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Unusual Problem With hot Water Venting From Expansion Pipe into Header Tank

Postby Rwginald » Wed Oct 09, 2019 7:49 pm

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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Postby ericmark » Thu Oct 10, 2019 10:48 pm

I am an electrician not a plumber, however I have lived in a house with solid fuel heating, and even with 32 mm pipes, well sure it was some imperial size, when the water boiled in the aga stove it produced a noise that was so loud one felt it needed rectifying quickly.

The circulating of the water from stack pipe to cold water tank can be cured by lifting the pipe up a little. Or any other method. But what is more to point is can a 15 mm pipe take the steam?

My son has under floor heating which is thermostatically controlled so as the temperature his a critical limit water is used to heat the floor.

This is a replacement for the oriental system where water was diverted to a radiator on the hull of his narrow boat, it is 12 volt and battery backed so power cut will not be a danger.

Mother's old house it was a case of drop the bars and rake out the fire onto a shovel on the quarry tiled floor. Even the narrow boat had a quarry tiled floor in front of the aga so it could be raked out.

Mothers house was dangerous, when the aga boiler the water it would overflow into the drive where any visitors may have been showered with boiling water, the auto feed did ensure there was always water to boil, but not really safe.

I know brother-in-law had a temperature gauge at the fire with back boiler to show if there was enough reserve in the twin tanks that stored the hot water, if it showed over a set limit than he could not light the fire.

Also of course a thermal plastic tank is not permitted.

Today HETAS approved installers are required in most areas to fit solid fuel fires, and you plumber would need to be HETAS approved, I don't think it's law, but most insurance companies would not accept work done by anyone not approved.
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Postby Rwginald » Sat Oct 12, 2019 4:32 pm

Thanks. That is very interesting. Your comment about the plastic pipework crossed my mind. The original 28mm is all copper pipe. The replacement expansion pipe is plastic and 15mm at that. I assume that plastic can take the heat of possibly boiling water if necessary. As for safety, there is already a dedicated safety valve on the back boiler and also in any case, as the water heats it hums, so we know it is heating. If it were ever to get loud, we would oabviosuly dampen down the fire, but it has never been necessary in the last thirty years - it is only used a few time a year in any case.

Thanks again.
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