My house is cold. It is regularly around 13 to 15°C (55 to 59°F) and it is only with a lot of (expensive) heating that I get it up to anything approaching "normal" room temperature. My sourdough starter is about three weeks old and is healthy enough (I've made several loaves with a decent texture and taste) but it does seem a little down in the dumps compared to many that I've seen online.
I've tried adding warm water rather than room temperature. I mix water half from my filter jug and half just boiled, which is about 53°C (127°F). I add it to my starter and the combined temperature is about 33°C (91°F).
This, along with several tea-towels for keeping that heat in, seems to help. The starter is significantly more bubbly and active and actually come close to doubling in size like everyone says it should!
But I'm aware that heat can kill yeast cells. Am I putting my starter at risk by this method? What do I need to be wary of?