Vented / unvented cylinders, pros and cons

[bernardgreen]

Reference to

Certification Scheme For Individual Unvented Hotwater Operatives

I've seen serveral imploded vented cylinders over the years. It's amazing what some people will do when there's limited tank(sorry cistern) head!! Never ever seen an imploded UVC.

Probably because they have a vacuum release valve / bursting disc for when a vacuum is created by
1 water being drawn off faster than the mains can re-supply the cylinder.
2 after a heat up period when the water has expanded, both the heat source and water supply are shut off, the water would start to cool, contracting in volume until a partial vacuum was created.

A vacuum can only develop in a vented tank if both feed and expansion pipes are blocked, either by a plug left in after work on the system or by freezing and water is drawn off.

Unvented cylinders have been around a good 30 years too

Only just 30 years, they were not permitted until 1986 if the NiC are correct

Traditional in this country, hot water supply has been based on a cold water feed from a storage cistern to hot water apparatus that is vented to atmosphere.
Prior to 1986 it was not permitted for hot water storage apparatus to be connected directly to cold water service pipes subjected to mains pressure.
The only exceptions to this bye-law were: -
Instantaneous water heaters and storage type water heaters with a capacity of less than 15 litres.
The reason for this was that there was a possibility of contamination of the water in the water main caused by back-syphonage of hot water into the cold water main or wastage due to expansion as in continental systems.

In 1974 the BACK-SYPHONAGE REPORT was published. It looked at the dangers of back syphonage, its causes and how it could be prevented. The one important conclusion of that report was that provided a satisfactory level of control is applied, mains connected unvented hot water systems could be permitted in the United Kingdom.

The whole document is of value when deciding on vented or unvented.

 

[Dan Robinson]

Probably because they have a vacuum release valve / bursting disc

Really? Common in domestic cylinders? I only know of one boiler that has one of those in a domestic environment.

water being drawn off faster than the mains can re-supply the cylinder.
2

Do elaborate please.

 

[Dan Robinson]

I see you, as always are ignoring previously made points.


Unvented cylinders are no less prone to problems than vented.

 

[bernardgreen]

water being drawn off faster than the mains can re-supply the cylinder

Do elaborate please.

You are joking or do you really not know how gravity pulling water out of a cylinder will create a negative pressure in the cylinder if the rate of water leaving the cylinder is greater than the rate of water going into the cylinder from the mains. ? Obviously this only happens when the cylinder is located higher than the tap that is taking water from the cylinder. Hence the preference to locate unvented cylinders below or at the same level as the lowest hot water tap or shower.

 

[Dan Robinson]

A mains fed cylinder.....

 

[Dan Robinson]

Hence the preference to locate unvented cylinders below or at the same level as the lowest hot water tap or shower.

Elaborate.

 

[bernardgreen]

I see you, as always are ignoring previously made points.

Unvented cylinders are no less prone to problems than vented.

Not ignoring them,but comparing them with points from other informed sources suggests they may not be as valid as they appear to be.

As already said any and all hot water systems are prone to problems. The type of problems unvented cylinders are prone to are different to those that vented cylinders are prone to.

The consequential effects resulting from a faulty system are also different and for the user much more important. Any competant plumber or DIYer can deal with the majoty of problems with a vented cylinder so repairs can be effected quickly. An unvented cylinder will be out of action until a G3 qualified technician can attend to it.

 

[Dan Robinson]

other informed sources suggests they may not be as valid as they appear to be.

Really. Cite these sources please.

 

[bernardgreen]

Elaborate.

from the same training document

In light gauge copper cylinders this reduction of pressure, below the external pressure (atmospheric pressure) is sufficient to cause the cylinder to collapse or implode. Higher pressure glass lined or stainless steel cylinders designed for unvented hot water systems are far less susceptible to implosion.
A vacuum relief valve is required to equalise the pressure within a storage vessel, should it be subjected to a vacuum condition. This would normally be incorporated within the thermal relief valve.
NOTE: If mains pressures are low, or are suspected of being variable it may be advantageous to site the unvented unit below the lowest draw off point,
e.g. GROUND LEVEL,. When this is not possible, consider fitting the pressure regulating/limiting valve on to the unit and install a separate (adequately sized) feed pipe to the unit.

 

[bernardgreen]

Really. Cite these sources please.

Some are e-mails but most are verbal conversations with a range of people who do have either or both qualifications and /or experience in their various trades and professions.

 

[Dan Robinson]

Some are e-mails but most are verbal conversations with a range of people who do have either or both qualifications and /or experience in their various trades and professions.

Thought so.

Where is this training document from? Looks almost as poorly written as that website you linked to a while ago.

 

[PullerGas]

Reference to

Certification Scheme For Individual Unvented Hotwater Operatives

I've seen serveral imploded vented cylinders over the years. It's amazing what some people will do when there's limited tank(sorry cistern) head!! Never ever seen an imploded UVC.

Probably because they have a vacuum release valve / bursting disc for when a vacuum is created by
1 water being drawn off faster than the mains can re-supply the cylinder.
2 after a heat up period when the water has expanded, both the heat source and water supply are shut off, the water would start to cool, contracting in volume until a partial vacuum was created.

A vacuum can only develop in a vented tank if both feed and expansion pipes are blocked, either by a plug left in after work on the system or by freezing and water is drawn off.

Unvented cylinders have been around a good 30 years too

Only just 30 years, they were not permitted until 1986 if the NiC are correct

Traditional in this country, hot water supply has been based on a cold water feed from a storage cistern to hot water apparatus that is vented to atmosphere.
Prior to 1986 it was not permitted for hot water storage apparatus to be connected directly to cold water service pipes subjected to mains pressure.
The only exceptions to this bye-law were: -
Instantaneous water heaters and storage type water heaters with a capacity of less than 15 litres.
The reason for this was that there was a possibility of contamination of the water in the water main caused by back-syphonage of hot water into the cold water main or wastage due to expansion as in continental systems.

In 1974 the BACK-SYPHONAGE REPORT was published. It looked at the dangers of back syphonage, its causes and how it could be prevented. The one important conclusion of that report was that provided a satisfactory level of control is applied, mains connected unvented hot water systems could be permitted in the United Kingdom.

The whole document is of value when deciding on vented or unvented.

I thought they stopped fitting anti-vac valves on UVCs some time ago??!! Jesus BG you do need to Google more up-to-date details!!!

Not too sure what you're doing here BG??

 

[bernardgreen]

Where is this training document from? Looks almost as poorly written as that website you linked to a while ago.

www.lomaxtraining.co.uk/media/guides/.../uvhw-training-section-2.doc

 

[bernardgreen]

I thought they stopped fitting anti-vac valves on UVCs some time ago??!! Jesus BG you do need to Google more up-to-date details!!!

Check with Worcester Bosch. Antivac is part of the G3 kit ( as of November 2015 )

 

[PullerGas]

I thought they stopped fitting anti-vac valves on UVCs some time ago??!! Jesus BG you do need to Google more up-to-date details!!!

Check with Worcester Bosch. Antivac is part of the G3 kit ( as of November 2015 )

Woofter Botch!!! Now you are having a laff!! Have you any idea how loathed they are as a manufacturer??!! They steal installers service work you know & that's just for starters. I wouldn't like to say anymore on a public forum.

C'mon BG you're a poor excuse for a Google junkie!!