I installed a brand new Enviro EF3 yesterday. Burns great for 15 minutes only!! It shuts down every 15 minutes.I appears to be on a timer that automatically shuts off after 15 minutes exactly. Can I change the timer settings somehow? The controls are not a circuit board model. I don’t know what to do to get the stove to run more than 15 minutes at a time. really at a loss here...please help if you can....