Navien Combi Boilers

[Deleted member 221031]

I understand that another method is to use the "suck" pressure ( a negative pressure ) from the venturi to operate a pressure control function in the gas valve. Gas rate is then set by a fixed orifice and the variable pressure from the gas valve.

As we call it “zero pressure governor”

 

[bernardgreen]

As we call it “zero pressure governor”

Is that the system where gas is "injected" at the venturi ( as in the image in post 174 ) ?

I was refering to a system where the air pressure (*) at the venturi control the gas rate with the gas being injected into the air flow after the venturi.

(*) possibly the difference in pressure between air before the venturi and air in the venturi though that would require two connections to the gas valve control.

 

[Deleted member 221031]

Ideal use a zero governor where the fan pulls gas through the gas valve and pulls air at the same time and mixes it in a Venturi and the pushes the mix through the burner.
This technology is nothing new!
This actually puts more friction in the pipework and would cause the gas pipe to require to be enlarged compared to an older type standard efficiency that requires positive pressure from meter.
That’s why we have a chart with acceptable pressure loss depending on boiler input.
Think on it this way.
The manometer is on a tee off the main route of gas and the positive inlet pressure is pushing the gas up one leg.
When the fan kicks in (the quicker the higher output) it pulls gas into the Venturi and this negative pressure also pulls the water down the leg of the manometer.
If the gas pipe is undersized then the gas can’t get through quick enough.
So your 15mm for 50kw is a complete lot of pish.

Shambolic describes it best.

 

[Dan Robinson]

Whoever wrote that rather poor description hadn't heard of modureg obviously. (not Shambo's)

 

[Dan Robinson]

I was wondering when Dannyboy was coming along.

Still avoiding the questions I see dipschit?

 

[hard-work]

Dannyboy is up to steam here, but not writing "muppet". Is this advancements in Dannyboy's literate? We shall see.

 

[bernardgreen]

hadn't heard of modureg obviously

Isn't that a generic term for gas valves that can modulate gas flow by various means ?

If not generic then you could describe the specific type of control mechanism in the valve.

 

[Razor900]

I realise that the professionals on this forum will be aware of this but for non professionals it may be worth posting. Sensible ( and polite ) comments are welcome

As I see it....

In the old days the amount of gas fed into the combustion area was fixed and determined by the size of the orifice in the nozzle and the pressure at the output of the gas valve. Air was drawn into the combustion area by the partial vacuum created by the hot combustion products convecting themselves up the flue. This simple system relied on the partial vacuum being greater than any pressure differential between internal air and external air.

The improvement that overcame problems caused by pressure differential between internal air and external air was to use a fan and balanced flue to ensure adequate air was fed into the combustion chamber, gas flow rate was still determined by the size of the orifice in the nozzle and the pressure at the output of the gas valve. No modulation possible.

The introduction of combi boilers required a means had to be found to modulate the heat ouput by varying the rates at which air and gas were fed to the combustion area.

Varying the rate of air was relatively simple, alter the speed of the fan.

In theory altering the pressure setting in the gas valve would be a simple way to vary the gas rate. In practise this was difficult to achieve while maintaining the fail safe features of the gas valve. ( Mechanically it became too complicated and too critical in assembly ).

With a fixed pressure at the output of the gas valve and a fixed orifice into the air flow the gas rate depended on the pressure of the air at the orifice. ( more accurately it depended on the difference between the pressure of the gas in the pipe and the pressure of the air around the orifice.

Increasing fan speed would increase air pressure around the orifice and thus reduce gas flow. The opposite of what was needed.

Placing the gas nozzle in the air intake of the fan did result in result in an increase of gas flow when the fan speed increased as air pressure in the fan's intake did reduce as speed increased.

Enter the venturi. The pressure of the air reduces when its velocity is increased by passing through a narrow length of tube.

A hole in the side of that narrow tube may suck air into the tube rather than allow air to escape.

A well designed venturi ( the narrow bit of tube ) will suck extra air into the tube at a rate directly proportional to the rate of air flowing through the venturi. There are limits to the maximum and minimum rates of air flow between which the ratio is maintained.

This can be used to "suck in" the correct rate of gas to suit the rate of air being blown into the combustion area. Difficult to design the venturi but once designed it's manufacture is simple machining and there are no moving parts. The range of modulation is limited.

I understand that another method is to use the "suck" pressure ( a negative pressure ) from the venturi to operate a pressure control function in the gas valve. Gas rate is then set by a fixed orifice and the variable pressure from the gas valve.

Nearly 20k posts in and you've finally made one I like; credit where it's due


It's weird cars have gone from carburettors to fuel injection for greater efficiency and in essence the combustion mechanisms and principles for boilers have gone the other way also for greater efficiency...........

 

[Dan Robinson]

Dannyboy is up to steam here, but not writing "muppet". Is this advancements in Dannyboy's literate? We shall see.

What?

 

[bernardgreen]

It's weird cars have gone from carburettors to fuel injection for greater efficiency.........

at the cost of extremely complex mechanisms and servicing procedures. My first car was a "sit up and beg" Ford Popular, side valve engine... Changing a head gasket was job that could be done in the lunch break when it blew on the way to work. Kept a spare gasket in the boot. I wouldn't tackle anything more than a wheel change on the 2015 Megane



Not my actual car but same model.

 

[Razor900]

Yes and no. A modern engine will do a quarter of a million miles with basic servicing. Not many Pops could match that

 

[Dan Robinson]

Ah - the ol' "it was better in my day" fallacy

 

[vulcancontinental]

Thank you! That’s what I’ve been saying. A negative pressure gas valve is the same as zero pressure governor.

Navien have made it sound like an “innovation”.

Fan speed alters gas and air input/output. Turn the temp control on a modern boiler and the fan will speed up or slow down. As you know.

It’s the way the fan works more than the gas valve.

Your boiler is classed as fan flued. It removes the products of combustion and has not direct control of the amount of gas used.

I'll have to come back to this thread as i'm going out but manufacturers pinch terms all the time; Viessmann with Lambda which with regard to boilers is the amount of excess air necessary to the minimum needed to burn the fuel. It makes it sound like they invented something but it's just marketing.

I look at the fan as making sure there is always enough combustion air available for best possible combustion at maximum gas rate as that's what determines the set speed of the fan for quantity; what pressure remains determines the flue length possible. Just my way of looking at things.

I was told the modulating gas valve set up with a modulating fan through a washed copper heat exchanger al la the Syntesi and Synergy boilers, really B rated Compact HE's with a modulating fan, can exceed the efficiencies produced with the current Band A boilers but it's manufacturing costs and emissions that are a problem there.

I can't prove it as I don't have the paperwork anymore but when working for Eaga who fitted loads of Isars and Icos's for the Warmfront scheme we were all given a sheet from Ideal saying the last 10' or 3m of gas supply could be in 15 as the fan would make up[ any loss through 'undersized' pipework.

 

[ianmcd]

Ideal use a zero governor where the fan pulls gas through the gas valve and pulls air at the same time and mixes it in a Venturi and the pushes the mix through the burner.
This technology is nothing new!
This actually puts more friction in the pipework and would cause the gas pipe to require to be enlarged compared to an older type standard efficiency that requires positive pressure from meter.
That’s why we have a chart with acceptable pressure loss depending on boiler input.
Think on it this way.
The manometer is on a tee off the main route of gas and the positive inlet pressure is pushing the gas up one leg.
When the fan kicks in (the quicker the higher output) it pulls gas into the Venturi and this negative pressure also pulls the water down the leg of the manometer.
If the gas pipe is undersized then the gas can’t get through quick enough.
So your 15mm for 50kw is a complete lot of pish.

He still wont listen though because Naviaen said they can over come the laws of Physics then it must be true, will say it again as we all know you can not get that amount of gas at that pressure through that pipework no matter who says it

 

[Dan Robinson]

a sheet from Ideal saying the last 10' or 3m of gas supply could be in 15 as the fan would make up[ any loss through 'undersized' pipework

Indeed VC - but that is for the boiler alone and not to BS . I remember back in the days of renting we had a TurboMax in our flat. If cooking and showering, the oven would often go out. Confused honourable MIL a few times that did when she came to stay

As you say though - Navien hasn't invented anything other than a branding image - which we have proven time and again can confuse Maintap as he ha nothing else to go on.