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.

Nerd Alert! Let's Calculate Proper Venting with Math!

So I have been posting a bunch lately about my under vented system. I've learned a lot and rather than guess and go through trial and error....I decided to use MATH!!!!

I can't claim that I figured this out on my own, all the math comes from "Balancing Steam Systems - Using a Venting Capacity Chart" by Gerry Gill and Steve Paiek.

A quick recap:
My near boiler piping is wrong, but it is what it is for now. My boiler is 37 years old and cycles on pressure....because I have hardly any venting! Main #1 doesn't have a vent.....but I am going to install a Barnes and Jones Big Mouth. Main #2 is a joke. It is hardly a main, it splits 2 feet from the boiler to two long runouts to feed two radiators....so I am going to treat them as such.

I want to vent my system quickly, and have it vent enough to heat at 1oz of pressure. The boiler is old and needs all the love it can get. I also want it to vent the runouts and radiators in 3 minutes, which according to the article above is a good target.

Radiator 1: (Tube Style)
1.25" pipe. 3.5' runout. 0.032cuft of air in the runout
EDR=30, Radiator Volume = 0.39cuft
Total cuft = 0.422
Required Vent Rate = 0.141cuft/min
Correct Vent: Gorton 6 @ 0.150cuft/min

Radiator 2: (Column Style)
1.25" pipe. 5.5' runout. 0.051cuft in the runout
EDR=28, Radiator Volume = 0.7cuft
Total cuft = 0.751cuft
Required Vent Rate = 0.250cuft/min
Correct Vent: Gorton 6 @ 0.150cuft/min
note: I could have gone with a Gorton C but it's in the same room as Radiator 1 and on the south side of the house.

Radiator 3: (Tube Style)
1" pipe. 3' runout. 0.018cuft in the runout
EDR=42, Radiator Volume = 0.55cuft
Total cuft = 0.564
Required Vent Rate = 0.188cuft/min
Correct Vent: Gorton 6 @ 0.150cuft/min
note: This radiator is close-ish to the thermostat, I don't want it to vent it too quick.

Radiator 4: (Tube Style)
1.25" pipe. 15' runout (upstairs). 0.138cuft in the runout
EDR=24, Radiator Volume = 0.312cuft
Total cuft = 0.45
Required Vent Rate = 0.150cuft/min
Correct Vent: Gorton 6 @ 0.150cuft/min

Now this is where it gets weird with the two long runouts that split right off the "main" 2' from the boiler.

Radiator 4: (Tube Style)
1.25" pipe. 12.5' runout. 0.115cuft in the runout
EDR=48, Radiator Volume = 0.624cuft
Total cuft = 0.739cuft
Required Vent Rate = 0.246cuft/min
Correct Vent: Gorton C @0.270cuft/min
note: this radiator is close to the main but I don't mind venting it quickly as it is at the staircase and sends heat upstairs.

Radiator 5: (Tube Style)
1.25" pipe. 35' runout. 0.322cuft in the runout
EDR=60, Radiator Volume = 0.78cuft
Total cuft = 1.102cuft
Required Vent Rate = 0.367cuft/min
Correct Vent: Heat Timer at 50% open = 0.34cuft/min

Is anyone still awake after reading that? If so what do you think? Right now there are NO main vents and I have Ventrite #1's on all the radiators. The Ventrite's can vent somewhere between 0.033-0.083cuft/min!!!!

That's crazy under vented compared to the calculations above! No wonder why my system is working so hard. Does this make any sense? Someone slap me if I'm wrong!


    EBEBRATT-Ed Member Posts: 13,834
    Yes. It makes perfect sense. When boilers were coal fired the fired all the time so who care how long it took to vent.......it didn't matter. The size of the flame was adjusted by the coal shovel.

    With oil and gas the flame must be sized for the coldest day of the year...........which only happens a few times a year. The makes the boiler cycle when the temperature is warmer than the design temp. So now when the boiler fires it has to vent quickly
  • AdmiralYoda
    AdmiralYoda Member Posts: 591
    I'm just really surprised my relatively small steam system could use THAT much venting!  

    I'm even more surprised how under vented my system is now.  Those little ventrites must be working their butt's off!
  • ethicalpaul
    ethicalpaul Member Posts: 4,822
    edited November 2020
    Your math is probably fine, but I wouldn't start there. You have overly fast vents on the radiators in my opinion. The #6 near the thermostat will likely be problematic, for example.

    The venting is a whole system, you can't really rely on calculations for individual radiators.

    If I were challenged to properly vent a house starting from scratch, I'd do the following (after getting the main venting working):

    * Buy a #4 Maid o Mist for each first floor radiator (easily the best bang for the buck of any vent)
    * Buy a #5 Maid o Mist for each 2nd floor radiator
    * Buy a couple more #5s and a couple #6s for later adjusting

    Run the system just like this for a couple weeks and keep a paper log noting any areas that feel cold.

    Starting with the area that feels most cold, bump up whatever vent it has by one size. Throw away the log.

    Run the system another week with a fresh log. Identify the coldest area and bump up its vent by one size. If there is an area that is really hot, bump down its size if possible. If an area is too hot with a #4 on its radiator, put a blanket on the radiator. Throw away the log.

    Repeat the step above as needed, but you won't need many iterations. The Maid O Mist make it easy to change the size, you just unscrew and swap out the little orifice on top of the vent, leaving the vent in place.

    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • AdmiralYoda
    AdmiralYoda Member Posts: 591
    Either way, I think I'm going to see a big improvement!  
  • The improvement will come after the main vent(s) installation.
    Start with the slowest orifices on all the rads, and adjust as necessary when it’s colder.
    This will prevent the closest radiator from getting steam first, and the end of the line last. They should all heat up together.—NBC
  • AdmiralYoda
    AdmiralYoda Member Posts: 591
    Well just ordered some Maid-o-Mist #5's, 6's, a C, and a D. As well as a Barnes and Jones Big Mouth for my main vent. I'm going big first and if I have to dial it back I need to.

    My VentRite #1's work fine, even after 13 years of hard use being venting the entire system. So I can always swap one back in if I need to while I order other vents to fine-tune.
  • Jamie Hall
    Jamie Hall Member Posts: 21,553
    Nice approach, there, @AdmiralYoda -- but don't be too surprised if it doesn't work out quite the way you expect it to! The factor which the cfm and math approach doesn't take into account is the time it takes for the moving steam in a pipe to heat that pipe enough to get past. If the pipe is insulated, it's not too much of an effect, but it will make a difference. Even a short length of uninsulated pipe, though, can really gum things up if it's in a cooler area.

    I think this is probably one of the reasons why the concept of slowing too hot radiators down, rather than trying to speed cool one up, when one is trying to balance works. It simply may not be possible to speed them up much.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England