Plus, in ten years, development and environmental emmisions requirements will have almost certainly forced better efficiency from both fuel economy and heat developed per g/kwh.

The differences may well be slight but the burner is regulated by its ability to heat the medium (water) and inefficiencies elsewhere such as heat loss through badly insulated ceilings, windows and walls will govern the burners activity….. i.e are your radiators on full just to warm the room because heat is being lost via a draghty old room with no insulation.

If the firebox of the burner and heat exchanger is in good clean condition, and your space is well insulated, then there is no reason why an old burner shouldnt be as good as a new one.

Sure, nozzle size and air/fuel mixture will alter the burner flame temperature but the whole thing is governed by the desired room temp thermostat and will keep going until the desired temperature is achieved.

Hope this is of use