Draining hot water tank slow

[JK0]

The immersion at the flat I'm working on seems to be blowing the trip, so I'm planning to replace it with a titanium one. As it is right at the bottom of the tank, I have had to drain the whole tank by putting a hosepipe on the drain cock and opening it. The hosepipe leads to a drain in the carpark.

So far, this has taken me over 7 hours, and the tank is still not empty!

Surely a professional plumber wouldn't wait around all that long. Do they pump out the water somehow, or am I missing a trick?

[Jon66]

I'm not sure, surely you just turn the water off and open the hot water taps to drain the hot water tank? Is this an early April fool joke? I must be missing something, I think I'll turn in 😭

[nukecad]

Taps are often higher than the bottom of the hot tank, especially in a flat (or bungalow).
Which is why you need a cold water header tank to give the pressure (head) to push the hot water through.
In any case the cold is usually fed into the bottom of the tank and pushes the hot out of the top, so opening the taps would not drain the hot tank and you need a drain cock as described.

Unless you have a valve between the header and the hot tank then you have to drain the header as well.

7 hours to drain though? It should only take 5 or 10 minutes.
You have turned off the mains supply to the header tank and it's not just filling up again as you empty it?

[Moderator2]

The clean water outlet for a gravity hot water cylinder is normally at the top of the tank. The drain at the bottom may be slow to empty if the tank has a significant accumulation of sludge. Since the outlet is probably at the top then it should not be necessary to drain the entire cylinder if you have turned off the water supply to the header tank, or stopped the ball valve from falling so the header valve does not open. You only need to drain enough water so that the hot water flow stops, then you can remove the immersion element even if the cylinder is largely full.

[JK0]

The clean water outlet for a gravity hot water cylinder is normally at the top of the tank. The drain at the bottom may be slow to empty if the tank has a significant accumulation of sludge. Since the outlet is probably at the top then it should not be necessary to drain the entire cylinder if you have turned off the water supply to the header tank, or stopped the ball valve from falling so the header valve does not open. You only need to drain enough water so that the hot water flow stops, then you can remove the immersion element even if the cylinder is largely full.

Really?

But what about the water that cascades out when I try to remove the immersion?

[JK0]

T
You have turned off the mains supply to the header tank and it's not just filling up again as you empty it?

Lol. Yeah, as far as I know.

[nukecad]

I think what Moderator2 is meaning is that most heaters are fitted from the top so can be changed with the tank full. Yours sounds different though.

[nukecad]

One other thought is that if you have closed a valve between the header and the hot tank (rather than turning off the feed to the header) then you should open the hot taps when draining from the bottom of the hot tank.

In that case opening the taps lets air in to replace the water and prevents the creation of a vacuum in the tank as the water drains out.
Having a vacuum in the tank will prevent the water from draining.

If you have turned off the feed to the header and so are draining that as well then there is no need to open the taps as the header tank is open to air anyway.
In which case sludge would be the best guess.

If you suspect that the drain cock is clogged with sludge or other deposits, you can try repeatedly stepping on the hose a couple of feet away from the tank to squirt water and air bubbles back into the tank. Hopefully this will break loose any obstruction.
If that doesn't work you could try carefully 'backflushing' the valve.
Make sure the hot tap(s) are open.
Connect the 'free' end of the drain hose to a cold tap, close the drain cock, and then open the cold tap to pressurise the hose.
Close the cold tap and then open the drain cock.
The pressure in the hose should backflush the obstruction.

[JK0]

Thanks Nukecad. Those are some good ideas.

Last thing last night I was putting my vax hose under the immersion while loosening it slightly.

[JK0]

Phew!

It was a long time draining. It was another long time filling up again!

I got the immersion out and replaced it by 2.30, and turned the water on again. At 4.10, I still had no water in the hot taps, but the water was still running into the header tank. I was beginning to think I had broken the shut off valve that feeds the cylinder. However, I've just been over there, and I have water in the hot taps at last.