It sounds like you may be mistaken as to how this is wired, or that perhaps I'm just not understanding your explanation. As others have mentioned, it's not possible to get 240 volts from a single pole in a 120/240V split phase system. Each tandem breaker provides 2 120 V circuits, this is true. However, if you measure between the terminals on a single tandem breaker, you'll get 0 volts. This is because the terminals are both powered from the same leg, and so are at the same voltage potential. If you measure from a terminal on the top tandem breaker to a terminal on the bottom one, then you'll measure 240 volts. This is because each breaker is connected to a different leg, which are each one half of a 240 volt circuit.

With all that said. For this setup to work, one appliance would have to be connected to both breaker. Something like this...

Notice that each appliance circuit has one wire connected to each of the tandem breakers. In this situation, you'd need a device like Speedy Petey shows.

Which ties the breaker handles together, to provide common trip characteristics.

Notice how the inner handles are tied together, and that the outer handles are also tied to each other. This way if either trip (or are turned off by the user), the entire circuit is shut off.

If this is wired the way you've explained, where the dryer is connected to the top tandem and the heater is connected to the bottom. Then there's some magic going on in those breakers.

The panel can indeed support a 240V breaker. The question you should ask is, is there space in the panel to physically accommodate additional breakers?

Based on the model number, your panel should look something like this.

Notice how this example image has lots of blank spaces. In a panel like this, there's no problem at all installing a new breaker (or double pole breaker in your case). However, it's not likely your panel looks like this. More probable, is your panel has all or nearly all the slots full.

If there are not two free spaces in the panel, you'll not be able to install a new breaker in this panel. You have a few options in this case.

• Install tandem breakers where possible to free up slots.
• Install a subpanel.

The other problem you could have, is that your current service will not handle the additional load. This panel can handle 100 amperes, but that does not mean you have 100 amp service. You'll have to check with the local utility to determine the size of the current service. If

Lastly, the panel might already be close to being overloaded. If you have all electric appliances (Range, Dryer, heat, water heater, etc.), adding a hot tub could cause the main breaker to trip from time to time. This will completely depend on your current consumption of power, which is not possible to know based on the information you've provided.

tl;dr

Contact a local licensed Electrician to determine what's required with installing a hot tub.