Commercial boiler trouble

[keh7486]

Hi got to go to a building Monday because of a boiler/extractor problem.
What I have been told is this
3 phase boiler
3 phase extractor fan
Pressure switch either side of the fan


Apparently when you isolate fan the boiler doesn't turn off and the same when the pressure drops from pressure switch the boiler stays on.


My question is how should this be wired and where do I start. Any help would be appreciated as I'm dreading this job.

 

[Gage]

If you want the boiler to drop out when there's no pressure and when the fans not on you can just wire those parameters in to the circuit, using contactors. Might explain more later with a drawing if I get time. What exactly have you been asked to do here

 

[snowhead]

The Boiler house has mechanical ventilation and air supply, linked to the control circuit for the boiler.
The boiler shopuld not run without the fans as there will be insufficient combustion air for the boiler.

Assuming it was once correctly working, the differential air pressure switch is connected into the boiler control circuit, no detected airflow across the fan should mean the air switch doesn't make.

Most likely fault is someone has linked it out either at the switch or in the boiler control panel.
Another possible is the tubes to the switch are cracked or have fallen off
Could be the pressure switch is faulty, you should be able to detect it switching when the fan starts.
Tubes could be on the wrong way round.
Fan might be running backwards from original design, so effectively no airflow.

 

[Gage]

So the circuit I recommended already exists? If so then yeah probably. Sounds like you have it under control tbh...Just find it if it did work before and find out what was altered and why and correct whatever lazyman's bodge has been done.

 

[Knobhead]

This really is one that if you don't know what you're doing you should leave well alone.

 

[big-steve]

Find out how it's controlled,and i take it the burner fan is 3 phase? The air flow has to be proved via the air diffriential switch, with tubes either side of the flue fan to prove flow, once flow is proved the signal should go to the boiler controller to enable on, and it should start the burners pre purge and ignition process

 

[keh7486]

Big Steve that's pretty much how it's set up. The cable from the air differential switch goes to the Bms panel. The Bms guy came out and said the controller isn't working in the Bms panel. So now there's talk of some how by passing the Bms and use a relay to switch the boiler feed using the air differential switch. This building is so old that the landlord doesn't want to spend any money on Bms repairs as the building is being knocked down at the end of the year