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Water Pressure Fluctuations

1265 Views 7 Replies 4 Participants Last post by  joecaption

Just moved into a new property where the house is approx. 70 yrs old.

I have an issue with the hot water where if I'm running the hot water on the main floor (sink/kitchen/shower) and the cold water tap is opened in the basement (laundry/laundry tub), then the hot water pressure cuts out to practically no flow.

I have 1/2" copper coming from my the bladder tank from the cistern out to all branches of the house. All plumbing is copper, however it is pretty old and there are a number of valves in the runs.

Any ideas what could cause this and how to fix?
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First of all: 1/2 inch for a main is ludicrous.

Second: what is the incoming pressure?

Third: Well or City?

Fourth: How high is second floor above incoming line?

Sounds like a generally poorly designed system in dire need of major upgrade to me, but impossible to help due to lack of info.
Main runs needed to be at least 3/4.
3/4 has close to double the flow of 1/2".
Also is there a pressure tank?

Old style gate valves can also have issues. There prone to not being able to shut off fully, stems can snap off inside the valve so the gate can not move.
Thanks for the responses!


First of all: 1/2 inch for a main is ludicrous. Agreed, would like to upgrade to pex and remove all valves/elbows possible.

Second: what is the incoming pressure? The cistern pressure tank is pressurized around 28 psi, and the pump kicks on when the gauge drops to 30 psi and shuts off at 50 psi

Third: Well or City? Cistern

Fourth: How high is second floor above incoming line? Standard height with 8 ft basment ceiling. Plumbing is run through the joists as typically done so approx. 4 ft or so from lines to the fixtures.


Yes there is a pressure tank as noted above. Is there any benefit to running 1" lines? I would like to do pex if possible to save in labour/material cost.

For the water heater line, would I run 3/4" cold directly from the cistern to the water heater, then 3/4" coming out of the water heater as a main trunk with 1/2" branching off to each fixture?
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Has Cistern been cleaned? Check my previous post re Cistern. Basically, You need bigger main lines.
Thanks - That's what I figured.

I planned on upgrading the plumbing it seems pretty messy at this point. Just a few quick questions;

1. Would it be best to run 3/4" trunks or 1" trunks for 1/2" branch off? Or 3/4" to each fixture then reduced to 1/2" to go into the fixture?

2. For optimum hot water distribution, is it best to run 3/4" from main trunk into the hot water tank, then 3/4" hot trunk out to 1/2" branches? What about a manifold for the hot water supply? Better service/pressure?

3. When running pex to fixtures, I have seen that the pex is fixed to copper lines for the last 1-2ft before it is connected to the fixture. Is this code? I would assume it is for the waterheater...

Copper pipe is not immune from arteriosclerosis after decades of use.

This can cause drastic drops in pressure when more than one sink/shower/etc. is in use at the same time.
You use one of these where it comes out of the wall so the valve is not just hanging from a flexable piece of tubing.

Also you can then just use a standard 3/8 X 1/2 compression stop.
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