Why are there so many iterations of the 2019 MCH-31 form -- -a, -b, -c, and -d?
The answer is that there are four variants of the MCH-31 because there are two ways to test Whole House Fan (WHF) airflow (call them A and B) and two ways to test WHF watt draw(call them X and Y). Because the results of each test are on the same form, it makes four possible combinations: A/X, A/Y, B,/X, and B/Y. So there is a variant for each of these. From the user perspective, they just have to answer the questions and the registry will determine which form variant to use. It's also only an issue if there are more than one WHF in the house, which will be relatively rare.