Steam Flooding

One of the most difficult service calls for many is the flooded boiler and the results. It’s not unusual for this problem to require more than one visit to the customer, so let them know up front that you may not get it right the first or second time, but you will get it right! Nobody likes surprises and if you take the ‘surprise factor’ out of the equation it sometimes may just make your day go a lot better. Many oil boilers still use tankless heaters, most conventional gas and electric boilers do not, so let’s start with that and then we can look at all boilers regardless of fuel.

Let’s look at the traditional boiler driven tankless heater, not the mini hangs-on-the-wall gas job. When a tankless, internal or external, has failed it will leak into the boiler causing the relief valve on the boiler to discharge. As this happens the tankless is merely taking the boiler to the incoming service water pressure. To test a tankless for internal leakage, isolate the tankless through the shut-off valves shown in Figure 1. With the valves closed tight and no air or water leaks present, install a test pressure gauge as shown in Figure 2 onto the tankless heater drain valve. Then, with the shut-off valves closed, open the drain valve. The test gauge should show the service water pressure and remain constant. If the pressure drops at all, you have a leak. After making sure the leak is not being caused by a leaking valve, gasket, seal or valve stem you can be pretty sure that you have a leaking coil. We sell those gauges on our website and also have a special low range gauge too!

In most cases, and without a tankless, the best place to start is the feed valve and determine if the feeder is at fault. Lanthier’s Second Law is ‘assume nothing,’ so don’t just assume that any manual shut-offs, Figure 3, are in fact working–check them. Although ball-valves tend to be pretty reliable, stop and waste and some gate valves have washers and if you check any manual feed valve you’ll know for sure.

Many years ago McDonnell & Miller showed the proper piping of automatic feeders for steam boilers and also how to do a ‘broken union test.’ In Figure 4 we have the procedure and drawing as it appears in their literature and if you can’t add anything, just shut up, so I will!

Although we spoke of this in an older article it’s important to note, once again, that if you have a steam boiler with a bouncing and surging water level the pH should be checked. A surging water condition may be the cause why the feeder may be going on for no reason. With a properly running steam boiler, the water level should stay fairly constant. When the water level surges up and down by a substantial amount it can trip the feeder and the result is a flooded boiler. When surging occurs each down-surge can trick the automatic water feeder into adding water that’s not really needed, eventually flooding the boiler. Although most boiler companies like to see the pH between 7 and 11, Figure 5, if the water is too alkaline, 11 or more, it will foam in the boiler and the foam will leave the boiler along with the steam. This action tricks the automatic feeder into adding unnecessary water. When the condensed foam returns, you get a flooded boiler. Check the water pH with either strips or a device like that shown in Figure 6 that is available on our website.


If you do feel surging may be the reason for the flooding, maybe you need to use something I’ve come to call ‘The Boston Trick.’ Although this can be used for troubleshooting, I’ve also left many on jobs as a safe guard. In Figure 7 we show a #11 switch that is used in many McDonnell & Miller low-water cutoffs. M&M recommends that power come in on terminal #2 and go to the burner on terminal #1 and the load wire to the feeder be wired to terminal #4, Figure 8. A jumper should be placed between terminals #2 and #3 to provide power to the feeder. The #11 switch is actually a three-position switch. With the proper amount of water in the boiler the switch is closed between terminals #1 and #2, Figure 9-A, allowing the burner to run and the feeder shut off. As the water level drops terminals #2 and #1 remain powered and now power flows from #2 to #3 and makes terminal #4, Figure 9-B. If the feeder is not present or fails to fill the boiler the switch between terminals #2 and #1 opens shutting off the burner, and terminals #3 and #4 remain closed energizing the feeder, Figure 9-C. Now, the problems and solution.

Did you know the average four section steam boiler currently holds about nine to 14 gallons to the working water line? Did you know that at 55 pounds per square inch of water pressure you can flow about 10 gallons of water per minute through a ½ inch pipe? That means that a gallon of water can be fed into that boiler in about six seconds, and that’s not a long time.

What happens if a wet return fails? Or a boiler fails, or any other leak? Well at 10 gpm you can put a lot of water on the floor in no time at all. So, years ago I was taught this great trick and I’ve gone on to teach it to thousands. Between terminals #2 and #3 put in a thermal fuse. That thermal fuse is nothing but a bare #18 AWG thermostat wire and it also fits in that ‘dog bone’ space nicely. The #18 bare wire can only carry between three to six amps. With both the burner and feeder running, your amperage draw is about three to six amps, so within a short time the amperage draw will burn out the wire. Essentially, you have a thermal link or fuse! That fuse will fail in about 30 to 40 seconds and instead of a flooded boiler or basement you only have between five or six gallons to mop up.

That pretty much covers all the reasons for a flooded steam boiler other than the customer overfilling it. We’ll be showing up here at Fuel Oil News on a more frequent basis again and so maybe we’ll look at what makes water boilers flood down the road.

By the way and if you haven’t heard although Firedragon is no longer involved with teaching any oilburner Codes including the Massachusetts Oilburner Code we will be bringing you Code updates on both 527CMR4.00 and NFPA31 as they happen and if you missed our last update get in touch with us.
See ya!

George Lanthier is the owner of Firedragon Academy, a Massachusetts Certified School and publishing firm. He is the author of over 60 books and manuals on gas and oil heating and HVAC subjects. He is a CETP, NATE, NORA, PMAA and PMEF Proctor and a Massachusetts Certified Instructor operating a NORA and State recognized school teaching gas and oil and related subjects. He can be reached at 608 Moose Hill Road, Leicester, MA 01524. His phone is 508-421-3490 and his website can be found at


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