1997 Outputs reduced by 11% in the UK Why is this?

This is because the test methods, accuracy of testing and design of the new test rooms were changed from the previous BS 3528 test criteria. The Technical Committee drew up a very tight specification for a new standardised test room which, coupled with changes to test methods and rating procedures, resulted in remarkable accurate and consistent results. There are currently four such test rooms, at BSRIA in Bracknell England, at Lyon, Stuttgart and Milan. These have reported repeatable results with an accuracy of +/-1%. Radiators tested under the new Standard are tested at a lower temperature difference than under old national standards.

This Delta T, the difference between the mean water temperature in the radiator and ambient air temperature in the test chambers, this was previously Delta T 60 degrees C, now it's 50 degrees C. While this dose not sound significant, outputs at Delta T 50 are over 20% lower than at Delta T 60. However, it's rather like testing fuel consumption of a car at 60mph instead of 50mph. Fuel consumption drops, but only because a different reference point has been taken.

UK installers have been used to brochure outputs derived at Delta T 60, so to prevent confusion, particularly during the introductory phase of the new Standard, manufacturers catalogues continued to show Delta T 60 outputs for the full spread of radiator models, with only the headline reference to the nominal outputs at Delta T 50. This means that the tested radiators at Delta T 50 were corrected back up to Delta T 60 output figures. (see paper on correction factors)

Reference to the headline figure is interesting; the four test centres could not possibly test all radiator models for all brands within such a short period of time, so a grace period of two years was allowed for manufacturers to test all models within their ranges. Until then one-meter long radiators from each height available in the range, were tested as a reference point. These reference points were used to calculate the outputs of the remaining radiators in the range!

Other changes included testing with Top Bottom Same End (TBSE) connections rather than the old, Top Bottom Opposite End (TBOE).

The new design of test room, particularly the elimination of water cooling to the rear wall of the test booth itself, resulted in outputs that are lower, regardless of the Delta T used.