Modern boilers - different type of Gas Valve?

[jackecole]

Hi - I have a query on boilers that I'd be greatful if someone could please explian?:

Do a lot of modern boilers now use a different type of Gas Valve (Zero Pressure type)?

Do these valves work in this manner: The fan draws gas through the fan and into a mixing chamber, the gas and air then mix and is blown down onto the heat exchanger where it is ignited at the burner?

Or am I way off with how it works? I know they have a gas/air ratio and you have to be specifically trained to be able to adjus this (if necessary).


Are these new type of Gas Valve's called Zero pressure Governor's?


If anyone could explain briefly I'd be so greatful!

 

[Agile]

Thats roughly correct.

A zero pressure regulator, as it says, does not produce a positive gas pressure so the gas is "sucked" out of it by a variable speed fan.

That kind of boiler needs to be set up by measuring the gases coming out of the flue.

We now have to do a special assessment module on using the gas analysers and have them calibrated every year. Yet more expense for registered gas engineers which helps to explain why we charge more than £10 to repair boilers.

Tony

 

[jackecole]

Thanks Agile for your reply and help.

Is the air/gas mixture then blown down onto/towars the burner, I was confused about it being blown through the heat excahnger?!!

Also on the gas valve, is there still a gas/air ratio point that you have to test or is it just done in the flue using a FGA then?

I know understanding boilers/fault finding/repairing them is very hard and knowledgable, hence why the fair prices and rightly so. I had a thermocouple changed last October on an old Potterton (first time I've had it replaced) how much would an average price have been for that in your opinion? I paid £110 in total, which I thought was fair.

 

[Paul Barker]

The fan (well actually it's a compressor but manufacturers seem happy to retain the name fan for the sake of most of their customers who won't appreciate the change of purpose) has an inlet which draws air from the boiler casing (which must be sealed to the room) whether via a fancy tube or directly, the air originates outside, usually through the outer concentric but some boilers have two pipe flues.

Usually in the centre of the air entry port of the compressor is a small venturi so the effect of the rushing air sucks gas from the gas valve which is more or less at neutral pressure. The gas an air ratio is normally adjusted in the gas valve so more or less gas is sucked by this venturi effect. Some boilers are only adjustable at high gas and low is just checked, but if it is out you would give priority to making sure max gas ratio is correct. In any case before adjusting untoward factors must be eliminated like cracked components which are not uncommon.

How to detect the ratio depends on the manufacturer. Buderus ask for a very fine resolution differential manometer to check the actual pressure of the gas valve , which they want to be between 0 and -.01mb so you set at -.005. Most manufacturers just ask for the CO2 in the flue gasses to be checked and ratio adjusted to get it to where they specify (which is different for all manufacturers, and sometimes across their range). Some ask for you to set fan speeds and some ask for you to monitor differential Pressure like Buderus but additionally and at the same time check CO2 for both max and min settings.

But the main principle is that the venturi effect at the inlet of the compressor ensures stoichiometric mix.

So the burner is fed a stoichiometric mix which is igighted by a glow device or a spark device as it is blasted into the combustion chamber, often downward firing but also radially firing, seldome upward firing these days due to condensation.

The speed of the fan essentially determines the size of the flame.

Some manufacturers are confident that they have set it all correctly as it left the factory and don't even want it monitored.

 

[jackecole]

Wow, thanks so much for that explanation, that is realy really top stuff. I understand it now (although it does seem confusing). And these boilers with this type of gas valve/operation are called ZPG's?

Do a lot of boilers now days have these, or do a lot still have the usual gas vale and fan / aps etc?

I guess fault finding/repair on these ZPG boilers must be a lot more technical!

 

[mehran]

nice explanation paul, you are wasted with them BG lot...

Do a lot of boilers now days have these, or do a lot still have the usual gas vale and fan / aps etc?

mostly every band B boiler is the old type "atmospheric burner" and most premix boilers are A rated.

they cant really get must more efficiency out of the older design as they have to burn the gas with a lot more air then it is needed to make sure it burn clean.

there is still plenty of this type of boiler out there(in fact most) but i think this will be the last year from them as all new boilers fitted will need to be A rated soon with the new part L

 

[Boilerman2]

Technically "Stoichiometric" Combustion means complete combustion of the fuel with Zero Excess air, this is not possible with any combustion process involving a boiler, due to flame chilling resistance of the heat exchanger etc etc, so we always set burners with an Excess air of 28-35%, this ensures that soot and CO will not be produced under variying conditions so not quite "stoichiometric"

 

[Agile]

The vast majority of the boilers currently being sold have a radial burner within the centre of a spiral stainless tube containing the system water to be heated.

All boilers mixing air/gas before the burner are called "premix" and thats how all future boilers will be designed. They only have just a few percent of excess air so are not heating air unnecessarily.

The reason boilers are not so close to being 100% efficient is because air is 79% nitrogen and that does not take any part in the boiler operation but it still wastes heat because its heated up and then just ejected from the flue.

The OP asked what others would charge for a boiler repair where a thermocouple needs to be changed. In our local area we charge a diagnostic fee of £84 plus parking and would have added £8 for the thermocouple.

At a 1967 boiler this evening, the thermocouple was in good order but the fault was the over heat stat which I managed to leave in good order so only cost £84.

Tony

 

[Nige F]

Thanks guys for interesting and useful posts - makes a change from the " it`s secret" replies

 

[Agile]

There is no problem explaining technicalities in general. What is not permitted is giving information or encouragement to do gas work to someone who is not qualified or experienced.

What I should have added to my posting above is that as there is so litttle excess air when the boiler is set up correctly, that makes the adjustment of the boiler quite critical.

Its no longer just seeing a nice bright blue flame! Now its seeing the makeup of the flue gases on the analyser. That obviously needs the equipment, needs someone who knows how to use it and interpret the results. I would guess that less than 10% of boiler installers actually have one and even fewer know how to interpret the results.

On boiler makes which I see often, I sometimes have a bit of fun with myself when I am not pushed for time and practise setting them up without the analyser and then see how close they were. Sometimes they are quite close but they can also be a long way out. Individual installations can sometimes be difficult to adjust even with the analyser.

Tony

 

[Gasman77]

I hope your not having fun at your customers expense tony! Don't think they are paying you to fanny about with their boiler for fun.

Also your comment about less than 10% of installers owning a FGA is silly and unfounded I would of thought.

And also stating that less than that know how to read the results? You really think over 90% of installers don't know what these results mean?

Why do you always run down others in the industry?

 

[Boilerman2]

Of course Oil engineers have had to analysers since the 1950'2 at least

 

[Happyplumber]

It is Agiles nature,to try and prove how good he is or maybe he has complex ,so only 10% know how to use them hey so why the hell they going on courses and having to sit the exams.

You only recently did yours as well if i remember correctly and when you are fafing around with peoples boilers and ""practising""" how to use a flue gas analyser,is this because you are not confident with using it correctly


Get off your fooking high horse and stop slagging off others in the industry.

That advice is free i wont charge you 84 quid for it numpty!!!

 

[sambotc]

Have to unfortunately agree with agile on this one. Most installers hang the boiler and leave, simple.

This is the creation of 3 day ACS courses to give a plumber a gassafe number IMO.

Must admit I found this post very imformative and useful, I won't pretend I knew what was written before I read it.

It really is frightening how little some people know, but are qualified to be able to install and maintain gas appliances legally.

Sam

 

[Gasman77]

Sam, no offence , but if you think less than 10 % of RGI's you really need to open your eyes, Agile is talking out of his rear end as usual, spouting facts and figures that only exist in his head.

The sad thing is , is that he really thinks they are true.