Cap on Chimney Linner

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We have an PE Alderlea T5, and about 5 years ago added a SS chimney liner. 2 years ago our chimney sweep mentioned that we would have better performance if we had capped the liner at the bottom. I was a bit shocked to find that it wasn't capped. The termination point is difficult to access from the bottom, but I managed to kind of get it capped. Unfortunately, it did not last, and I don't believe given the difficult access, that it can be successfully capped from the bottom.

I decided to contact the installer and ask why they didn't cap it, and how much it would cost to put a cap on. He replied that MA code for liners does not allow for caps to be installed. Does anyone know if that is the case, or have any thoughts or suggestions on this? It just doesn't seem like an open ended liner would draw from the stove the same as a closed system would.

Is the liner directly connected to the stove? If not, that's a serious issue.

Or, by capping, do you mean a block-off plate sealing the damper area? If so, the sweep is spot on and the dealer is not.
There is a tee at the bottom of the liner where the stove (which sits in front of the fireplace) is connected to the stove. The photo is from the hearth looking up through the damper. I manufactured some sheet metal that I installed in the hearth, and then seal with silicon to create a tight seal there. I was certain you would respond BeGreen, lol! So, that bottom should be capped, and there is no way MA code would prevent this?
That helps. I was confused because the T5 is a top vent only. Can you post a picture that includes the tee and stove?
The bottom of the tee must be capped to avoid dilution of the draft and cooling of the flue gases. And there should be a block-off plate in the damper area to keep the heat in the room.
👍 I would add an extension onto the tee to carry it through the block-off plate, then cap the extension. That will make cleaning easier.