Air Vents on a Two Pipe System

Magnehelic

Hi All......Happy Heating Season.

I have a customer (a large Cathedral here in Spokane) that has a steam system.  The boilers were cycling at 10-12 psi for the last many years.  I have cranked the pressure down to a nice 3 psi cut in and 6 psi cut out, with no problems getting the steam to  the furthest runs, and plenty of capacity for everything needed. (I know that is higher than the 2 psi max, but the system cycles like crazy when I try to lower it any further) My question is this.  About 14 years ago, a contractor added a long circular run all the way around the complex (maybe 80 feet on each wall, all around all four walls) and installed fin tube rads, with thermostatic rad valves (Danfoss type) and an AIR VENT right at the outlet of the trv's (the trv's are piped on the inlet of the rads).  My wuestion is WHY?  Is it to break vacuum on shutdown?  These rads have SERIOUS condensate return problems, and yes, I have verified that pitch, as well as most likely sizing too, is a problem, and will be addressing that.  As we take these things apart, for leak repair etc, can we leave the vents out and just plug the 1/8" tap?  They fail miserable, lasting MAYBE 1 season (in most cases not......maybe it won't be as bad now that I cranked the pressure down a bit) and I can think of no good reason fro them to be there.  We only have a few "experts" on steam systems, and they are in "semi-retirement" phase.....in other words, good luck getting any of their time (overall systems.....lots of burner specialists) and I'm thinking this was just a botched install all the way around.



Whaddaya Think!

jpf321

2 things ..

1) for what it's worth .. in Danfoss' document here on page 2 .. http://danfoss.ipapercms.dk/Heating/AutoGen/VDSXA122/ .. it says they are not recommended for Copper Fin Tube rads.



2) Rad air vents do not belong on 2-pipe systems as you may well be aware .. perhaps they were put in place instead of functioning traps either F&T or Radiator Traps. Perhaps the last guy didn't know he was using Danfoss 1-pipe TRVs. Maybe he just went to the supply house and asked for danfoss TRV.



2.5) you say you checked the pitch .. they are pitched toward the RETURN end right.. not the supply end. do they have Radiator Steam Traps?

Magnehelic

Air Vents on a Two Pipe System

The rad valves that are installed ar NOT the Danfoss single pipe type with the vent connection right at the outlet.  The air vents are piped in separately, right after the outlet of the trv's (a coupler with a 1/8" threaded female fitting at the top).

jpf321

how bout ..

a picture? 

jpf321

2-pipe TRVs should

2-pipe TRVs should be piped at the supply inlet valve. not at a radiator vent tapping. perhaps i'm still not clear and why a picture would be helpful. after re-reading again are you suggesting that there is a TRV at the inlet, followed by a coupling with a 1/8th vent followed by the radiator? and the TRV is not at the rad vent tapping .. that does sound strange indeed.



and what about the pitch? toward the return side and not the inlet side?

nevermind about the pitch .. i re-read your top post and realized that you are saying that you verified that the pitch is problematic.



this maybe one case where the boiler should not be sized to installed EDR .. being that the copper fin tube have such a small volume compared to EDR, it's no wonder that the unit cycles so much at a lower pressure. it is probably grossly oversized .. i'm not sure if orifices would help in this situation, but i'm open to discussion, although I think orifices are called for mainly for condensate management.



the Macon TRV's are only rated for 2psi Max as per their spec sheet. I'm not sure about the Danfoss units. most air valves are max of 10psi if I recall correctly and really prefer 3 or less i believe.



possibility to downfire this beast?

JStar

Air.

Would you say that the fin-tube zone is now the majority of the system piping? Is there any way for the air in those lines to get out on start-up? Is there an F+T trap at the end of the line? Are there any unwanted water seals after the trap? I would suspect all of the air in that piping to put quite a bit of resistance on the steam, and possibly causing the short cycling at lower pressures. Keeping the pressure higher than 2 is just compressing more of the air, and masking the real issue.



I would get rid of the air vents on the TRV's. They shouldn't be there on a two-pipe system with traps. Piped on the supply valve, they would just close too early anyway. The fact that they are there at all tells me that there isn't  proper venting on this newer piping.

Steamhead

Are there traps on these fin-tube units?

and can the air get out of the dry return? 

Magnehelic

Yes.....there are radiator traps, and f&t's at the end of line

So......yes, rad traps.....yes f&t's at eol.  I was thinking that the vents might be there to let air out at startup (and/or break vacuum at shutdown).  I think we can turn the beast down quite a bit.  I hesitate only slightly because the load in this facility is DRAMATICALLY diverse.  There is 1000's of EDR on the other radiator loops, a million and a half btu heat exchanger with about 7 diverse zones (a combination of both radiators AND floor loops) another heat exchanger that does the floor loop for the sanctuary, and several air handler coils ranging from half a million btuh to 1.5 million.  I will have to go up there on a chilly morning with a couple technicians to turn it down, crank the pressure down some more, and see how she does.  there are no other significan problems with this system other than the "probably unnecessary" vents, and the fact that in the runs with those rads there is condensate return problems, causing constant leaks due to the high acidity of standing condensate.  The boilers are sized for total redundancy, and 90% of the time, only one of them cycle.  I would LOVE to be able to turn this system down to 1.5 psi..........and it would probably save them a ton of money.  A stupid question, but say me and two of my guys spend oh......4 hours there at 100 bucks a piece for a total of $1,200.00 bucks, doing nothing but turning the boilers and pressure down, and making sure everything still works, and we are getting enough steam out to the furthest devices under a good load.  Do you think I could promise the customer that it will pay for itself in THIS heating season?  AND couldn't this also possibly have a dramatic affect on the types and amounts of chemical treatment they are doing? (we don't get into that stuff, but work closely with the company that does)  When I go up, I'll snap a bunch of pictures and post them about the fin tube vents and such.