The newer Hydroflames just have a simple on/off toggle switch on the outside, since most controls work through the thermostat.
When you find the thermostat, it should have two levers: one on top and one on the bottom. The one on the bottom should slide across the width of the thermostat and adjusts the temperature. The one on top should only slide a distance of about 1/4" is all and turns the furnace on and off. It is a little tough, and it will snap into the position. There should be a short delay before the fan comes on, and then it will take a minute or two for the actual furnace to kick on. Then you can adjust the temperature however you wish. The furnace should cycle on and off as needed.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9473.8 (as of 6/18/21)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR