Old steam boiler: Pressuretrol not making adjustment

Peter Zelchenko_3

That's exactly right, it's an M&M. And you're right: the sight-glass fittings are 1/2 and the blow-down valve is 3/4. That explains the lower flow. But what if the flow were constricted somewhere inside the 1/2 inch, by some blockage there? Wouldn't that reduce the total pressure against the pressure switch and cause the symptoms I'm seeing? I can't think of any other explanation.

Peter Zelchenko

Pressuretrol not switching in and out

Okay, I am working on a very old (single-pipe) steam boiler in a three-story building. There are 16 radiators fed by a 700 sq. ft. Peerless boiler that is probably from about 1960, and that is probably also the age of most of the controls and other trim. The wet-end piping is encrusted with mineral corrosion and leaking water in two places when under pressure. The system apparently has not been serviced in many years, except for a new automatic water feeder replaced recently. I know a new boiler and careful cleaning are needed, but that will need to wait a few weeks for various reasons. I just need to get the system heating properly in the interim.

All of the radiators on the third floor are shut down because it is not being used, so only 10 lower radiators are operating. In spite of this, it is still taking about 2.5 pounds to get 10 radiators and average piping to full heat, which is probably high. There is some recent hammering in one of the runs. One run is taking an unusual amount of time to heat up. The vents are old and should be replaced with adjustable ones. There has been no cleaning on the system, possibly in decades.

The high-low pressure range did not seem to be working right. A cycle has typically lasted about 4 hours: typically from -2 pounds of vacuum and a ridiculous 5 to 7 pounds of pressure before the Pressuretrol would finally cut in or out. Honeywell simply said it sounded like a faulty Pressuretrol, which incensed me because any technician should know that it could just as easily be something up the line somewhere. Indeed, Dan Holohan has suggested looking at the pigtail when that is the case, and it makes perfect sense. So I did, and it was totally blocked. I thought my problems were over. But I replaced it and still showed the same problem. Honeywell was right: I would need a new Pressuretrol.

In order to get the system through the night before I could get the control, I tried to bend the old Pressuretrol's tabs in such a way that it would be sure to cut in and out at about 1 lb. and 2 lbs., but it simply would not do so. I stayed in the boiler room most of the night cycling the system manually every hour or so, and feeling like an idiot.

I put in a brand-new Honeywell PA404A (1033) and set it to range from 1 pound to 2.5 pounds, but it functioned exactly as the old (and now mangled) one, suggesting that the control was not to blame, and that replacing even a blocked pigtail seemed to have no noticeable effect. Even setting the new control to the minimum cut-in and cut-out pressures showed no change.

The next device in the line toward the boiler is the low-water cutoff. I removed the Pressuretrol and pigtail. Inserting a screwdriver into that port I felt something yielding, probably the float going up and down. But there was no apparent blockage. Is it possible that there is a blockage further back, between the cutoff and the boiler, causing insufficient pressure up the line? Would the low-water cutoff even work if there were that much of a clog between the boiler and the cutoff? What's going on?

Here's a question: Shouldn't the low-water cutoff, when opened up, be sending water out at an even (and heavy) rate for a long period, even at no pressure? It seems to slow down after a few moments, suggesting that the device's diaphragm has dumped out what it had in it (perhaps 16 ounces) and it's taking its time to fill up because there is a clog beyond it. It's coming out from this M&M Series 67 device at about 1.67 gpm at 0 lbs. That seems slower than the rate I might see through the 3/4" pipe coming out of the boiler, with 50 to 100 pounds of water pressing down on it. But could that alone be responsible for the total lack of function of the Pressuretrol?

ed wallace

pressuretrol

when you flushrd the low water cut off was the boiler running? if so did the boiler shut off? when you renoved the pigtail did you add water to the system to see if water entered the low watercut off ?

Peter Zelchenko_3

Yes, the cut-off seems to test normal when I open the valve. The flame cuts out and the feeder runs. After the feeder stops it starts up again. Everything there seems normal. Water comes out of the low-water cut-off. I'm just beginning to think that it is flowing in so slowly (because of blockage) that it is cutting the pressure to the Pressuretrol to a fraction of what it should be. If a blocked area before the Pressuretrol is smaller than the pigtail, wouldn't it lead to a lower pressure on the Pressuretrol than the actual pressure?

Steamhead (in transit)

If your LWCO

is one of the McDonnell & Miller units that mount to the sight glass fittings on the boiler, that flow pattern is normal.

The reason is that the blow-down (drain) valve is 3/4-inch, and the sight-glass fittings are 1/2-inch. The blow-down will flow at full capacity as the LWCO's float chamber empties, then slow down when the chamber is empty and water is flowing in thru the 1/2-inch fittings.

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John_201

Someone suggested priming the pigtail with water before installing the Pressuretrol. Is that a must?

Steamhead (in transit)

Not likely

since the sight glass and LWCO have two connections to the boiler, and one is (or should be) always above the water level.

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John_201

Someone suggested priming the pigtail with water before installing the Pressuretrol. Is that a must, and if so, how much would need to go in? I have never heard that mentioned anywhere before.

boiler tech

check for blockage ahead of the pressuretrol

i've been in the boiler business for a long time and have never primed a pigtail, that should accure naturally as the steam condences in the pigtail. Does the pig tail go directly in the boiler? or is there some kind of control header that the control mounts on, either way i would verify that there are no blockage in the boiler fitting or control piping.