Steam coil issues in a small school

JHK

We are working on a steam system at a small school. We have had 1 day on the job so we are still getting information but I wanted to run some of the general information we got so far by the members.
The system has steam radiators, unit ventilators with steam coils 3-4 rows with return bends and unit ventilators with non-freeze 1 row steam coils. The radiators and coils are controlled by modulating valves and the steam pressure at the boilers is controlled by the outdoor air temperature ( range 1 psig to 6 psig ).
The problem the school called about is the four new units. They are classroom unit ventilators with 1 row non-freeze steam coils 5/8” tubing, and they barely get hot.
All the steam coils old and new have thermostatic radiator traps and no vacuum breakers or air vents ( the return line is slightly below the condensate receiver ). The old 3-4 row steam coils do not have an issue according to the school.
We worked on one of the four new units. The installers had the trap right at the outlet of the coil. On this one we dropped the existing thermostatic trap down as low as we could go in the cabinet which was 10” and installed a vacuum breaker in between the coil outlet and the trap. The boiler was at 4 psig on that day and we had the same issue with only the very beginning of the coil getting hot. We are going to have to install a tee on the inlet to the coil to see what is the actual inlet steam pressure.
My questions are:

Should the steam coils have F&T traps instead of thermostatic radiator traps?

A varying steam pressure for a system with steam coils is not the best way to operate this system it should be a fixed pressure of 5-6 psig?

The installers reduced the 1” npt to 3/4” for supply and return on the coils should it be changed to 1” on supply and return?

Thank you for your assistance.

Steamhead

How does the air get out of those coils?

EBEBRATT-Ed

Non freeze steam coils can freeze. I hate to say this but I would suspect a frozen coil a non freeze steam coil will freeze and break internally and you will see no water leak. If the supply header is hot on both the steam inlet and the condensate outlet check the far end of the coil where the tubing sticks out and see if they are hot or cold. Hot you still ok cold you have a busted coil. Sometimes at the far end you will find some tubes hot and some tubes cold. This shows that the cold tubes have frozen. Non freeze coils with 1" tubing are available the 5/ 8 tubing is more of an issue but both can freeze.

I have been through these many times.

Non freeze coils have a tube withing a tube design,

I have been on jobs where several technicians , plumbers and steamfitters have preceded me and could not find the issue.

Many believe if there is no leak the coil couldn't have frozen but it can.

Believe it or not. I would use an F & T trap.

I would like to see a sketch of the piping.

Does the unit have a freeze stat?

Most times this is caused by condensate not draining.

JHK

Sorry for the delay. I did not return this week to the job, customer wants to have the installing contractor to fix it under warranty.
They do have a freeze stat installed. I do not know how air is getting out, but the other older 3-4 row coils that are tied into the same return line are heating up and they have no air vents either. I will post before and after pictures. We removed the copper on the return line and used black iron and moved the existing trap lower and added a vacuum breaker.

EBEBRATT-Ed

Thats fine but what I am getting at is the non-freeze steam coils can freeze and break internally. You won't see any leaking water but they will not heat. If this is what happened with the uni vents the contractor will probably have no clue.

Most people in the business have no clue about it either. They check the steam inlet and its hot and they check the trap and its hot so the say the coil is working and it isn't.

JHK

Ok, I know what you are saying. Thank you for the input.