If it's something that it part of my living expenses (home maintenance, car repairs/replacement fund, etc.), then I budget for it spread out over the year, or years as it may be. So I guess that does decrease the savings rate I see, but I feel like it gives me a more complete version of our spending than if I just took it out of savings and hadn't planned for it. For the example you gave, if I was confident it was going to save money long term, then I'd probably do it sooner. Just make sure you've accounted for all of the costs like chimney cleanings, increased home insurance, the wood itself, maybe more I'm not aware of.
For bigger things that aren't part of our typical spending, we evaluate how much it's worth it to us. There's a few things for which we've reduced our saving rate, like our daughter's college fund, but to us it's worth working longer for. We're not entirely sacrificing our retirement savings in order to save up that fund quickly, but we think it's better for us to be saving for both now. I'm not 100% sure it's optimal, but it seems good for us.