Is it better to plumb them up in parallel or series. …? ? --172.250.x.xx

Is one a temper tank? What method of control is used? Why are 2 used? Fuel source? --76.190.xxx.xxx

The plumber piped them in series in my house --71.174.xxx.xxx

I've installed them in series before. This was to enable the first tank to temper the water before it entered the second one. Doing this enable me to use a couple of 40 gallon gas fired water heaters to supply adequate HW to a 4 unit property. When I was able to do a major remodel I changes things around so each unit had their own water heater as well as their own gas bill. --72.93.xxx.xx

Stack'em. --24.183.xxx.xxx

1 inch run then to three quarter or half at fixtures --172.250.x.xx

It would be more efficient to install a wall mounted demand water heater then vent direectly through the wall no chimney then no recirculation loop. A demand water heater or some call tankless willl put more hot water then a direct fired tank water heater where at maximum firing rate 200,000 Btu. Dirct fired gas hot water tanks have constant heat loss when water heater is not firing then must fire to keep water hot. A demand water heater only fires according to load where minimal when filling up bathroom basin. --68.69.xxx.xxx

Series. Water will take the course of least resistance, one additional elbow on one of the two water heaters will mean way less water flowing through that branch if it's in parallel. Thus when you fill up the large spa tub you won't be using all the hot water in both tanks. When he plumbs them have him pipe in a bypass valves on each water heater so if one fails you can simply change the valves to bypass it and still have hot water in the house. Takes the emergency out of a hot water heater failure. --209.205.xxx.xx

series, I replaced HW tank in multifamily I couldn't separate, w 2 40 gal in series. 2nd one refills first one so water stays hot for everyone. Also worked out because humping 2 smaller eaters down a hatch was easier than a colossal one. --72.10.xxx.xxx

Tankless is only efficient if the hot water isn't used that often. If it's used several times during the day, then it's more efficient to maintain temp with tank water heater than to flash heat multiple times per day. Tankless is really great for vacation homes that sit vacant a lot. --108.69.xxx.xxx

If using a indiredt hot water tank the standby loss is 0.6 degress per hour where a direct fired hot water tank has a open flue where 7 to 12 degrees goes up the stack when no hot water is used. So if demand hot water heater is used then can turn on recirculation pump where hot water generation will start. The efficiency of a condensing demand water heater is 95 per cent where a direct fired hot water tank is around 55 per cent which is a significant difference. If have a high efficiency gas boiler then a indirect hot water tank is the best option as the standby loss is really low. In the EU they do not use inefficient hot water tanks. Some citiees in the EU have district heating where the city or town has a heating plant where in houses and buildings there is indirect hot water tank in the basement then radiaters or radiant floor heating in newer buildings. In the house the demant water heater is 81 per cent efficient where as long as working it would not worthwhile to install 95 cent efficient one. Eventually condensing hot water tanks will be norm which have a lower standby loss along with greater efficiency when hot water is being generated. The differene between standard effciency and high efficiency are two heat exchangers where the second is made of stainless steel as condensate is acidic. --68.69.xxx.xxx