The solution is very simple - you should replace the O-Rings on the filter every few years. The O-Ring Kit costs only a few bucks (available HERE) and we recommend that everyone have an extra O-Ring kit on hand. These O-Rings just wear out; they get flattened over time and it gets harder and harder to get a good seal. The O-Ring Kit installs in seconds and stops leaks!
More details below:
The filter has Two O-Rings located here:
It is very simple to replace the O-Rings when cleaning the filter cartridge. Just unscrew the clear housing and you will see where the O-Ring needs to be placed.
Also, there is a smaller O-Ring that needs to be replaced in the gray valve at the bottom. This O-Ring does not wear out as fast as the larger one at the top, but you may as well replace it at the same time since the O-Ring kit provides both O-Rings.
Sometimes, when the O-Ring at the top fails, it seems logical to attempt to screw on the clear housing tighter. DO NOT DO THIS! It may crack the black hub at the top. If your filter is leaking, you should inspect this black hub for cracks.
This is what the black hub looks like:
If this black hub is split or cracked, it must be replaced. This takes more time than simply replacing O-Rings. Be sure to replace your O-Rings every few years in the future to prevent this from happening again.
If you don't have a filter...
You need one. Here's why: Outdoor boilers are all open to the atmosphere. Therefore, you will get sediment in your system. Sediment will build up in your heat exchangers and destroy them, AND sediment is the "Secret Killer" of outdoor wood boilers. If you don't have a filter, your heat exchangers will fail after some years and need to be replaced.
We have seen some boilers fail after only a few years from sediment - this of course depends upon your water conditions too. Don't make the mistake of thinking "Everything is fine! I don't need one." What you can't see WILL hurt your boiler system.