Should Fill Valves Be Left Open or Closed?

Now there’s a question for you. The feed valve’s job is to, well, feed. You figure out the height of the hydronic system from the point where the feed valve is to the highest hydronic component or pipe. You take that height in feet, divide it by 2.31 to convert it to pounds per square inch, and that tells you how much water pressure you need to fill the system to the top. And since you’ll need some pressure at the top so you can vent air from that point, you’ll add a couple of psi to what you just came up with. Simple, and something you probably already know. I mention it just in case you don’t.

But here’s the thing: Since hydronic systems are closed to the atmosphere (or at least they’re supposed to be), you shouldn’t need that feed valve again unless you drain water from the system, which may make you wonder why you bought the darn thing in the first place, right? I mean some of the feed-valve manufacturers tell you to shut the valve once you’re done filling the system. In fact, they do more than tell you; they plaster their installation-and-operation instructions with WARNINGS, which threaten you and your customer with portents of personal injury and/or death should the valve remain open. And let’s face it, that’s enough to make even Chuck Norris nervous.

But what if you close the feed valve and the air vents finally finish doing their job on those far-out circuits (such as those long, secondary radiant circuits that we love so much) and the system pressure drops a bit. Now you have to go back to fill the system by hand, and that costs money. And since you’ll be adding cold water to the system by doing this, and since cold water contains air, you’ll probably have to go back more than once. Oh, the misery of it all! What’s a contractor to do?

You don’t want to leave the fill valve open and defy the manufacturer’s instructions (and all that implies); but you also don’t want to have to keep going back to the job because that’s just going to eat up your profit, aggravate your customer, and make you look and feel like an idiot.

Flip a coin?

But before you do, here’s an example of why some of the manufacturers issue those warnings about leaving the feed valve open. This is a true story. A house on a slab had baseboard heat and part of the copper loop dipped into the concrete to get by the front door. You’ve probably seen a thousand jobs like this one. That pipe that went into the concrete floor developed a leak, which sent the water down, not up. It leaked for a good long time with nobody noticing because there was no vapor barrier under that slab. The water just drained away. The fuel bill went up and so did the water bill, but the homeowners just took that in stride because it happened gradually, as do so many things in life.

One day, the local water company decided to work on the main in the street. They went around the neighborhood, leaving notes in all the mailboxes, explaining that the water would be off for the day. The couple living in the house on the slab were at work and they didn’t get the word. It was winter and the circulator was running while they were at work. There was no low-water cutoff on this boiler. The installer had depended on the open feed valve to keep the boiler and the system full, but with the city water shut off, the boiler ran dry. The thermostat kept calling for heat, and the burner kept running. Soon there was hardly any water in the boiler. The burner kept running, though, and the boiler got hotter than the hinges of Hell.

At the end of the day, the guys working on the water main were done and they opened that water main. Cold water spewed from the feed valve, hit the red-hot metal, flashed into steam, blew up the boiler and took down the house. No one was home, which was a blessing, but this is one of the reasons why you’ll see those warnings in the feed-valve instructions.

We learned from events such as this that all boilers need (and now must have ) low-water cutoffs. It’s insane not to use a low-water cutoff, and I know they sometimes lead to nuisance callbacks, but I think that if a low-water cutoff trips a burner, it’s time for a professional to take a look at that heating system. I’m hoping you agree with me on that.

We were kicking around this open-the-valve or close-the-valve issue on the Wall at HeatingHelp.com. Lots of people had plenty of points of view, and I read and considered each of them. And then Bob “Hot Rod” Rohr wrote this: 

“If you have a rubber-tube radiant system, you either have a working fill valve or you may have to return every heating season to add a shot of fill water. For some reason, many of those systems tend to need a boost of pressure every year.

“I've tested some to 100-psi (tube only) for 24 hours and they held, but over the summer they drop enough to prevent boilers with low pressure switches from operating.

“I suspect that for every disaster story of a fill valve left on, you could find one for a fill valve left off. Vacation homes are a classic case of freeze-up due to low-pressure lockouts that could have been prevented if the fill valve was allowed to do what it was designed and intended to do.

“It’s your choice and there are plenty of arguments either way you go. It's a sad day when lawyers dictate hydronic installations. Which is the lesser of the two issues?

“Some auto-fill valves come with a knob to regulate flow. Once the system fills and purges you can adjust the flow to provide some make-up without flowing 4 GPM in the event of a break, and maybe that’s a compromise?

“A large radiant job can take days to purge all the microbubbles, I'd advise leaving the fill on for a few days or a week, or you may be returning every day to top it off, only to meet an unhappy, no-heat customer.

“Perspective varies depending on whether you’re manufacturing, selling, or on the receiving end of the unhappy customer calls. They just want the reliable heat and domestic hot water you promised and billed them for.

“Maybe the tank-fill systems with alarm contacts tied into a phone dialer would be another answer. But whose phone number should we use?” 

Good question, and a bad situation anyway you look at it. Let’s keep the conversation going, okay? I’d love to hear your thoughts on this.