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Unvented Hot Water Systems - Plumbing Tips

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Published on Aug 8, 2011

***You MUST use a qualified plumber for the installation of an unvented cylinder***
This video briefly explains the basics of an unvented hot water system or heater. We also cover the safety features of an unvented hot water system and how they work. For more information please visit http://www.plumberparts.co.uk***VISIT OUR SPONSORS***
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Welcome to this plumberparts.co.uk video. Today, we're gonna look at unvented hot water cylinders. The particular one we're looking at today is an indirect cylinder. That means it has a hot water coil that goes through it that doesn't mix with the hot water that comes out the taps, and that provides heated hot water from the boiler that goes through the coil, transfers its heat into the hot water here, and then goes off to your taps like that. Let's have a quick look.
So, here we go, this is the unvented hot water cylinder that we're talking about. The control for the hot water coil coming here, this here is the flow from the boiler. Now, that is controlled by this two-port valve, here, that opens and closes according to whether the stat on the side of the cylinder is calling for heat, and whether the programmer is calling for hot water to be on in the first place. What happens, if we have a look 'round here, there's a small diagram, and that relates to all the inlets and outlets on this particular tank. So, the hot water comes in from here, from the boiler, goes in here, through a coil, round, and round, and round again, and then back out here, cold, transferring its heat into the main body of hot water, ready to go off to your taps.
Also, at the bottom, we have a connection here, this is the cold water feed, okay, from the mains, and that goes into the bottom of the tank, picks up heated water from either the immersion here or the coil up here, goes out at the top, through here, off to your taps.
There are a few safety features that you have to consider having installed when putting in an unvented hot water cylinder. For a start, you need to have somewhere for the water to expand to. Obviously, when water is cold and it gets heated up, the molecules and atoms in that water make more movement, and therefor the water expands. Now, it needs somewhere to expand to, so what we provide is an expansion vessel. This is up here. There is the expansion vessel. Now, all that is, is a diaphragm here. On this side, we have water, on the top side up there, we have air. Now, the rubber diaphragm then expands into that part there, giving the water somewhere to expand to.
The next safety requirement you need to have is a high-pressure relief valve. That's in the case that, say the diaphragm in the expansion vessel fails and the hot water has nowhere to expand to, it needs, if it should get to a point of, say, 6.5 bar, which is a measurement of pressure, if it gets over that particular pressure, the pressure relief valve will open and relieve that pressure to a visible and safe drain-off point, so you know there's something wrong, and also, when it does start to relieve the pressure, it doesn't burst hot water all over someone's head. In this case, the high-pressure relief valve is here, and that is set to six and 1/2 bar.
We've got a small tundish here so you can see if it's starting to let by, and then it goes into the copper pipe down here, where it's terminated downstairs, outside, in a safe place. When we look at the cold feed of an indirect hot water cylinder, we have to recognise that there has to be a balanced cold feed that goes off to all the same services that the hot water cylinder supplies. If you have a shower valve, you'll have the mains pressure hot water coming out, so therefore, to make it so they're even you'll have to have a mains pressure cold water, as well. How we do this, we have the mains pressure cold water coming up through this valve here, ignore this bit there, that's nothing to do with this, and it comes down into this balancing valve. That sets the pressure at three and 1/2 bar, so we can either have water go down into the hot water tank, or come out the other side of this valve here and go off to our cold water taps. That means you have an equal balance of pressure on all the hot and cold feeds throughout the house. This is especially important if you have shower valves that mix the water, because, obviously, if one side, the hot or the cold side is of different pressures, it won't mix properly.
So let's have a look at the fundamentals of an unvented hot water system. So, we have our unvented hot water tank, we have our cold main in, and our hot out at the top. There is a balancing valve here that also draws a balanced cold water main, and they will go off to a shower valve, and then you've got the balanced water at the top of the shower valve.

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