Originally Posted By: tanstaafl.
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My hot water heater has two propane burners and two small (11-liter each) tanks, the tanks are connected in series so that the first burner/tank preheats the water for the second. This seems like a reasonable compromise all around, with the small tanks reducing the heat loss between usage yet enough of a buffer to handle heavier usage such as showers. It allows the use of smaller burners as well, which may (or perhaps not?) add to efficiency.

The question? How to set the thermostats (each burner/tank has its own) to minimize propane consumption and still provide enough hot water to meet demand. Both at maximum temperature? Both at the minimum temperature that satisfies? Some intermediate setting? Low on the first burner, high on the second?

There are a lot of possible permutations here.

The technicians who serviced the heater last week (new thermostats, new pilot lights, new igniters and new pressure relief valve) just cranked both thermostats to maximum hot and left it at that. Is that the best way to go?

In the next few months I will be adding a passive solar system to pre-heat the water before it goes into the water heater, so perhaps it doesn't matter too much anyway. ..
My first question would be, what thermostat settings does the manufactuerr of your refurbished two stage water heater recommend?

With large flow hot water consumption (shower, bathtub filling) what is important is the flow rate at which the heater system can continuously deliver additional freshly cooked 'hot' water. How much water is stored in the twin 11 liter tanks is less important than the rate at which the dual burners can make more of it as your shower duration progresses.

For small hot water draws, such as hand washing, the water in the tank(s) is enough capacity and burner heat output (aka recovery rate) is not so important.

If you need more than 11 liters of full temperature hot water at a time, then either the second stage heater can add enough heat to the incoming water flow to maintain the target outlet temperature or the hot water gets less hot after the first 11 liters are delivered. This may be why the technicians default is to set both tanks to the same high heat setting. This defers the potential temperature drop until after all 22 liters have flowed through and been delivered.

The flow rate at the point of use also affects matters. The faster the newly arrived water is moving through the tank(s) the less time the burner has to impart heat energy into the newly arrived water before that water has moved on.

I would be guessing, but I imagine the near optimal overall energy efficiency would occur with staggered thermostat settings. Preheat the water inside the first tank to some fraction of the second tank's target temperature. The thinking is that the parasitic heat loss from the standing (semi-hot) water in the first tank is lessened by the lower temperature delta relative to the surrounding air.

The second (fully hot) tank would have a (somewhat) greater hourly heat loss rate when just sitting idle, but that is true of all 'tank' type hot water heaters.

When your solar water preheating system is in place, the ideal might be to split the difference between the incoming water temperature (preheated by solar) and the target output hot water temperature and set the first stage tank to the mid-point.

Much depends on whether the propane tank stages can deliver a satisfactory water heating rate during those times you are continuously drawing hot water (more than 11 or 22 liters).

I presume the solar system will circulate water through the collectors and then into a storage tank of some significant capacity. The larger the solar storage tank, the more total energy can be stored. Of course, a larger tank takes up more space and needs to be properly insulated.

Part of the win from having preheated water on hand is to deliver that preheated water directly to the propane heating stage whenever demand for hot water use occurs. Cooled water sitting in the intervening pipes between solar tank and propane heaters is to be avoided as much as possible.