Vacuum breaker at boiler, or not?

History_Engineer

Hello, I'm a facilities coordinator for a school in New England built in 1959. I'm also an engineer. This year we've been vigilant in bringing back the classroom unit ventilators to bring in the maximum amount of fresh air. This has, not surprisingly, driven us into the boiler room to discover years of tragic "improvements."

In the boiler room is a NG forced draft Burnham V1109 (circa 2002), condensate pump/boiler feed storage tank (just recently added the tank to get 50 gal. + storage capacity) all feeding the original 2-pipe steam system with dry returns. The square footage rating of the boiler is in line with the mechanical engineer's original specifications from 1959. As you can imagine, however, this boiler will go from a cold start to full steam in under 10 minutes. We're 100% convinced that the original boiler could not do this!

The good news is that with watching hours upon hours of Dan Holohan's highly entertaining and incredibly educational seminars, we've "un-improved" probably 85% of the "improvements" made and installed a proper boiler feed control (Hydrolevel 250) and auxiliary condensate storage tank.

The remaining Gremlins we now have to overcome is the fact that someone removed the original thermostatic traps on all 14 classroom Herman-Nelson unit ventilators and replaced them all with F&T 15's. Based on what I've learned watching Dan, these things are horribly oversized. Of course, each of these F&T traps have a thermostatically controlled air valve in them. Our thought is that once they cool down enough at the end of a heating cycle, they allow air back into the supply side of the system. The individual coil valves are another concern and are not a factor at the moment. Probably only 60% of them are working, so you can assume that the steam side of the system can get air back into it once thermostatic vents re-open.

Sorry about the long setup to the problem, but it was important so that we don't add a vacuum breaker that may not be necessary. That's the conservative engineer talking... ;-) As the boiler cools, we are hearing a "glug, glug, glug" sound in the condensate return pipe coming from the pump. We are also seeing a rise on the sight glass at the boiler. As you can imagine, we installed the boiler feed control and auxilary tank to keep the boiler from flooding at the end of a heating cycle. Clearly the boiler is now drawing water from the condensate receiver/storage tank as the vacuum develops. What that says to us is that this is happening quicker than the thermostatic air valves in F&T's are cooling down, as this would allow the air to run backwards through the steam mains and break the vacuum in the boiler.

Ideally, we're going to put back all of the original thermostatic traps specified by Herman-Nelson in 1959, but that's not happening this year due to fuzzy COVID funding and the presence of winter. Is there any harm in putting in a Hoffman #62 above the boiler to allow the air back in upon cool down? Our main reservation, and reason for writing this, is that we don't want to create an "air sandwich" that can't be expelled if the boiler comes back on (say 20 minutes or so) when the thermostatic air vents in the F&T's in the classrooms are still closed. This would result in no heat until the whole system was shut down and allowed to cool enough to reopen the thermostatic air vents at the unit ventilators.

Many thanks to all in advance!

Jamie Hall

Well... yeah. You should have the correct thermostatic traps on all the convectors, radiators, and what have you. But I can see where you might be with budget...

In the meantime, no, there's no harm to putting a vent on the steam mains at the boiler. It will, of course, close very quickly. That said, you should take some time to figure out how this thing was vented when it was installed. Disregard the F&Ts for the moment. There really are only two types of true two pipe systems, based on venting: those which were vented at the ends of the mains, and those where the mains were vented with crossover traps into the dry returns and all the venting was done at the boiler, where the dry returns joined before dropping to the wet return and the boiler.

In either case, you must have venting on the dry returns; otherwise air can't get out of the radiators or what have you. If there were crossover traps, those must be checked to be operational -- they are the key to fast, even distribution of steam. If they are gone, or the system never had them, then you do need venting on the mains, at or very near their ends.

F&Ts are not vents. They do have a thermostatic air release in them, but they aren't vents. You are quite correct in thinking that they are slow to reopen, and if you are depending on them as vents the system will work remarkably poorly. You are also quite correct in thinking that if the only change you make is to put a vent near the boiler as a vacuum breaker, you will get air in the steam mains which has nowhere to go -- and the system will operate poorly.

If, however, you replace or repair the crossovers -- if you ever had them -- or place proper vents at the ends of the mains, and if you place a vent or vent cluster for the dry returns, you and the system will likely be much happier.

History_Engineer

Jamie, can't thank you enough! Your response made the lightbulb go on!

Yes to: "those where the mains were vented with crossover traps into the dry returns and all the venting was done at the boiler, where the dry returns joined before dropping to the wet return and the boiler."

The only proper vent we can find in the entire system is the one coming off the condensate receiver that the dry returns all drop into. At start-up, you can definitely feel the air wooshing out of it.

In the boiler room, there is a 10" diameter drop header suspended from the ceiling. You can tell that the "dead men" designed this as it does exactly what Dan preaches about with the turn down of the near boiler piping. The drop header has a 1" drain at the end which is with an F&T 15. There are three (3) steam mains (all with their own king valves) that go straight up off this header. It's a beautiful sight!

At the far end of the building, there is an F&T trap for each of the three mains, which are 2" diameter at their ends with 3/4" F&T's on them. Back in the boiler room there are indeed three (3) crossover traps. All of these pipes are 1" and have 3/4" F&T's on them. We have disassembled, inspected, and cleaned or replaced all seven (7) of the above noted traps. We also cleaned and/or replaced all of the Y-strainers up stream of them.

The "lightbulb" that came on in my head was that it if we were to install a vacuum breaker, the ideal place would be on the steam side, but just in front of the crossover traps. This way, if you put air back into a system that was going into vacuum, it would be in the crossover returns, and have to travel a long way to get back to the boiler.

My fear in this is, and question is.... How come the dead men didn't do that? Or did they, and somebody "improved" the system?

Jamie Hall

I think somebody "improved" the system... but your lightbulb fix sounds good to me. Only I'd use Gorton #2 vents, rather than strictly a vacuum breaker. They vent a lot, and open reliably (unless you overpressure them -- then they can stick shut) and quickly.

History_Engineer

Thanks Jamie! As far as I can tell, the only way the system vents today at start-up is through the thermostatic air release in each F&T vent. As you stated:

"They do have a thermostatic air release in them, but they aren't vents."

Now, at the far end of each main, it did not appear that the original F&T's had been replaced or that somebody removed the vents since the original installation in 1959. Original paint, fitting manufacturer was consistent throughout, original gate valves, etc. The crossover traps in the boiler room are a different story as it looks like these could have been replaced 30-40 years ago. Not sure of that, but maybe.....

So, if you were doing this, would you install a Gorton #2 at each of the crossovers in the boiler room (for a total of 3), or would you also install them at the far end of each main (for a total of 6)? Like I said, it doesn't look like they were ever at the ends of the mains to begin with.

I do have the original specifications for the system, but the mechanical sheets of the contract drawings are long gone. Forensic engineering, my favorite!

Jamie Hall

I think in your setup I'd go with the 6 -- bit more expense and hassle, but pretty well goof proof. Well... as things are ever goof proof.

I wonder what they were originally contemplating for mains venting?

EBEBRATT-Ed

I have run into this before.

Maybe a little different situation. A new boiler was installed with a water line lower than the old boiler. This caused water to syphon from the boiler feed tank to the boiler through the boiler feed water line when the boiler shut down.


We solved it by piping the boiler feed water off the pump up to the boiler room ceiling and then back down to the boiler with a vacuum breaker at the top of the loop

History_Engineer

Thank you for the tip on this. This may be a viable option for us as well, as the original 1959 feed line is still there and in service. Wouldn't you know it, it's hung just below the ceiling!

I got involved in this mess back in May and the first OMG moment came when I saw that the pump was cycling all the time due to a failed check valve. There hadn't been a call for heat in probably a month! Anyway, the only way to replace the check valve was to drain the boiler far enough to break the siphon.

The good news, I guess, is that when they made the "improvements," they skipped the part on Burnham's near-boiler piping diagram that showed the feed line hitting the equalizer below the normal water line in the boiler. May there was a little bit of silver lining in the "knucklehead moves?" ;-)

Thank you and Jamie for all of your help!