What type of thermostat are you using? It could be the thermostat. There's really no reason your tubs can't stay up to temp unless your house is really cold. I have 3 foot tubs for my adult females and I'm able to keep them warm. But, I do heat the room with a space heater until it gets pretty warm outside, I keep it between 70-75 in there. What do you have your UTH set at? You are going to loose a couple degrees from the UTH to the tub. I keep mine around 92 and my tubs are 90 hot side 75-80 cool side. They are also in racks which helps hold in the heat. When I have a tub outside of the rack for whatever reason, I put a towel over and around the tub to help hold in the heat if it's chilly in the house.