Positive Pressure Boilers

[0161coop]

Shouldn't all the modureg condensing boilers get added to the list of positive pressure boilers in the FAQ sticky?

 

[namsag]

WHY.????

 

[Paul Barker]

Only the ones with a compressor/zero gosvener gas valve.

 

[0161coop]

Yes - all the positive pressure ones !!!

 

[swbjackson]

All those that I have worked on are not positive pressure in the way that the old boilers are. On the old boilers the fan is on the air intake and pressurises the whole casing. on condensing boilers the fan draws air from within the casing so the main casing is still negative pressure.

 

[namsag]

COOP. would you like to explain why you think they are positive pressure and what models

 

[Paul Barker]

All those that I have worked on are not positive pressure in the way that the old boilers are. On the old boilers the fan is on the air intake and pressurises the whole casing. on condensing boilers the fan draws air from within the casing so the main casing is still negative pressure.

you are quite right but the problem is that any break in the integrity of the inner casing forces stoichiometric mix of gas and air into the outer casing at positive pressure. Now suddenly the integrity of the outer case is significant. Yet is is sometimes made of plastic.

In the case of Ideal Glowworm and Buderus there may be others I am unaware of and we may yet discover more, this has become a significant issue, ranging from customers smelling gas to burned out boilers. only a miracle a house hasn't burned down yet.

Sadly ktchen fitters and other cupboard builders are not gas trained, so are blissfullly unaware of the importance of this insignificant looking outer casing seal. So they carve it up in order to be able to remove the case without having to carve up that thing which must be worshiped the kitchen cupboard.

 

[mogget]

I would have thought that with your average design of premix boiler, as long as the compressor is blowing at least something out the flue, the outer case is still at net negative pressure. There could be a leak, say of air/gas mixture but it will be recirculated within the boiler and not into the room. (I'm thinking specifically of vaillant / glowworm / worcester etc, and don't know an awful lot about other makes).

edit - swbjackson, I should have read the whole thread from the beginning

 

[Agile]

There cannot be a leak of gas/air mixture because that would mean no flame and would be shut down within 5 seconds on flame failure.

There could be a leak of POC but even then as stated above a small leak would be recirculated.

Because the combustion conditions are so sensitive the flame would not burn if there was a very large recirculation as the O² would be insufficient.

Tony

 

[Paul Barker]

There cannot be a leak of gas/air mixture because that would mean no flame and would be shut down within 5 seconds on flame failure.

But there is one in the Buderus customer reports smelling gas, put a gas sensor on it and it hits the roof. On the Buderus there are two red rubberish seals through which a couple of electrical items pass I think one is the glow electrode the other the sense electrode. So what must be happening is that some of the unburned gas air mix escapes before it reaches the burner in the buderus example.

There could be a leak of POC but even then as stated above a small leak would be recirculated.

The boilers mentioned with this fault had no insignificant leak of poc they had flames. In the case of the Ideals the burner box made of alloy is burned right through. The Gowworms are also trashed if left alone.

Because the combustion conditions are so sensitive the flame would not burn if there was a very large recirculation as the O² would be insufficient.

Tony

But this is not the case in practice.

I have pictures of an Ideal burner box burned through I actually went to one.

I have also been to a Buderus with gas/air escape.

The manufacturers have dealt with these issues for the most part in the boilers mentioned.

The significant point i am trying to make is that these boilers require that second sealed chamber for safety. not all members of our trade and certainly no joiner is aware of this significance. It is not uncommon to find one hacked away at.

As for whether it is a positive chamber yes it can become positive as easily as you can call it negative it all depends on the size of the break in integrity of the inner casing seal and the relative air flow resistances. It has to be treated as positive for safety conscerns.

 

[Paul Barker]

I should also add that on the Buderus which was gassy the large round seal was dried up and shrunk also. As I haven't found another Buderus with that problem I can only suspect that the changed combustion environment created by the leaky seals affects other parts.

I have also had a Buderus with corroded heat ex coil, but can't be sure it wasn't system water. Though it passed a british Gas termidity test (could see all the rings, almost clear actually, I didn't send it away for analysis as that is costly. I haven't encountered this on any other Buderuses. Anyway I powerflushed that one when I installed the new heat exchanger.

The buderuses haven't been as trouble free as I expected them to be when i introduced them into my area. For the most part it was only a couple of installers who trusted me that installed them. Now I hang my head in shame. i will say however I don't mind going out to fix them as they are nice and easy to work on.