Try not to panic.
It's likely what they are eating or drinking that is causing your chicks to be sick. Chicks that young are not very likely to pick up either Marek's or coccidiosis unless they have come from a breeder or hatchery with tons of health violations. Chilling may also be causing these deaths due to some chicks not getting enough heat.
I'll start with the feed.
Dumor chick feed has on occasion been the cause of baby chick problems here on BYC over the years I've been here. Not often, but enough where I wouldn't buy it for my chicks. The problem with it runs from outdated and rancid feed, which can make chicks very sick, to feed spoiled by mold, which can mimic Marek's symptoms.
Contaminated water can also kill chicks. You may have developed a resistance to contaminants in your tap water, but your chicks could be susceptible. Bad water has also been a killer of chicks here on BYC over the years I've been here.
So what I suggest is to get rid of the Dumor, and give your chicks only bottled water. If Dumor is the only chick feed you can find, you may substitute Purina Flock Raiser for chick feed. It has all of the same ingredients in practically the same proportions as chick feed.
Now, let's talk about the heat.
Since you're not using a heat lamp, I feel it's unlikely a heating problem unless your chicks are getting chilled. Place a thermometer on the brooder floor under the heat plate and if it's not reading at least 80F, your chicks may not be getting enough heat. Also, fifteen chicks is quite a lot. Make sure they all can get under the heat plate without some being squeezed out into the cold, which can cause chilling and death.