Ok, a bit of follow-up. The contractor is required to submit copies of O&M manuals as part of the project. I looked at the Trane installation instructions for the fan coil unit, it has a very clear diagram similar to the one paul posted, with somple calcs to determine the depth of the trap. Apparently there is negative pressure on the equipment side of the trap, so the instructions say “dimension should at least equal two times the maximum negative static pressure that can occur in the system”.
We have a bit of a convoluted system, i have to get the blessing of the local maintenance manager before closing the project. If they find fault with any part of my project they won’t sign off and accept it. So i turned the tables: told him what i see wrong with the installation and gave a copy of the diagram, and said if he’s ok with it he needs to sign the turnover form.
The installer sent a dozen pictures, and in one of them i found a j-box with an open knockout. I pointed that out and told the maintenance guy it needs to be plugged. He said it’s not this installer responsibility, it was an existing condition. I said too bad, he touched it, he has to make it right. I inherited this project from another PM who suddenly left one friday, and I don’t think i am making friends with the local FM’s.