UNVENTED HOT WATER TANK COMBINATION VALVES – How they work – Plumbing Tips

Posted on: January 13, 2017 by in Plumbing Videos
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UNVENTED HOT WATER TANK COMBINATION VALVES – How they work – Plumbing Tips


All about Un-vented hot and cold water combination valves and pressure reducing valves.. We show you how they work, how to replace one that is faulty on your system and how to strip a combination valve down. We even have hand drawn diagrams all about how they reduce pressure, vent high pressure and have a balanced cold water feed out for showers and mixer taps.

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So, this is just a really quick video to tell you about what combination valves do, how they work, and their basic use in an unvented system. We’ve got one here that, as you can see, is leaking from the pressure regular top spring on here, and basically what we’re gonna do now is to swap that over for a new one in a second. But before we do that, I’m just gonna show you how they work and the basic principle of what they do. So, coming into the top here, we’ve got mains cold water coming in at mains pressure, whatever that may be. Now, remember that the mains pressure can fluctuate up and down according to what time of day it is. Generally it goes up at night because, obviously, less people are using the water. And, obviously, during the day it goes down a little bit. So we want to protect our whole hot water system, and our tank here, from getting so over-pressurized that it bursts. Especially if you live in a mains water area where the pressure can over seven bar at night, that’s way too much for these beasts. So the first principle thing that a combination valve does, is regulate the amount of pressure that’s going into the system. And if you look on the end here, you’ll see that it says three bar on this. So we know now that the pressure going into the system is three bar. So another they can do, which is really good, is you can remove this nipple here, unscrew that plastic bung, and then screw a pressure gauge in so you can see what the pressure is coming into your system. So that’s really handy. And then the next thing they can do on the way a bit down here, we have an eight bar pressure relief valve. Obviously, it’s pretty self-explanatory what they do. If the pressure goes over eight bar in the system, ie. this fails, that pressure relief valve is gonna start dumping water into this tundish here, and then running away outside. The system that this combination valve is fitted on at the moment is a mega-flow. So they’ve got their air pocket and the their expansion in the top of the tank itself. But a lot of other unvented tanks have the expansion vessel remotely fitted, just like you would do on any corner heating system with a braided hose. And that is always fit it on this piece here. You can whip this out, pop a braided hose in, and then have your expansion vessel which is kind of a big white bulbous thing, hanging next to it. Obviously, the expansion vessel takes up expansion as well, but also I like to see it as a mini accumulator as well. Gives the system a little bit of a boost, couple of litres of water when you open up that water tap. One last thing they do as well, and it depends on the actual combination valve you’ve got, you can pipe it up on here. But you can have a balanced cold water out. So, say you’ve got thermostatic mix of valves like on a shower, and it wasn’t balanced. You might have some slight variation in the pressures on the hot and cold side. If you pipe up the cold off the back end of the combination valve here, and the hot as well, then that’ll make sure that the pressures of both three bar are equal, and you won’t have that problem. So, to recap, the principle use of a combination valve is to regulate the pressure going into your hot water system. It’s also to provide a safety drop-off for high pressure should the combination valve fail. It also allows you to manifold to attach an expansion vessel to the system. And also, it allows you to have a balanced cold feed out to anywhere else in the system, especially if you’ve got thermostatic mix of valves that rely on the hot and cold being equal pressures.

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