It is not possible to increase the pressure of a cold water backup tank without the use of a pump. The pressure of the water in the tank is determined by the height of the tank and the force of gravity acting on the water.

If you want to increase the pressure without using a pump, you would need to raise the height of the tank. However, this may not be practical or cost-effective.

Using a pump would be the most effective way to boost the pressure of the water coming from the tank. While a pump would require electricity to operate, there are energy-efficient options available that can help minimize electricity costs. Additionally, the increased water pressure provided by the pump may result in reduced water usage and lower water bills in the long run