Published: June 8, 2015

Categories: Steam

Fig 166 Water Gauge

I once met a guy in the fuel oil business who tried to make things easy for his steam customers. They’d call in the spring and complain that they didn’t have enough domestic hot water.

He knew why: The boiler water covered only part of the tankless coil. He’d tell them to add water to the boiler until it was near the top of the gauge glass.

“Oh, I’m not sure how to do that,” they’d say. “What if I make a mistake? Will my boiler explode? Will I have a flood? Can’t you send someone over to do it for me? I have a service contract, you know.”

So, afraid he’d lose the account if he didn’t, he’d send a serviceman over to put some water in the boiler. Of course, he never charged for these calls. “Customer Relations,” he called it.

Unfortunately, as the years rolled by, “Customer Relations” began to grow hair and threatened to put him out of business. So on replacement steam-boiler jobs he got into the habit of installing the low-water cutoff near the top of the gauge glass instead of at the bottom of the gauge glass where it belongs.

This way, he reasoned, water will cover the coil all year round. And to make the job “fully automatic,” he installed an automatic water feeder on every steam boiler he put in – to keep the water level nice and high. “There!” he’d say, “That should keep them in hot water all year long.”

But then the phone calls started to come in: “I don’t have any heat today and the banging in the pipes is terrible,” they’d say. “I never had this problem before you put that new boiler in my basement. Send someone right away! And don’t think I’m going to pay for the call. I have a service contract, you know!”

“How’s your hot water?” he’d ask.

“Oh, there’s plenty of hot water,” they’d say. “We just don’t have any heat! Get over here, you bum!”

So he’d send out a serviceman who would drain some water out of the boiler and cut the firing rate. But within a week or so he’d be back again, playing with the fire and losing his mind – as well as his customer.

This is the subtle side of near-boiler piping that’s often overlooked. You now know how important that 24-inch height between the water line and the header is. Without it, steam will pull water out of the boiler and dump it into the header. That creates wet steam and callbacks.

But even if you follow the manufacturer’s instructions to the letter and allow for that 24 inches, you’re dead if you raise the water line by raising the low-water cutoff.

Think about it.

The manufacturer wants 24 inches between what he calls the “normal” water line (about halfway up the gauge glass) and the bottom of the header.

But that water line is only “normal” when the burner is off. When the burner is firing, the “normal” water line should be very near the bottom of the gauge glass. That’s because water, in the form of steam, is out in the system.

That’s why we install low-water cutoffs so low. They’re there to protect against a minimum water level. The same goes for automatic water feeders; they’re designed to maintain a safe minimum water line. An automatic water feeder is a back-up safety device. It’s not a convenience item.

The minimum water line is just slightly above the bottom of the gauge glass. No one ever intended for water feeders to maintain a center-of-the-gauge-glass water line when the boiler is running. If they did, where would the returning condensate go when it came back from the system after the burner shut down? You’d have a flooded boiler for sure.

So picture the poor fuel-oil dealer. He sets everything up so the water rides high and proud in the gauge glass and covers the coil all year long. Let’s watch what happens:

He begins from a cold start; the gauge glass is about three-quarters full. Now here’s something he probably never considered: Depending on the size of his boiler, the water line he sets “cold” can rise nearly three-quarters of an inch by the time he gets steam. That’s because boiling water takes up nearly 5% more space than cold water. And as the water expands, up goes his water line!

Now he’s nearly at the top of the gauge glass. His water line is getting dangerously close to the steam outlet. And then the boiler begins to steam.

Remember, you’ll get your highest steam velocity at this point. Steam will be roaring out of the boiler, and since the water line is so high, it will be pulling water up with it. This is when you really need that 24-inch rise to the header if you’re going to try to separate the water from the steam before it hits the header.

He thinks he has the 24 inches. He even measured from the center of the gauge glass to the bottom of the header. It looks like he has it.

But he doesn’t have it, does he? Oh, he may have it externally, but internally, where it really counts, he doesn’t have 24 inches. He raised the low-water cutoff, installed the automatic feeder and, along with both of those things, he also raised the water line.

So his new boiler gets sucked practically dry before it has a chance to heat the house. The automatic feeder bangs on because that’s what it’s being paid to do. And, finally, the condensate returns. Now he has a flooded boiler and a cold customer.

But hey, the customer has plenty of domestic hot water!

Obviously, this is not the way to get the job done. Like near-boiler piping, you have to consider low-water cutoffs and automatic water feeders a part of the boiler nowadays. Install them where the manufacturer tells you to install them. If you have to submerge a tankless coil in the summertime, do it by hand. There’s no way around that. Steam heat is strictly a hands-on proposition.