Two pipe steam system issues

Lars

I have replaced an old HRT steam boiler with a new LGB and I think I have created a problem because my new boiler is now sitting lower than my condensate return tank. When the new boiler fires  up it is fine with exception of air bubbling into the new condenstae tank. The building heats up fine but when the boiler shuts down a large vacume is created in the sytem somehow, and Over a few hours the water gets sucked out of the tank .......Eventually the boiler and tank flood. Has anyone come accross this before?

Jamie Hall

Haven't encountered

that particular problem before but... at the risk of stating the obvious, somewhere in the old system there was a vent which isn't in the new system.  The condensate return tank shouldn't have air bubbling into it, nor should any newer system pull any sort of vacuum (with the exception, of course, of the few systems which are designed to do so!).



Must admit, too, that I'm a little worried about the condensate return tank sitting higher than the boiler water line -- sounds to me as though that's a pretty well guaranteed recipe for flooding -- particularly if there is a missing vent and the condensate can't get back into the boiler pretty well as fast as it drains into the tank.

Steamhead

If the system has zone valves

they can cause the boiler to go into vacuum when they close, which will pull water out of the tank.



But I also have to ask, if the tank is above the boiler, is it really needed?

Lars

two pipe seam issues

Yes I too looked for a main vent but could not find one, every rad in this building has it's own trap and I took a select few apart to see if they were in good shape, they were like new. I didn't want to go through the entire building and start changing traps when I feel it's something else. I have done these types of installes before and the only difference here is the odvious condensate tank level being about 12" above the boiler level..........But the wet pipe return runs over below the level of the new boiler and I piped it up into the new tank.

Lars

no zone valves

There are no zone valves, there were no steam vents on the rads so I installed about 30 of them and the bubbling stopped into the tank. The tank didn't overflow last night but the boiler did............maybe I need to tie in the condensate return line lower in the boiler loop so the water can hold back the syphon.

Jamie Hall

Something

is surely amiss, and it probably has to do with the new water levels here and there.  Nice that adding the radiator vents stopped the bubbling into the tank -- but except for a few rather exotic old systems, two pipe steam neither needs nor wants to have radiator vents.  Not that they will necessarily hurt anything, actually, but that they are quite unnecessary.



You mention that every radiator in the building has a trap, and it looks like they are in good shape.  Good!  That's something not to have to worry about, anyway.  The way it's supposed to work, though (as I say, unless it's one of the exotic oddities which, if there are radiator traps, it isn't) is that air and condensate are supposed to exit through the trap while the steam comes in the other side.  Then the air has to go somewhere, and it had to have been going somewhere before the new boiler.  Also, the condensate has to go somewhere, and that somewhere was the condensate return tank and then into the boiler -- at the same level.



I'd be inclined to say to go back to square one, if you have the information available (I take pictures of the original set up before I start changing stuff, since I'm older and my memory isn't so good as it once was!), and try and figure what was happening, and at what water levels.  Where was the air getting out of the system?  What were the original water levels?  Was there originally a condensate return tank?  How was it vented?  How did the water get back to the boiler before?  What pressure did it run at?



Then... take a look at the new system, and figure out how to recreate the old, as closely as possible while using the new boiler, of course.  Something about those new water levels has messed up the old system, and until you can figure out what it is and what to do about it, the problems will persist in one form or another!  I might add that it is rare for a changed water level at the boiler to not create a problem -- changing the water level is just asking for trouble.

Steamhead

I think it's time for a pro

to look at this. Where are you located? Have you tried the Find a Professional page of this site?



And is your LGB hooked up for low-high-low firing?

ttekushan_3

is the tank new?

air bubbling in the tank sounds like its higher than the dry returns. As others have said, what else did you change? There should be no vents on the radiators if there are traps. As far as the boiler back filling the tank, install a motorized zone valve in the boiler feed line. The feed control on the boiler will open the valve and the contacts on the valve will operate the feed pump through a relay. I've seen this done on several installations and it works out fine. Water can't be sucked into the boiler this way, either.



Bear in mind that the water level in the tank must be below the lowest point of the dry returns as this tank is a vented receiver to vent the system's air and provide vacuum relief during cool-down.



Is there a condensate pump somewhere in the building that you're unaware of? If one of those fails, you can have all sorts of weird problems that depend on its location and elevation.

Was there a vacuum breaker on the old boiler? Here's why I ask: Its possible that the original tank installation height was screwed up from the get-go and higher steam pressure was used to drive the air through the water in the tank. The high water level of the HRT would actually have to be held back from the feed tank by the in-line check valve. In this scenario, a vacuum breaker on or near the boiler might have existed to deal with the incorrect height of the receiver tank. In addition to all of this, the absolute strangest reason for such a hypothetical arrangement would be that the tank was elevated to condense steam from the returns caused by both high supply pressures and bad steam traps! Stranger things have happened. And it all sort of works until you change the boiler and leave out a tiny vacuum breaker that was there to deal with a symptom. Hypothetically.



A pro out there as Steamhead says should be able to figure it out.



-Terry