Pressuretrol boiler tap pressure vs internal siphon gauge tap pressure

JimOhnig

I've got a older Paragon Oil Steam Boiler Model FCS9-815 boiler that has two different gauge taps: 1) Siphon Pressure Gauge Tap and 2) Pressuretrol tap. When calling for heat, the Siphon gauge starts at ~ minus 5 psi and rises to 8-12 psi while the Pressuretrol tap is reading 0-3 psi at the same time. The pressuretrol is set at 0.5psi with a 1 psi differential.
The system is a closed 2-pipe system that has a Hoffman #35 air vent on the return line above the boiler. There is valve that can be opened or closed to isolate the vent. When the vent is closed the Pressuretrol tap will reach a cutout at about 2.5-3 psi and will cycle back on around 1 psi 2 minutes later. when the vent is opened, it never reaches cutout pressure. All the while the steam is screaming through the radiators which make be believe that the radiators are seeing much higher pressures.
Note this boiler is configured to provide both domestic Hot Water and Steam Heat.

Couple of questions:
1) Why do I see different pressure readings on gauges off these two taps? (note the Pressuretrol and series 0-15psi gauge are new, the siphon -15 to 30 psi siphon gauge predates our purchase of the house in 1989).
2) Why doesn't the pressuretrol side reach pressure when the hoffman vent is engaged?
3) Should the pressuretrol be connected to the same tap as the siphon gauge?

Fred

Most likely the old gauge isn't working properly. The 0 30PSI gauges are required on boilers by local codes but they are virtually worthless as a useful tool to see what is happening.
The Pressuretrol should shut the boiler down somewhere around 1.5 to 2 PSI. While it is new, they usually need to be properly calibrated to make them a bit more accurate. I have posted the re-calibration procedures below.
The isolation valve on near the Main vent should always be open, except maybe when you are removing/servicing the vent. In reality, that Hoffman 35 vent is probably way to small for your system. Posting some Pictures of the system components will help to determine what type system you have and how best to vent it. The reason pressure builds when the isolation valve is closed is because you have eliminated the path for air to escape. Essentially you are compressing the air in the system when adding steam. The end result is pressure.
Pressuretrol Calibration procedure:
Inside the Pressuretrol, right below the micro switch, there is a pivot arm. At the end of that arm you will see a screw pin that is activated by the diaphragm at the bottom of the Pressuretrol. If you look very carefully at that screw pin, you will see it actually has a tiny (I mean tiny) hex head on it. It takes a .050 hex wrench and you can turn it clockwise (Towards the bottom of the Pressuretrol to decrease the Cut-out pressure or counter clockwise to increase the cut-out pressure (which none of us want to do but who knows, your Pressuretrol may be really screwed up!). Turn the power to the unit off first. You may find the first attempt to turn that screw a little bit stubborn (relatively speaking) because it has some Locktite on it but it does turn. Don't turn too much, a fraction of a turn goes a long way towards getting it adjusted where you want it (maybe 1/32 inch turn to start with). You may need to play with it to get it exactly where you want cut out to be.

JimOhnig

Fred,
Thanks for the quick response>
Here's some added details:
The house is vintage 1920 Boston area construction. I believe that the system is designed as a 2-pipe, dry return vacuum system. Hence a cold boiler pressure reading on the siphon pressure gauge of minus 8 to 10 psi. I have attached a photos of the hot readings for the two pressure gauges, cold reading of the siphon pressure gauge, the Moline vapor trap and associated air vent piping located at the end of the return piping above the boiler, and radiator inlet and outlet valves and steam trap.

JimOhnig

Fred,
I also took a picture of the Pressuretrol to confirm with you that the hex nut adjustment is the lower yellow coated nut above the pressuretrol shaft and not the screws on the brown body located below the power lead connectors.

Fred

@JimOhnig , Yes the screw with the yellow gunk on it. Boy they really loaded that screw up with locktite! Also, on that system, you should be using a Vaporstat and the Cut-out pressure should be no more than 8 to 12 ounces. You can't get that kind of control out of a Pressuretrol. Use a 0 to 16 Ounce Vaporstat. keep the Pressuretrol on the system and wired in series with the Vaporstat and other safety devices as a back up device. When you install the Vaporstat, set the Pressuretrol for 2 PSI cut-out. That way if the Vaporstat ever fails, the Pressuretrol will still control pressure.

Jamie Hall

As @Fred says, you need a vapourstat on that system. With some ingenuity and a few fittings, you can probably add it and a 0 to 3 psi gauge -- which will actually be useful -- all onto the same pigtail and fitting group where the pressurestat and existing gauge are located.

That being so, I wouldn't worry much about trying to calibrate the pressurestat.

On the other hand, once you get the 0 to 3 psi gauge on there, it may be possible to recalibrate the existing gauge, which is most likely way out of adjustment. As a first cut, even before you get the new gauge, if the system is really cold the existing gauge should read zero; very few systems will hold a vacuum for more than an hour or two!

And incidentally, while the pressure side of that gauge is in psi, the vacuum side is in inches of mercury.

EBEBRATT-Ed

Just my opinion but who cares what the pressure is as long as it's not over 2 psi? Vaporstats are expensive and a good pressure gage is a more accurate reference as to the boilers pressure than the pressure control setting that are only approximate.

If the firing rate is correct (radiation load) x 1.25 assumes 80% boiler efficiency x( 1.33 piping & pickup)= firing rate then your boiler may never build pressure that you can read on a 0-30 gage.

You do not want the boiler to cycle on a pressure control. The pressure control is a safety control (a back-up if you will) and on a residential job the thermostat is the operating control.

Commercial jobs are different that why commercial jobs have multiple pressure controls

Fred

The OP has a two pipe vapor/vacuum system. Pressures need to be maintained below 12 ounces or steam may blow past some of his traps.

Jamie Hall

@EBEBRATT-Ed -- you are right, of course; you really don't want the boiler to be cycling on the pressure control if you can help it. Unfortunately, many systems are slightly oversize, for a variety of reasons, and will cycle -- eventually (the system I care for takes almost an hour to get to the 10 ounce cutout).

But @Fred is also right -- systems which are vapour type systems may get very unhappy if the pressure goes over 10 to 12 ounces. The kind of unhappy depends on the system type (no surprise there!). Orifice control systems just get steam in the returns, as do some types of systems with orifice or water seal type traps. Some others -- such as Hoffman equipped -- simply shut down (if a differential loop trips, the pressure in the dry returns rises to the pressure in the steam mains). Either way, you are wasting fuel and money.

So a vapourstat can be a very handy gadget...

EBEBRATT-Ed

@Jamie Hall @Fred

Good point, understood. Was just trying to point out that if a pressure control or vapourstat is used as an operating control (cycles the boiler regurlarly) then an additional pressure control should be installed to act as a limit control.

Normal residential jobs without an oversized boiler the thermostat would be the operating control and the pressure control would be the safety

Jamie Hall

EBEBRATT-Ed said:

@Jamie Hall @Fred

Good point, understood. Was just trying to point out that if a pressure control or vapourstat is used as an operating control (cycles the boiler regurlarly) then an additional pressure control should be installed to act as a limit control.

Good heavens yes. The darn things are so inexpensive that in my mind there's no excuse for not having an additional pressure control on there. Cedric -- the friendly dragon I care for -- has a total of 3: a vapourstat (it's a Hoffman equipped system) at 10 ounces, a normal pressuretrol at 2 pounds -- and a manual reset pressuretrol at 3 pounds. 2 LWCOs, too -- one regular and one manual reset.

Proper old belt and braces chap, I am.