Added Main Vents, System Now Out of Balance

AdmiralYoda

Hi all, thanks for your help so far. I recently added a Big Mouth main vent on the end of my main and my system is now a bit out of balance. The radiators are heating up much faster, but one stays cold until all the others reach pressure. Then it heats up evenly and gets hot.

The problem is that this radiator is in the room where the thermostat is, so the other rooms are really hot until this radiator warms the room and the thermostat shuts off.

I replaced all my radiator vents with Maid-O-Mist vents sized based on calculations. Air volume in the runout and in the radiator based on EDR, proper venting rates, etc.

A little description of the setup:
The main is 2" and is about 35 feet long. There are 4 radiators on the main.

• Radiator 1 is closest to the boiler, about 15' up the main. This is the one having the problem. Calculations say it wants a MoM #5.
  
• Radiator 2 is another 4' down the main and heats fine. Its a big radiator and overheats the room. I have it dialed down to a MoM #4. It heats quickly and evenly.
  
• Radiator 3 is 2' from the end of the main. It is at the far end of the same room Radiator 2 is in. Calculations say a MoM #6 would be best. I have a #5 on it so that room doesn't get too hot.
  
• Radiator 4 is off the very end of the main. It goes upstairs and has a 15' runout. Calculations say a MoM #6 and thats what is installed there.
  

I have tried reducing the vent rate further for radiators 2-4 but radiator 1 is still the last to heat up. I even removed the cap on the MoM #5 and replaced it with a #6, a #C, even a #D....and finally just removed the cap so its wide open! Still the last radiator to heat up.

So what's next? Any ideas? I suppose I could swap out the entire MoM vent and put a new one on there just in case it is faulty. I could also just remove the vent while the system is heating and see if steam comes out. After that what.......maybe the radiator valve is clogged or partially closed?

Jamie Hall

Calculation is all very fine, but the real world doesn't run on calculators.

I like the idea of seeing what happens when you take the vent off completely to see what happens -- just be ready to shut the system off when it does. If it does...

Otherwise, if we assume that the mains heat quickly and evenly -- within reason; no matter what vents you have the far end won't get hot as soon as the near end does -- what is needed is to adjust the venting on the radiators so they are even, too. And this will require some experimentation -- which is why adjustable vents are so nice to have.

So... having found out that the steam does get to that radiator... sometime... you would start off by putting your calculated vents on all the radiators. Then start slowing them down. This is an iterative process -- particularly on smaller systems -- as a change in one vent will affect the balance of all of the system. Patience. But the key is to slow down the fast ones, not try to speed up the slow ones (except as a last resort!)..

CantabHeat

Final fine tuning is more art than science. Try making making adjustments gradually, note the impact, and then try again. Vents with interchangeable sizes are ideal for this but I’ve seen people even just carefully place tape partially over the vent to adjust its capacity. Blowing into the vent is a half decent way to “measure” the venting capability of an adjusted vent like that. 

As others have said, if you have fast mains venting focus on slowing down the fast rads vs trying to speed up the slow ones. Slowing those down leaves more steam to go elsewhere in the system. There’s only so much steam to go around at any given moment so as you’ve discovered if it’s already going elsewhere even removing the vent may not get steam to a slow rad.

It can take some experimentation, but you’ll get there eventually. 

AdmiralYoda

Great advice, thanks!  The reason I'm stumped is the radiator CLOSEST to the boiler is taking the longest to heat up which is counterintuitive.

The next heating cycle I'm going to remove the vent and see what happens.  Will report back!

ethicalpaul

It's not really counterintuitive...the steam doesn't care how far away it is, it will go to the lowest pressure. Distance can affect that, but not against large vents it won't.

I hate love to say I told you so :

https://forum.heatinghelp.com/discussion/comment/1625539/#Comment_1625539

https://forum.heatinghelp.com/discussion/comment/1626683/#Comment_1626683

AdmiralYoda

My official stance is that steam heat is 25% science, 50% art and 25% black magic.

What you think will make sense never will.  It comes down to fine tuning and experience.

ethicalpaul

You're exactly right except the science is 100% and the other two are imaginary for stuff that is not (yet) understood (or has been forgotten) by one or by the industry

Jamie Hall

Oh no. 100% black magic... at least for most people it seems so!

EBEBRATT-Ed

@AdmiralYoda

I would check that radiator valve on the problem rad to see if it is fully open and that the disk isn't broken. Check the size of each run out versus the size of the radiator....check all of them.

Your run outs may all be large enough for the connected radiator but the problem radiator .......it's run out may be large enough but may be undersized compared to other run outs.

Also check the run out and radiator pitch which you probably already have

AdmiralYoda

Thanks again everyone!  The run-outs for the radiators that are heating fine are 1.25" pipe.  The radiator that is taking too long to heat up is 1.0", but only about 18" of pipe runout.

The system heated evenly before with NO main vents and relatively small Ventrite adjustable valves.

BUT I THINK I MAY HAVE FOUND THE CULPRIT!
When the system was heating and the other radiators were hot and the problem radiator was just starting to get hot.....I took the valve off and what did I see?  Steam!

It wasn't like a steam locomotive but I could see (and hear) a light stream of steam for 6 or so inches.

I'm going to replace the valve this afternoon with a new one and will report back!

AdmiralYoda

Problem solved!!!!

I took the MoM #6 off and gave it a shake.  It went "thud-thud" instead of "clink-clink".  The water that drained out was rusty looking too.

Put a new one on and all is good!  Everything is roughly in balance again.  Now just the fine tuning to make it perfect.