Positive Pressure Cases

[Gordon Millar]

2nd post from newly qualified engineer out on his own, so please be patient. In general will MI's state if a boiler is working under positive pressure and therefore the need to check for POC's around the case seal when refitted. Or will they assume the engineer will know it is under positive pressure. I know there is a list of positive pressure boilers available however I am not sure how accurate or up to date it is, and aside from infamous ones like the Potterton Netaheat how can you categorically determine a positive pressure system.

To be honest at college we had one such system and spent around half a day on it, not enough in my opinion for such an important area. Have purchased the Hayes Smoke Pen for testing such a scenario as I have read it is the preferred method over a Telegan, taper or smoke match test. Any comments from anyone with experience of testing using different methods would also be welcome, and which way was found to be most reliable.

 

[Billy Bob Bob]

ldf is a good tester also if you have room co on your fga that would help most older rs boilers can be positive pressured ,ferroli 77 ff combis for instance are one example

 

[GrahamM]

The list of boilers in for example recent corgi/gas safe book will be fairly accurate as there have been no positive pressure boilers produced in recent years due to the safety issues.
Basically what you are looking for is the fan pulling air in rather than pulling combustion gasses out.
The smoke pens are very good and show a clear indication, much better than a lighted mach or taper and as per gas man spend a few minuets running your FGA round any suspect seal and hold above the boiler for few mins.
LDF is not an approved/recognised media for testing a combustion seal though although may give an indication.

 

[< 0.004 = :-)]

worth bearing in mind a lot of the modern condensers are positive pressure , pushing the air/gas mixture into the burner , rather than pulling the poc's out .

a cold mirror is another way to check around seals , if you get condensation forming on the mirror , you ve got a leak , simples .

 

[GrahamM]

worth bearing in mind a lot of the modern condensers are positive pressure , pushing the air/gas mixture into the burner , rather than pulling the poc's out .

a cold mirror is another way to check around seals , if you get condensation forming on the mirror , you ve got a leak , simples .

Had a discussion bout this before.
Modern condensing boiler is not positive pressure in the conventional sense?
The outer case is still under negative pressure as the fan draws from here and forces air/gas into the sealed burner. If worst came to worst and the burner seal was to leak, POC's would just be recycled back into the burner. Unlikely any POC's would spill out the outer case.

 

[diesel_ltd]

Had a discussion bout this before.
Modern condensing boiler is not positive pressure in the conventional sense?
The outer case is still under negative pressure as the fan draws from here and forces air/gas into the sealed burner. If worst came to worst and the burner seal was to leak, POC's would just be recycled back into the burner. Unlikely any POC's would spill out the outer case.

this would show as a high co reading on fga

 

[< 0.004 = :-)]

Had a discussion bout this before.
Modern condensing boiler is not positive pressure in the conventional sense?
The outer case is still under negative pressure as the fan draws from here and forces air/gas into the sealed burner. If worst came to worst and the burner seal was to leak, POC's would just be recycled back into the burner. Unlikely any POC's would spill out the outer case.

know what you mean big G . not in the econventional sense like a netaheat.
senario - potterton envoy heat only condenser, flue hood not reseated correctly on HE gasket after a strip down service, fine jet of pocs being directed on to air box cover seal, heat and acid eats away at the foam seal, POCs leaking into room, seen this last week.

 

[Pssst]

An Engineer should be able to discern whether a boiler has a positive pressure arrangement either by knowledge of the appliance or by a visual assessment of the fan position/type/orientation.

Sealing?

Visually inspect the seal and the casing for wear/distortion. Check the outer edges of the case and the back panel for signs of heat/staining.

Fire the boiler and feel around the case for positive pressure release and also visually inspect for signs of condensation/heat release.

The Netaheat is a positive pressure appliance and also IMHO one of the best boilers made of that particular era. Solid and dependable but for the occasional explosion caused by dicky White/Rogers valves.

 

[grahamgas]

When I am testing profiles, primas neatheats etc, I turn off gas, put a smoke bomb in the air inlet part of flue, and put boiler on. Any case seal leakage WILL show up.

Graham

 

[Billy Bob Bob]

When I am testing profiles, primas neatheats etc, I turn off gas, put a smoke bomb in the air inlet part of flue, and put boiler on. Any case seal leakage WILL show up.
i also do this,its a good test,boiler looks funny though when the flames turn green

 

[mfgs]

When I am testing profiles, primas neatheats etc, I turn off gas, put a smoke bomb in the air inlet part of flue, and put boiler on. Any case seal leakage WILL show up.

Graham

Interesting way of doing it. I was always taught to run a match around the case seals and any leak with distort the flame. I always run my analyser around them aswell.

 

[grahamgas]

I do that as well. The only reason I test with smoke bomb, is I have lost count of the number of positive pressure appliances I have seen wedged tight into a corner, with no chance of getting a smoke match/pen in to test

Graham

 

[gasmarc]

I had this problem last week and could only get access to the top of the boiler as the kitchen units were a rizla paper away from the sides and no access to flue

 

[Plucky]

You should be able to tell if a boiler is positive case seal by just looking at the fan. If you can see the square outlet then it's 99% positive.

Once you get working with different boilers you'll soon learn

 

[gasmarc]

been doing the job for years and still learning :rockon: just bought a CO detector have to start using it