  We analyse those objects in the Brightest 55 sample of clusters of galaxies
which have a short central cooling time and a central temperature drop. Such
clusters are likely to require some form of heating. Where clear radio bubbles
are observed in these clusters, their energy injection is compared to the X-ray
cooling rate. Of the 20 clusters requiring heating, at least 14 have clear
bubbles, implying a duty cycle for the bubbling activity of at least 70 per
cent. The average distance out to which the bubbles can offset the X-ray
cooling, r_heat is given by r_heat/r_cool=0.86+/-0.11 where r_cool is defined
as the radius as which the radiative cooling time is 3 Gyr. 10 out of 16
clusters have r_heat/r_cool>1, but there is a large range in values. The
clusters which require heating but show no clear bubbles were combined with
those clusters which have a radio core to form a second sub-sample. Using
r_heat=0.86 r_cool we calculate the size of an average bubble expected in these
clusters. In five cases (3C129.1, A2063, A2204, A3112 and A3391) the radio
morphology is bi-lobed and its extent similar to the expected bubble sizes. A
comparison between the actual bubble size and the maximum expected if they were
to offset the X-ray cooling exactly, R_max, shows a peak at R_bubble ~ 0.7
R_max with a tail extending to larger R_bubble/R_max. The offset from the
expected value of R_bubble ~ R_max may indicate the presence of a non-thermal
component in the innermost ICM of most clusters, with a pressure comparable to
the thermal pressure.
