Hi, I was wondering if you have a certified FD30 fire door and replace the intumescent strips with combined intumescent smoke strips (essentially upgrading the fire door to a smoke door), does this negate the certification of the door or does it simply become a certified FD30S door?

Probably not, but try bat wings they are far more robust

The point about a certificated fire door is that the label on the top edge of the door leaf enables you to find the certificate of approval at  https://www.warringtonfire.com/certified-companies/check-a-certificate   Read the certificate to find out the requirements for the smoke seals.

In this way the door retains it's certification and becomes FD30(s).

When using smoke seals care must be taken to read both the door and seal certificate.

CF218   Page 5

However, CF341

So yes they are allowed, but at least one fin must be continuous