Hoffman Vapor Vacuum Question

alexd

I'm gradually working through a series of improvements suggested by several of you to improve the operation of my 2-pipe system. After installing a vaporstat and draining/re-pitching a few rads, system is running much better.

However, there are a number of rads that still have Hoffman air vents that were installed as a workaround at some point. I know I need to remove these, but have a question.

Whenever he boiler shuts down (always from tstat, not pressure prior to installing vaporstat) these vents start hissing. I held a lighter over one to verify that they are sucking air in, not blowing it out. After a longer run (30 mins plus), this hissing is followed by the automatic water feed turning on and overfilling the boiler.

I'm still wrapping my head around how a vacuum system works, but curious if anyone can shed light on this. For references my syestem is very similar to the one @Jamie Hall takes care of, including working Hartford Loop.

nicholas bonham-carter

You can turn the Hoffman vents upside down, and then they won't vent. As the boiler fires, see what sort of vent on the mains allowed the air to escape, and check whether there is a vacuum check in it. Valve off the auto feed, after verifying the LWCO shuts down on low water.
That main vent could have been low capacity, due to the fact that vacuum, once established meant no more air to push out.
You may have the opportunity to experiment with a sub-atmospheric system.--NBC

alexd

Ok, I'll try turning all the vents upside down. I have two vents on the mains. At the end of one main there is a Hoffman 75h, and directly above the Hoffman Differential Loop is a Hoffman 11 vapor vacuum. A few minutes after the system turns on this one starts pushing a lot of air out and makes a light rattling noise, but goes quiet after a while.

Also, the LWCO shuts down on water already. What's the purpose of valving off the?

I'll have to read up on the sub-atmospheric operations. Sounds complicated.

Jamie Hall

Check two things. The main vent or vents, just above the Differential Loop. Make sure they are operating -- and you might note what they are (it is) and come back to us on that.

Then, make sure the crossover traps are working. They are really critical, and if they aren't working it might explain why air vents were installed on the radiators (those are going to have to come off, by the way). If they aren't working, replace or repair them (you can probably just replace the guts -- you don't have to take them off the piping).

alexd

Jamie Hall said:

Check two things. The main vent or vents, just above the Differential Loop. Make sure they are operating -- and you might note what they are (it is) and come back to us on that.

Then, make sure the crossover traps are working. They are really critical, and if they aren't working it might explain why air vents were installed on the radiators (those are going to have to come off, by the way). If they aren't working, replace or repair them (you can probably just replace the guts -- you don't have to take them off the piping).

I can verify that the main vent is working (though probably undersized, it's still the original Hoffman No 11) There is a second vent out at the end of the main which you said previously shouldn't be there, that's the 75h, which is also working.

The crossover trap is definitely working.

Jamie Hall

I'm not sure that crossover trap was open at any time... I would have expected steam hot to the trap body, then cool.

And you do need a bigger main vent!

nicholas bonham-carter

By valving off the auto feed, you can see how quickly the water comes back after disappearing. The fact that you end up with an overfilled boiler, leads me to believe that the water is hiding somewhere, and the job is to find out where/why.
Next step might be to remove the ancient main vent from the differential loop and fire the boiler to see if steam gets that far. Removing the Hoffman 75H, and plugging the hole would return things to as installed. The 75H is for high pressure situations, and has low capacity at these low pressures.--NBC

Jamie Hall

Plugging the hole? Ah... um... no. Put a decent main vent there -- a Gorton #2 or a Hoffman 75 (not 75H) or 76. Steam should never get to it -- unless the pressure is high enough (over 8 to 10 ounces) to trigger the Differential Loop.

If there are other vents on the system, they should be removed. Presence of other vents -- other than at the loop -- will completely defeat the purpose and function of the loop, and may result in water backing into dry returns and, not incidentally in the present instance, being remarkably slow getting back to the boiler.

nicholas bonham-carter

Yes as i said, removing the 75H from the main, and plugging the hole should return the system to its as installed venting.
Remove the vent on the differential loop for observation while you fire the boiler. if the crossover trap is working, it should prevent steam from coming out.--NBC

alexd

Yeah, the 75H is way out on the end of the main, @Jamie Hall, which I think you also said shouldn't be there. So what I'm hearing is that I should remove that completely. Then, I should install a Gorton #2 in place of the Hoffman 11 that currently sits on top of the differential loop. Will I lose anything by replacing that Hoffman 11, which is a "vapor vacuum" vent, with a gorton #2? Would a system this size benefit from two Gorton's (29 rads, 275BTU boiler)?

@nicholas bonham-carter, I'll try removing the vent as you suggest and valving off the water feed.

I ran the system for over an hour last night and it never built enough pressure for the vaporstat (set at 8 oz) or the diff loop to trip. The vent on top of the loop was pushing out air most of that time, though did get quieter towards the end. It sounds a little bit like there is water in that vent, unless the rattling noise is normal for the #11.

Once again, when the boiler cuts off, the water level drops and the manual feed comes on, overfilling the boiler. I drained it to get it back to a reasonable water level, but seems like I'll need to get the venting right in order to keep this from happening when the boiler runs for a while. From what I understand, it really shouldn't run for this long without building enough pressure to trip, and shorter cycles are more efficient?

Thanks for your help!

-Alex

Jamie Hall

Ah. I misunderstood, sorry. And yes, that 75H should go. Make sure that the crossover traps really are functioning, though it sounds like they are. If the 11 is holding against vacuum, you will lose that when you change it out; some folks would be horrified, but IMHO that is not a problem. Try one Gorton #2; if the pressure still doesn't rise more than an ounce or two, that's adequate.

And no -- you don't want the thing to build pressure. Not if you can help it. If it isn't, then you are doing just fine as you are. Cycling on pressure is not more efficient than just going along!

nicholas bonham-carter

With the auto feed valved off, how long does it take for the water level to return to normal?--NBC

alexd

Turns out the auto feed is calling for water when sight glass is 3/4 full. What could cause this? Just faulty? Boiler stil shuts down when I blow down and when water is acutally low, which seems to be just from slow returns.

When the boiler runs for a while and the aternoj e drops, the LWCO is coming on and flooding the boiler. This started happening after I replaced the pressuretrol with a vaporstat earlier this week. Don't know why the two would be related but it wasn't happening before.

BobC

If it's a float type of LWCO there could be crud in the float chamber not allowing the float to move freely. What pressure is the boiler operating at now?

Bob

alexd

Operating at 10 oz. here's a pic of vaporstat and auto feeder. It's a McDonnell & Miller Uni-March Universal Water Feeder. It's not coming on randomly right now, but when the boiler cycles off (always via tstat) the water level drops slowly for about 30 seconds.

BobC

Does the water feeder have an adjustment to delay feeding water? If you can delay the feeding of water you may be able to get the system to stop overfilling till someone can find the cause of slow returning condensate.

Bob

Fred

Sounds like you need to have someone look at thgat LWCO and feeder. It is possible the float in the LWCO has a small leak in it and that will allow water into the flaot and weigh it down creating a false low water situation. Does the water in the sight glass drop when you blow down the LWCO? If not,it is also possible the port for the sight glass is clogged and the water level in the boiler is different than what the sight glass shows.
Changing the Pressuretrol out for a Vaporstat should not affect the water feeder but check all of you connections between the Vaporstat and the LWCO and make sure they are good and that no wire is shorting to another terminal.