When I have to feed late in the season, it is important to give them warm syrup.
The easiest way to heat syrup is an immersion heater.
I use a 120 volt heater. It is made to drop it in the syrup. It is not to be used with honey, as it is not food safe.
The syrup warms quickly, caution is required not to leave it unattended when in a plastic pail.
The warm syrup is put on the hive and is taken down very fast by the bees.