Jan 1st safety codes are changing for hot water tanks and furnaces for new installs and converting over. I believe they are no more low efficiency replacements they have to be upgraded to mid efficiency or better and for new installs they have to be high effiency. Sounds like you have mid effiency on your furnace and mid effiency on your hot water tank.(there is a small fan on top of the hot water tank exhaust vent? )
Up grading you are limited in on the size of your tank by the diameter of the exhaust vent so measure it and check with home despot or a plumber first on what you can upgrade to , unless you want to change the size of the b-vent all the way to the roof.
If you use the shower like four times a day a load of laundry and the dishwasher every two days or so you want a tank . If you use no hot water for a week then use a hundred gallons in an hour then tank less is more efficient.
I just Drained and flushed ours, used the garden hose and drained and flushed for over an hour lots of sediment came out and then turned the temperature setting on the tank down two settings cause it was now hotter. Flush and drain it again till it comes out clear.
The really good hot water solar heaters cost $20,000 for the unit on the roof and with the glycol pumps they can provide heat for up %80 of your home. The problem is you need a heat sink to get rid of excess heat , er read outside hot tub
for the system to work properly