is high, or viciated air (incomplete combustion) you'll have this chemical equation:
Ch4 + 1o2 =
Co (carbon Monoxide [commonly known as soot]) h2o (water)
Methane is carbon based hence the c.