Fitting a combi and unvented systems

[Bernie2]

Here is an interesting point that came up in a mail and one I have never heard a proper answer too yet.

"If you have to be licenced to fit unvented cylinders over 15 litres capacity, what about combi systems?"

Most people seem to ignore this and as I said, I have never heard it properly defined. I know combi's themselves don't have much water in them but the system does. And if it goes bang it all goes bang.

Anybody know the answers?

 

[Plucky]

When I done my G3 ticket they said combi's dont come into because of the low water content.

Even the boilers with a thermal store, such as worcester heatslaves, the store is primary water not the secondary.

 

[Puddle]

Oh great,lets find another area to require a ticket for
I will send a e mail now to citb,bet they can have a £150 course up and running within the week

 

[Bernie2]

I agree with you Puddle.

If you add the water content up of a combi system it soon leaves 15 litres behind. But they have been installing them unticketed for over 20 odd years now and not many have blown up that I know about.

So why require a ticket?

 

[Plucky]

I agree with you Puddle.

If you add the water content up of a combi system it soon leaves 15 litres behind. But they have been installing them unticketed for over 20 odd years now and not many have blown up that I know about.

So why require a ticket?

Not so sure I understand you.

The volume of water that you are looking to measure is what would be in the DHW heat exchanger, which at a guess would be anywhere up to 2litres max.

 

[Bernie2]

Hi! Plucky

It applies to the whole system under pressure, the expansion vessel serves the whole system not just the boiler.

So if you had an uncontrolled overheat situation, the entire system water content would turn to steam and expand to occupy a space 1700 times bigger than it does cold.

An average rad holds anything from about 2 litres upward, the average house has say 6 or 7 rads plus the pipework and it piles up. Plus of course what ever the boiler holds.

Lets be honest all a central heating system is is one big circle containing water, near enough the same as an unvented cylinder when its pressurised. And its the danger of water overheating beyond 100C, the system springing a leak and it all turning to steam that is the problem.

 

[Plucky]

Hi! Plucky

It applies to the whole system under pressure, the expansion vessel serves the whole system not just the boiler.

So if you had an uncontrolled overheat situation, the entire system water content would turn to steam and expand to occupy a space 1700 times bigger than it does cold.

An average rad holds anything from about 2 litres upward, the average house has say 6 or 7 rads plus the pipework and it piles up. Plus of course what ever the boiler holds.

Lets be honest all a central heating system is is one big circle containing water, near enough the same as an unvented cylinder when its pressurised. And its the danger of water overheating beyond 100C, the system springing a leak and it all turning to steam that is the problem.

I see what your getting at now, your talking of any sealed system not just a combi.

I guess that when you do you CCN and CEN trainging and assesments you prove to the assesors that you understand how safety devices work. I,e. boiler stat, over heat stats, pressure relief valves.

Another thing to consider that may catorgorize it differently. A boiler is directly heating the water in question. Where as with a cylinder it is a vessel that is being indirectly heated.