The following was taken from the April 2003 issue of "Heating & Ventilating Review":
Poor Installation Standards Undermine Large Capacity Unvented Systems
Unvented hot water systems have become extremely common for both new and retrofit applications but it is quite alarming how often we come across shoddy installation practices that lead to poor performance, unnecessary water loss and, in some extreme cases, a real potential hazard.
BS6700 Section 2.4.2.c recommends that the safety devices of an unvented system be fitted by the manufacturer so putting the onus on them to ensure the component parts are correctly sized, BS6700 also recommends that storage vessels be manufactured to one of the following standards, BS699, BS853, BS1566 Parts 1&2, BS3198 or BS7206 as appropriate. Some manufacturers pay lip service to this requirement or offer commercial vessels with no explanation as to the design parameters used in determining the shell thickness.
It is also not uncommon to see quotations specifying ISO9000 as a mechanical design standard! If too many corners are cut premature vessel failure may occur, sometimes within the first four years of service.
A correctly installed unvented system offers the following advantages over a traditional vented system:
- Predetermined operating pressure irrespective of the height of the building.
- It is more suitable for fittings requiring higher operating pressures such as mixer units.
- In some circumstances smaller bore pipework may be used.
Of all the components in an unvented system, correct sizing of the expansion vessel can not only result in trouble-free operation, but will also provide a cost effective solution when procuring storage vessels. When designing an unvented system, the engineer must first determine the required cold feed pressure. This must take account of the necessary normal operating pressure at the highest point in the system. The final maximum hot working pressure is determined by the system temperature rise, system volume and expansion vessel size.
The following example highlights the effect on a vessel shell:
The vessel in example A requires a thicker shell than that of example B. Material is available in pre-set thicknesses e.g. 3, 3.5, 4 mm etc. Increasing the shell thickness from 3 to 3.5 mm equates to a 16% increase in shell material cost.
| Item | Example A | Example B |
|---|---|---|
| System Vol (storage + system) | 2000 litres | 2000 litres |
| Cold feed pressure | 3 bar g | 3 bar g |
| Expansion vessel size | 300 litres | 500 litres |
| Final hot working pressure | 3.8 bar g | 3.4 bar g |
Special care must be taken when dealing with low pressure unvented systems. Too narrow a gap between the working and design pressure can lead to nuisance safety valve lifting. BS853 Section 10.2.1.4 calls for a 10% or .35 bar difference between the maximum working and design pressure. For example a 1.5 bar maximum working pressure would result in a 1.85 bar design. BS6700 Section 2.4.3 and 2.4.2.4 however recommends a minimum difference of 10% or 0.5 bar. For the example cited this would result in a design pressure of 2 bar g. This slight increase can be a real benefit if you consider that the expansion vessel must be charged extremely accurately to avoid over pressurisation problems.
The expansion relief valve offers protection against failure of the expansion vessel – this is an aspect that is often overlooked.
The pipework layout should be installed in such a manner as to ensure the heating input source cannot be isolated from this valve. Expansion relief valves should not be less than 20 mm and set to the system design pressure.
The storage calorifier should be fitted with a pressure temperature relief valve sized in accordance with BS6283. It is a requirement of G3 Building Regulations that this valve be tested in accordance with BS6283 Part Two or Part Three, which has recently been superseded by BSEN1490. G3 Building Regulations are currently under review.
Applications incorporating large instantaneous water heaters are frequently installed with no regard to the G3 Building Regulations as they are deemed to have no storage volume. This is not good practice. The potential for transferring energy still remains in a large distribution system. Some of these instantaneous machines are rated in MW; the capacity for over heating is considerable so temperature and pressure relief valves should be sized against the rated duty of the heater.
Another common mistake with unvented systems is incorrectly sized pipework. This is not generally associated with the vessel inlet and outlet diameters but rather those parts of the system that are exposed to constant volume flow rates like flow lines feeding semi-storage tube bundles or plate exchangers. Copper pipework velocities should be limited to 1.5 to 1.8 m/s depending upon the actual diameter in question. Plate heat exchanger connections may be as high as 7 m/s thus requiring swift adjustment in connection pipe diameters.
Unvented systems should be installed by a competent person. G3 Building Regulations outline this as "one holding a current registered operative identity card for the installation of unvented domestic hot water storage systems issued by the Construction Industry Training Board, The Institute of Plumbing, The Association of Installers of Unvented Hot Water Systems ..." etc.
It is for circumstances like this that Ormandy was set up two years ago.
By gathering a group of engineers with extensive design and manufacturing expertise, we felt we could meet a growing client demand for complete packaged solutions that ensure all necessary criteria are met.
The assemblies we produce in our Halifax factory are fully tested before despatch to site so that installation is swift and easy requiring only minimal mechanical and electrical hook up.
All unvented systems are manufactured by coded welders using approved weld procedures.
Full compliance with the PER is assured and packages can also carry the CE mark if appropriate.