open vent ch?

[Stephen]

Hello,

I spoke over the phone to a plumber to arrange a boiler service. The plumber tried to sell me a new boiler. Apparently this model is about

20 years old and is relatively inefficient compared to new models. He tried to sell me a replacement with the argument that the oil saved will pay for a new boiler in no time, and with the price of oil, he may have a point!

He said that a new boiler may require changes to the pipework to avoid an open vent system. He said these were bad because a boiler malfunction would boil the header tank, which would melt and kill people. I remember a report in the news about that happening.

What is the alternative to open vent systems. Are open vent systems really discouraged? How much can I DIY regarding either changing the pipework or the boiler?

Thanks, Stephen.

[Huge]

Where "no time" ~= 5 to 10 years.

[Dave Plowman (News)]

Which was caused by an immersion heater - not boiler. You'd need a number of faults before a boiler could produce boiling water. And even then, a properly designed open system should cope without the above sort of incident.

First - are you talking about open vent primary or secondary?

I've just DIY replaced my gas boiler and changed the primary to pressurized, but kept the secondary (hot water) open vented.

[EricP]

The plumber was a double glazing salesman in his previous job. He has used the lowest of scare tactics on you to sell a new system.

[cynic]

He is an opportunist bastard posing as a salesman! The case in question was an electric immersion heater with a one stage (failed on) thermostat. The tenants ignored the bubling and banging (were they deaf or simple?) The header tank was not fully supported but rather balanced on two joists so it sagged and split pouring boiling water onto the ceiling which softened and collapsed with a cot beneath it. If your oil boiler does not have a back-up overheat thermostat it can easily be sorted. If it has tell him to sod off. It would be sensible to make sure any plastic header tank has a full supported base. The relative efficiency is another matter but if you had told us what model you have it would have enabled us to estimate the likely benefits. Oh and one last point -most plumbers have no idea about how an oil boiler works!

[Phil L]

He may also have an ulterior motive

Everybody does, which is why, having already sold you the boiler with his opening sentence, he's now in the process of bumping the price up with scare tactics.

If the boiler does what it's supposed to, IE give you HW and CH, then leave it as it is

[Ed Sirett]

Pay back is likely to be a number of years, but less than the life the new system.

Read the SealedCH FAQ.

If the installer has to resort to scaring you it does not bode well.

[Stephen]

Thanks. I had heard of the story but didn't know the full details.

Well I'm not sure what open vent means! I think both halves of my system are open vent because both have header tanks. There's no pressurisation like there is on say a combi boiler. What was the advantage of pressurising your CH?

[Stephen]

As it happens I occasionally hear the odd bubble first thing in the morning when we use Economy seven to heat the water. The immersion heater is set to 60C. Is an odd bubble ok or does it mean the thermostat is a bit iffy?

I would have thought a plastic tank would melt at high temperature no matter how it was supported but I can see that in that scenario, the lack of support may have speeded things up. I am reassured that our tank sits on a sheet of plywood over a number of joists.

It is a Mistral M1A. The manufacturer cannot find the serial number in records for the last fifteen years, so they think it is 18-20 years old. The manufacturer told me that the boiler would have been 65-70% efficient when new but loses 1% per year so is now about 45-50% efficient.

Of course, like the plumber, they have a commercial interest to sell me a new one. Does the above ring true or does it sound a little fishy?

When I bought oil last time (Jan 08) it was £400 for 1000L. I heard someone bought 1000L recently for £800. The manufacturer (Mistral) said that a new boiler would be 95% efficient and mine is 50%. Will my old boiler will use twice as much oil (95/50=about 2) or is it not as simple as comparing efficiencies?

If so, I was thinking that in three years the cost of a new boiler (£1200) would equal the cost of the saved oil. Or are my maths wrong?

I searched through the Yellow Pages to try and find an OFTEC one, so I had hoped this chap knew more than most.

[Stephen]

[Dave Plowman (News)]

Aren't many kettles plastic?

[Dave Plowman (News)]

They're elsewhere in the thread. Of course the meja made a sensation out of it and blamed the wrong things.

Correct.

Think you're getting confused - a combi boiler simply heats the hot water directly. Pressurization doesn't come into it - although most will be.

Basically I bought a system boiler which has the pressure vessel built in. Advantages are (in no particular order) easy filling with no airlocks. Any leaks restricted to the content of the system - with a header tank it could continue being topped up. The ability to have rads higher than where a tank could go. Easier and cheaper installation costs - and possibly space saving.

[meow2222]

A case of the truth but not the whole truth. The price of the new boiler isnt what you really pay. You'll pay: purchase price installation cost asstd mods to installation to get it to meet newer regs repeat repair costs as you find the new boiler unreliable boiler replacement after a surprisingly short life

Vented header tank installs are preferable in most cases, since theyre more reliable. Pressurised systems are better in flats, where space for even a little header tank is a problem.

This is really a problem with electric rather than gas heating. For a gas boiler to do that multiple faults would be needed, and afaik it never has happened. Electric heating is a different fish of kettle, and just one common fault has the potential to cause such an accident on some systems.

Why make life harder

NT

[Dave Plowman (News)]

Since a pressure vessel isn't exactly rocket science, is this the case? Header tanks - or rather the ball valves - do give trouble too.

[meow2222]

Pipework is pressurised, so more prone to leaks.

Open headers can be turned down to a slow dribble, yes they can overflow, but it doesnt stop the system working and an exterior dribble is not too urgent to fix. And chems are easy to add... I'll stay with headers anyway.

NT

[Dave Plowman (News)]

So your cold water main leaks? Oh - a header tank system is pressurized too by the head of the tank.

The point was if you have a leak the open system will continue to supply water for that leak - the pressurized one can't. It will also prompt you to look for a leak whereas with an open vented type might well go unnoticed.

You might find that when new boiler time comes around you have no choice anyway.

[<me9]

More reliable?? The ball valve regularly seizes resulting in no water in upstairs radiators. Small leaks aren't detected leading to loss of inhibitor and corroding radiators. Small undetected leaks can lead to permanentlky soaked carpets and flooring resulting in extensive damage. I could go on.....

Pressurised systems do have some downsides, but IMHO are *always* better.

[<me9]

It /can be run with similar pressures to that with a header tank. although it usually isn't. Higher pressure means less cavitation and less boiler noise (less relavent on a condensing system run at lower temps).

Which are detected after a small quantity of water is released, rather than when the floor collapses with rotten chipboard.

[Michael Chare]

I replaced my oil fired boiler 2 years ago. All DIY. My new boiler does use less oil.

I kept my plastic header tank. If the boiler generated boiling water I think that it would be very obvious that something was wrong! For this to happen both the normal and the emergency cut thermostats on the boiler would have had to fail, and probably either the H/W or the room thermostat. I do recall a story in the press but IIRC the problem was caused by a immersion heater staying on, and the occupants of the house did ignore some warning signs.

One reason for not having a sealed system is that they are pressurized, and the wisdom of preasurizing old radiators is debatable.

The main change to the plumbing was a conversion to a fully pumped system. For this the supply pipe from the header tank now connects to the boiler return pipe next to the boiler, rather than the loop to the hot water tank coil.

[Dave Plowman (News)]

The amount of pressure is small. If it caused the rads to leak they soon would anyway. What it might do is find weeping seals on valves, etc.