{"id":495,"date":"2026-10-09T08:30:16","date_gmt":"2026-10-09T00:30:16","guid":{"rendered":"http:\/\/www.fcsteeltech.com\/blog\/?p=495"},"modified":"2026-10-09T08:30:16","modified_gmt":"2026-10-09T00:30:16","slug":"what-is-the-difference-between-a-normally-open-and-a-normally-closed-check-valve-4791-896373","status":"publish","type":"post","link":"http:\/\/www.fcsteeltech.com\/blog\/2026\/10\/09\/what-is-the-difference-between-a-normally-open-and-a-normally-closed-check-valve-4791-896373\/","title":{"rendered":"What is the difference between a normally open and a normally closed check valve?"},"content":{"rendered":"

If you\u2019ve ever spent time working with fluid systems\u2014whether you\u2019re a maintenance tech at a factory, a plumber troubleshooting a home\u2019s water heater, or even someone who helps spec out parts for irrigation projects\u2014you\u2019ve definitely heard the term \u201ccheck valve.\u201d But when you start digging into the details, there\u2019s a split that trips up a lot of folks: normally open vs. normally closed check valves. And as a check valve supplier, I talk this stuff through with customers every single week, so I get how easy it is to mix these up. No fancy jargon here\u2014let\u2019s break this down like we\u2019re chatting over a coffee (or a toolbox, if that\u2019s your vibe). Check Valve<\/a><\/p>\n

<\/p>\n

First, let\u2019s start with the basics: what even is a check valve? It\u2019s the unsung hero of any pipe system. Its whole job is to stop fluid (that\u2019s liquid or gas, by the way) from flowing backward. Think of it like a one-way door for water, air, whatever you\u2019re moving. The thing that makes normally open and normally closed different boils down to what the valve does when there\u2019s zero pressure on it\u2014like when the system\u2019s turned off, no pump running, nothing pushing fluid around. That \u201crest state\u201d is the key, not how it works when pressure\u2019s active.<\/p>\n

Let\u2019s start with normally open check valves, because these are the ones people run into more often in everyday systems, honestly. A normally open (often shortened to NO, because we\u2019re not writing \u201cnormally open\u201d every time) valve is wide open when there\u2019s no pressure on it. Like, the door\u2019s propped open. Why would you want that? Mostly because you don\u2019t want flow blocked unless you specifically need to stop it. Let\u2019s use a super relatable example: the water line to your washing machine. Most of those use a NO check valve? Wait, no, wait\u2014don\u2019t get ahead of me, let\u2019s pick a better one. Irrigation systems! If you have a sprinkler system in your yard, when you turn the whole system off, the lines drain a little. A lot of NO check valves sit at the lowest point of each zone, right? When the pump\u2019s off, there\u2019s no pressure, so the valve is open. That lets any leftover water drain out instead of sitting in the lines and freezing when winter hits (freezing water expands, breaks pipes\u2014we don\u2019t want that). The second you turn the pump back on, pressure builds on the inlet side of the valve, and that pushes the flap or disc or ball (depending on the valve type) closed, so water flows forward to the sprinklers and won\u2019t go sloshing back down the line.<\/p>\n

Wait, but NO valves aren\u2019t just for irrigation. Let\u2019s say you have a compressed air system in a small machine shop. If the compressor turns off, you don\u2019t want the line to the air tools to stay blocked\u2014you need any trapped air to vent, or you might have pressure building somewhere unintended. A NO check valve there keeps the line open when the system\u2019s idle, so it stays safe until the compressor kicks back on. Common designs for NO valves? Swing check valves are super popular for this, because the hinged disc just falls open when there\u2019s no pressure, flips closed when flow comes. Also, some ball check valves can be NO if the ball is held away from the seat by a tiny spring or gravity when there\u2019s no force on it.<\/p>\n

Now, the other side: normally closed check valves (NC for short, obviously). These are the ones that are sealed shut when there\u2019s zero pressure. The door\u2019s locked tight, not open. When would you want that? Any time you don\u2019t want any backward flow at all, even when the system\u2019s off, and you need to make sure it stays that way. Let\u2019s take the backflow preventer under your kitchen sink. When your city\u2019s water main is working, pressure pushes water forward to your faucet, and that opens the NC check valve. But if there\u2019s a sudden pressure drop in the main\u2014say, a fire department turns on a hydrant a block away\u2014you don\u2019t want dirty, backward-flowing water from your sink drain (or worse, a neighbor\u2019s) getting sucked back into the clean water supply. The NC check valve? It\u2019s already closed by default, no pressure needed. The only time it opens is when forward pressure is high enough to push the disc off the seat, and if the pressure drops even a little, it slams shut immediately. That\u2019s non-negotiable for potable water systems, right? Health codes basically require NC check valves there.<\/p>\n

Another good example: a chemical processing line at a pharmaceutical plant. If the system\u2019s not running, you can\u2019t have any of the expensive, sensitive drug ingredients sloshing backward through the pipe and contaminating a different line, or leaking out into storage. An NC check valve keeps that line sealed tight when there\u2019s no flow, so you don\u2019t have to shut down the whole system to fix a backflow issue that never should\u2019ve happened. Common NC valve designs? Dual plate check valves often work here, because they have tight, precise seats that don\u2019t leak when closed. And spring-loaded check valves are super common for NC, because the small spring pushes the disc right up against the seat when there\u2019s no pressure\u2014no way it\u2019s open on its own.<\/p>\n

Wait, let\u2019s clear up a big myth here: neither of these is \u201cbetter\u201d than the other. It all depends on what your system needs. That\u2019s where I, as a check valve supplier, come in\u2014 I don\u2019t just sell parts, I help people figure out which one they actually need, because picking the wrong one is a headache. Let\u2019s use a concrete mix-up example: if you put a NO valve on a backflow preventer for your home, when the city\u2019s pressure drops, the valve\u2019s open (its normal state) and there\u2019s nothing stopping dirty water from getting into your drinking water. That\u2019s a hazard. If you put an NC valve on your sprinkler system\u2019s low point drain? When you shut the system off, the valve stays closed, water sits in the line, freezes, and cracks the pipe. Oops.<\/p>\n

Another thing that surprises a lot of people: sometimes folks mix up these terms with normally open \/ normally closed solenoid valves, but check valves don\u2019t have an actuation signal\u2014they\u2019re mechanical, no electricity needed. That\u2019s a big difference. Solenoids use power to switch, check valves use pressure. So when someone says \u201cmy check valve is NO,\u201d they\u2019re not talking about an electronic switch\u2014they\u2019re talking about its resting state. That\u2019s a super common mistake I catch when customers call in, asking if they need to wire a NO check valve. Nope, no wiring required, just pressure.<\/p>\n

Let\u2019s talk about performance and reliability too, because that\u2019s what matters when you\u2019re dealing with systems that can\u2019t go down. NO valves tend to have less pressure drop when they\u2019re open, right? Because they don\u2019t have a spring holding the disc tight to the seat, so flow can move through easier. That\u2019s a win for systems that need efficient flow, like large irrigation lines or compressed air systems. NC valves, though, have that spring (usually) holding the seat tight, so they seal better\u2014no leaks, which is critical for potable water, hazardous chemicals, or systems where even a tiny backflow could cause damage. The trade-off with NC is a little more pressure drop when open, because you have to push against that spring to open the valve. For most small to medium systems, that\u2019s negligible, but for huge industrial lines, we\u2019ll adjust the spring tension or recommend a different design to minimize that.<\/p>\n

Wait, also\u2014what about different industries? Let\u2019s break that down to make it relatable. If you\u2019re in HVAC: normally closed check valves are standard on condensate lines. When the AC is off, the line\u2019s closed so sewer gases can\u2019t come back up into your house. When the AC runs, condensation flows forward, pressure pushes the valve open, so water drains out. That\u2019s NC 101. If you\u2019re in automotive? Wait, cars use check valves too\u2014fuel lines, brake lines. The one-way valve in the brake booster is usually NC, because you don\u2019t want vacuum leaking back when the engine\u2019s off. And the valve in the fuel pump line is often NO, so fuel can flow when the pump\u2019s on and the rest is blocked when it\u2019s off? No, wait, maybe don\u2019t overcomplicate that\u2014stick to stuff people know.<\/p>\n

As a check valve supplier, I tell every customer to ask themselves three questions before picking one: 1) What does my system do when it\u2019s not running? 2) Could backward flow cause safety hazards, damage to equipment, or contamination? 3) Do I need maximum flow efficiency when the valve is open? If your system\u2019s idle needs to drain excess fluid, and backflow only matters when the system\u2019s active, go NO. If your system needs to stay sealed when idle to keep water safe, prevent leaks, or stop contamination, go NC.<\/p>\n

I can\u2019t tell you how many times I\u2019ve gotten a call from a guy who installed a NO valve on his pool\u2019s return line and now has algae growing back into the filter, or a small factory owner who put an NC valve on their air compressor drain line and now has standing water in the lines because it won\u2019t drain when the system\u2019s off. Those are avoidable mistakes, and that\u2019s why I\u2019m here\u2014not just to ship a valve, but to make sure it\u2019s the right one for your job.<\/p>\n

Wait, one more quick point: testing. When we test these valves at our shop, we check their resting state first. For NO, we apply zero pressure, confirm they\u2019re open, then apply forward pressure to make sure they close. For NC, zero pressure means they\u2019re closed, we apply forward pressure to open them, and check the seal on backflow pressure. That\u2019s the QA step that prevents the kind of installs that go wrong later.<\/p>\n

<\/p>\n

At the end of the day, it all comes down to resting state, nothing else. Normally open is open when no pressure is applied, normally closed is closed when no pressure is applied. The rest is just adapting that core to your system\u2019s needs. If you\u2019re not sure which one you need for your next project, or you\u2019re replacing a failed valve and don\u2019t want to guess, hit me up to chat through your setup. I don\u2019t do pushy sales talk\u2014just straight answers, because a happy customer who has a valve that works is the one that comes back, or refers a friend. No matter if you\u2019re a homeowner with a small sprinkler system, a mechanic with a shop air line, or an industrial engineer working on a chemical plant, I can help you sort out what fits.<\/p>\n

Y-type Filter<\/a> Now, as promised, the references. These are the standard industry sources for check valve specs and definitions, no sketchy sites:<\/p>\n

    \n
  1. American Water Works Association. (2020). Backflow Prevention and Cross-Connection Control Manual. AWWA.<\/li>\n
  2. Fluid Controls Institute. (2018). Industrial Check Valve Selection and Application Guide. FCI.<\/li>\n
  3. International Code Council. (2021). International Plumbing Code (IPC). ICC.<\/li>\n
  4. Valve Manufacturers Association. (2019). Standard Terminology for Valves and Actuators. VMA.<\/li>\n<\/ol>\n
    \n

    Xiongxiang Valve Group Co., Ltd.<\/a>
    As one of the most professional check valve manufacturers and suppliers in China, we’re featured by quality products and good price. Please rest assured to wholesale cheap check valve for sale here from our factory. Contact us for more details.
    Address: Building 1, No.306, 12th Binhai Road, Jinkai District, Wenzhou City, Zhejiang Province, China
    E-mail: info@xiongxiangvalve.com.cn
    WebSite:     https:\/\/www.steelvalvegroup.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

    If you\u2019ve ever spent time working with fluid systems\u2014whether you\u2019re a maintenance tech at a factory, … What is the difference between a normally open and a normally closed check valve?<\/span>Read more<\/a><\/p>\n","protected":false},"author":47,"featured_media":495,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[458],"class_list":["post-495","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-check-valve-4928-89ac32"],"_links":{"self":[{"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/posts\/495","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/users\/47"}],"replies":[{"embeddable":true,"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/comments?post=495"}],"version-history":[{"count":0,"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/posts\/495\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/posts\/495"}],"wp:attachment":[{"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/media?parent=495"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/categories?post=495"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.fcsteeltech.com\/blog\/wp-json\/wp\/v2\/tags?post=495"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}