I already burnt out an 8-amp heating thermostat. A 15A model is in the mail
That is an important piece of information that is easily missed (done it myself a few times and I should know better), when using switches such as relays (which is what is probably what is in your thermostat) it is critical to use switches
rated far beyond the expected run load.
- First off is that the power ratings for most equipment is the running load and does NOT account for any start up surge they may cause, which can be a couple times the run load. This is normally of too short a duration to account for with wiring and breakers, but it does affect switches (more so for mechanical, but also for solid state). So if your run load is 5A, you should have at least a 10-15A rated switch. This is especially important for inductive loads such as motors (fans, pumps, etc).
- Second, for power loads like mains, a mechanical switch will arc every time they close (and sometimes when opening). Over time, if the contacts are not rated high enough such as operating it near its rated max, a mechanical relay will eventually fuse in the closed position, this can happen right away, or take days, weeks, months...but it will happen. And without a doubt at the worst possible time (I know this from too much experience).
My rule of thumb is to be sure my switches are rated for at least twice the expected run load, more if the load is inductive. If I am not trying to build commercial products cost effectiveness is not really an issue, so I normally error on the side of caution and exceed that just to be safe. After all, what is a additional couple of bucks to protect your trees? The closer you operate a mechanical switch to its rated limit the shorter its life will be.
Here is a nice little article that I found which is easy to follow and understand.
https://relaypros.com/choosing_proper_amperage.htm
While this is is for mechanical relays, for the most part this applies to any switching devices including solid state relays, triacs, SCR's, etc.
BTW inductive loads can create additional concerns for solid state switches that I won't go into here. But if you are going to use them, I advise reading up on it. I keep a smoked SCR on my desk as a reminder to myself.
FYI 1500W, assuming a 115vac supply and nothing else is running off the same supply, it is a 10A run load. It can be double or even triple on start due to the fan motor in a space heater. So to be safe (or at least reliable) you should anticipate at least a 20A load. And if you think that is only theoretical...Just a few years ago I had a 1500W space heater controlled by a 10A relay (it is all I had on hand and I was being lazy) and it fused shut after only a couple of 2 months of operation.
I hope this is helpful.