I had though you had just a tub. Racks really need to be in correct ambient temps they simply cannot heat ambients effectively any other way with out heroic efforts. Flexwatt UTH are radiant heat sources and by definition radiant heats objects not air or water. They do not change ambient air temps much. (objects heated release some heat to the air).

I have a rack in a room that is quite cool as well (below 68ºF) I have managed to hold correct ambient air temps but it is not easy. I have hot side and low wattage cool side flexwatt (4 and 17 inch) This hold surface temps but not ambient air temps. I found that the top two and lower three slots needed different set points so I have a herpstat pro (replaced by the HS4 now) Running upper and lower hot and cool heat tape. I also routerd slots on the bottom of each shelf the slots are covered by the flexwatt and have holes drilled in from the top of the one below each level. This creates a channel for air to flow from the ceiling front to the back ceiling of each level it runs in contact with flexwatt all the way. This added 2 maybe 3º to ambient air temps. I insulated the top, sides, back, and bottom with 1/2 inch blue foam insulation. This added yet another 3-4º (5-7º total so far) I have also changed the air flow patterns of the rack so the tubs are vented about 1 inch from the top on three sides. The closed side is to the room this gives the air a chance to warm a touch more. I got one degree more this way (6-8 total over ambient room temps) I also tape in the winter thermal reflective material to the sides of the of each tub (not blocking vent holes) this was good for another 2-3º (total is 8-11º over the ambient room temp) My rack holds 90º warm and 80º cool with an ambient temp of 77-83º . I had to increase the room temp to around 69ºF but past that I have no issues anymore.

I run failsafes as well as the tub bottoms are covered by heaters the 17 flexwatt does not get as hot as regular (low wattage) so it is much safer. In an wide open test (no snakes in the rack) I ran the flexwatt maxed out for 4 hours and it only hit 87º.