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No water in Hot Water Tank?

Postby Kelanen » Wed Apr 08, 2009 11:34 pm

Hi there,

I have a problem... lol

I'm afraid I'm going to err on the side of giving too much information, rather than too little. My experience in my own field (IT) shows this to be the most helpful.

[b][u]About me:[/b][/u]

The problem is in my own home, not a business, not a landlord, etc. I'm fairly practical and handy, good with carpentry etc, okay with electronics, but plumbing is pretty new to me. I thought I understood the basics of how central heating works, but I've obviously missed something, and I suspect it's basic! I realise that anything done by other than by a professional may be dangerous, invalidate insurace, etc...

[b][u]Background to Problem:[/b][/u]

Basically my boiler is dead, and needs replacing. I've explored that elsewhere, but it's not relevant here (I think). Should it have any significance, it's a Potterton Netaheat 16/22, non-combi gas boiler, and it's ~25 years old.

Essentially this has died, and I don't have any money for engineers or a replacement (unemployed, borderline bankrupt, threatened repossession. etc) so I'm seeing what I can do about things myself. Tinkering with the boiler has shown I can get about an hours use out of it every 3 days at some random point, but it consumes gas the whole time, even when not lit. This doesn't seem a terribly good idea to me, so mostly it's switched off now. I use electric heaters where necessary (thankfully the weather has warmed up now), and kettles for baths (an hour long process).

[b][u]The Problem:[/b][/u]

What struck me, was in a previous house I had an immersion heater in the tank, , which when the gas failed provided not loads of hot water, but enough. Checking my hot water tank, it did indeed have one, but no hot water, so it's not working some where along the line.

The hot water tank is what I have identified (I believe) as an indirect vented tank. I have a large and a small water tank up in the loft, with open overflow pipes going into them etc. It's a plain copper tank with a red foam jacket around it. I suspect it's 25 years old like the rest of the CH system. Water is very hard if it makes a difference, although the previous occupants had a softener on the mains.


Having decided I couldn't do much more with the boiler, I looked at the tank and immersion heater. First of all I checked all the fuses (house distribution board, and three at the tank) but they are all fine.

This led me to think the next stage was replacing either the immersion heater, or the thermostat in it. I figured probably the whole unit was safest. I couldn't get the large nut undone though, and went to B&Q's, and got a special tool. I didn't know what length immersion heater I needed either so I figured I'd drain the tank, and get mine out, and see what it was.

I switched off the only stop cock I could find (actually the wrong one - I think it's hot water circulating from boiler (Pipe 6)), ran the hot taps upstairs, but it didn't stop at all. After a while I turned them off, and figured I'd try anyway.

[b][i]Problem 1: [/b][/i]Even with WD40, a hell of a lot of effort and swearing, I can't undo that immersion heater screw. It may turn out being irrelevant (see below), but is there any trick, or is it just the application of brute force? I just ended up rocking the tank back and forth...

[b][i]Problem 2:[/b][/i] The ease with which the tank rocked, coupled with the noise of tools on the copper made me think it's empty. Some knocking later and I'm convinced.

I've taken the thermostat rod out of the immersion heater, and yes it's dry, no sign of water, etc. Clearly the tank did have water when I was getting some hot water from the boiler, because I checked and felt the tank heating in these times. I have switched the boiler off, CH clock off, and house thermostat off - would any of these cause the tank to drain itself?

[b][i]Question 1:[/b][/i] How is the hot water tank empty?

Now obviously if the tank was empty, the thermostat or immersion heater could have burnt itself out, but the empty tank is only a recent event, so I don't think that's it. If I could get the tank full, my next plan was still to replace the immersion heater, unless someone competent has better advice?

Now, here's the bit that I really don't understand...

[b][i]Question 2:[/b][/i] How come with an empty tank, the hot water taps still have an unlimited supply of (cold) water?

I mapped out the pipes besides the tank, followed them into the loft etc, and I have labelled them in the pictures:



Now, when I turn off the tap on Pipe 1 up in the loft, after a short time the water to the hot taps stops. Now this is the pipe that feeds the hot water tank, so this behaviours is what I expected, BUT, I expected the cold water to flow through the hot water tank. As I understand it, the cold water is pushed into the tank at the bottom, which then displaces hot water at the top, sending it to the taps, etc. Is that right? What am I missing?

[b][u]Other details:[/b][/u]

Pipe 2 clearly is the one that takes the hot water out of the top of the tank, and distributes it to the taps, shower, etc I have just realised that I have marked Pipe 3 on the loft photo twice, The high pipe which hangs over the top of the tank is actually Pipe 2. I assume this is a safety overflow for steam/hot water?

Pipe3 - I have absolutely no idea what this is or what it does? It has a stopcock in the loft, and it doesn't enter the hot water tank at all. Is it possible that cold tap water upstairs runs from this rather than mains? My childhood home worked that way (much older house), and cold water pressure is very low upstairs, and very high downstairs, so this is my best guess?

Pipe 4 - this is mains water refilling the tank by the ballcock.

Pipe 5 - No idea. It splits into two rising pipes, just where the lagging obscures it in the photo.

Pipe 6 and 7 - the input/output of hot water from the boiler I assume, and thus the normal heating method when the whole system is working. I assume the thermostat on the immersion heater lets that kick in, when there's not enough hot water at the top of the tank?

The grey box labelled control box was replaced 15 months ago.

I don't know what the red metal box behind it does, but previously it made noises when water was running - some kind of pump/valve?

Sorry if that was very long. I've just been trying to provide all the possibly useful information in case any of my terminology or assumptions have been incorrect.

Can anyone shed any light on:

a) Why is the tank empty?

b) When it wasn't, why wasn't the immersion heater working? (I'm assuming limescale/corrosion/burnt out)

c) Why do I get cold water direct to the hot taps, now the tank is empty?

d) How do I get that damned immersion heater out? lol

Thanks all for your time reading this! My gf would be so pleased if she could wash her hair without using kettles! Especially when she has to be up at 5am...

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Simply Build It

Postby plumbbob » Thu Apr 09, 2009 8:10 pm

Right, where do we start on this one? Not sure I can answer everything in one post, but here goes.

Firstly, stop messing with the boiler. It is better to be bankrupt than dead or in prison.

Anyway, the first main point. If you have water coming out of the hot taps, the tank is full. It must be, because the water leaves the tank out of the top pipe 2 in the photo as you thought. Move the jacket to one side and thump the tank on the side with your fist. If it feels solid and sort of wobbles like a jelly, it is full. An empty tank rings almost like a bell.

I can see from your photo, pipes 6 and 7 are the primary coil heating pipes which circulate the CH water inside the tank to heat the hot water. There will only be one other pipe connected to the cylinder (pipe 1?) and that will be the cold feed from the header tank. Normally there is a valve in this pipe which allows the hot to be turned off.

Now the immersion. Firstly, the rod thermostat is in a sleeve, so should be dry. It is reasonable to suggest the element is broken, but have you actually identified this for certain?

To remove the element, you need a box spanner. ... on-Spanner

It comes with a paltry little tommy bar that wouldn't shift anything. You need to have a good 2 feet of leverage to undo it. What you need to take on board, is the tank is THIN COPPER and will bend, buckle or split given half a chance. When you put force on the bar, it must be exactly at right angles to the box spanner. The longer bar will allow you to exert an EVEN force without straining or the possibility of slipping. Don't allow the tank to tip, rock or turn. It will break. Then you get gallons of water escaping that you can't stop! EEk.

Oh, pipes 5 are the heating feed and expansion, pipe 4 is the cold mains to the header tanks and pipe 3 is probably cold feed to a shower or bidet(?).

A last point. Even if the hot taps are dry, a pint or so of water will escape when undoing the immersion nut so be ready.
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