We are designing a three storey, timber frame house that needs FD30(s) fire doors but we are also fitting an MVHR system which needs a 10mm ventilation gap under the doors which conflict with Building Regulations.

We have looked around for doors and spoken to many companies and all have not come across this before. MVHR in 3 storey cannot be new?

One company suggested we use a sprinkler system.

Any ideas would be welcome!

For FD30s doors, Building Regulations ADB require compliance with BS 8214:2016 in terms of the threshold gap.

Where did you get, you require FD30s in a dwelling house my reading of Approved Document B (Fire Safety) Vol 1  Appendix C, table C 1, only the door between a dwelling house and an  integral garage requires a FD30s fire door with self closer. 

As FD 20 are not readily available so you could use FD 30 fire doors and this could allow you some options regarding the ventilation.

11.   In a dwelling house:                                                           Euro                   BS 476 p 22
 
 a. between a dwelling house and a garage.                        FD 30 S a(2)       FD 30 S(2) 
 b. forming part of the enclosures to a protected
     stairway in a single family dwelling house                       E 20                    FD 20 
 c. within any fire resisting construction in a 
    dwelling house not described elsewhere in this table.    E 20                   FD 20 
 

So if restricted cold smoke spread is not a requirement for your design, FD30 doors may be used instead of FD30s doors.  Many FD30 doors permit a 10mm gap at the threshold, check with the particular fire door manufacturer.