In SharePoint 2013 there is new feature called Request Management Service. This service is some what like Load Balancer. It distributes web request as per the throttling rule. You can set multiple rules for Request Management Service.
I found that if your Central Administration Server is hosted on the APP server the RMS add the APP server in the Request Management target server. This effect the request as APP server will not have the web site if it is only used for Application. As a result the web request may fail. So to avoid this you can make the APP server unavailable from the routing target so that no web request is send to the APP server.
Here is the command that you can run to make it unavailable. You need to run this for each app server in your farm
1. Run the below command to find the status of servers in the Target routing
Get-SPWebApplication | Get-SPRequestManagementSettings | Get-SPRoutingMachineInfo –Availability Available
2. Run the below command to make your APP server unavailable for each web application
$wa =Get-SPWebApplication -Identity <URL of the Web Application>
$rm=Get-SPRequestManagementSettings -Identity $wa
$m =Get-SPRoutingMachineInfo -RequestManagementSettings $rm -Name <Name of the APP server>
Set-SPRoutingMachineInfo -Identity $m -Availability Unavailable