I have an unvented hot water cylinder in my apartment (OSO Hotwater, Direct 20 RD)

I need to drain and disconnect it, but I’m just wondering the how I can drain the system without too much wasted water/spillages.
I’ve identified the connections that I would need to disconnect in order to move it (circled in red).
I can also see a drain at the bottom of the cylinder (bottom circle, below), but was wondering if I would be able to mostly drain the water by opening a hot tap and shutting off the inlet valve (circled in blue, below)?

After fully draining the cylinder, what becomes the best way of filling it back up with water and getting back to an appropriate air gap?
I read that these types of cylinders typically have an air gap inside them to help maintain pressure, and don’t want to drain the system and let air in that I can’t get out again after filling back up with water.

Image of cylinder with highlighted areas

Image of make/model

It's worth noting that the central heating is completely separate; I know some systems are Y-plan or S-plan, but my system don't have any overlap.

Here's a link to the manual

  • Key question: Is this on the same level of your house as sinks/etc., on an upper level or on a lower level? Commented Dec 5, 2023 at 15:27
  • The same level, I live in an apartment
    – Harrison
    Commented Dec 5, 2023 at 15:29
  • What country is this? Is this an electric resistance water heater (WH)? Are you replacing it with another water heater and if so what kind? Commented Dec 5, 2023 at 15:57
  • I'm in the UK, I only need to move the tank for a short while, before putting it back (I need to get to pipes in the bottom right). I believe the tank uses a thermostat to control its temperature, and the water pressure comes from the cold mains supply
    – Harrison
    Commented Dec 5, 2023 at 16:00
  • 2
    I gather that at one time in the UK water heaters were not pressurized and were said to be vented because they were at atmospheric pressure so that delivery of hot water was by gravity flow. The term "unvented" is not used in the US because AFIK all water heaters in the US are pressurized to at least 50 psig or so. Commented Dec 5, 2023 at 16:05

1 Answer 1


You'll probably not get a lot of water out with the cold supply shut off. It's unlikely that there's an "air gap" in that, as well. So when you shut the supply (which provides both water and pressure), hot pressure will drop to nil in a few hundred milliliters.

You may have some success if you identify the lowest and highest hot water outlets, and open them both so that air can flow in from the highest and water can flow out the lowest. Eventually you'll have to resort to the drain valve (leave the hot taps open for venting.)

The main thing when you refill it is to be sure you have gotten all the air out of it (run the hot taps and expect a lot of air, and then water and bursts of air, and then water and bubbles...) before you turn it back on. It appears to be an electric heater, and you can burn an element out in seconds if there's not sufficient water to cover it before it's powered on.

  • Thank you, your tip about keeping the taps open for venting definitely helped. After I drained the tank and disconnected the pipes, I was able to move the heater over as needed. The next day the tank was fully operational again (and actually better than before as we now get higher hot water pressure than before)
    – Harrison
    Commented Dec 15, 2023 at 9:36

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