Frequent low water cutoff during warm-up

lpsteam · November 2016

Dear Sirs: I have a 2-pipe steam system with approximately 140 radiators attached to it. I recently had the badly leaking 2-million BTU Weil McLain boiler replaced with a new Burnham of similar heat output. Before replacing the boiler, I limped through one heating season and noticed that about 1/4 of the building was not getting much (if any heat), but the old boiler seemed to perform well, reaching 2 psi or higher pressures quickly and uneventfully. When the boiler was replaced several steam strainers and traps which were clogged were replaced with new ones. Now the entire building's radiators receive steam. However, after it is turned on, the new boiler frequently shuts off the burner when the water level approaches the level that has been set for condensate feed pump activation, which causes the burner to cut off, perform a pre-purge, then reignite, until the low water cutoff is then tripped again 5 or 10 minutes later. This cycle may repeat for over an hour until the system is stable at it's set modulation pressure at 1.5 psi. There appears be only be 1-2 inches separation between the heights for feed pump activation and low water cutoff activation. The feed pump is the same as used on the old boiler. The water in the gauge glass does tend to bounce about one inch when the boiler is under the start-up load. The boiler is level. There is a typical pattern for feed pump activation before the low water cutoff engages -- the pump comes on for several seconds, then off for several; this happens again within several seconds, then the low water cutoff kicks in and the pump runs continuously for a while as the boiler pre-purges. The water level in the boiler rises visibly within about 10 seconds as the feed pump runs -- and then may rise dramatically as the pre-purge continues, even with the feed pump off.

The only time I ever witnessed the fluid in the gauge glass not bounce or the low water cutoff be activated was right after a cleaning agent was added to the boiler fluid -- which leads me to believe that I am getting foaming under high load that is tripping the low water cutoff. The dramatic rise in the gauge glass when the cutoff is activated, even when the feed pump has ceased operation also makes me wonder if foam within the boiler is collapsing when the flame as been cut off. I note that since the cleaning agent was added I have performed numerous blow-downs and alkalinized the boiler fluid to a pH of 10.5. I have never seen foam in the gauge glass (which I do see in my other boilers if I over-alkalinize the boiler fluid).

Thank-you in advance for your thoughts!

Jamie Hall · November 2016

Was the new boiler thoroughly skimmed after installation? If not, even with cleaning agents and blowdowns you may still have enough oil in there to cause unstable boiling -- not really foaming, but problems with really big bubbles forming and bursting. It might be worth trying -- even if it was done once.

Are there any wet returns which feed into the boiler without going through the condensate tank? If so, you might have a problem with those being slow.

The symptom of the dramatic rise in water level even after the pump stops suggests the possibility that the increased pressure when the system runs is backing water out of the boiler into something -- a return, maybe? -- which should be always full of water but isn't or which should never have water in it -- but now does under pressure -- which leads me to one of my favourite questions: is the water line of the new boiler within an inch or two of the water line of the old boiler? And if not, have you checked all the piping to make sure that what is supposed to be wet really is, and stays that way, and that what is supposed to be dry really is, and stays that way?

Last thought -- might there be a way to either increase the distance between the LWCO and the condensate return pump operating levels? Or to set the sensor (is it a probe? Float) for the condensate return in a standpipe with throttled connections to the boiler so that it isn't subject to as much bouncing of the water level? Or preferably both?

lpsteam · November 2016

Thank-you for your comments. The only source of water for the boiler is the feed from the pump on the condensate tank. The pump is connected to near the bottom of the condensate tank, then passes overhead, well above the water line of the boiler, through a check valve, then connects to the bottom of a loop on the back of a boiler. The loop connects the upper (steam containing) to the lower (water containing) portions of just that boiler. I believe this would qualify it as a dry return.

The condensate tank is vented to the atmosphere.

nicholas bonham-carter · November 2016

Some pictures of your boiler piping would be useful for us to comment on the problem.
A good low pressure gauge would help to keep the pressure honest, as the supplied manufacturer-supplied 0-0 psi gauge is not very accurate at the low pressures, (ounces), these systems require.
See if this description is a possibility:
When the boiler fires, its pressure will attempt to raise the level of water in the wet return piping 1&3/4 inches per OUNCE of boiler pressure. This water is hiding from the boiler, and causes the LWCO/feed pump to supply more water to compensate for the lower boiler water level.
When firing has finished, the hidden water comes back, and raises the boiler level, causing a higher water line.
Some aspect of the return or supply piping could be wrong, exacerbating this condition. Were any of the installers suffering from Dunning-Kruger syndrome? Keeping the pressure low (ounces), with a vaporstat would help.
Also try a good skimming, with no additives in the water, unless you know you have a water quality problem.
I would think the LWCO should be able to feed water without cutting off the burner.
How was this boiler sized-by radiator EDR, or by guess-and-by golly? How old was the original boiler, and why did it fail?
If you had this to do again, I would suggest a twin boiler setup, properly sized, controlled by pressure, with gravity returns.--NBC

Pumpguy · November 2016

How does the new boiler compare with the old boiler with respect to total water volume capacity?

If it is less than the old boiler, which is frequently the case, you may need a larger capacity boiler feed tank.

Have you calculated how much steaming time your boiler feed tank will provide? I like to recommend the boiler feed tank have enough water storage capacity to provide the boiler with 20 minutes of steaming time.

Frequent low water cutoff during warm-up

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