just be aware crap in equals crap out and the blame always dumps on your shoulders not the app maker. I reckon they are a waste of time, as you need to understand how it arrives at the answer to know the figures you get are within reason, so why not do it manually, it doesnt take long, and you know you are doing it right if you work it yourself.
As chalked said i use all the mr combi ones and the rad app is accurate , ive been using it on new builds for a while and theyre bang on, i even tried it against the company the merchants use for heat loss calcs and exact same results,
The rad app is there or there abouts,
I dont have a spreadsheet like some and the paperwork way is a faff.
Gas pipe sizing its a piece of cake doing it manually