dry return vents ????

raz

i went on a job where a new install had been done and found vents on the ends of the dry returns where they drop into the hartford ..... my question is .... are they supposed to be there or not ?????

nicholas bonham-carter

Vent location

The best place for the main vents is at the end of the dry returns. They should be big vents, but not however be mounted right on top of the drop down to wet, in the "hammer zone" where they can be quickly destroyed in the first few firings; but instead isolated on an antler with a few elbows to absorb the force of the slugs of water which may be flung up the drop. They should be mounted as high as possible so as not to become water-logged as the pressure rises with steaming(keep that pressure low).--NBC

Hap_Hazzard

Two Schools of Thought

You'll find there are two schools of thought on this question. I've never seen the need to allow steam into the dry return. It will end up getting pretty hot anyway as the condensate gets hotter, but there's no reason I can see to divert steam from the radiators. To me the best place for a main vent is after the last runout.

nicholas bonham-carter

Venting dry returns?

My reason for preferring the vents at the end of the dry returns is convenience in checking them. Finding out if there is a problem with a vent is easier when they are all in the same area. There may also be a factor in the production of carbonic acid as the condensate rolls down an air-filled pipe.

However H-H is right, the vents just after the last radiator will work.--NBC

crash2009

Main vents

are usually installed 18" after the last takeoff or 15" before the drop to the Hartford, or anywhere in-between.

I am a big fan of having them at the end of the line for a couple reasons. 

1- I have been convinced that vents at the end will assist in returning condensate quicker, In a perfect world this doesn't matter, but in my case (oversized boiler) I want every drop of condensate returned to the Hartford as quickly as possible.

2- More convienent to observe the operation of, and maintain the system.

Jamie Hall

And furthermore...

There are a number of vapour steam configurations where the main vents -- indeed, the only vents -- simply have to be at the location where the dry returns tie together and drop.  A Hoffmann system with a differential loop comes to mind, and I know some Tranes require this.  If there are vents anywhere else on the system, the device -- whatever it is -- that is part of the system which protects against excess pressure simply won't work.



Systems of that type -- and any other system with the main vents at the boiler on the dry returns -- are normally (always?) provided with crossover traps at the ends of the steam mains, which allow air to escape very rapidly (these puppies are huge compared to any vent) into the dry return, but which close on steam.  An elegant solution.

EEngineer

Slightly different, but that's what I did

My system has a large loop in the basement. All of the radiators are taken off of this loop. The bottom of the loop is about 1 foot lower than the top and the dry return comes off of that to return to the boiler.



I thought long and hard when I replumbed this system but, in the end, I put the vent on the dry return about 6 inches from where it dropped out of the bottom of the loop. The entire loop is about 50 feet but the vent is only about 4 feet from where steam enters the loop. I reasoned like this: steam rises; the bottom of the loop doesn't see any steam until the loop is full; once I start getting steam into the dry return, the rest of the loop should be full of steam and it is time to stop venting so that's where I put the vent. I put the vent on top of a 10" pipe 1/2" in diameter so it won't see any water hammer.



It works like a champ (3 seasons now). It takes about 2 minutes to fill the loop, heat the vent and stop venting and about 15-20 minutes to fill all the radiators, at which point the thermostat is satisfied on all but the coldest days.



It makes a lot of sense to me to vent the mains at the dry returns. They are the lowest point of the system where there is still steam. Maybe not the very end, but, face it, the dry returns will end up filled with steam anyway so what difference does it make whether at the beginning or end of the dry returns?