One pipe steam - slow returns

alano9999

Hi - I noticed the following: When the boiler initially starts firing and the water in the sight glass is at the normal filled level all seems well. Then when a period of time goes by the boiler momentarily shuts off when the pressure reaches 2.5 lbs and then restarts after the pressure drops to approximately 1lb. (to me this is all normal) . When I look at the sight glass after the boiler shuts off at 2.5 libs of pressure , the sight glass water seems to be pulled down about an inch . Then when the boiler re-fires after the pressure is reduced, the boiler water rises again. Normally I had not been concerned but I also noticed that all my boiler return piping (which is visible and above ground) appears to be very slow in replenishing the boiler water. Thus during a prolonged cold spell, with the boiler constantly firing and re-firing, I am concerned that eventually low water will be hit and the boiler will shut off completely until the returns refill the boiler. Now the boiler return piping is old and am wondering if it is partially clogged or is some sort of vacuum being created in the system between boiler firings? Is there a good way to tell if my return pipes are partially clogged or some type of vacuum is being created?
Thanks

Jamie Hall

Eventually low water will be hit.

Part of the problem is the pressure. Indeed, that may be all of the problem. As pressure is raised in the boiler, water in various returns (where the pressure is much lower) has to rise to balance it.

So... as a very simple first step, try lowering the cutout pressure of the boiler to 1.5 psi, which is more normal for residential (and even large commercial) systems. You will need to check both the differential, which is the white wheel inside on a standard sort of pressuretrol, and the cutin, which is the scale on the outside of a standard pressuretrol (some pressuretrol designs have the differential scale and the cutout scale on the outside). Whichever flavour of pressuretrol you have, it should be set to cut in at just above 0.5 psi and cutout at just above 1.5 psi.

See if that helps with the water level problem.

CantabHeat

That pressure buildup in the boiler is forcing water back out the return. The water level isn’t dropping so much as it’s being blown backwards out of the boiler.

Picture blowing pressure into a straw. If that’s the boiler the liquid gets pushed out the bottom of the straw until you remove the pressure then the water rushes back in. If you can get the boiler to run at much lower pressures (less than 1 lb is good and staying in the low single digit ounces is great) then I think you’ll stop seeing the water level “dropping.”

alano9999

Hi Jaime and CantabHeat
Thank you for your replies.
I checked my boiler pressuretrol (Honeywell 404A).
The Cut in is set for 0.5. I opened it up and the differential wheel is set for 1.0 where I was expecting it to be set for 2.0 since the boiler seems to cut out at 2.5.
Does this mean that the pressuretrol is bad or the boiler gauge is innacurate? I heard somewhere that the 1.5 pressure of the setting of the pressuretrol should correspond to the PSI of the last radiator? Does this make any sense?
I also noticed that each of the two mains off of the boiler have GORTON D radiator vents on them instead of standard main air vents. Could this also be contributing to the confusion? Hmmm..

Jamie Hall

Well, the pressuretrol is set correctly. Without a low pressure gauge it's almost impossible to be sure what the system pressures really are.

The pressure at the boiler will, inherently, be greater than the pressure at the last radiator. Piping was sized, however, for standard steam systems so that -- in theory -- 1.5 psi at the boiler would be enough to get steam quickly to the last radiator. Perhaps that is what you were thinking?

As to the Gorton Ds -- it's a matter of balance. You do what you have to do -- if those radiators need Ds to balance properly, so be it -- though it probably means that you need more main venting.

alano9999

Hi Jaime
OK - This however leaves me in an uncertain position as what actions to take. In my original post, the suggestion was to lower the boiler pressure via the pressuretrol. However, the pressuretrol settings seem correct (0.5 cut in and 1.0 on the differential wheel) and perhaps the implication was that the pressure gauge was slightly off.
Now if the pressuretrol settings are correct, how can I address the problem I'm seeing with the boiler water being sucked down about and inch in the sight glass immediately after the boiler shuts off because of pressure, and rises after the boiler fires again Because my boiler returns are slow, at some point during a cold spell I could see the low water cutoff coming into play and shutting down the boiler entirely? Can I set the pressuretrol differential wheel below 1.0 of which there are no markings or could you recommend another appropriate action to take at this point? Thanks

Jamie Hall

I expect the very first thing to do is to verify what pressures the pressuretrol is actually operating at -- and the easiest way to do that is to get a low pressure gauge (0 to 5 psi) and T it onto the pigtail with the pressuretrol. While you're at that, make sure that the pigtail for the pressuretrol really is clear.

And no, so far as I know setting the differential below one -- or the cutin below 0.5 -- probably won't change matters much, and runs a risk of linkages getting disengaged in the mechanism, which is a bore. So leave that alone, at least until we make sure the pressuretrol is more or less in calibration.

On the slow returns. One really has to pay attention to what the boiler water levels are across a whole cycle and until the boiler is cool again to find out what's happening and where.

A normal sequence is for the water level drop some -- the amount varies with the boiler and the system, but an inch or so is quite reasonable -- as the boiler comes up to steam. It should hold at that, though, while it is steaming. Now if the water level continues to drop through a cycle, that is what suggests slow returns -- but if it just drops and then holds steady, the returns are returning condensate as fast as it is being produced, and than the only concern is that the operating water level -- whatever it is -- is above the low water cutoff. Again, when the boiler shuts down, the normal sequence is for the water level to rise over the next few minutes to where it was when you began the whole thing, as remaining condensate in radiators and pipes finds its way back to the boiler. A quick drop when the boiler shuts down is, admittedly, a little odd, as the only way that can happen is for water to be sucked out of the boiler by a vacuum forming somewhere in the system which doesn't also make its way back to the steam main and thus the boiler (if it did, the pressure would be the same on the return and on the water in the boiler, and the water level wouldn't change). Honestly, on a one pipe system I can't easily envisage a scenario where that would happen... I'll think about it.

On worrying about a low water shutdown, however, several questions. First, does the water level return to where it started after the end of a cycle, given time for things to cool? And, related, does it always seem to drop while in operation (or immediately after) to the same lower level? If so, and that lower level is adequately above the low water cutoff level, it shouldn't be a problem. Second, is the low water cutoff the automatic reset type? Most are. If so, the worst that could happen is that the boiler would shut down until the water came back, and then restart. Third, is there an automatic water feeder tied into the low water sensor? If so, that should feed if the water gets low. A caution here: resist the temptation to set it for a quick feed, or to feed until it's happy. Rather, set it for a fixed volume -- say one gallon -- and to do nothing at all unless the low water condition persists -- perhaps as long as 10 minutes. Otherwise there is a risk of the automatic water feeder getting too enthusiastic and causing an overfill condition. Not wanted.

Now if your low water cutoff is the manual reset variety, it should be located so as to be very near the bottom of the safe operating level (say an inch or two above the bottom of the sight glass), and the boiler operator -- that's you -- has the responsibility to maintain the water level in the sight glass so that it doesn't drop that far when operating. Perhaps half way up the glass when cold. Perhaps even higher -- two thirds of the way up won't hurt.

Don't get me wrong -- I like manual reset low water cutoffs. Provided they are a backup, and provided that there is an automatic one set higher as well, preferably with a controllable automatic feed.

alano9999

Hi Jaime
Thank you for all your responses. I was wondering in lieu of installing another temperature gauge that I can see what pressure the boiler cuts out on and then what pressure it starts back up. i.e. My boiler seems to stop when a pressure reaches about 2.5 on the 30 lb. gauge. Then it seems to re-fire when the gauge reads about 1.5. The pressuretrol is set for .5 and 1 for the differential. So if I read my gauge rather than 2.5 as 1.5 and 1.5 as .5 it seems to me that it puts the readings in a more consistent range..i.e. my gauge may be 1lb off. Does this sound reasonable?
I also ran my boiler for approx. 3 hours starting with a full mark on the sight glass.
During the 3 hours, with each boiler cycle of shutting off and on from pressure, I had noticed the sight glass water drop when the boiler shut off due to pressure and rise after the boiler re-firing. With each boiler cycle, however I noticed that the sight glass water was slightly lower each time.
After 3 hours, I manually shut off the boiler. It looks like it took approx. 12 minutes for the returns to refill the boiler from the returns .The sight glass water was down about an inch at the time after the 3 hours.
Since each boiler cycle is shorter than the 12 minutes, it seems possible that I could hit low water to shut off the boiler.
I do have an automatic water feed and which has a maximum delay of 10 minutes but it did not come into play here..
If however low water is hit because of the boiler pressure recycling, it seems that the automatic feed would then put in excess water that will flood the boiler.
Also, I still don't understand - if the pressuretrol is correct, (boiler running at 1.5 lbs max) , why I see the sight glass water seemingly lowered after the boiler shuts off from pressure and then as the boiler re-fires the water seems to rise again. The discussion originally said that this is symptom of too much boiler pressure. Is 1.5 lbs. pressure bad?
Thanks

neilc

how old is the boiler ?
maybe you're leaking.
or do you eventually flood?

alano9999

Hi Neilc
Boiler is a Weil-Mclain about 10 years old.
I have tested the boiler for leaks by flooding it up to the main and there are no apparant leaks.
I also have not seen any flooding or water anywhere..
It seems that after a long cycle of the boiler refiring and stopping because of pressure , that after the boiler cools down that the sight glass water level eventually returns to where it started.
Is the pulling of water down an inch or so down the sight glass when the boiler shuts off because of pressure, and rises again when the boiler refires a sign of possible leaking?
Thanks

Jamie Hall

No, not if it returns eventually to where it started. The behaviour you are seeing is almost certainly an odd slight pressure imbalance somewhere in the system -- and without studying the whole system, I wouldn't care to venture a guess.

alano9999

Hi Jaime
Do you think that a contributing factor would be that I have Gorton D's on both of my mains. With an 1/8 nipple extending from each main to a Gorton D, I am probably undervented. I don't know who set this up..