gas furnace with no replaceable filter?

Even permanent filters must be cleaned.

Condsider the possiblity that the furnace has been run with no filter and maybe that's why you aren't finding an old filter to change.

the problem is that there is no access place to put a filter. it is completely sealed!

Look harder.

And it should be cleaned and serviced by a pro, Even an old filter can clog things up reducing efficency dramaticly. No filter caused a friends unit to reduce efficiency by maybe 60% from a clogged AC coil. Get a pro out.

A 'permanent' filter would require cleaning.

If you can give make and model number it would make it easier.

Are you sure the filter is not on the air inlet or at the intake duct.?

If you are renting you could ask the landlord.

As a renter, he should contact landlord as to what action to take. Landlord should be happy to have conscientious tenant, and inquire of installer. Who he should know.

Just because you can't find it doesn't mean much.

Then too there is budget-conscious cost-cutting, as in D.C. Contractor could have said "they won't miss it if they can't find it."

HTH, J