There is another sizing misconception that we would like to shed light on today regarding NFPA 13D pumps. Many times, when designing and building a development or series of houses in the same area, a pump calculation from the first house will be used to size the pumps for all of the houses - the reasoning is that the houses are all similar size and design. This reasoning can lead to many problems including undersized pumps, a non-functioning system and a big waste of money!
One way the pump size can change from house to house within the development is if the development is built on a hillside. It is very possible that the change in elevation from the house on the bottom of the hill to the house on top of the hill is enough of a change to cause a difference in water pressure coming into the house. As we stated last time, pressure is a big factor in correctly sizing a pump - where it changes so does the pump.
Another way the pump size can change from house to house can be found in the building options. Let's say that the model house in the development is built using the standard design plans and the residential pump is sized correctly for that design. However homeowners of the next house built in the development decide on a vaulted ceiling option for their living room. The sprinkler system design is now changed and it is very probable that the pressure requirements of that system have changed as well. This can mean that the pump needs to be re-sized as the pump that worked in the model may not meet the demands of the home with the vaulted ceiling. Conversely, the pump could be over-sized and money can be saved by purchasing a smaller model!
In summary, don't assume - calculate! Very often the pumps will be identical from house to house in the same development. But just because the houses are next to each other does not mean that they always have the same pump at the heart of their residential sprinkler systems. The best way to proceed is to contact the residential fire sprinkler pump experts at General Air Products!