Vacuum -vs- Flatplate

Both systems have their fans. There are pros and cons for both systems;

A flatplate collector consists of a large metal plate with water tubes running through it, placed inside a box with glass on top of the plate and insulation underneath it. Sunlight passes through the glass, heating the water inside the tubes, and this water is plumbed back to a coil in your hot water cylinder.

There are various types of vacuum tube, but ours is effectively a thermos flask made up of two layers of glass with a vacuum between the layers. Because there are so few air molecules in the vacuum, there is virtually no heat loss between the two glass layers, so it is far better insulated than a flatplate, which loses heat through the glazing. Heat is transferred to the cylinder via a heat pipe. Read more here..

Performance

On a hot sunny day, flatplates will out-perform vacuum tubes of the same total area. However, it could be argued that on hot sunny days, either sytem will provide plenty of hot water. On cloudy cold, windy days, a vaccum flask will outperform flatplates. Thus vacuum flasks can be useful if you want to extend the effective season over which the panel operates. e.g. A vaccuum tube system will give you more heat in the late Autumn and Winter.

Ease of Installation

Flatplates ideally should be fitted to roof timbers and then the roof should be slated or tiled, with appropriate flashings around the panel. On a new house this is a relatively simple operation. Vaccum tubes are always fitted on top of slates or tiles, so are easier to retrofit.

Flatplates are usually a lot heavier to lift into place, and this can be a hazardous operation for DIY enthusiasts. Again on a new house, especially in a housing development, lifting gear will normally be available and this should be used. NEVER install a flatplate by lifting it up two ladders as is common practice

Durability

Vacuum tubes are made of lighter glass, though this is borosilicate (often tradenamed as Pyrex) and quite tough. It will break if hit by a hard ball or rock. In the event of this happening, the panel will continue to function on its other tubes, and it is a relatively simple operation to replace the broken flask.

Flatplates are usually made with toughened glass, and while this is far more difficult to break, in the event of it breaking, you will have a bigger job on your hands to replace the panel.

Cost

Vaccum flasks used to be a lot more expensive. Often, Chinese made panels were re-badged and sold at inflated prices (virtually all flask systems are manufactured in China..). However, prices of flask systems have come down dramatically and both systems are about the same price generally.

Aesthetics

Some architects will not like the appearance of vacuum flask systems on a rooftop and will insist on flatplates, regardless of the implications in terms of efficiency.