Two pipe steam system with recessed convectors retrofit from central steam plant supply

SenorSteamHeat

Background Information:
The house is a 1941 stone center hall. The steam system was originally attached to the Overbrook Central Steam System and an oil boiler was installed when the plant shutdown. The original steam main still runs through the basement.
We purchased the home in 2017 from an estate sale, so no background information was available on the functioning of the steam system. Due to water damage and mold, we demoed the finished basement and removed the asbestos insulation prior to moving in.
The oil company informed us that the house typically consumed 1200 gallons/year of heating oil. This seemed high to us since the house is only 2700 sqft. We consulted with neighbors who have homes built by the same builders around the same time with the same basic layout of our home. Our bill is unusually high, although others have converted to hot water/forced air and may not be a reliable comparison.
The entire house has convectors recessed into the walls with removable metal covers.
I have only found vents on the main and return at the end of the line prior to returning to the boiler. Both appear to function but may need to be replaced by large vents, possibly on a tree.
I have not been able to identify any traps in the system. My experience is limited to bulky bucket traps though. I would not be able to identify a trap built-in to the convector. The returns do not heat up during operation, so something is keeping the steam out of the returns.

Problems:
1. 2 radiators do not heat up due to what I believe are frozen shutoff valves. I removed the packing nut and thoroughly covered them in penetrating oil and left a rag wrapped around the stem for hours. They won’t budge and any further attempt to turn risks breaking off the stem. The union nut appears to have been set inline with the floor making this difficult to access without cutting up the floor.
2. Center Hall radiator bangs (heavy hammer like banging, suspect water hammer or possibly air?) when steam initially enters the riser and works its way into the convector. It does not bang if the boiler is running frequently. It typically bangs after a period of without the boiler running ie. Afternoon. Turning off the convector did not appear to stop the banging. Issue may be related to riser having to extend 2 stories vertically and approximately 15 ft horizontally. Pitch appears correct for the exposed portions of the riser. Upon removing convector cover, I was able to identify a plug in the upstream side of the convector. I believe there was a vent here at one time since the first-floor convectors that I have inspected do not appear to have this port on them.
3. Radiator in bedroom over garage bangs (loud aggressive hammer like sound). I have not observed a timing for this one since it is intermittent, and we aren’t typically in the room.
4. Unusually high energy bills
5. Steam pressure gets to 6psig before cutting out. Thermostat only calls 1cycle/hour. Cut in is at .5 lbs
a. Typical behavior is:
i. 10 minutes boiler firing before cutting out
ii. 5 minute pause (to allow pressure drop?)
iii. 10 minutes of boiler firing, convectors heating up and affected radiators banging if system is cold. System cuts out after the banging of approx. 20-30 seconds
iv. 5 minute pause (to allow pressure drop?)
v. 2-10 more minutes of boiler sending steam to the convectors before cutting off from thermostat

Steps taken:
1in fiberglass insulation added to main and risers accessible from basement. This is to replace the asbestos which was professionally removed prior to moving into the home.

If anyone has any ideas on what steps I would need to take in order to eliminate the banging, and get the system running efficiently again, I would greatly appreciate your input.
My current hypothesis is that the single vents on the main and return need to be upgraded and the terminal convectors on the two longer riser runs need to be vented. If this is the case, how do I size them all?

Many thanks!

Jamie Hall

Step one is going to be to figure if the cutoff pressure really is 6 psi, and if so, why. You need no more than 2 psi and perhaps even a little less. It may be the gauge is wonky. It may be that the pressuretrol is not adjusted properly. It may be that the pigtail is clogged. It could be all three...

Then. I don't see on your diagram a main vent on the steam main -- which seems to go completely around the basement? Does it start high at the boiler and go around and eventually drop into a wet return? Or... ? A main vent is going to be needed, and it would normally be at the end of the steam main..

The banging -- in both situations -- is almost certainly water hammer and is equally almost certainly due to a sag or inadequate pitch in a horizontal section of the runout. All horizontal pipe runs should be checked for proper pitch -- and not just end to end. A sag in the middle can be even worse.

If the convectors you mention which don't seem to have vents really don't have vents... they'll need them. But usually a convector with no vent won't heat at all, or heat very little, so perhaps there is one somewhere creative?

Do not turn off the inlet valve to a one pipe radiator or convector -- they almost never seal tight, and condensate will collect behind them. And gurgle or bang or otherwise make a nuisance.

Your complaint about fuel usage may be correct. First thing to do, though, is that pressure question. Second thing is to make sure the boiler and burner have been cleaned and adjusted correctly, recently, by someone with the proper test equipment and the knowledge to use it. Third thing, of course, is to make sure that you are comparing apples to apples -- is your house insulated the same? Good windows? No draughts?

SenorSteamHeat

Jamie,
To respond to your questions/ suggestions:

That sounds about right in terms of the cutoff pressure. I will clean the pigtail and have the pressuretrol and gauge verified by the steam guy.

The steam main and condensate return both circle basement and both have vents at the end prior to reaching the boiler at the end of their loop. The loop for the steam main and condensate start about a foot higher than they finish before returning to the boiler.

Sag in the vertical runouts of the risers would be possible. Both of the banging branches have significant horizontal segments. Unfortunately they are inaccessible. One runs through the floor and the other runs up through a wall and then through the garage. I wouldn't be able to verify pitch in this situation.

None of the convectors have vents and only one so far even has a port for a vent. The convector covers were painted shut so I am reasonably confident the previous owner did not touch them ever.

shut-off valves are all open except in the case where I cannot open them due to being unmovable and requiring maintenance by a pro.

I will go through the steps you suggested with a professional and follow up. The house has full replacement windows and is better insulated than the neighbors. Many of the people I have spoke with do not have any insulation.

Jamie Hall

Ah -- this is two pipe, then? My bad -- I assumed one pipe since you spoke of vents on the radiators. There is then an inlet pipe (most likely with a valve) and an outlet, low down on the other side? What does that outlet look like? Could you post a picture please?

Even more -- if this is really two pipe -- you need to get that pressure down, and the vents on both the steam main and the condensate return (overhead, I presume?) need to be good sized -- it's almost impossible to over vent in this situation, although there is an optimum between vents and money...

If this really is two pipe, do get a picture of a typical inlet and outlet to a radiator. And if it really is, then the radiators don't need vents except in one rare situation, which will be seen from the pictures.

SenorSteamHeat

Jamie,
My original post has photos of the vents, convectors etc. I actually put together a pdf called writeupHH.pdf which is attached to the initial post and this one. It has all of the pictures and my narrative. I have been thinking about this problem for a while since I have a two pipe steam system with main vents only and I can't ID a trap either. I put a photo in there in case you can ID it.

nicholas bonham-carter

Those 2 Hoffman 75 main vents are certainly not big enough, and each should be replaced by a Big Mouth vent, or 2.
Get the pressure down, and verify the function of the pressuretrol with a 0-3 psi gauge, from Valworx on a tee.
Maybe someone else will be able to tell if the trap is built into the convector.
The near boiler piping is incorrect, and is no doubt throwing a lot of water up into the supplies.--NBC

Jamie Hall

Sorry. I didn't look at the pdf at first -- very informative and thank you.

And no, I don't see anything like a trap on the outlet -- and I'd be very surprised if there was one built in. Which leads me to wonder if this system -- like many vapour (very low pressure) systems -- depends on an orifice at the inlet to control the steam flow to the convector. Can you operate the valves to the two units which bang, and, if so, are they fully open? And -- I'm sort of fishing here -- can you measure the outlet temperature of several convectors (an IR thermometer is ideal), including the two problem children, when the system is well along on a cycle -- but after you get the pressure under control?

SenorSteamHeat

NBC,
That sounds like a solid plan of action. Verify and test with lower pressure will be the first step.
I have been told that piping is incorrect and likely throwing a lot of water up with the steam. I will need to get an estimate to get that changed around.
Thanks

SenorSteamHeat

Jamie,
I agree, I think this is one of those vapour systems with orifices.

The two cold convectors have shut-offs that are unmoveable. I did the typical removal of the packing nut and soaked it in wd40. Still stuck. I am going to have a pro replace them. The one I have worked on most is the one in the living room. I used a IR thermometer to verify that steam isn't even entering the riser to it, but risers later in the sequence of flow get hot so it isn't a steam supply issue.

The plan is to lower the pressure and get those shut-offs replaced. I will report back how that goes.
Thanks