Question on using 15 amp rated switch on 20 amp circuit?

The switch onl;y has to be sized to the connected load.

What does "sized to the connected load" mean?

What is the wattage for the heater? Sure they ask for a 20 amp circuit, what does it draw. Size the switch to the load. Most residential grade switches are going to have connectivity issues breaking a 1500 watt or greater load.

Electric heater, bathroom, smells a lot like a GFCI circuit to me. But then again I am paranoid about water and electricity. I will not swim in a pool if the light is on.

It sounds like the load is 20 Amps on this combination heater/fan. There are toggle type isolation switches available for this type of load but they are usually quite a bit larger than a standard 15A toggle switch and I don't think you will be able to gang one in a two gang box with another device. Will most likely need it's own separate box.

if you can fit a couple of contacters in the fan then any switch will work seperate control fuse on the switches would be nice

Simple stuff, Bob. Look on the device and see how many watts the heater draws. Divide the watts by 120 volts and you should get pretty close to the amperage. If it is less than 15 amps, you are O.K. with a 15 amp switch. They do make that double single pole switch in a twenty amp model, and I recommend a 20 amp because of the inrush current you will get from a resistive load like a heater. It will just be a bit more robust and last longer. What is left after that is the light, which is nothing to concern yourself over on a 15 amp switch. It probably draws about a half an amp. The reason they ask for a 20 amp circuit is that you are not allowed to load your circuit over 80%. If you check out the load of that unit, it may be below 15 amps, but still over 80% of 15 amps. (12) Even 15 amp switches come in different grades. I would get a "Spec Grade" switch for that load. Sometimes Home Depot and the like will have, "Contractor, or Professional Grade" which is a cut above the residential crap. Make sure you don't use any "push-in" connections on the project, and good luck.

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If the box can accept two switches there is nothing wrong with installing a switch RATED FOR THE LOAD. Just because the manufacture says use a 20 amp circuit it does not mean that the unit draws 20 amps. Which if it really did you would need a 30 amp circuit.

Do a search on toggle switches by manufacture. Look at the ratings.

You can get 2 of these together in a box for 2 devices. It becomes harder if the wire fill approaches the max. But do-able.

The heaters in my bathrooms are 2kW (I was surprised when I pulled one to tile floor). The "switches" are thermostats rated for 25A, IIRC, though on 20A circuits.

These aren't on GFCIs. I don't think they have to be since there is nothing exposed. You'd have to try pretty hard to get at 'em (under the vanity).