Physics of water pressure based on size of pipe

nickg5 used Ask the Experts™
If we slide a 1/2 inside diameter pipe into a 3/4 inside diameter pipe how much watch pressure do we lose. Would that cause an issue when the house was sold and the buyer inspector noticed the weaker water pressure. They don't find everything because my inspector failed to find a gas leak as well as the fact that the roof had been in a fire years before. The water heater had propane fittings when the gas was natural gas and required natural gas fittings and the inspector missed that. Maybe they and the buyer think the water pressure through half inch pipe is normal.
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With no flow, the pressure will be the same.  However, once water starts moving, the resistance of the 1/2" pipe will be more than with the 3/4" pipe, so the pressure drop will be greater.

The actual loss depends on a number of factors, primarily the length of the pipe and the internal roughness.  There are many online calculators for this.  One that appears to be handy is:
Dr. KlahnPrincipal Software Engineer
Half inch water pipe is too small unless the device being served is very low flow, e.g. a bidet.  A good inspector would catch this and the resulting offer to buy would then include "pipes shall be replaced where necessary to minimum 3/4", copper, no-lead solder or the price shall be discounted by $xxxx to reflect this issue."  I'd discount an offer at $50 per foot of pipe to be replaced.  Nor would I accept PEX or PVC.  Copper will last centuries.
It's serving a single family home with an average water bill every month of $11.xx and around 850 gallons a month. The pipe leaked 24 years ago and the line was replaced with black flexible pipe. As the tree grew it's roots have caused a new leak.
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If this is the main supply line, I wouldn't worry about pressure.  I would worry about supply.

The classic:  flush the toilet when someone is taking a shower.

Even if the inspectors are idiots, the would still probably see the tie-in of the supply line only being 1/2 inch. If you add enough foam/tape/stuff to try and cover that up, it will likely be a red flag that could draw attention.

If tree roots caused issues with the 3/4" pipe, how long until a 1/2" wrapped in a 3/4" pipe also has problems?  They only flex so much even with a wrapper.
All repairs of the existing issue will be under ground. Repair - replace, cover and let the grass grow. There is a huge maple tree that may die due to all the roots we have had to cut trying to find the existing line. We found a hole accepting water about 8 feet from the meter. Digging between there and the faucet on the front of the house has not found the existing line. We are in the wrong place or not deep enough.
The City only has diagrams of water and sewer to their meter and no onto anyone's property.

The line leaving the house will be tied into the blue flex coming out of the meter with 3/4 black. In fact the City inspector saw that the existing line is blue flex out the meter and about 2 feet from there the clamped connection with 3/4 black which was a similar repair 24 years ago.
Dr. KlahnPrincipal Software Engineer

Buried pipes should always be wrapped in a generous amount of biobarrier such as Typar so that this issue does not occur.
The leak was found about 14 inches closer to the house where the previous water hole was found. Ax and chain saw were needed to remove the large root system above the pipe which uncovered the leak. That was repaired with black flexible, 3/4 ISD. The curvature of the pipe shows myself and the other guy were digging in the wrong places but we did not know that. He worked as a plumber many years and said the line should have gone as straight as they could get to the outside faucet. It looks like it made a curve around the area between house and tree and maybe headed under the driveway to the laundry room area where the water heater is located. So no 1/2 inch inside of 3/4 and one of the guys today who removed all that root who is in construction work as well as a pastor said "copper" is not required today.
Any way the leak is fixed and where the extra 2000 gallons went from September 10 to October 9th I don't know. The abnormal water usage may have started after the 9th not sure. So over a period of 30 days or less 2000 gallons went somewhere as there was no wet ground anywhere. And toilets don't normally leak 65 gallons a day. That was eliminated days ago.

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