First off,A tank type water heater begins with a full vessel of hot water and that water is being diluted by the cold supply water entering the tank, therefore the first hour rating of a water heater determines how much hot water the heater can deliver when you consider both the amount of BTU energy in the stored water and the amount of energy being added by the burner.

It's Sunday morning and I don't have all my notes here at the house but if memory serves me, the first hour rating is typically considered to be the amount of water the tank can deliver at a temperature that is 10degF below the tank set temperature, and it is typically computed at 70% of the total tank volume. I.E. A 100gal tank set at 125degF could supply 70gal at 100 degF for the first hour. (This is assuming the incoming water is at 55degF)

A second hour rating is the amount of hot water a water heater can produce when heating all the water from cold startup. Given that a tankless unit has no capacity to store energy it then stands that you cannot assign a first hour rating to a tankless.

I am going to jump ahead and answer the last question next: can copper pipe handle 70psi day in and day out? Yes, although the plumbing codes limit distribution water pressure to 85psi or less, the same copper pipe is also used to convey liquid in the HVAC industry and the working pressures are rated to a test pressure in excess of 350psi, however at pressues in excess of 100psi it requires brazing the joints instead of soldering.

The plumbing Codes state that in instances where the supply pressure exceeds 85psi, even momentarily, we are required to install a Pressure Reducing Valve (PRV) which will limit the pressure to 85psi or less. In turn, ALL fixtures designed for use on a potable water syste are required to be able to withstand 85psi. (Most have a high pressure test rating in excess of 150psi)

Now in regards to pressure. Most people confuse Volume(gallons per minute) with pressure. The total volume of any line is determined by the smallest orifice the liquid must pass through and varying the supply pressure will have very little effect on volume. Think of this in terms of an exit ramp on the interstate. If the ramp can handle 20 cars a minute, it doesnt matter how fast the cars travel on the interstate or how many cars are attempting to exit, it still handles 20cars a minute and the excess traffic simply backs up on the interstate roadway. The same is true of a water line. You can demonstrate this for yourself with a garden hose attached to a hose Bibb. Adjust your hose nozzle for a medium stream of water, then open the hose Bibb 1/2 way and time how long it takes to fill a 1 gallon jug. Now point the nozzle straight up in the air and note how high the water goes. The vertical height is approximately equal to the vertical head pressure of the water at the nozzle. Now without moving the nozzle adjustment, time how long it takes to fill a second jug with the hose bibb fully open, and you will see that it takes the same lenght of time but if you point the nozzle straight up in the air the water will rise about 25 to 50% higher because the pressure at the nozzle is greater when the valve is fully open. The point here is that while the volume of flow remains constant, the velocity of flow is much greater at a lower pressure. This explains why a flow restrictor such as those used in a water saver shower head or a water meter has the same end effect regardless of what the supply pressure is (at least within the range of pressures available in water distribution piping).

Just as we use Drainage Fixture Units to determine the size of Drain,waste & Vent lines, the codes also give us tables of "Fixture Units" to determine the proper size of Supply and Distribution piping.

I computed all the fixtures listed above and the total "Fixture Units" for this structure is 54FU. This would require a 1-1/4 Supply line, and the main Hot and Cold branches should be 1".

When connecting two or more water heaters in a bank it is most fuel effective to connect them in series. In this manner during periods of low demand when the first unit can meet the demand the water simply passes through the secondary or tertiary units without causing the burners to come on. In instances of peak demand when the primary unit cannot meet the load the water will enter the next water heater below the set point and the burner will kick in to add additional heat to meet the load.

When selecting your water heater one problem you will encounter is that most water heaters of 70gal or less only have 3/4" inlet & outlet ports but your system requires a 1" primary branch line. In order to get the required 1" ports you may need to select an 85gal+ water heater or a commercial high volume water heater. (Installing a water heater with a 3/4" port would have the same effect as installing a flow restrictor.)

The alternative would be to install two water heaters in parallel. (two 3/4" lines have approximately the same volume as one 1" line.)

(1 x 1) / (.75 x .75)= 1/ .5625 = 1.77

Keep in mind that the 54fixture units for the structure is the total combined load if all faucetts were open at the same time. Generally when a single family dwellling has multiple bathrooms such as yours we can derate that demand by 40% for actual maximum working load.

One gallon of water occupies 231cu.in of volume.

The cross sectional area of a pipe is determined by multiplying the Pi x the radius squared.

To determine the volume of a pipe we must multiply the cross sectional area x the length.

Now let us consider a 1/2 diameter pipe.

The diameter is 1/2", therefore the radius is 1/2 of the diameter or .5 / 2 = .25" thus the cross sectional area equals:
3.1416 x (.25 x .25)=
3.1416 x 0.0625 = 0.1963sq.in.

Dividing the volume of one gallon of water, 231cu.in, by the cross sectional area of the pipe equals:

Thus one gallon of water equals 231 / 0.1963 = 1176 linear inches of pipe.

Dividing 1176 linear inches by 12" per foot we can then say that 98 linear feet of 1/2" pipe will contain one gallon of water.

Now if you have a 2.5gpm shower head we could then compute the time required to bleed off one gallon of water as 60seconds per minute divided by 2.5gal/min.

60 / 2.5 = 24 seconds.

Now 24 seconds doesn't seem like a very long time, but when you are standing naked under a stream of cold water that can seem like a lifetime and in those instances where someone has erroneously run a 3/4" line from the water heater to the shower mixer the time is doubled therefore the delay is 48seconds which then makes for a rather rude eye opener for your morning shower.

Now, while the toe operated faucett would certainly provide a solution, in my opinion it would be far from the best solution. A recirculation pump is by far a much more practical solution to the problem because it doesn't waste any water.

Many people are reluctant to install the recirculation pump because of the perceived notion that it wastes energy, however this is not true, in fact a properly installed recirculation pump will actually save energy.

The most effiecent method of installing a recirculation system, and the method that we are required to install it in new construction is to install a dedicated return line from the furthest point of hot water demand back to the water heater. The pump is then generally installed on the return end of the return line near the water heater and the code requires that all hot water lines and the return line must be insulated. Most people are under the illusion that the pump runs full time, which is the manner that it is often done in large commercial systems, but for residential applications the more practical solution is to install a thermostat on the return line. We then set the thermostat for about 10degF less than the output temperature of the water heater. Typically the water heater is set for 125degF so the return pump thermostat is set to turn the pump on when the line cools to 115degF. Now let us assume that you have a 90' run from the water heater to the furthest point of hot water demand. This means you have 90' of supply piping and another 90' of return pipe plus the short risers from the top of the water heater to the basement ceiling and another up to the shower mixer, so from the previous computations we can readily see that the pipe loop contains about 2 gallon of water. The pumps move about 4gal/min so it would only require about 30seconds to move the necessary 2 gallons of water. If the lines are properly insulated this action only occurs about once or twice per hour for a total operating time of 30 seconds to one minute per hour. (For uninsulated piping in a colder climate the pump may cycle 3 to 4 times per hour or a total of 2minutes per hour.)

Let us consider the operating cost of the pump. Typically the pumps are 1/24horse power and 760watts of electrical energy equals one horsepower of mechanical energy, so the pumps actualy draw 760/24 = 31watts or about 1/2 the wattage of an average lightbulb for one to two minutes per hour.

Now let us examine the energy savings at the water heater. One BTU (British thermal unit) is the amount of energy required to raise one pound of water one degree of fahrenheit.

We must remember that for every gallon of water we take from the water heater we must supply the heater one additional gallon of water. If we were to open a faucett and let the water run down the drain it would then pull and equal amount of cold water into the tank.

The water is typically entering the tank at about 45 to 55degF and we heat it to 125degF for a total differential of 70degF. One gallon of water weighs 8.34lbs therefore we would need to apply 70degF x 8.34lbs = 583.4BTU to replace the hot water in the tank.

If we install the return loop the water is then re-entering the tank at 110degF for a total differential of 15degF. The energy loss can then be computed as 15degF x 8.34lbs per gallon = 125.1BTU for a savings of 458.3BTU per gallon.

Now if one were to be really frugal you could install a timer on the pump so that it will not cycle during the off demand period during the nite while everyone is asleep but in truth if we consider the cost of the timer versus the amount of energy actually saved the payback for the timer is then 3 to 4 years.

A dedicated return line loop system is easy to install during new construction but when retrofitting an existing system it can present some formidable challenges. In this case, they now make a small self contained pump with a builtin thermostat that can be installed by simply cutting the 3/8" supply tubes to a kitchen or lavatory faucett and connecting the pump lines by means of 3/8" compression Tee's which are supplied with the pump. You then plug the pump unit into a common 120v outlet and your problem is solved. The only down side here is that this system uses the cold water line as the return line and there is some slight loss of thermal efficiency, but here again, it is only moving 2 to 4 gal/hr.

It was soon realized that keeping the water in constant motion is a rather inefficient method because it requires energy to keep the pump running full time as well as increasing the amount of heat energy lost to radiation from both the supply and return piping.

In order to reduce the radiation losses the Plumbing codes now require that all supply and return lines on a recirc system must be insulated.

If the hot water supply lines are properly insulated the pump should only need to run about twice per hour and then only long enough to exchange the amount of water in the line, which would be one gallon or less in most cases.

Most of the recirc systems with a dedicated return line have a thermostat control on the return line to activate the pump when the water in the line cools below a set point. Both the pump and sensing element are generally installed in the utility space near the water heater. This has the advantage of isolating any operation noise from the pump to the utility space where it is usually not objectionable. Keep in mind that these pumps are extremely small and barely make any noise when running. This is also the most economical method because the hot water, which has only cooled 5 to 8deg is then returned back to the water heater where it only requires a minimal amount of energy to reheat it to the desired temperature by example, one gallon of water weighs approximately 8lbs and one BTU will raise one pound of water one degree of farenheit so the required enery is 8lbs x 5deg to 8deg = 40 to 64BTU. Allowing two cycles per hour that is 24 cycles per day for an estimated total heat loss of 24 x 40 to 60 = 960 to 1536BTU/day.

In recent years they came out with the end use pumps that are mounted under a sink cabinet near the furthest fixture and use the cold water line as the return line. When they first came out the pumps ran full time and it resulted in keeping the cold water line warm as well.

The new end use pumps have a built in thermostat and they only operate when the hot water cools below the set point. The pump then only runs long enough to pull hot water from the water heater to the pump, and as soon as the thermostat senses the hot water it shuts the pump off. The actual volume of water that must be moved is very small, by example, a 1/2" water line contains one gallon of water for each 98 linear feet of run. There is some energy inefficiency in this application because the hot water is returned to the cold line so the water heater must draw an equal volume of cold makeup water. The cold water is entering the tank at approximately 55degF. If you water heater is then set at 125degF the differential is 70degF x 8lbs per gallon = 560BTU per cycle. Estimating two cycles per hour we can then estimate the daily energy loss at 24 cycles x 560BTU's = 13,440BTU/day.