3/4" compression coupling leaking for alpine burnham U.S. boiler

Question

My 3/4" compression coupler, which is attached between the alpine burnham boiler's condensate trap stub and the pvc pipe that takes condensate to the neutralizer is leaking. I replaced this compression coupler with a new 3/4" COTS coupler hoping the leak would subside; unfortunately the issue still persists. The leak appears to be happening from the point of contact between the trap stub and compression coupler. Is there a particular coupler that I should use instead of the COTS I can get from any hardware store?
One thing I noticed is the compression coupler's gasket on the condensate trap stub does not sit snug and on the other side(PVC pipe to the neutralizer), the gasket appears to be snug.
Any recommendation is greatly appreciated.

Vivek · Accepted Answer

I was able to resolve this using two 3/4" flexible coupling(a.k.a sometimes known as fernco) and small piece of pvc pipe. Take a look at the picture:

I had to tighten a little more on the side of fernco coupling that was attached to the drain trap stub from the boiler.

Programmer66 · Answer

If it is leaking at the point I highlighted in the photo below.  The drain stub is most likely not a 3/4 PVC pipe or is designed for a different type of coupler.  You would need to check that it is a smooth 3/4 PVC pipe exiting the boiler or is it a barb that requires a push on and locking clamp.
Just to clarify where the leak is and ask for more details on where the leak is.  See questions and requests in the picture below:

Edited to add additional information:
From your new pictures, it appears that the original drainpipe coming out of the boiler has been modified and is no longer a 3/4" PVC pipe. See diagram from manual below:

Your current connection as shown in your third picture appears to be a soft connection hose.  You would have to open the front panel and see how that clear hose is connected to the trap.  You might be able to connect a 3/4" PVC pipe in the trap area come out the outlet hole and match up to your Compression coupler.
A view of the pipe inside the trap area might help us provide a better answer on how to repair the leak.
You at least know why it is leaking.

Added details on how to fix the leak or make a connection.  
As stated earlier, the draintrap pipe and validated by your new picture indicates that the drainpipe is a soft push-on hose.  From your picture of the trap in the boiler, the type of hose clamps used for connections (marked in the picture) shows that the trap and piping is a soft but stiff hose.  A compression coupler will not be able to assert enough pressure on the hose to prevent a leak.

Below is one way to provide a better connection to the soft hose from the boiler.

Insert a short ½” PVC pipe into the hose from the boiler and secure with a wire or ring clamp.
Attach a  ¾”-1/2” PVC reducing couple to the end of the ½” pipe
Attach a short ¾” pipe to the other end of the reducing coupler (long enough to reach the union.
Attach the other end of the union to the existing ¾” PVC pipe.

3/4" compression coupling leaking for alpine burnham U.S. boiler

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