I think you would generally approach this the same way you'd approach any subjective research, which is heavy on asking questions on a survey. You could research traditional qualitative research processes to learn more about how they're conducted.
You'd be surprised by the emotions that can be hiding in an otherwise 'boring' purchase like a boiler, however. If it's the middle of winter in Chicago and snowing outside on a Tuesday night and your boiler goes out and you have a baby crying in the house and a dog that needs to get walked - I can guarantee you're feeling a ton of emotions (frustration, anxiety, stress) when you open your phone, google "chicago 24/7 furnace repair" and finally get somebody on the phone to come out that night and fix it.
Even in a B2B purchase, there are emotions and motivations that are hidden under the surface, such as picking the more expensive furnace repair vendor that has a better reputation because you need to use up your end of year budget in order to keep it for next year, and you can always justify the choice based upon their reputation or even just a Yelp score.
SO - the hard part is capturing that emotion on a survey form or getting an authentic customer response shortly after purchase, or getting somebody to be truthful with you about these motivations that may be embarrassing or incriminating or whatever.
I think you're best off doing some research on general emotions, and then creating a list of all of the considerations that go into purchasing your product (free shipping? good warranty? website looks trustworthy? my wife will like the color? etc.). Once you have each of those lists, brainstorm how various emotions will factor into each of the purchase considerations, and how it will change depending on your type of customer (b2b vs b2c, enterprise vs SMB, senior exec vs intern, etc.). This may yield a lot of ideas, or not many at all, depending on the nature of what you're selling.
I think once you complete this inventory across each product, each customer type, each purchase consideration, and each emotion - you'll have a pretty darn good idea of how emotions can be factored into your marketing program, such as how to adjust the copy on your product/service pages, and how to create entertaining or educational content that reflects these emotions.
Hopefully that helps, but let me know if I can assist further!