Why convert from a vented expansion header tank to a pressurised unvented system?

[Stephen]

My In-laws have an oil fired boiler that was fitted 40 years ago.

This is based on a pump with two motorised 2 port valves, one for the hot water cylinder and the central heating.

There were of course two plastic tanks, one being the cold water tank and the other being the expansion tank, both in the loft.

Now they hired a plumber to move the hotwater cylinder from one end of the landing to the other.

The plumber has removed the expansion tank, put in a filling loop and an expansion vessel so effectively converting the system to a pressurised sealed system. There is of course a pressurised relief valve.

The 40 year old boiler is still present and still in use. The boiler in question is a simple system boiler so none of this condensing or combi stuff.

WHat I'd like to know is why did the plumber do this? What benefit is there to be gained by doing such a conversion?

More to the point, is a 40 year old oil fired boiler designed to cope with a pressurised system?

Stephen.

[Stephen]

P.S. the hot water cylinder still remains so all the hot water taps is on tank pressure and the cold taps are on either tank pressure or mains pressure.

[NY]

I'm not sure what the advantages and disadvantages are for converting the central heating system to a pressurised system, but there are certainly advantages in having the hot water system (whether hot water cylinder or combi heat-on-demand) fed from the rising main rather than a tank in the loft: it means that the hot and cold water are at the same pressure as they come out of the taps which makes it a *lot* easier to mix them; with a tank-fed system you often need the hot tap full on and the cold tap barely open to get an acceptable combination for warm water.

I'm not sure whether new houses with hot water fed from the rising main need larger 22 mm pipes for the hot water system, compared with 15 mm for cold, so that saves on the cost of copper needed. That advantage doesn't apply if an *existing* system is later converted, because it's not cost-effective to rip out all the 22 mm and put in 15 mm!

Are new houses ever fitted with header-tanks for hot water and/or central heating, or is it standard nowadays to fit pressurised?

As regards an existing boiler being suitable, I wonder how the maximum pressure of water in a pressurised system (before the pressure relief valve trips) compares with the head of water from the expansion tank which will act as a pressure relief valve in that water won't flow into the expansion tank until the system pressure exceeds that head of water pressure.

[Roger Mills]

You'd have to ask the plumber why he chose to do that. It may be that, with the hot water cylinder in a different place, it might have been difficult to connect the fill and vent pipes in a way which would ensure that pumping over wouldn't occur.

I've still got a vented primary system (with a 25-year-old gas boiler which would be happy with vented or unvented) and have no plans to convert it any time soon.

Unvented systems do have certain advantages:

- no danger of pumping over

- less risk of getting air in the system

- less risk of cavitation

- less risk of a flood if there's a leak[1]

However, it's slightly easier to add inhibitor to vented systems - and you don't have the complication of filling loops, expansion vessels and PRVs.

You pays your money . . .

[1] In practice, the ball valves in a vented system's fill & expansion tank often seize up due to infrequent use, so there may not be a flood even when there is a leak.

[Tim Streater]

No pipework in the loft to freeze.

[Dave Plowman (News)]

For a start, much easier to bleed the rads. Any leak limited to the contents of the system, unlike an open tank fed from a ball valve. A sealed system also can't pump over and introduce air - and air in the water speeds up corrosion.

No reason why the pressure should be much greater than from a header tank.

[DerbyBorn]

Don't assume the plumber gave it any thought - he probably just did what he normally does with the bits he has in his van.

[Roger Mills]

It *is* likely to be higher. It will probably be around 2 bar when hot. The header tank would need to be about 60' in the air to achieve the same pressure. Even so, the boiler is unlikely to mind.

[Dave Liquorice]

Could be but assuming that the primary return is still available where the cylinder was they could have just been T'd into that with

6" or so of separation. Though TBH I'd be happier with the expansion connected to the flow from the boiler and the feed into the return, both at the boiler. One assumes that they were connectd to the flow/return of the primary at the cylinder.

What the plumber has done was almost certainly the quickest for him. Time is money...

The boiler is probably specced up to 10 bar or was 40 years ago. Might be worth a look on the SEBUK site for that boiler and find it's notional effciency. It might be one that qualifies for a subsidised replacement. You can get condensing system boilers but the gotcha is that to run in condensing mode the primary has to be at a lower temperature which may mean the radiators are now a bit undersized.

[Dave Plowman (News)]

It would depend on lots of things. But my Viessmann boiler instuctions were quite specific - it had to be used in a sealed system. Which meant running in a cold supply to it, so more work than sticking with the existing header tank.

[John Rumm]

From the sealed system FAQ:

"The advantages of sealed systems are at least the following:

They are simpler to install since there is no header tank in the roof space and any of its possible problems such as over-flowing or freezing. They are easier to fill since air is expelled under pressure when bleeding the radiators. There can be no problems that stem from a number of installation faults occurring on open systems that end up drawing air into the system causing air locks, corrosion and sludge. The increased pressure raises the boiling point of water in the boiler. In an older boiler hot-spots cause localised boiling that often make kettle-like noises or even loud clanks or bangs. In the unlikely event of major damage to the heating system the resulting flood is limited. In contrast with a conventional system where the automatic top-up on the header tank will contribute an indefinite amount. They save space in flats where there is usually no good location for the header tank anyway. The system can be flushed out under pressure using the mains water supply.

In fairness the following are slight draw backs:

It is a bit harder to introduce chemicals (such as inhibitors and cleansers). The system pressure needs checking from time to time.

It will be fine.

[NY]

Digressing slightly, is there a similar list of advantages and disadvantages for a hot water system as opposed to a central heating system, comparing those fed from the rising main (ie hot water comes out of tap at rising main pressure) versus those that use a header tank, either in the loft or (in the case of my first house) in a tank that is joined on to the top of the hot water cylinder.

What is normal practice / building regs for a new house: are mains-fed hot water and sealed central heating more or less standard these days, or are any new houses (or brand new installations in existing houses) still fitted with header/expansion tanks?

[John Rumm]

You mean like:

Unvented systems are certainly more popular in the mid to high end new build market. Combis are popular in the low end. I don't know how many conventional gravity / vented systems still get installed.