In Magento you may need to process the email text for an email but sometimes those templates are default ones, other times they can be a customized one set in the admin settings area. A customized email template from system > transactional emails menu will set in your core_config_data a numeric value for the customized email template.  There are 2 methods that load email templates one is for if the template is numeric (customized) other for if it’s just a default one from the config.

To Properly load in either case, you can use the code below:


// large array of email variables, just one here for example
$firstname = 'Joe';
$emailTemplateVariables = array('firstname'=>$firstname);
$emailTemplate = Mage::getModel('core/email_template');
// config is the config path to the email template, here is for the new order as example
$emailTemplateId = Mage::getStoreConfig('sales_email/order/template');
if (is_numeric($emailTemplateId)) {
$emailTemplate->load($emailTemplateId);
} else {
$emailTemplate->loadDefault($emailTemplateId);
}
$processedTemplate = $emailTemplate->getProcessedTemplate($emailTemplateVariables);