[regularfry]

If you know the rectangle then you know something about the minimum possible length of pipe. If heat loss is proportional to total pipe length, it gives you an order of magnitude for the total.

[friend_and_foe]

OK, but what toes that have to do with dividing the area traced by the pipe with the floor area of the place?

[regularfry]

That gives you a ratio that you can compare between properties.