EmailTemplate et = [SELECT Id FROM EmailTemplate WHERE DeveloperName =:emailTemplateName];
List<string> toAddress = new List<string>();
toAddress.add(primaryEmail);
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    mail.setTemplateId(et.Id);
    mail.setToAddresses(toAddress);
    mail.setSubject('test subject');
    mail.setTargetObjectId(primaryContact);
    mail.setWhatId(primaryAccount);
    mail.setSaveAsActivity(false);
    mail.setUseSignature(false);
List<Messaging.SingleEmailMessage> allmsg = new List<Messaging.SingleEmailMessage>();
allmsg.add(mail);

try {
    Messaging.sendEmail(allmsg,false);
    return;
} catch (Exception e) {
    System.debug(e.getMessage());
}

mail.setWhatId(ContactId);