I have a firebird Popular 90 installed in 2007 and have lots of condensation leaking from where flue joins boiler. The flue is lined with a flexiable flue liner

The condensate pipe must be piped into an interior drainage pipe or it will freeze. Turn off the red emergency burner switch to reset the boiler. Also make sure that your flue pipe is clear of snow and doesn't have residual condensate freezing up and blocking it. If it is not pitched back to the blr, it can freeze and clog the flue. This will cause your press switch to cut out on high pressure.

There are several questions that need to be answered about what type of system you have, but i found this posting that may help:
As your gas furnace runs, is there water leaking at the base? Is the furnace's air filter wet, too?There are a few things that could be happening here. But if you have a high-efficiency furnace, here's the most common cause: a condensation leak.
We'll explain why a furnace creates condensation, and what's causing it to leak out of your furnace.
Note: You'll know if you have a high-efficiency furnace if:

• The yellow energy guide tag says your furnace's efficiency is 90% or higher AFUE.
• The vent/flue pipe is white PVC as opposed to metal.

Why a high-efficiency furnace creates condensation
How in the heck does a high efficiency furnace create condensation/water anyway?
It's all about how the furnace extracts heat from combustion gases.
A lower-efficiency furnace extracts some heat from combustion gases and then quickly vents them out the flue pipe.
But a high-efficiency furnace, to extract more heat, has 2 heat exchangers. These allow the furnace to extract heat from the gases for a longer period of time. This causes the combustion gases to cool and then condense.
That condensation then exits out your home through a drain. But if that water is pooling around the furnace, there's a problem preventing it from draining properly.
Causes of a condensation leak around a high-efficiency furnace

• Clogged condensation tubing
• Clogged condensation drain
• Breaks in the condensation line
• Issues with the condensate pump (if you have one)

You'll need a furnace technician to diagnose which of these is the issue and then fix it.
Other causes of leaking water around a furnace
If you have a conventional, standard-efficiency furnace...then its flue pipe may be incorrectly sized.
If you have a whole-house humidifier connected to the furnace...there may be an issue which is causing the humidifier to leak into your furnace. Your humidifier should be getting annual maintenance from a professional to prevent this.

If you are installing a new boiler, there will be detailed instructions within the installation booklet shipped with the boiler. Being high efficiency, it sounds like it will be vented with pvc pipe. If so, it could be either a one pipe or a two pipe system. Make sure you follow specifically the instructions concerning the slope of the flu as there will be some moisture condense inside the pipe. Most manufacturers recommend a minimum fall in the horizontal runs BACK to the furnace, some do not.

There should never be a 'sag' in the horizontal runs caused by supports spread too far apart. The two pipe system draws combustion air from outside the home and minimizes any chance of negative pressure in the case of an extremely tight home. That negative pressure is usually never an issue for a normal home, because of leakages around doors and windows and the normal opening and closing of the doors on a routine day.

Again, follow the instructions provided with the boiler and there will be no problem.

The condensation will destroy your heat exchanger and should be dealt with immediately. If you have a 90% efficiency or better (condensing furnace), check the pvc drainage plumbing for a clog. Less than 90% would indicate the furnace isn't burning hot enough, and in that case you should call a qualified technician to at least inspect the heat exchanger and the flame. The condensate is very acidic, and in time eat through the metal of the heat exchanger

water vapour is one the main products of combustion of gas. With a standard efficiency boiler, upon initial heat up when the flue is cold this vapour can condensate on the inner wall of the flue but should stop once the flue heats up, to stop this condense dripping back on to the heat exchanger the flue should be tilted towards the outlet and thus on initial heat up water can drip from the flue. with high efficiency (condensing) boilers, condensation will occur in the flue and secondary heat exchanger during normal operation, dispense of this condensate is accommodated for in the boiler and therefor the flue should be tilted towards the boiler, if not the flue will drip constantly.

Basic checks:-
water pressure in boiler
gas supply on
Turn boiler on then:-
does the fan come on - u can hear this at the top of boiler
does the gas valve click on
has there been a burning smell near the lower front [check the pcb]
check the co-axial flue is ok with no gaps between inner & outer [rare but not unknown]
If it's a condensating boiler check the condensate outlet tube for ice blockage especially in current weather

airswitch is faulty

Had the same problem today. What I did to fix this was to disconnect the unit and shake it so that the float became free. Once this was done, I reconnected the power and drain tube and PVC lines and primed the pump with about 32 oz of water to activate the pump. All works fine now.