In the last year I have gained 20 lbs. Without any change in diet or exercise. I am 71 and have had cardiovascular issues including CHF. No edema.??

Check labs. followup with your doctor to get routine labs including a thyroid panel. you may also benefit from BNP and echocardiogram if you have CHF.
Fluid Retention. Is important in patients with CHF. It can mean fluid retention that can mean early sign of decompensated CHF. Weight monitoring is important in patients with CHF. Follow up with your doctor about this.