The Bosch Aquastar125 series included the 125NG (natural gas) and the 125LP (liquefied petroleum).
Although the 125x series is now discontinued I did manage to find a downloadable copy of the installation manual in PDF format at:
Tankless Water Heaters from Bosch, Takagi Flash TKJr, TK3, T-M1 - fueled by Gas & Propane and Powerstream, PowerStar and Ariston Electric Tankless and Electric Mini-Tank Hot WaterHeaters and On Demand Water Heaters
In reviewing the manual I came across a number of details that would warrant attention.
PERFORMANCE:
The AQ125xng is rated for 3.7gpm @ a 45degF rise. Assuming you desire the maximum code rated hot water temp of 125degF the incoming cold water would have to be 85degF to achieve the maximum flow rate. This might be practical in mid summer in the Sunbelt where the water lines are buried rather shallow and the soil is heated by solar energy, however, in Northern climates where the water lines are buried a minimum of 6 below the average frost depth in mid summer we can expect the cold water to come in at the earths natural geothermal temperature of 50-55degF. On the other hand, in mid winter we can expect the incoming cold water to range in the 35 to 40degF range. With an incoming temperature of 35degF and a hot water calling temperature of 125 we have a 90degF differential, and at 90degF differential the AQ125xng is rated for a maximum flow rate of 1.8gpm. This may be okay for those such as myself who enjoy a moderate shower at around 90degF but for those who enjoy a very hot shower 1.8gpm will not meet the needs of even a water saver shower head (2.5gpm), not to mention the cold spike you that would occur if someone were to start the dishwasher, laundry or even wash their hands at a lavatory in another bathroom.
VENTING: (See Local codes)
The venting must be in compliance with the national gas code and all local codes for a category 1 appliance rated at 116,000btu.
Requires Type B (double wall) vent pipe.
The minimum vent height must be 6 provided there are no elbows and the point of termination must be 2 above any obstruction within a 10 radius.
When a horizontal run of vent is required, the vent must have as much vertical rise as possible (minimum 12) before turning horizontal.
All horizontal venting must have a Ό per foot pitch upwards towards the exterior stack.
The maximum length of a horizontal run may not exceed 100% of the total vertical rise. (When a horizontal pipe is used to exit the structure, the length of the vertical riser stack on the exterior must be equal to or longer than the horizontal length.)
The vertical riser must terminate at a point at least 2 higher than any obstruction within a 10 radius of the point of termination and the top of the riser must be equipped with an approved weather cap.
GAS SUPPLY (PIPE SIZE):
The AQ125xng may be supplied by a ½ black iron pipe, provided the maximum developed length of the pipe from the gas meter to the water heater inlet does not exceed 10. If the maximum developed length of pipe from the meter to the water heater is greater than 10 but equal to or less than 40 it may be supplied with a Ύ gas line and if the maximum developed length is greater than 40 but less than 150 it requires a 1 line.
This is assuming that the AQ125x water heater is the only appliance attached to the gas line.
When we have multiple appliances on a common gas line we must first total the BTU requirements of all gas appliances, then convert the BTU requirement to cubic feet of gas per hour.
By example; let us assume a house that has a gas furnace (150,000btu), A freestanding gas range in the kitchen (35,000btu), a gas laundry dryer (35,000btu) and the AQ125xng (116,000btu).
The total BTU requirement for the structure is then 336,000btu. The codes state that we are to then contact the local gas supplier and get the actual BTU/cu.ft value of the gas, but in instances when that information is not available we may use the average value of 1100btu/cu.ft
Dividing the total BTU requirement (336,000btu) by 1,100btu/cu.ft we get 305cu.ft
We must then compute the total developed length of pipe from the gas meter to the furthest appliance.
NOTE: The total developed length of the pipe is the actual length of the pipe including rises and drops plus the fitting insertion losses, which are expressed in lengths of pipe. By example, a ½ black iron pipe 90 elbow has an insertion loss equal to .8 (9.5) of pipe.
For illustration let us assume that the total developed length to the furthest fixture is 70. Per IRC table T2413.4(1), With a load of 305cu.ft and an overall length of 70 we would need to begin at the gas meter with a 1 pipe and continue running 1 until the first appliance tees off the line. From that tee we would deduct the BTU rating of that appliance, then compute the balance to cu.ft as before and determine the maximum developed length from that tee to the last fixture to determine the size of pipe from that tee to the next fixture tee. We then repeat the process again and again until we have sized all the pipes by the Cu.ft table.
MAINTENANCE REQUIREMENTS
Per instructions in the AQ125xng installation manual we must inspect the Main burner, Pilot Assembly, Water screen assembly, and vent assembly annually. The water valve must be lubricated at least once every 2 years and the water valve must be rebuilt one every 3-5years,