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Improving water flow by larger pipes (C) Carson Dunlop Associates Pipe Diameter Affects Water Flow
Small changes in pipe diameter make a big difference

How to get more water "pressure" or how to improve water flow rate (gpm or lpm).

This article series explains how to improve water pressure or flow rate and how to improve the total quantity of hot water available from your water heating system.

Page top sketch illustrating the effects of changes in pipe diameter, courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool firm.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Install Larger Diameter Water Supply Piping to Improve Hot Water Pressure & Flow

Effect of installing larger water supply pipes to the water heater

As we discussed at WATER PIPE CLOG REPAIR, and as we illustrate with Carson Dunlop Associates' sketch shown here, installing larger diameter water supply piping makes a large difference in the water flow rate.

In fact you can improve hot water flow in a building by replacing only part of the supply piping - perhaps that portion which is easily accessible. Installing larger water supply piping feeding the water heater may alone improve the hot water pressure and flow in the building.

[Click to enlarge any image]

Watch out:  if your hot water is provided by a tankless coil, increasing water pipe diameter may not be of much use, and it could make matters worse!

A tankless coil (and also an instantaneous water heater) is normally rated by its manufacturer as capable of increasing water temperature to a desired level only if water flow through the coil is limited to a specific rate, perhaps 5 gpm. Flowing water through the tankless coil or instantaneous water heater too fast will mean that the water temperature may be too low at the fixture.

In sum, larger diameter piping increases water pressure and flow. It does not increase the total volume of hot water that is available from a water heater. This improvement may be of most value where water pressure is poor and where water piping has previously become clogged by rust or mineral deposits.

You may regain some of this loss by insulating hot water supply piping or by setting water heater or boiler temperatures higher as well as by an adjustment at

the HOT WATER MIXING VALVE.

Impact of Changing Pipe Diameter on Water Flow Rate

PEX Clamp fitting installation © D Friedman at InspectApedia.com Question: how much water pressure will we lose if we replace copper pipe with PEX of the same nominal size

I have municipal water and a combination furnace. The bathroom shower has separate hot and cold faucets and a diverter from tub spout to shower head.

Recently I noticed that I was using less and less cold water to moderate the hot but only in the shower. The hot in the bathroom sink was still scalding so I knew it wasn't the thermostatic control valve at the furnace.

The only thing I could think it could be was the shower control unit. My thought was that even though the cold was off there was still some cold water somehow mixing with the hot. It had got to the point where you could comfortably take a shower just in hot water while the hot in the sink would scald my hand after a second.

The only logical explanation I could come up with was a failing control as suggested above.

So I have cut out the old unit and replaced it with a Glacier Bay (Home Depot) similar unit. I also replaced the cut out copper with pex and used shark bite fittings to complete the job.

Well, the hot water problem has gone but now I have lower pressure from the shower. I haven't changed the shower head itself, just a few feet of copper to pex, shark bites instead of soldered unions and the fitting itself. I can't say if the tub spout has also been negatively affected.

Do shark bites or pex piping reduce flow ?

Could the new fitting reduce flow, after all, it's a cheapo from Home depot ?

If the water is too hot, should I adjust the thermostatic control at the furnace so I can turn the hot water up in the shower and so increase the volume at the shower head ? That said, it was higher pressure in the old unit so where has that pressure gone ?

Reply:

Shark Bites won't make a notable difference in flow rate; they very slightly crimp the tubing. But as we'll discuss, the smaller ID of PEX tubing may constrict water flow up to ytour shower head.

If the internal diameter of the Pex piping were as large or larger than the piping that you removed and I would not expect it to make a difference in the flow rate. But of course we know PEX has a smaller ID than copper of the same nominal size.

Generally, before ripping out the PEX to go to the next larger size you may want to be sure other easier obvious fixes have been done.

Have you checked the shower head itself? Often mineral scale or debris clog the shower head; cleaning that may be enough.

Reader follow-up:

2020/01/02 Will said:

I have not, it didn't really make sense to me that when changing the fitting the shower would lose pressure because of a part I didn't change.

I'll put a new one on tonight and report back :-)

Also, PEX 1/2" seems to maybe have a slightly smaller internal diameter than copper 1/2" due to the thickness of the pipe wall.

Moderator reply: Calculating the reduction in pipe cross-sectional area when changing from Copper to PEX = Reduction in Water Flow Rate

Will

Thank you for asking a great question: how much difference to water flow does changing from Copper to PEX make if the nominal pipe sizes remain the same.

At WATER PIPE CLOG REPAIR we explain that as illustrated with Carson Dunlop Associates' sketch, installing larger diameter water supply piping makes a big difference in the water flow rate.

I am (sorry to say) far too familiar with the effects on water flow rate (popularly called "water pressure") of reductions in the diameter of a supply pipe, thanks to a lazy local Poughkeepsie plumber who used 1/2" instead of 3/4" ID PEX on a job.

That reduction in diameter when changing to PEX or when including ANY PEX in the piping run can show up as a complaint of a weaker shower flow just as you’ve suggested.

In general, the greater the percentage of smaller diameter piping in a water system the greater the reduction in flow rate, all other factors (such as pressure, total piping length, number of elbows, valves, etc. ) remaining the same).

The ID of copper pipes is ROUGHLY 1/8 less than the OD. But the actual copper piping inside diameter (ID) number varies as across types L and M the OD stays the same.

"Type K tube has thicker walls than Type L tube, and Type L walls are thicker than Type M, for any given diameter. All inside diameters depend on tube size and wall thickness." - copper.org cited below

Percent decrease in cross sectional area going from copper to PEX

Percent decrease in cross sectional area going from copper to PEX is about the same as the percent reduction in water flow through the piping, if all other factors are kept equal:

Pressure Drop When Using a Smaller Pipe Size

sketch of water pipe diameter effectsPexuniverse gives nominal pressure drosp in psi per 100 ft. of tubing length for several flow rates from which we excerpt below.

At Flow Rate of 1 GPM

This is very significant as it illustrates that a reduction of 1/4" in the ID of PEX going from 3/4" down to 1/2" tubing will give a net change in pressure drop of (1.70 - 0.34) or 1.36 gpm over a 100 foot run (all else unchanged).

That's a 400% greater pressure drop per 100 ft. of run when going to one nominal smaller pipe diameter smaller. The numbers for copper piping pressure drop as diameter changes will be similar.

Ilustration courtesy of Carson Dunlop Associates discussed further at CLOGGED SUPPLY PIPES, REPAIR

Research on Plumbing Pipe Size vs Flow Rates

 


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-12-28 by InspectApedia Publisher (mod) - Calculate how much improvement acheived by replacing part of my piping with a larger size?

@Mostly replacing small with larger pipe,

I think that the improvement you'll see from replacing just a percentage of the total pipe run with a larger diameter pipe is equal to that percentage (of pipe length) x the increase in flow rate if all of the piping were at the larger diameter.

If I understand your note correctly, you're replacing about 6 metres out of a total of 9 metres; that's a 2/3 or 66% replacement.

So if you have already found that you expect a 300% flow improvement if all of the pipe were replaced, I think replacing 2.3 of it gives you about a 2 x improvement in flow rate. (66% of 3).

On 2023-12-28 by Mostly replacing small with larger pipe

I have a tank of water based product, only under its own pressure, with a 15mm speedfit pipe at the bottom, travelling about 3 metres, that is redirected where needed, no more than 6 metres.

It is 2 metres tall when full. If I cut the 15mm (roughtly 12mm internal diameter) back as far as I can and replace with almost 5.90 metres of 19mm internal diameter hose, will I get much of a flow rate boost?

Calculations of free flow with a completely renewed pipe run show almost a 3x flow rate increase. even a double flow rate in litres would be good.

Will the small constriction of the old 12.5mm bore speedfit make much of a difference to the calculation of 3x flow? I only need a rough answer please.

On 2023-07-26 by InspectApedia DF (mod) - diagnose a problem with house water flow rate by looking at pipe sizes

@Roger Mulligan,

As you'll see above on this page, even a small change in water pipe diameter makes a big change in flow rate.

We had a plumber who, while I was away from the job, installed a smaller diameter pex bringing water to a second floor bath that is only a short distance above the water supply source. The flow right there is noticeably worse than in other parts of the building.

Unfortunately we didn't spot it until after the piping had been enclosed with spray foam insulation and then sheet rocked in a ceiling so replacing it would be a costly ordeal. An option would be to install a booster pump or to live with the reduced flow.

You'll see in this article that replacing any accessible smaller diameter piping with a larger size will help the overall flow rate so you might get that a try.

On 2023-07-26 y Roger Mulligan

I am trying to determine if there is a problem with my house water flow rate - in the mech room basement, there is a 1" pex line that is T'd off of the 1-1/4" Pex line from the PRV. The flow rate of this line is about 15 gpm at 60 psi

This 1" line tine T's off to 3/4" lines running to various parts of the house - The master bath is some 100' of line from the mech room feed with 3/4" then reduced to 1/2" to feed the Tub filler (from beneath the sink) . We are only getting about 5gpm at this point.

(this is two floor up as well - with a measured drop of 10 psi from the basement mech room ie. it is 50psi on the top floor and 60psi in the basement)

this seems to be a very large drop in flow rate from 15gpm in the basement mech room to 5gpm two floors up all apparent due to 10psi pressure drop and pipe reductions and resistive losses, as well as likely a dozen pex 90 degree fittings.

Does this all seem reasonable? no sure how to calculate if this is the expected result. The 1/2" pex line is about 50% smaller than then 1" line so all else equal, one might expect 7.5gpm. so maybe there is another 2.5gpm loss from the two floors of head (10psi loss) and frictional losses...

On 2023-04-24 2 by InspectApedia Editor (mod) - yes you can install a smaller diameter pipe to balance water flow

@Dave,

Thank you, that's a helpful question on the relationship between water "pressure" and pipe size.

No, reducing the pipe diameter will not improve the "pressure" you see at the end of the line to your garage.

Pipe engineers explain that increasing all or even just part of a stretch of water pipe to from smaller up to a larger diameter will always give you better total flow rate - in gpm - at the end of the line. That "flow rate" is what normal people mean by "water pressure".

It's the pump that produces the "pressure" that's forcing water through the piping, not the pipe size, but it's the larger pipe size that gives more flow rate or "water pressure" at the end of the line.


So to IMPROVE the flow rate or "water pressure" at your garage you want to look for any obstructions or constrictions in the piping that you can eliminate, such as un-necessary elbows, valves, or other clogs.

Beyond that, for really great pressure at the garage at the end of what's a rather long run, you could

- increase the 100 ft of pipe to a larger diameter - say 1 1/2" -
or maybe less costly if you've got electrical power at the garage,

you could

- install a pressure booster pump such as the Scala 2 that does not need a separate water pressure tank.

On 2023-04-24 by Dave

Hi, i am on a well water system ,

I have a low pressure issue on a line going to my garage
-well pipe is 1" id
- house piping is 3/4 " including one garden hose which has lots of pressure and sprays very strong.

- the problem is the line going to my garage , this line begins from the 3/4 "
House line, then expands to 1 " pipe for 100feet then reduces to 1/2 " final for a sink and garden hose, this line has very low pressure ,

i think the solution is to reduce the 100feet of 1 " to 3/4 " , would this be right ? Thanks !

On 2023-01-08 by Anonymous

@InspectApedia (Editor), Thanks for the reply!

On 2022-12-19 by InspectApedia (Editor) (mod)

@Anonymous,

Latest update (prior to our comments today) was in July 2022, original work goes back to the mid 1980s

Happily the science of calculating pipe or duct flow rates has not changed.

On 2022-12-19 by Anonymous

What date was this article published in?

On 2022-07-08 by InspectApedia-911 (mod)

@Brendan,

I wish for commenting, I'll review the article to see how we can clarify the text.

On 2022-07-08 by Brendan

@Brendan, Hahaha, i just realized you were using radius lol, my mistake

...

In one of the images you use, you are denoting the change in area from a 0.5 in pipe to a 0.75 in pipe however, you don't actually show this. For some reason you seem to use 0.375 which doesn't make much sense...

On 2022-03-28 y Inspectapedia Com Moderator (mod) - working to recycle hot water to reduce loss and get true "instant" hot water

@Anonymous,

The Bosch in Mexico is plumbed all with copper; at a similar installation in Minnesota recently we plumbed the building in PEX but ran copper into and out of the on-demand water heater, as per the manufacturer's advice: you need some distance - maybe 18" of copper near the heater - varying by brand and model - before going to PEX.

The pressure boosting pump is for the entire building's water supply system as otherwise the gravity-feed is rather modest;

It is critical to match the instant-water heater model to the water flow rate, that is, Bosch makes different heater models for different flow ranges; a gravity-fed water instant water heater expects an incoming cold supply that runs at a lower pressure - flow rate.

If you feed that heater under pressure it can't keep up. Conversely, if you feed a gravity-only water supply into an instant water heater designed for higher flow rates it'll overheat or shut down.

Your plan sounds interesting and thoughtful and probably right but honestly the best step would be to confirm your design withe the heater's manufacturer's technical support engineers. They'll probably be thrilled to hear about your installation.

You may already know that the usual way buildings provide instant hot water - such as in hotels - is by a hot water circulating system that keeps hot water in the whole hot water loop all the time.

On 2022-03-27 by Anonymous

@Inspectapedia Com Moderator,
Interesting. I have been working to recycle hot water to reduce loss and get true "instant" hot water when I first turn on the shower in the morning.

I tried a pressure sensing pump at the tankless heater with a thermal shut off valve at a remote vanity. Worked OK but not what I was hoping for. I found a PROZRTED pump on Amazon which has a builtin thermal cutoff and wireless remote activator.

Worked really well. Had to add a solenoid valve on the cold water side to make it work properly. It is now quite amazing as I can get hot water in the shower in a few seconds. I still have the pressure activated pump and am now thinking of reinstalling it.

How did you plumb your Bosch tankless, copper or pex? I have a Honeywell/LAARS combi. It states that DHW should have 3/4 piping in and out. I am using 3/4 PEX which has an ID more like 5/8, smaller than 3/4 copper.

Was your boost pump on the inbound side of your Bosch or on the outbound side? Mine was on the outbound side. The smaller ID input to the DHW makes me concerned that I might be "starving" the heating elements of the DHW as the input rate of flow is less because of the PEX inbound.

I have 1' PEX that I used for heating (approved for drinking water). The ID of this is about 7/8" so the volume into the DHW boiler would optimum. Adding the pump on the outbound side of the DHW using 3/4 PEX should be no problem.

The increased hot water flow should take care of my problems. Does this sound like good thinking? Not cheap but I have all the necessary parts to do the job sitting around.

On 2022-03-21 by Inspectapedia Com Moderator (mod) - reduce the affect of the washing machine on my shower.

@Phil,

That's an interesting idea, but in my OPINION because your data is incomplete, lacking pressure and flow measurements, and because the effect of other used of hot water on hot water delivery at a shower head is affected by other variables, even the individual shower head design AND just how much hot- and cold- water is turned ON at the shower vs. at other fixtures (that is, how far the fixture valves are open),

because of all that the result of your plan is too unpredictable and un-stable to be worth the trouble.

Simplest is to simply stop the washer from filling during any time that someone is in the shower.

We do that in home in NY by just pushing in the washer control to turn off that appliance until use of the shower is finished.

At our office in Mexico we installed a gas-fired tankless water heater, a Bosch model, and a water pressure booster pump.

As additional users of plumbing fixtures open the supply valves, the booster pump actually ups the delivery flow rate in the system to meet the demand, and when hot water is running, the tankless water heater itself monitors the water flow rate through the water heater, increasing or boosting the flame, and thus the BTUh input rate at the water heater to meet the added demand.

In that installation if someone turns on hot water at a second, or even a third or fourth fixture when someone is in the shower, both water pressure AND hot water heating rates are automagically increased. The result is very good: the person in the shower isn't hit by a shot of cold water.

But that's a pretty expensive solution that I'd only recommend if you're already considering changing how your hot water is delivered.

On 2022-03-21 by Phil

I am hoping to reduce the affect of the washing machine on my shower. The supply line main is 3/4" pex now with 1/2" pex "T"s to all fixtures. The shower is in line before the washing machine. I have an 11' run that I can affect.

If I increase the HOT main from 3/4" to 1" for 8' before the shower T (1/2" to shower) followed by 3' of 1" that is then decreased back to 3/4" just before the "T" to the washing machine have an effect? The pressure will be reduced going into the 1".

But the volume will be increased over the 11' of the 1" run. The 1/2" T up to the shower will increase the pressure (?) to the shower.

The greater volume with additional 3' of 1" between shower and restriction to 3/4" which quickly T's to 1/2' to the washing machine reduces the overall effect of HOT water draw when the washing machine line is opened to fill the machine. Or do I just tell my wife not to use the washing machine while I am in the shower?

On 2021-04-28 by danjoefriedman (mod) - will going from 3/4" pipe to 1/2" for a foot back to 3/4" decrease the GPM vs the whole stretch being 3/4"

@jay,

Yes, as you'll read in the article above - but note that the total length of reduced- or enlarged- water pipe diameter is very important in assessing the total effect of that change.

On 2021-04-27 by jay

will going from 3/4" pipe to 1/2" for a foot back to 3/4" decrease the GPM vs the whole stretch being 3/4"

On 2020-05-18 by Gowtham

I need a fogging calculations . With 14 por 12 fogging nozzels 80 littes of water has to be consumed for 9 hours.which motor and nozzels are adviced .
Presently i used half hp submercible pump 6mm numatic pipes and 0.2mm 7 nozels.its consuming 80 ltrs of water by 6.30 hrs.
I have done many experiments on this .now i want to change to 1/4 hp motor .i thought this may increase more time.

Can someone suggest me.
Iam doing it for disinfection tunnel.
Disinfection liquid is costly so iam trying it do a fog .

On 2020-02-10 - by (mod) -

Will

Thanks for the follow-up as that will help other readers;

On 2020-02-10 by Will

OK, so I finally got around to at least changing the shower head for the cheap plastic one that came with the fitting. Before putting it on I drilled out the restrictor and it's made a huge improvement. So perhaps the PEX isn't an issue after all :-)

On 2020-01-07 - by (mod) -

If you take a look at the explanation above on this page you'll see the effect of replacing part of water piping with a larger diameter.

Let's take it step by step doing the easy things first as we've been discussing.

On 2020-01-07 by Will

You said "About going to 3/4" copper throughout, that gives much greater delivery rate of water but may not be necessary; As Carson Dunlop's sketch on this argues, often replacing just the accessible part of supply piping with larger diameter makes a big difference"

Does that statement suggest that if I take the 1/2" hot water supply from the furnace and a couple of feet away from that connection to the furnace I convert the 1/2" to 3/4" I will see a noticeable difference if I keep that 3/4" all the way to the shower fitting where it has to go back to 1/2" to connect to the shower fitting ?

Doesn't the fact that the supply from the furnace starts out as 1/2" cause a bottleneck that isn't corrected when the diameter increases further down the line then decreases again at the fitting.....or doesn't water pressure/flow work that way ?

Meantime, I'll remove the restrictor, hadn't even occurred to me. Thanks

On 2020-01-06 - by (mod) -

Will

You can also stay with PEX but go to the next larger size, with proper adapters to connect it to your existing piping.

About the shower head: it is probably a low-flow shower head as most are currently; at low water supply pressure systems where going to a low-flow shower head gives unacceptable water flow for bathers, I either pull out the restrictor in the shower head (usually it's right in the screw-on fitting) or if necesssary I drill out the opening into the shower head at its connector.

About going to 3/4" copper throughout, that gives much greater delivery rate of water but may not be necessary; As Carson Dunlop's sketch on this argues, often replacing just the accessible part of supply piping with larger diameter makes a big difference. I'd go step by step.

On 2020-01-06 by Will

And, furthermore, would you recommend swapping out all hot and cold water lines for 3/4" copper ? I might have a problem doing that with the hot as it comes from a combi furnace and I believe the only hot outlet is 1/2" so making it 3/4" a foot from the 1/2" outlet and then maintain 3/4" all the way to the faucets/shower would be a waste of time, correct ?
In any event, the shower fitting itself is 1/2" so wouldn't that create it's own bottleneck anyway ?

Thank you for the assistance and the subsequent comments on the theory that the PEX is causing a lower pressure at the shower head. I have now since swapped out the shower head for a brand new head, albeit low budget, from Home depot.

If anything the pressure is even lower than before. I have to assume that the PEX is causing a reduction in volume which manifests itself as low pressure at the shower head.

I guess I'm getting some soldering practice in the near future. Low shower pressure is a pet peeve of mine.


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