Estou usando o seguinte para forçar o HTTPS no meu site principal.

### Force SSL
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Se você quiser ver o arquivo htaccess completo, coloquei abaixo:

<IfModule mod_rewrite.c>

    RewriteEngine On
    RewriteBase /

    ### Blacklist via Referrers

    RewriteCond %{HTTP_REFERER} removed\.net [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.removed\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.net [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.sx [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} removed\.org [NC]
    RewriteRule ^(.*)$ - [F,L]

    ### Force SSL
    #RewriteCond %{HTTPS} !=on
    #RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

    ### Canonicalize codeigniter URLs

    # If your default controller is something other than
    # "welcome" you should probably change this
    RewriteRule ^(welcome(/index)?|index(\.php)?)/?$ / [L,R=301]
    RewriteRule ^(.*)/index/?$ $1 [L,R=301]

    # Removes trailing slashes (prevents SEO duplicate content issues)
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^(.+)/$ $1 [L,R=301]

    # Enforce NO www
    RewriteCond %{HTTP_HOST} ^www [NC]
    RewriteRule ^(.*)$ https://removed.com/$1 [L,R=301]

    # Removes access to the system folder by users.
    # Additionally this will allow you to create a System.php controller,
    # previously this would not have been possible.
    # 'system' can be replaced if you have renamed your system folder.
    RewriteCond %{REQUEST_URI} ^system.*
    RewriteRule ^(.*)$ /index.php/$1 [L]

    # Checks to see if the user is attempting to access a valid file,
    # such as an image or css document, if this isn't true it sends the
    # request to index.php
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^(.*)$ index.php/$1 [L]

</IfModule>

<IfModule !mod_rewrite.c>

    # Without mod_rewrite, route 404's to the front controller
    ErrorDocument 404 /index.php

</IfModule>

Estou lutando para descobrir como o loop de redirecionamento está acontecendo.

Atualizar

Não deixei claro que meu servidor web (Apache) está configurado apenas com um host virtual na porta 80 que recebe o tráfego HTTP e HTTPS direcionado a ele pelo balanceador de carga. O balanceador de carga lida com a conexão SSL.