3/4" meter and 3/4" line to existing house, hooking up a new 1.5" inch line for new construction. Adverse effects on main house?

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vickyseattle

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Hi everyone, I am currently in the process of building an additional 2bath house on my property. Current house is a 1bath unit with 18 WFSU. We have a 3/4" meter with a 3/4" line to the existing house with 80 psi pressure at the first hose bib.

New addition: The GC is planning to tie in a 1.5" inch line to the existing 3/4" line before it enters the existing house, and take this 1.5" to the new contruction about 100 feet in. Then it will reduce to 1inch in the additional house. The new construction will also have 18 WFSU.

My questions are:
1. With a 1.5" line added to the existing 3/4" meter and 3/4inch existing house line, will we see an adverse impact to the current house water flow? Will the magnitude of this impact be a lot?
2. Can a 1.5" line be added to the current 3/4" line? The WFSU charts show a max of 1 1/4" line for fixture count calculation.

Thank you for your time!
 

Jeff H Young

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Sure you can add as big a branch yoiu want only issue is if you have a high demand on that branch in which case its a no brainer of cource it will rob the house of water but as long as the 1 1/2 line isnt running no effect.
Im assuming you are governed by building codes you need to follow those as well
 

Jeff H Young

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reading it over again . It might be a good idea to replace the entire water main and possibly resize your meter , you can get away with a lot and its not part of the scope of work in the end the homeowner pays for everything so maybe he t rying to keep the price down so you can enjoy the addition and you can run a new main later, if wanted.
 

John Gayewski

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18 fixture units is pushing it for 3/4". Adding another 18 units will most likely cause a problem as you now have 36 units supplied by one 3/4" pipe. You need to go back to the main and run a bigger pipe to feed these.
 

Jeff H Young

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Actualy took another look why would he run 1 1/2 to a 18 F.U. house? anyway the answer is still the same 2 houses on a 3/4 that you plan is undersize
 

vickyseattle

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Thank you everyone for your inputs. I used this chart to come with the total load calculations: https://kingcounty.gov/en/dept/dph/...ping/installations/water-supply-fixture-units
It does say with 3/4 meter and 1 1/4 supply line, the water flow should be fine for up till 39 units. We made sure new house has a 1 1/2 supply line, which tapers to 3/4 in house. But biggest concern is for the existing house which has a 3/4 supply line. Now the work is almost done so let's see what the impact is, will update in about 3 weeks.

Actualy took another look why would he run 1 1/2 to a 18 F.U. house? anyway the answer is still the same 2 houses on a 3/4 that you plan is undersize

The reason for a larger diameter is to reduce the pressure loss as the run up is 150 feet from the meter to new house. We branch off right at the meter.
As long as water flow to existing house isn't affected much, I think this should work out. Worst case scenario would be to update the water main to existing house. Lets hope it doesn't come to that.
 

John Gayewski

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Thank you everyone for your inputs. I used this chart to come with the total load calculations: https://kingcounty.gov/en/dept/dph/...ping/installations/water-supply-fixture-units
It does say with 3/4 meter and 1 1/4 supply line, the water flow should be fine for up till 39 units. We made sure new house has a 1 1/2 supply line, which tapers to 3/4 in house. But biggest concern is for the existing house which has a 3/4 supply line. Now the work is almost done so let's see what the impact is, will update in about 3 weeks.



The reason for a larger diameter is to reduce the pressure loss as the run up is 150 feet from the meter to new house. We branch off right at the meter.
As long as water flow to existing house isn't affected much, I think this should work out. Worst case scenario would be to update the water main to existing house. Lets hope it doesn't come to that.
This isn't going to effect one house or the other is going to affect both. From what your describing your getting your piping segments in the wrong order. Serving one house with a larger pipe doesn't effect things backwards and make the pipe serving both of them need to be smaller.
 

Jeff H Young

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I guess if you are digging a trench might as well put the right size new pipe in the trench, you dont undersize piping just to match existing
 
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