Steam pressure higher than before

LudgerHentschel

Recently, I noticed that our boiler, attached to an old Dunham 2-pipe steam radiator system, runs at higher pressure than before. In the past, the boiler would make steam at 2oz or 3oz per square inch. Now, the pressure peaks at 4oz to 6oz. The system seems to heat just as well as before.

Should I look into this or leave it alone?

I think the pressure increases started during this heating season. The boiler began short cycling because the VaporStat was set to cut out at 4oz or 5 oz. I have raised the cutout on the VaporStat to 8oz. The boiler does not reach this pressure -- for now.

The system heats a single-family home on 3 levels.

We have run this steam system for a few years. After some initial restoration -- with advice from the good folks here -- there have been no physical changes to the system. The most recent maintenance was the annual checkup on the burner and nozzle. The new nozzle is the same model as last year.

There have been no changes to the pipes or radiators.

Based on the higher pressure, I have replaced the vents on the steam mains and the central returns. The old ones seemed fine. The new ones are larger. (Barnes & Jones "Big Mouth" vs Gorton #1.) The boiler operating pressure remains higher than before.

I don't think the VaporStat has gone bad. The pressure on the VaporStat agrees with a low-pressure gauge I have mounted in parallel to the VaporStat.

I have cleaned out the pig tail leading to the VaporStat and pressure gauge. There was some water in the bend. Otherwise it was clear. There was no change in the pressure readings before and after the cleaning.

Previously, the boiler would build pressure quite gradually during a cycle. Now, the steam pressure on the gauge seems to rise from 0oz to 4oz in a matter of seconds, once the boiler starts making steam.

I am curious:
Is the new operating state better or worse than the old state?
Is there a likely cause for the increase in operating pressure?

Thank you,

Ludger

ethicalpaul

I find it a little coincidental that you raised your vaporstat setting and now the pressure is running higher, but you don't think that's related? You're sure it's not cutting out? I mean it must have been cutting out before because that's why you raised it, so I'm not sure why you're ruling that out.

A deeper question: Why would you raise the vaporstat? I know you said because it's short cycling, but what is the harm in that?

EBEBRATT-Ed

@LudgerHentschel

Sounds like there is nothing wrong with it. Leave it be. Raising the vapor stat setting slightly does no harm as you say it doesn't reach the vapor stat setting.

Things often change for seemingly no reason. The fuel changes the combustion air temp changes the outside weather changes

Something has to change enough to be a problem before you have a problem

LudgerHentschel

Sorry if I was not clear about the VaporStat.

Last year, the VaporStat cut out was 5oz. The boiler essentially never cut out on pressure because it ran at 2oz or 3oz. This year, the boiler runs at 4oz to 6oz and started cutting out on pressure and short cycling.

After a few days of this short cycling, I finally worked out that the higher operating pressure was the cause. To stop the short cycling, I raised the VaporStat cut out pressure to 8oz. Now the boiler runs without cutting out but at higher pressure than it reached last year.

If I return the VaporStat to a 5oz cut out, the boiler cuts out on pressure and cycles on and off every 30 seconds, or so. That seems inefficient. So I am operating at the higher cut out setting, for now.

ethicalpaul

Ahh I see, sorry for my confusion, that makes total sense.

I don't know nearly as much about two pipe systems, but it seems that your issue is happening before the steam even gets to the main vent, so I guess the two-pipe aspect doesn't really come into play.

I'm trying to think what could change that would result in higher pressures in the first 30 seconds of a firing cycle. The main venting for sure is venting during this time?

Jamie Hall

Did the maintenance on the boiler include cleaning? And are you sure it's the same nozzle and oil pressure? Either one could have increased the steaming rate of the boiler a bit, which would account for the difference in pressure.

So could one of your main vents getting a bit sticky...

LudgerHentschel

I have replaced the main vents with bigger ones. The new (and old) vents appear blameless.

I too suspect that the higher pressure comes about as the steam leaves the boiler, long before it gets to the main vents.

The higher burn rate seems plausible but has no obvious cause.

The annual maintenance does include cleaning of the combustion chamber. But that was true in prior years. New guy though. Maybe he was much more thorough.

The service report notes the nozzle specs. The report claims the specs are the same as last year. I might check the actual nozzle.

Also, the steam pressure increase became apparent a month or more after the burner maintenance.

I am not aware of any adjustments to the oil pressure. But I may have to call the service company to check that the pressure is still within specs.

Is it possible for something to obstruct the steam on the way out of the boiler if there were no changes to the system?

Jamie Hall

Anything is possible... but such an obstruction would be pretty darn close.

Dan_NJ

Does oil vary at all in the BTUs it generates when burned like NG does? If so, any chance you filled your tank around the time this started?

LudgerHentschel

The tank gets refilled automatically. It surely was refilled not too long before I noticed the steam pressure increase but I don't think they are closely linked in time.

I will keep an eye on what happens after the next fill -- assuming the pressure is still up before then.

Jamie Hall

Fuel oil BTU value is very constant, unlike gas.

LudgerHentschel

The steam pressure has returned to its previous level.

By pure coincidence, the oil tank was refilled from nearly empty. Typically, the tank gets refilled from about half empty. But the fuel service comes automatically, so I don't tightly control when the tank gets refilled.

I am not aware of any other changes to the system.

I have learned that some oil companies deliver different mixtures of #1 and #2 oil at different times of the year. There is a "winter mix" in colder climates. We live just outside New York city. However, it looks as though the winter mix has some #1 oil to be less viscous and that this comes with slightly lower BTU content. Maybe the usual delivery is winter mix but the previous delivery was regular #2. I have not spoken to the oil company. As best as I can tell, any of this mixing would change BTU content by well less than 10%.

A nearly empty (old) tank may have more sludge, which is more viscous. Maybe that has higher BTU content? I think the more common interpretation is that the sludge is noncombustible and may include water.

I am a little annoyed by the mystery of the pressure changes. However, I am relieved that the pressure has dropped again. My concern was that some underlying problem might grow and produce additional pressure increases.

Thank you,

Ludger

LS123

@LudgerHentschel , As I have learned from the forum, oil nozzle size, and PSI burner set for makes a significant difference how fast the steam is produced. Link below, page 4 from Beckett... most nozzles probably are comparable ( not sure not a pro on this)
https://www.beckettcorp.com/support/tech-bulletins/the-effect-of-high-oil-viscosity-on-oil-burner-performance/

If are handy and know how to add a oil pressure gauge to your burner.... you can find something that may fit your burner... New boilers come with casing around the burners, so you may not be able to get one installed, but consul your service provider...

below is an affordable oil pressure gauge from supplyhouse.com
https://www.supplyhouse.com/Winters-Instruments-PFQ807-2-1-2-Stainless-Steel-Liquid-Filled-Gauge-1-4-Bottom-NPT-w-Brass-Internals-0-300-PSI?gclid=EAIaIQobChMIj53ylb7h9QIVXfbjBx2D8w4AEAQYCiABEgJJwPD_BwE

They come really useful and I find it helpful to learn from the settings that may give best performance...

LS123

@LudgerHentschel the chart I meant to post



This is from page 4 I was referring to....

LudgerHentschel

Thank you all for your suggestions.

If I come to a definite explanation of this pressure change, I will post an update.

Thank you,

Ludger

LS123

@LudgerHentschel .... you don't have to give me nor others explanation.... unless you feel its valuable to share, its all up to you and and many people in the forum is so experienced and they can help you understand and determine a course of action plan, be knowledgeable of what is going on...some things can be done by self, somethings could be done by a certified pros... regardless.... I hope you find a solution to make your heating system to be fine tuned to its best...

EdwinD

After replacing the water gauge set, my boiler had a similar increase in pressure. Make sure both gauge valves are fully open.

LudgerHentschel

The operating pressure has returned to the previous norm, 2oz or so, and remained there.

Unfortunately, the exact causes for the increase and return to "normal" remain a mystery to me. The comments from the good people here were helpful and reassuring. Unless the pressure changes return, I am not sure I will be able to determine the cause(s).

Thank you,

Ludger

ethicalpaul

Good luck in your operation and any search that might result from future pressure changes. It's an interesting puzzle for sure

Chris_L

Have you measured the pressure with a manometer on the boiler, or just the pressure gauge? If not, I'd suggest trying the manometer.

You can make one by attaching some clear tubing to a valve below the water line, and measuring the difference in height between the water in your tubing and and the water in the gauge glass. (Be sure to keep the tubing high enough so the boiler water doesn't come out.)