Frozen ground, it's best to use a full on stove in my opinion. Honestly, I've used bottomless only a few times myself, and mostly with the sibling stoves. With the sibling I have both just used it on dirt, or dug out underneath. I wont get hours of burn either way as it is just too small. Bigger stoves I can get better burn time, but I would not consider it "reliable" . There are a lot of variables.
I have heard several people say Eco Logs work well to extend the burn when you are not packing in. I have not tried them myself, but i have heard 3 reports on it. Eco Logs are compressed wood with no additional additives.
I would avoid coal / charcoal use. Not there have been documented issues in our stoves, but because there have been documented cases in Wall Tent / Stove combo's using these, thus I would avoid it.
