The old man has a bit of heart failure and is usually short of breath, especially on cloudy days. Is there any good way?

This situation requires long-term oxygen inhalation. The longer the inhalation time, the better the condition. My old man has a bad heart. I originally bought a three-liter oxygen concentrator, but when the old man couldn't breathe, The three-liter machine didn't work at all, and I had to go to the hospital. Later, the doctor asked me to buy a five-liter machine, which can be turned on for a long time. It usually breathes oxygen at a low flow rate. When the elderly are wheezing or holding back, the flow can be increased to quickly relieve the symptoms of holding breath. Now The old man rarely gets sick all year round