The boiler is a saunier duval, and it`s possible the pump is not running. Reasons for this could be stuck as you say the property has been empty, or it could be the capacitor on the pump, the no. 1 micro-switch,or the motor but that is the last resort, I`ve just seen this problem so I would presume that this is water under the bridge by now. Sorry if too late.
alan, March 2007