Should you repair or replace your old air conditioning unit when it starts giving you trouble? It's a dilemma so many of us face when warmer weather comes and we pop on our AC unit only to find it's not working.

At my home, we had an AC unit that was out of Freon and had a crack in the line. So then came that difficult choice -- should we fix it or replace it, based on the age of the unit?

This year we faced an additional challenge. We are in the midst of a spike in the cost of older refrigerant that's been in phase-out for a number of years. If you're unit empties out, you may find it cost prohibitive to put the refrigerant back in.

These factors can change the math on when you bail out of an old unit versus when you try to limp along a bit longer.

The AC industry is making far more efficient units than even just several years ago. Efficiency is measured by the Seasonal Energy Efficiency Ratio (SEER). A unit with today's standard of SEER 15 is much cheaper to run than an older one with a SEER 10 rating. You can figure that your power bill will be reduced by roughly a third with the new units.

I'm not automatically suggesting you prematurely dump an old unit that's working well.  But when it does break down, the math may favor replacing, not repairing.

I know it's not an easy choice when you're faced with the huge expense to replace versus the really big expense to repair. But just remember, every month that power bill comes like clockwork!