Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Mains get hot same time, but radiators on long side take forever

samstu Member Posts: 14
edited November 2019 in Strictly Steam
Hey all,

I recently had a boiler replaced in a 3 story 6 unit property. The short main feeds the back 3 units of the building (1 apt per floor) and the long main feeds the front 3 units (1 apt per floor).

I noticed right away that the units on the long main took much much longer to heat up than those on the short main. The short main heated up crazy fast, and there isn't even master venting on that side yet.

A) First step I took was to add master venting on the long main (Two Gorton #2's). This caused the ends of both mains to heat up at the same exact time (8 minutes from cold start. Prior to adding the venting the long main took 12)

However, the radiators on the long main side are still taking a crazy amount of time to heat up after those on the short side are fully hot.


-All the units are identical and have brand new radiators sized the same in all of them
-All new valves, fully open
-All new Gorton vents, with bigger ones on the long side (C's/D's)

I have temperature sensors in all the units, and I have it set for boiler to run until long side hits 72. By the time long side hits 72, short side is already 83! (Same insulation, new windows etc on both sides).

Any ideas on what could be causing this? I can't tell if it has anything to do with the new install because the last time the old boiler ran was before my time dealing with this property. If the ends of both mains are getting hot at the same time, could anything at the boiler be causing the radiators on the long side to take so much longer to heat up?

Thank you for your time!

PS On a side note, is there any point in me installing master venting to the short main at this time or would that only serve to widen the gap to the long main?


  • gfrbrookline
    gfrbrookline Member Posts: 753
    I had the same issue. The C and D's were the culprit. They caused the radiators to heat on the top and sides but never heated through. Replaced them with 5's, the radiators heated fully and the units heated evenly.

    You want to vent the mains as fast as possible but the radiators should be vented slowly. I vent my small radiators with G4's, 6 fins with G5's and my sole 8 fin with a G6.
  • ethicalpaul
    ethicalpaul Member Posts: 5,735
    I like what @gfrbrookline is saying. You should be able to verify it by putting hands on the rads on the long main during start of cycle. I don’t think you mentioned what vents were on the short fast main’s radiators, but I would put G4s.

    Actually I’d put MoMists everywhere so I could swap orifices at will, at least for now. They have the same sizes as Gorton

    I would hold off main venting the short line for now. It surely won’t help the long line heat faster.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el