[LazyPup]

A boost pump will not resolve your problem.

Under the IRC (International Residential Code) a boost pump & pressure tank is to be installed only when the municipal supply “Static Head Pressure” to the structure is less than 40psi. Ref:IRC-2903.3

Under the UPC (Uniform Plumbing Code) a boost pump & pressure tank is only to be installed when the municipal supply static head pressure is less than 15psi - ref: UPC-608.1 (20psi when the shower has a pressure balance control.)

A pressure-reducing valve is required whenever the static head pressure exceeds or is likely to exceed 80psi.

You post states that water enters your house at 45 to 50psi but that is not true. The static head pressure is 45 to 50psi however it must be noted that “Static Head pressure” only exists when there is no flow, hence the term “Static”. The moment that flow begins the static head pressure immediately drops to “Dynamic Head Pressure”; or what is commonly referred to as “Working Pressure”.

“Dynamic Head Pressure” is Static head pressure less “Friction Head Loss” and “Vertical Static Head Loss”.

“Vertical Static Head loss” is the opposition to flow in a vertical riser caused by the physical weight of the water in a vertical column and VSHL can be computed as 0.434psi/ft of vertical rise from the physical source of pressure or the “Prime Mover” to the physical elevation of the outlet. (In the case of a municipal water supply the prime mover is the municipal water main).

Example. Let us assume the municipal water main to be 10’ below grade and you have a bathroom faucet on the second floor, which is 15’ above grade. The vertical differential from the prime mover to the faucet would then be 25’ therefore the Vertical Static Head Loss would be 0.434psi/ft x 25’ = 10.85psi loss.

While we were able to use a constant to compute “Vertical Static Head Loss”, computing “Friction Head Loss” is not so easy. Friction head loss is the physical loss of pressure, which results from friction between the flowing liquid and the interior pipe wall. Obviously there is no friction when the liquid is standing still but the moment that any faucet or other outlet is opened and flow begins, the liquid passing through the pipe exhibits friction loss from rubbing the interior of the pipe, and the faster the rate of flow (velocity of flow) the greater the Friction Head Loss. In addition to the problems associated with pressure drop in the system we must also consider pipe wall erosion. In copper pipe, pipe wall erosion will occur when the velocity of flow exceeds 10ft/sec and in plastic pipes erosion occurs when the velocity exceeds 15ft/sec, therefore the codes require that we must compute the volume of flow and select a pipe large enough to keep the velocity of flow under 10ft/sec in copper pipe or 12ft/sec in plastic pipe.

To determine the proper pipe size we must first make a list of all fixtures connected to a line, then we refer to the code tables to determine the required volume of flow for each fixture. From this we can then compute the total volume of flow required for a worst-case scenario where all faucets and fixtures on that line are open at the same time. Once we know the total volume of flow required to meet the needs on any line, we then consult the published “Friction Head Loss Tables” for the type of pipe we intend to use.

By example, let us consider a simple house with one bathroom group, (Lavatory, Bathtub/shower & Tank Type Watercloset), a kitchen with a sink & Dishwasher, a laundry with just a washing machine hookup and one outside hose bib.

Under the IRC we would then consult table: IRC-T2903.1 to find the minimum fixture requirements:

FIXTURE GPM Flow PSI
Lavatory………………..2 …………………………….8
Bathtub/shower……….4…………………………...…8
Watercloset……………3 ……………………………..8

Kitchen sink…………...2.5 …………………………...8
Dishwasher …………..2.75 …………………………..8

Washing Machine ……4 ………………………………8

Hose Bib ………………5………………………………8
TOTAL 23.25gpm

Let us now assume that we are installing Polyethylene SDR9 pipe for the house main water line (line from the municipal main to the structure main water shutoff).

Consulting the PE pipe Friction loss table we find that with a flow rate of 23gpm we could use the code minimum line size of ¾” pipe however if we do so, the Friction Head Loss for the house main water line would be 41.32psi/100ft. (0.413psi/ft).

Let us now assume that our house is 50ft from the municipal water main. If we were to open all the valves in the house simultaneously the Friction Head Loss in the house main water line from the municipal main to the structure would then be 50’ x 0.413psi/ft = 20.65psi. This means that if we had the afore mentioned 50psi static head, when all the valves are open the resultant Dynamic Head at the Main Water Shutoff valve in your house would drop to 50psi – 20.65psi = 29.35psi.

If we were to then deduct the vertical static head to the lavatory in the upstairs bathroom (computed above) we would have a net pressure of 29.35psi – 10.85psi vertical static head = 18.35psi at the lavatory faucet but then we would need to go back and compute the friction head loss for the pipe from the Main water shutoff valve in the basement up to the lavatory on the second floor and once we factor that in it is very likely that we could not maintain the code required 8psi minimum at the faucet.

Now I would wholeheartedly agree that it is very unlikely that you would ever have all the faucets and valves in the house turned on at the same time, but none-the-less, for design purposes we must engineer for the worst-case scenario.

For the example at hand the simple solution would be to increase the size of the house main water line from the code minimum of ¾” to 1”. By doing so we would reduce the Friction head Loss in the house main from 20.65psi to 6.38psi loss. When we consider that the cost of installing the main water line is primarily labor with only a mere fractional cost of upgrading from ¾” to 1” pipe, it then goes without saying that we should select the 1” pipe.

One of the most common problems we see in the plumbing trade is when homeowners buy an older home that no doubt has the code minimum size of water main, then they rehab the bathrooms, adding fixtures and in many cases adding additional bathrooms without giving a moments thought to the size of the main water line.

If you could post a list of all the fixtures in your house along with the estimated distance from the municipal main tap point to the furthest fixture in the house I would be glad to work out the math to see if your lines are sized correctly but in almost all cases when people have the type of pressure problems you are reporting, the problem is improperly sized lines and a boost pump will not solve the problem.