Hayzee,,,the system you have deescribed will work, however there is a much more fuel effiecient method of solving your problem.
Typically a "Hot Shower" will require 90 to 100 deg water,however in order to insure that no one would be scalded the plumbing codes require that the hot water be regulated to not more than 120degF. Normally this is accomplished by setting the water heater t/stats to 120degF.
The problem here is that the incoming cold makeup water entering the tank is approximately 50degF. This means that the incoming water will instantly begin diluting the hot water stored in the tank. Often the rate of flow is great enough that the dilution rate is higher than the water heaters ability to make up the loss.
Now let us consider a 100 degF shower demand. With hot water at 120 degF it requires a mix at the shower of about 80% hot water and 20% cold water or less. In this instance with a 2.5gal/min flow rate at the shower we are now demanding 2gpm from the water heater. Often 2gpm is above the water heaters first hour recovery rate, which means the tank simply cannot heat the water as fast as it is demanded.
One solution is to install a "Whole House Tempering Valve" on the output of the water heater to regulate the distribution to 120degF, then we can turn the water heater up to 180degF. The differential between the water in the storage vessel and the makeup water is now 180deg to 50 rather than 120degF to 50degF. By increasing the differential we dramatically reduce the rate of dilution which means there is more available water in the tank to meet demand and the heating unit is able to keep pace with the demand. In most instance the installation of the "Whole House Tempering Valve" and increasing the water heater temperature is sufficient to meet most residential demands. The cost advantage here is that a "Whole House Tempering Valve" is only about $40 to $50 as opposed to $250 or $300 for a second water heater, not to mention a substantial savings in installation labor cost.
In years past it was argued that leaving the water heaters at 180degF increased the amount of residual standby losses from the water heaters, which reduced fuel efficiency. That was true with the older water heaters that typically only had a 1/2layer of fiberglass insulation which was often R-5 or less. In recent years the plumbing codes have been ammended to require all water heaters to have a minimum of R-12 insulation, which could be achieved by adding an insultion blanket on the older heaters.
In order to meet the new specifications almost all water heater manufacturers have now switched to cast in foam insulations that provide the R-12 and in some instances as much as R-30 or more. With the new insulations the standby losses minimized to a point where they are almost negligible.
Given that the "Whole House Tempering Valve" regulates the water temperature to the code standard of 120degF, the standby losses on the distribution system remain the same. This can also be further reduced by insulating all hot water lines in the strutcher.
In a worst case scenario it may still be necessary to install a second water heater. In this case, if both water heaters are installed in parallel the high rate of dilution occurs in both tanks, which means both heating elements will come on with each demand.
A more efficient solution is to install the second water heater in series with the first heater. The temperature of the secondary tank is set to 120degF to meet the code for distribution temperature. The primary tank is then set 5 to 10degF hotter.
In this configuration the water entering the second tank is normally above the calling temp of the second tanks heating unit so the burner does not come on. This means that he second burner only comes on occasional to satisfy standby loss which is minimal.
Normally in conditions of relatively low demand such as a bathroom lavatory, kitchen sink, dishwasher or washing machine the dilution only occurs in the primary tank, which in turn is able to satisfy the demand, therefore the secondary tank is only acting as a storage vessel. In a worst case scenario such as multiple showers or filling a large garden tub or soaker tub as the water in the primaary tank begins to drop below the desired temp and the primary heating element is not able to keep up with demand, the water begins to enter the secondary tank only slightly below desire temp which greatly redusces the dillution rate, therefore the secondary heater is able to meet the demand and you are afforded an almost limitless supply of hot water.
In an absolute worst case scenario we can then set up two water heaters in series with a whole house tempering valve and increase the tank temperatures.
Linear Mode
