Anyone use RESFEN to calculate energy costs?

I just downloaded RESFEN 5.0 and input my data for the current windows (double pane, assumed U-factor of .49) and local electricity and gas rates. The annual heating/cooling cost was a little low but close enough.

Then I changed all the windows to .35 U-factor and recalculated. The annual cost dropped less than 3%.

Does that seem right? If so, there is almost no payback for new, more energy efficient windows.

 

Hi IMac and welcome to the “enlightened”. The program is taking all of your heat loss into consideration and calculating what portion of the total is attributable to just windows, and then estimates the savings for the upgrade. 3% is on the low side, but 10% would have been high. It also depends upon what you fed into the program.

Here is a rough estimate. There are 5 major areas of heat loss. If we assume each is accounting for an equal share, then the window portion would be 20%. With your numbers, U=.49 upgrading to U=.35, and my rough estimate, you would save about 6%. So in your model the windows are a smaller portion of the overall heating cost, but the savings are small in either situation which is typical for window upgrades.

Other factors can play into the decision, such as drafts, single pane windows, the wife wants new ones, or the mother-in-law has offered to pay, but in terms of a payback there are often much better places to put your energy improvement dollars.

Good Luck.
Bud

 

Thanks Bud. My window area was less that 10% of total surface (by the program calculation) so I guess I should have expected a small savings. In my case, the upgrade is as much because my old casement windows don't operate well and most can't be closed fully without a push from the outside. The only real draft I get is from the patio slider and that will be the first thing replaced.