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Rethinking 1-pipe steam venting strategy
I'm doing a complete revent tomorrow of a third building. 19 unit building, 4 rads per unit. I'll be changing out 76 vents. On the first 2 buildings I did last month, an typical venting strategy has caused me some balancing problems. I'm using MOM vents on the rads and Gorton 2s and 1s as needed on the mains. Yes, I quickly vent all the mains, the longest mains with multiple G2s. Mains fill with steam within 4 minutes. Just about as fast with an open pipe at the vent. But for 1-pipe systems with this many radiators the typical venting idea that most rads get 5s, larger rads get 6s, far/cold rads get C and small rads or rads in rooms with the thermostat get 4s, and maybe far/cold/rads on the third floor might need to be stepped up one step to vent the riser, the amount of radiator venting exceeds the venting available at the mains.
Just as an example, I added up the main venting on one building. total venting on the mains is 5 G2 and 2 G1. This is 6.16 CFM at 1 oz.
The 76 vents on the radiators vent at 7.216 CFM at 1 oz. And if you calculate the radiator venting at 3oz it comes out to 21.702 CFM. I believe that this is too much venting on the radiators. I have decided to try a more conservative (aka slower) venting strategy on the radiators on this third building. All radiators will be getting MOM #4 with the following exceptions. Add 1 size for 3rd floor rads. Add 1 size for "very large rads". That's it. On this building I'm doing tomorrow, that give me total radiator venting of 2.08 CFM at 1oz and 10.6CFM at 3oz.
With the additional "backpressure" I should be able to get steam to go where I want it to, not where it wants to.
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