Dunkirk boiler, tankless area full of water

[manuforti]

I have a Dunkirk oil boiler that is 20 years old. A month ago I noticed water coming out of the pressure release valve and the expansion tank was full of water. Also, at the same time I noticed water leaking from the seal around the plate of the tankless coil area. I replaced the pressure relief valve and the expansion tank. After doing that everything seemed fine for 2 weeks and then the leaking started again. I also heard some air in one of the zones, I bled both of the zones for the boiler. After doing that I had no leaks again for 2 days and it started again. I forgot to mention that I do not use the tankless for hot water, I have an electric hot water heater.
I purchased a blank plate to cover plate to put over the tankless part of the boiler. When I pulled the existing plate on the boiler, the tankless area was full of water, at least 4 gallons. Why would that much water be in that area of the boiler? Most of the water spilled out and I used my wet vac to get the rest out. Don't know what my next step should be? Thanks in advance for any help anyone can give me.

 

[Fitter30]

Whats the model of the boiler?

 

[Sponsor]

 

[Dana]

I replaced the pressure relief valve and the expansion tank. After doing that everything seemed fine for 2 weeks and then the leaking started again. I also heard some air in one of the zones, I bled both of the zones for the boiler. After doing that I had no leaks again for 2 days and it started again.

If the replacement expansion tank wasn't properly pre- charged to the system pressure during the installation it could still be the PRV going off. What is your system pressure? Does the system pressure slowly rise over days/weeks?

I purchased a blank plate to cover plate to put over the tankless part of the boiler. When I pulled the existing plate on the boiler, the tankless area was full of water, at least 4 gallons. Why would that much water be in that area of the boiler?

A tankless coil needs to be immersed in boiler water to have effective heat transfer, so the tankless chamber SHOULD be full of water if it's inside the pressurized part of the boiler, not just the sheet metal jacket around the boiler.