The ST799 replaced a 25 year old ST798 central heating timer that had failed. It fitted directly into the old back box with no modification. The 799 was an upgrade allowing daily timing. The unit arrived very quickly at a competitive price. Although this unit was a modern unit, the quality of build was lower than that of the original unit, the alignment of the sliders with the fixed lines was not precise. This seems to be common with a lot of modern equipment, efficiency savings seem to go too far sometimes.