2 water heaters?
I have a 3 bath home with just 3 of us living here. My main water line runs into a 30 gal gas unit. It then runs out of the 30 gal unit and into a 50 gal gas unit and then out to the house. There is a line spliced into the outgoing line over to a circulating pump and into the hot water coil at the furnace. It then comes out of the furnace and into the drain valve on the 30 gal unit.
Do I possibly need this much hot water? Both of my units are over 20 years old and I am concerned about their lifespan. I would like to replace both units with a single if possible. Is it really a bigger draw on the water heater since the hot water is circulating through the water coil and right back into the tank?
Do you have a boiler or a hot air furnace? Usually, if you are using a boiler, you'd install an indirect water heater rather than a separately fueled WH. A coil in a furnace could be for space heating, but that normally isn't run off of a WH. A clarification might help here.
Now, whether you need both WH is totally user dependent. Were they both put in at the same time? OR, did you start with a 30g, and then find out it was too small, then add a 50g to it? There's less standby loss with a single tank. Recirculation is a convenience issue that helps you from wasting water waiting for the hot to get there. Unless you use two WH, and place them each closer to where the hot water is necessary, having two usually isn't a benefit, other than the increased volume available.
Depending on how you use water, your 'need' might be met with a smaller tank. If three people are all showering at the same time, what you have in total volume might be necessary. There are ways to minimize this by installing a waste water heat recovery system (if you have the vertical room for one). This will not work if you use the hot for say a tub, since you need all the hot to fill the tub, and there's nothing going down the drain WHILE you're using it to help offset the energy going in.
At 20-years, you're well over the norm for needing a replacement.
It is a hot air furnace. This system was in the house when we bought it. The 30gal is labeled 1991 and the 50g is labeled 1994. I wouldn't think that the furnace would ever knock the hot water system down much because it is circulating back to the heater. I'm thinking of putting in a 50 gal. 40,000 btu single unit. The FHR would be above 80. Do you think that would be enough. Even when we have company there would only possibly be 2 showers going at the same time.
Per the "70%" rule, a 50 gallon, 40K unit produces about 35 gallons of showerable temp water. At 4 gpm ( 2 showers) that would be 8 minutes, then a long wait for recovery.
Millions of homes in this area, including mine, are surviving quite well on 50 gallon gas or electric water heaters, and since gas is far superior to electric that makes your situation doable with a single 50 gallon gas heater.