You may have a 'generous' 20 amp breaker on the circuit, and the heater may be a little low in power. Devices draw the amperage they require, so changing to a 30 A breaker will not increase the output. There is not really a hazard in using an undersized breaker. If you have #10 conductors you can switch to a 30 A breaker but the preformance will remain the same.

Glad to hear it - that was a drastic idea and not really a solution. Since you're putting a new circuit in (here in BC I don't think you're allowed to use a 15a circuit for power, it's only allowed for lighting) I'd suggest you put the printer on the 20a and keep the computer on the 15. I too wonder about the accuracy of the printer's draw but at the same time suggest you do consider replacing it, if you can verify that excessive draw. The 80% you were mentioning is more a thing of good housekeeping - for safety's sake, a circuit should always be de-rated as it were - just as with electronic components like capacitors, they're usually rated far above the operating voltage you should actually use. For example, I got a collection of capacitors from a surplus store very cheap because they were odd values - one is rated at 100,000 microfarads which is huge but rated only at 15 volts dc - I couldn't use it say, in a car as a noise suppressor in my stereo, as car alternators can put out as much as 13.8 to 14 volts, which would be too close to its peak rated voltage....its absolute limit would probably be 12 volts, but in practice, it'd be better for 6 or 8 volts. The other thing to note in the life of circuit breakers is they do not have an indefinite life, I've heard talk they should be replaced as soon as they've been tripped a few times, since they can start to act up like yours is, and not trip when they should....also, if you're running them at or above their peak, the likelihood of them failing to trip is increased. A really safety-conscious person probably ought to replace ALL their breakers every few years, although I doubt many do.

OK, to answer the question, you can't tell about the loads without doing a proper analysis. What used to be connected to the 30A breaker. Seaborn: The way it works is that the panel is appropriately sized for the service and there are limitations on what you can put in the "top" section. That's where we run into a problem. The code that allowed these panels in the first place requires that all the lighting and appliance e branch circuits (the ones it sound like he wants to add) CAN NOT be connected to the top section (mostly the understanding is you don't know what someone might connect to them as opposed to the things in the top section which are fixed items: furnace, stove, dryer, hot water heater. This requirement persists in the modern code. 408.36 covers it. It was formerly 384-16. There were some exceptions but they don't seem to apply here. So the answer is NO, YOU CAN'T DO THIS by code. ProSource is correct. You could connect a subpanel to that 30A breaker and put your new branch circuits on that.

You can't know without doing a load calculation. A lot of it depends on what you are using for heating, hot water, cooking, clothes drying. At one point they were pushing 400A services for larger homes, however, things have gotten more efficient over time. While I have 400A service, checking the load from time to time shows that I never get anywhere near that. As pointed out the "panel" itself is only part of the problem. You won't get 100A out of the power company unless the service is upgrade. Frankly, if this is an existing home, and you've not had any problems, the upgrade is of dubious merit. I disagree with Live_Wire's statement. "It will never be cheaper to do than now" is nonsensical." I've seen nothing that has made service upgrades cheaper or more expensive in years (other than normal inflation, and just because it might cost more in the future due to inflation doesn't mean that its a good idea to do it now when the money could be used for something else).