Navien Combi Boilers

[shambolic]

Probably a typo... should read 1k pascals.

on my electronic gauge but it is 1.1mbar as it’s the offset reading on the outlet with the fan pulling through.
Try it some time on an Isar
Ps how would my intermediate slamshut cope on my meter?
Every time the boiler sucked that amount the thing would trip

 

[shambolic]

That sounds rather high, 500mm water.

And that seems low, 1.1mm water. Maybe it's enough for your burner.

Nope.

 

[fixitflav]

Nope.

In #187 you said 11Pa. That's 0.11mbar (BTW the French call a mbar a hectopascal) = 1.1mm wg. Now you say 1.1mbar (= 11mm wg)

 

[fixitflav]

I’m really hungover so I can’t quite understand the difference between that and a negative pressure fan set up an most modern boilers.

Negative pressure is created by the fan. It sucks gas at the desired rate, set by demand/fan speed.

In my opinion Navien’s gas valve works the same as a zero pressure one.

There’s no difference.

I found this on the web
http://www.a1gassafe.co.uk/pdf/tb189_air_gas_ration_top_10.pdf
which explains the differences. It implies negative pressure valves are common nowadays, rather than something of an innovation from Navien. It has diagrams of venturis etc which older systems don't have.
It refers to the need for modulating fan and gas valve. I can see the advantage in modulating the fan, not just the gas supply, as otherwise excess air is being heated, and not all the heat recovered. But if you have a modulating gas valve and a variable-speed fan, the same can be be achieved with a traditional set-up, as far as I can see.

 

[fixitflav]

And its still working? Even Ideal said it was crap. Time for new one soon.

Yes, I've read some scathing comments! But mine is still working fine. I've seen positive things about the Ideal Logik on this forum. Can you confirm?

 

[Deleted member 221031]

I found this on the web
http://www.a1gassafe.co.uk/pdf/tb189_air_gas_ration_top_10.pdf
which explains the differences. It implies negative pressure valves are common nowadays, rather than something of an innovation from Navien. It has diagrams of venturis etc which older systems don't have.
It refers to the need for modulating fan and gas valve. I can see the advantage in modulating the fan, not just the gas supply, as otherwise excess air is being heated, and not all the heat recovered. But if you have a modulating gas valve and a variable-speed fan, the same can be be achieved with a traditional set-up, as far as I can see.

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.

 

[Deleted member 221031]

Yes, I've read some scathing comments! But mine is still working fine. I've seen positive things about the Ideal Logik on this forum. Can you confirm?

I’m a Baxi and ideal installer. The logic is a good little boiler.

 

[fixitflav]

I’m a Baxi and ideal installer. The logic is a good little boiler.

OK thanks

 

[fixitflav]

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

Perhaps some of us have been getting at cross purposes. I thought you were saying that a negative pressure gas valve system is no different from older type (like mine, for example) which it clearly is as mine does not have a venturi, and needs 8.6mbar at the valve outlet.

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

That's right, it has an On/Off gas valve and a fixed-speed fan. It may not be the latest thing, but it's simple, and has been reliable.

 

[Deleted member 221031]

Perhaps some of us have been getting at cross purposes. I thought you were saying that a negative pressure gas valve system is no different from older type (like mine, for example) which it clearly is as mine does not have a venturi, and needs 8.6mbar at the valve outlet.

That's right, it has an On/Off gas valve and a fixed-speed fan. It may not be the latest thing, but it's simple, and has been reliable.

Your boiler is classed as “fan flued/standard efficiency” SE

Newer boilers, the ones I’m referencing are “premix”. They are the zero pressure governor type.

If we take it that Navien haven’t invented a new gas valve. That leaves your the older type. And the more modern ones.

Position of the fan is a good indicator

 

[Dan Robinson]

The Minimiser like the Response is a kind of half way house.

Still, not a zero/negative pressure system.

Maintap hasn't a clue what he is wittering on about others he would have explained himself and answered the pretty basic questions asked of him.

It's pretty sad really.

The Navien combustion train is nothing new. It's a simple down firing burner, like Atag. Except is doesn't have a ceramic burner head from what I can see.

 

[hard-work]

Yes, I've read some scathing comments! But mine is still working fine. I've seen positive things about the Ideal Logik on this forum. Can you confirm?

I avoid Ideal like they were the plague.

 

[hard-work]

That's right, it has an On/Off gas valve and a fixed-speed fan. It may not be the latest thing, but it's simple, and has been reliable.

Make sure you clean the heat exchanger once a year. In real use the efficiency was no better than a decent non-condensing boiler.

 

[hard-work]

The Minimiser like the Response is a kind of half way house.

Still, not a zero/negative pressure system.

Maintap hasn't a clue what he is wittering on about others he would have explained himself and answered the pretty basic questions asked of him.

It's pretty sad really.

The Navien combustion train is nothing new. It's a simple down firing burner, like Atag. Except is doesn't have a ceramic burner head from what I can see.

I was wondering when Dannyboy was coming along.

 

[bernardgreen]

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.