Hot water failure

[John Andrews]

Dear all

I have a conventional heating system, comprising an Ideal gas boiler, with gravity flow and return (hot water circuit) and pumped flow and return (central heating circuit).

I'm getting no hot water out of the taps at present, central heating is ok. Can anyone advise me on how to fix this please?

Thermostat on boiler is ok, and the control panel is asking for hot water as it should. The gravity flow pipe from the boiler is not heating up. So I guess it might be the heat exhanger that's the problem?

Yours in hope John

[IMM]

It looks like two points:

1. sludge in the gravity flow and returns

1. An air lock ( is there an air vent above the gravity pipe to the cylinder)

Get rid of the gravity DHw and make the system more efficient by fully pumping.

[Set Square]

Does the CH use entirely separate pipes from the HW, connected to the opposite side of the boiler?

Do you have a small fill & expansion tank in the attic, with pipes connected into the HW circuit? Does it have any water in it?

There is a possibility that the ball valve may have stuck, allowing the level to drop through evaporation/leaks etc. to the point where there's an air lock in the HW circuit.

If this is the case, freeing the ball valve should fix it.

Failing that, are there any controls on the HW circuit - such as a motorised valve driven by a cylinder stat to stop the HW from getting too hot?

[Andrew Gabriel]

1. Ball valve has got stuck up in header tank (smaller tank), and water level has dropped below the top of the hot water cylinder coil so it no longer circulates. Check the small header tank.

[IMM]

Good point. Usually the highest rad is above the coil, and if this was the case the upper rads would not work. Maybe that the coil is above the highest rads.

[John]

You got to it first Andrew. It was (is) a common problem with gravity primary hot water and pumped CH systems. It sometimes occurred with an intermediate stage where the HW only warmed up when the heating was on and provided just enough water movement within the boiler to persuade the primary to move sluggishly before giving up entirely.

[Set Square]

[My post suggesting the same thing was actually sent 12 minutes earlier than Andrew's. Let's hope we both got it right!]

[John Andrews]

Good idea, I hadn't thought of that. But no, that's not the problem. Water level and ball valve are fine - in fact I replaced the ball valve assembly about six months ago. The important point is that the hot water supply to the cylinder is not working - i.e. cold.

Cheers John

[John Andrews]

Thanks for your ideas. I guess with (1) I can have a go at disassembling to check it out. There is a vent, so that takes (2) out of the picture, right?

Excuse my ignorance, but why is a pumped system more efficient? Surely by definition an unpowered system, if it's working ok (which it's not obviously), is more efficient?

Cheers John

[John Andrews]

Yes and yes. Gravity flow and pumped return go in one side, pumped flow and gravity return in the other.

Yes and yes. Ball valve/expansion tank are A-OK.

...

There's nowt apart from what's in the boiler unit itself. I'm beginning to feel like I should take this apart and see, but as much steer as I can get from the knowledgable folks out there the better!

Cheers

[Dave Plowman (News)]

No - the heat exchanger is the same one as for the heating. Only different feeds off it.

Has it got provision for bleeding? Any air in there? Has the system been worked on recently necessitating draining down?

If the system once worked normally, then it can only be:-

Airlock caused by the water level being too low at some time. Blocked pipe.

[Dave Plowman (News)]

These can block. Check there is water there by partially dismantling.

[Set Square]

I think that you may have misunderstood the point. You should have *two* header tanks - a big one and a little one.

The big one feeds cold water into the bottom on the cylinder expelling hot water out of the top to your hot taps. Is this the one with the new ball valve?

The small one keeps the primary heating circuit full of water. This is the water which circulates from the boiler by gravity to the heating coil inside the hot cylinder, and pumped action to the radiators. If the water in *that* circuit becomes depleted for some reason, the first thing to suffer will be the gravity circulation to the heating coil - followed by the central heating.

Are you *sure* that you have located the *small* tank, and that it has water in it?

[John]

Your other post still hasn't surfaced to me - ain't usenet grand:-)

[John]

Its worth checking that the "fill" pipe from the tank to the primary loop isn't blocked with crud. This is not too rare IME

[John Andrews]

Bingo! You read my tiny mind - ballcock in the little one was floating in the air! I'll keep an eye on water heating working less efficiently than before - I guess I might have some air in the system now - but normal service has been resumed so I'm happy.

Thanks to everyone for their help - very much appreciated.