Discuss VENTED central heating system - water circulating through header tank in the Central Heating Forum area at PlumbersForums.net

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Two years ago, I requested a fully qualified plumber to change the location of the hot water cylinder in a vented central heating system. I wanted to move it from the first floor to the loft, which he did. During the last two years, I have often noticed when I have been up in the loft that the header tank in the loft just above the hot water cylinder was warm. Due to past experience when I discovered a leak in the boiler itself which had been leaking probably for years, I deliberately do not have a ball valve in the header tank, but simply a hand operated valve to fill it, so that I know if there is ever a leak in the system. As a result, I check the water level of the water tank visually from time to time.

I have however been surprised at how often I have had to re-fill the header tank since the hot water cylinder was moved. I have checked for leaks very thoroughly all over the house but can find nothing. So I put it down to evaporation during this hot summer.

As the evenings have been getting colder, I turned the central heating on last week. I then thought that I ought to just check the level of the header tank at the same time. When I went into the loft, I could hear water trickling constantly. So I naturally investigated, to discover that warm water was flowing out of the expansion pipe. It was not hot enough to be boiling, and therefore it could not have been due to normal expansion. It had to be pumped water. If I put my finger just lightly over the end of the 15 mm expansion pipe, the flow stopped and there was no build-up of pressure. The water level in the tank was also normal.

Having a basic understanding of central heating plumbing, I started to investigate further. When I went down to where the old hot water cylinder used to be, I soon discovered that the expansion pipe which had originally been 28mm copper had been reduced by the plumber to 15mm plastic. I was not happy about this and immediately called him to come and take a look.

I should explain at this point that the expansion pipe is probably EVEN more critical in my house than in most, because, in addition to an oil boiler, we have a wood-burning stove with a back-boiler to provide hot water when there are power cuts - we live in a very remote rural location. Power cuts are frequent, and as we had a disabled daughter who loved a hot bath, we could go without everything except hot water !

So the wood-burner is connected (through separate isolating valves and pressure relief valves) to the hot water cylinder as well as the oil boiler. But obviously a wood-burner cannot have thermostatic cut-offs unlike an oil boiler, and therefore it was imperative that all pipework in the domestic hot water circuit from both the oil boiler and the wood-burner is run in 28 mm, including the expansion pipe. This system has been in place for thirty years and has always worked excellently - until now.

[As an aside, one of the advantages of 28 mm pipe is that the pump does not have to be circulating the water for the hot water cylinder to get hot, as a result of simple gravity circulation. Therefore to turn the central heating off, all I have to do is to switch the pump off, and the boiler still continues to heat the domestic hot water cylinder.]

I can only assume that the reduced size of the expansion pipe from 28 mm to 15 mm has reduced the resistance in the circuit, causing the central heating pump to pump hot water up the expansion pipe to the header tank, which it never used to do before. As I probably never checked the level in the header tank in the past when the pump was on, I never of course heard the water flowing out of the expansion pipe, splashing into the tank during the last two years.

So anyway, my plumber has said that it will mean a lot of work to drain down the system, open up where the pipes run into the loft and replace the 15 mm pipe with 28 mm. I do sympathise but then, he is the specialist and should have realised what he was doing was wrong. What he has proposed is to fit a pressure relief valve over the end of the 15mm expansion pipe, which would allow the expansion pipe to release water into the header tank if it were to over-heat or over-pressurise for any other reason.

I am pretty certain that it is just the reduction in expansion pipe size that has caused the problem, and nothing more dramatic. But it must be rectified, one way or another.

My plumber's solution does sound logical, it is a very simple fix and it would avoid a lot of disruption. But is it a safe solution ? That is where I am not qualified to judge, and would appreciate other opinions.

Apologies for such a long post and many thanks.
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I would doubt your plumbers advice.

An vented cylinder in the loft, unless you have a significant headroom above it, would be a very difficult installation to complete effectively. Presumably you have a header tank above the top of the cylinder and the unvented feed tank above the cylinder too?

The pipe diameter and loop height up, over and return to the header tank are critical to the safe and effective operation of the system.

You may not be in England - but if you are the Building Regulations require the expansion loop to be unimpeded. The header tank must also be free filling through a valve and not require manual intervention.

Having said that the issue you currently have that is causing over topping is the configuration (relative elevations) of your heating system.

Advice - change your plumber to one who knows the rules ( assuming you are in England) and why ( the technical reasons) are in place.


Gas Engineer
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The 15mm expansion pipe does sound wrong, however pumping over is usually a symptom of a blockage in the heating pipe work, either due to sludge or air. Also reducing the size of the pipe would increase the frictional resistance not decrease it.

Think of it this way, water takes the path of least resistance, currently this is up to your loft via a 15mm pipe and then up and into the tank... somethings not right here, it should be going round the radiators. He may also have positioned the pump, feed or vent "wrong"

Too be honest I think you need a second opinion.
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Thanks both of you for the reply. Firstly, I am of course in England. Secondly, the word "reduce" was my choice not the plumbers, so he is not to blame for not saying "increase".

There is no blockage anywhere in the system. It was thoroughly flushed out after leaving the cleansing fluid which was added to the water for circulating for several weeks in the spring when the heating was still in use, and then drained and flushed twice. This was when I thought the warm water could have been due to something like that. I saw the water flushing out and it was not that dirty, but after the second flushing it came out completely clean.

The only thing that has changed in the installation otherwise is the location of the hot water cylinder which is now in the loft whereas it was on the first floor. The header tank is the same and is of course above the hot water cylinder, above which is the expansion pipe the expansion pipe. What is different is the size of the expansion pipe which is now half the size it was.

I did think about changing from an open system with a header tank to an unvented system, but then I thought that maybe that would not be suitable with the back boiler on the wood-burner. Am I wrong about that, or could I get the system changed to a closed system because it might involve less pipework change ? There is plenty of room for a diaphragm in the loft - there is one there already for the domestic water of course which is pressurised, with a new indirect cylinder (two years old). Only the central heating is unpressurised.

Until the cylinder was moved up into the loft, the system had worked without issue for more years than I can remember.

I am grateful for your comments because obviously I am now in a better position to argue with my plumber about it. He is fully qualified and has NVQ level 2 Diploma in plumbing and heating, which is why I used him in the first place. It would be helpful to know whether the idea of a closed system is worth thinking about.
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Irrespective of the regulations I would only use an uncontrolled heat source with a properly installed vented system and preferably with a heat dump too
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