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Hot-water zone off a steam boiler?


Dan Holohan
July 16, 2009
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I once got a call from this contractor who was at his local wholesaler. He had the wholesaler's heating man on the phone as well. The three of us talked about how far you can push a steam boiler.

The wholesaler sounded pretty confident. "He wants to pipe a hot water zone from below the water line of the new steam boiler. He wants to heat some baseboard on the first floor with it. I told him he can do it without a heat exchanger, but he's never done it before. That's why we're calling."

"I've never done it before," the contractor said.

"There's nothing to it!" the wholesaler said. "Am I right, Dan. There's nothing to it."

"You can do it," I told them both. "It's done all the time. Just make sure you use a bypass line to blend some return water into the supply water so the water in the radiator doesn't get any hotter than 180 degrees. If it's too hot, it will flash to steam when the circulator shuts off. The key is the bypass line."

"How much load can he put on the zone," the wholesaler asked.

"Well, a lot depends on the size of the boiler," I said. "Usually, you can get away with a 3/4" line and a flow rate of about 4 GPM. That will deliver 40,000 BTUH to the zone. That's enough to heat a good size zone."

"Can he run another zone to an indirect domestic hot water heater," the wholesaler asked.

"Again, it depends on the size of the boiler," I explained. "You can't get more out of the boiler than you put in. It's like an ATM machine. If it's not there in the first place, you can't possibly take it out."

"What if the boiler's borderline?" the wholesaler asked. "Will it still work."

"Well, you can wire it for priority," I said. "You know, don't let the indirect heater run when the other hot water zone is on. Or shut them both off when the steam zone calls. Just don't try to take out more than what's there."

"Like the ATM machine," the wholesaler said.

"Right. If you try to take out too much with these satellite hot water zones, you won't be able to make steam when the time comes. That's the danger."

"You getting all this?" the wholesaler asked the contractor.

"I think so."

"Should we go over it again?" I offered.

"You want him to go over it again?" the wholesaler asked.

"No," the contractor said. "I think I'm okay now."

"Yeah, we're okay now," the wholesaler added. "Thanks."

"But guys," I immediately added, "don't oversize the boiler when you're sizing it. If you do, you'll have more problems than you need."

"What kind of problems?" the contractor asked.

"Short-cycling, carry-over, water hammer. Things like that," I told him.

"I got it," the wholesaler said. "We'll keep it in mind."

A few weeks later, we were all back on the phone having this conversation:

"I think the boiler might be too big," the contractor told me. "I mean, it's almost the same size as the old boiler, physically, that is. That's what's bothering me. Replacement steam boilers are usually smaller than the old ones, aren't they?"

"Yes, they are," I said. "They're more efficient. They hold less water. That's why they're smaller."

"This one's big," he said. "Real big. The label says the load is more than what the old one was."

That's when I told him it probably wasn't going to work.

"What's gonna happen?" he asked.

"It's gonna short-cycle," I told him. "Just like I told you on the phone a couple of weeks ago. It'll run for a minute and go off for a minute. Then it will come back on. It will carry water over into the system piping, and that water will hammer like crazy."

"I'm screwed," he said.

"How did you guys let this happen?" I asked. "You remember we talked about this, don't you? I thought you guys understood."

"Well, we did," the contractor said, "but then we got nervous when we were sizing it. You know, we were nervous about the extra load we were adding with the hot water zones," he said.

"So what did you do?" I asked.

"We figured the steam load, plus the pick-up factor, and then we added a bit. Just to be sure. "

"How much did you add?" I asked.

"You know...a bit," he said.

"How much?"

"About fifty percent," he admitted.

"That's a bit?" I asked.

"Well, we were nervous, you know? Didn't want to come up short."

"So it's fifty percent oversized?" I asked.

"Well, not really. We also added the load for the hot water zone. That was about 40,000 Btuh. Then we tacked on the load for the indirect heater. That was another 160,000 Btuh."

"So you sized the steam boiler by measuring the connected radiation, you then added a pick-up factor, and then you added fifty percent to that?"


"And then you dumped another 200,000 Btuh on top of that?"

"Yeah," he said.

"It's too big."

"Ohhhhhhh," he moaned. "What am I gonna doooo? Can I down-fire it?"

"I think you're beyond down-firing at this point. If you finish putting it in and try to down-fire to get rid of the short-cycling, the flue gases will probably condense in the boiler."


"I think you're going to have to put in a smaller boiler."

"Ohhhhh. You don't understand," he continued. "The guy' who owns this house is rich. I can't admit I made a mistake. I just can't."

I don't know how this one worked out. He never called back.

Here's what I wish the wholesaler and contractor had heard me say:

You can run a hot water zone off a steam boiler, as long as you blend return water into supply to temper the water and keep the maximum supply temperature under 180 degrees.

If you pipe it without vents (use purge valves instead) you can install the hot water zone as high as 30 feet above the boiler water line without benefit of a heat exchanger.

When you use one of these hot water zones, you'll be playing with the boiler's pick-up factor. The pick-up load is usually a third to a half of the total net load, depending on how you sized the boiler.

The purpose of the pick-up load is to give you the "extra" capacity the boiler needs to heat the pipes on the way to the radiators. The pick-up load becomes available to your hot water zone only after the steam pipes have heated up.

The pick-up load (and the rest of the boiler load) is obviously available to your hot water zone when you're not making steam for the rest of the building.

The pick-up load sets the limit for what you can do with a hot water zone off of a steam boiler.

Never add the hot-water-zone load to the steam load when you're sizing the steam boiler. This is a game of subtraction, not addition. Size the steam boiler first. Base it on the connected radiation load, plus a suitable pick-up factor. Then work with the available pick-up load to size your hot water zone or zones. You can't take out more than what you put in. Think ATM.

If you over-size the boiler, you'll have lots of problems with the steam side of the system.

The contractor in this story got nervous. He wanted to make sure he had enough power in the boiler to heat the rich guy's house. He figured bigger was always better, and that the safest course was the conservative one.

The trouble is "conservative" doesn't work when you're playing this game. In this case, less is more.