Household water pressure....what's safe?

asoloMay 18, 2008

Noticed increased water pressure at my residence a couple of weeks ago. Used to be 55psi. After local utility service upgrade, now entire neighborhood, including me, is getting 105-127psi. The utility says this their intended psi.

Neither I nor any of the many houses around me are having problems. However, that seems awful high to me. Considering having pressure regulator installed to bring it down.

Am I over-reacting? Is this reasonable "nominal" household pressure?

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The maximum permissible static head pressure is 85psi.

In any case where the pressure at a residence serviced by a municipal main exceeds 85psi, whether the excess pressure is constant or only ocassional we are required to install a PRV(Pressure reducing valve) limiting the pressure to a maximum of 85psi on the water supply at the main water shutoff valve location and an expansion tank in the near proximity of the water heater.

    Bookmark   May 18, 2008 at 1:41PM
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Thanks for responding.

Are you saying 85psi is from a code? (I don't know plumbing codes) Is it the city's responsibility to manage this or is it my own responsibility to install PRV for my own home?

Will be discussing with city utility tomorrow but would like to be better informed.

    Bookmark   May 18, 2008 at 3:32PM
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The 85psi maximum is from the Plumbing Code.

It is the property owners responsibility to provide a PRV when necessary.

    Bookmark   May 18, 2008 at 3:44PM
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Thanks, lazypup. I will deal with it appropriately.

Annoying state of affairs, but I'll get it handled.

    Bookmark   May 18, 2008 at 3:59PM
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----After local utility service upgrade, now entire neighborhood, including me, is getting 105-127psi. The utility says this their intended psi.----

We live 1/4 mile from water tower and same thing happened, they provided all houses on the steeet with pressure regulators and installed them. Ask and ye shall receive, doesn't hurt to ask, the most they can do is say no, but, they created this and they should fix it. IMO

    Bookmark   May 19, 2008 at 7:29AM
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Hendricus, the operative word in your reply is IMO, however you will find that in most cases our opinions have very little validity in a court of law.

By law, a municipal water service provider is required to supply a minimum volume of flow determined by the type of structure, by example, a 4br house would require considerably more volume than an office structure at a used car lot that only has one 1/2bath for their employees or a 100 unit apartment complex across the street.

They are also required to provide the code minimum static head pressure at the point of connection to the water service providers utility main, which under the IRC (International Residential Code)is 35psi, whereas under the UPC (Uniform Plumbing Code)it is only 15psi.

In nearly all cases the water service providers utility line pressure is a result of static head pressure created by gravity fall, whether the fall is from a water tower or in a hilly region it may be from a ground mounted tank placed upon a high hill in the community.

Over the years as our communities expand it places more and more demand upon the existing water infrastructure and as the demand increases, the velocity of flow in the pipes must increase to meet the demand, however as the velocity of flow increases the "Friction Head" in the pipes will increase proportionally thus the service provider may be forced to increase the line size or run additional lines and divide the region into zones. In either case the end result would be a substantial increase in the line pressure on any given line in the system.

The physical terrain conditions of the community can also play a significant role in water pressures; by example, in the community where I live we have hills which represent as much as 150' of change in elevation within the short distance of a city block. In this case the static head of a line running parallel to the street would change by approximately 65psi from the bottom of the hill to the top, thus in order to meet the local code demand of 35psi to the structures on the top of the hill the pressure on the line at the bottom of the hill would need to be 35psi required static head + 65psi vertical static head loss = 100psi.

As our communities expand it may involve zoning new construction areas up the hill, in which case the water service provider would be required to build a taller water tower to meet the code minimum of those on the top of the hill, which in turn would further increase the pressure to the existing structures at the bottom of the hill.

Now, while the water service provider is required to provide the code minimum volume and pressure at the point of connection to the service main, in instances where the resultant pressure at any structure exceeds code maximum it is the responsibility of the property owner to provide the required PRV (Pressure Reducing Valve). On the other hand, if you were to elect to build a house on the top of the hill beyond the region designated by your local zoning commission as the responsibility of your water service provider you would be required to supply your own boost pump to provide the code minimum pressure at your structure.

    Bookmark   May 19, 2008 at 9:28AM
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85PSI, might be nice for some but, when installing the PRV, you will find most standard units will max out at 75PSI when turning it up to the max.

Irregardless, 75PSI is plenty unless you desire a pressure washer shower.

    Bookmark   May 19, 2008 at 9:37AM
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"...pressure washer shower..."

Actually, that's how I noticed the difference the first time.

    Bookmark   May 19, 2008 at 10:19AM
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PRV being installed tomorrow. Thanks to all who responded, especially you, LAZYPUP. Everything you said has been confirmed by city utility and a few plumbing professionals of my acquaintance. Good on ya!

You may have saved many from future grief. There are more than a hundred houses in my subdivision. The information was new to the HOA supervisor and he was grateful to have been made aware of it. Apparently my notice of the pressure change was the first.

    Bookmark   May 19, 2008 at 1:27PM
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be advised that after the PRV is installed your house will now be a "closed" system. That means that thermal expansion from the water heater can NO LONGER expand back into the mains outside your home. There can be pressure spikes well over 120lbs/in2

That can cause premature WH failure and T&P valve discharge not to mention leaking faucets all over the house and that high a pressure might make your Kinetico very unhappy.

More than likely you'll need to add a thermal expansion tank in the cold water line at your water heater.

click here to learn about thermal expansion

    Bookmark   May 19, 2008 at 3:04PM
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Thanks for the additional info, justalurker. Actually, I was waiting on the line with PRV installer to address this issue when your post popped up. I hate this stuff but I do want to handle it once and with gusto.

    Bookmark   May 19, 2008 at 3:16PM
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No gusto required... pipe wrenches, channel locks, and solder.

The PRV will resolve the pressure problem and the thermal expansion tank is a no maintenance item.

Luckily, you caught the pressure increase. Many in my area get "improved" water service and learn about thermal expansion the hard, wet, and expensive way.

    Bookmark   May 19, 2008 at 3:39PM
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"...hard, wet, and expensive..."

Sorry to say I am personally acquainted with that way. Do not care for any repeats if they can be avoided.

Again, my thanks for fast, informative, and accurate responses.

    Bookmark   May 19, 2008 at 4:08PM
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Thanks lazypup, good answer. I still say it doesn't hurt to ASK and I never even thought of a court and lawsuit.

    Bookmark   May 20, 2008 at 10:46PM
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FYI........PRV and expansion tank installed today. They did everything. I did nothing. I wasn't even there. Licensed, bonded, insured plumbing outfit with whom I have had many previous dealings. Total for all $522.00.

    Bookmark   May 20, 2008 at 11:43PM
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Additional thought.....

I had my water heater blow out some months ago after only six years. Now wondering if this increased pressure, undetected then, may have been responsible for that. Guess I don't get to know, but the close association in time of the event makes me wonder.

Too soon old, too late smart. On we go. At least I know it's done right now.

    Bookmark   May 21, 2008 at 4:03PM
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We used to live in an area that had high pressure. It was kind of a PITA: it ruptured garden hoses, and when non-clued-in guests turned on a faucet to wash their hands, they got soaked.

    Bookmark   May 23, 2008 at 1:42PM
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Tom Pultz

I'll be installing a PRV to drop our pressure down below its current 105 psi. I believe the PRV will come preset to 45 psi. Is that enough or would it be advisable to raise it... and if so, to what?

We've been living with the high pressure, which I thought was normal, for quite a few years. I like the high pressure for the outdoor spigots, but the faucets can be a bit tempermental :-)


    Bookmark   May 23, 2008 at 6:53PM
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Hi tom in seattle (I used to live in Madrona/Leschi area).....

Before recent surge, I lived quite happily at 55psi. Noticed new install set on 50psi...which seems fine. Don't know if that was factory pre-set (I doubt it) or if that's just what the plumber left me with. Do know it is adjustable. Previously did not know about 80-85psi code maximum before lazypup set me straight. His advice confirmed by city water and my plumber.

    Bookmark   May 23, 2008 at 7:12PM
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Tom Pultz

Thanks for the data point asolo. We actually live in Sammamish (at the bottom of a hill, which is probably why our pressure is higher than normal), but nobody knows where that is so I just say Seattle for convenience :-)

I'll probably try the factory 45 psi preset to begin with and see how that is. When I install the PRV I may try and replumb the outside spigot at the rear of the house for full pressure.


    Bookmark   May 27, 2008 at 12:31PM
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The concept of pressure as it relates to the residential water distribution system is without question the most miss-understood topic in all of plumbing.

We constantly see questions posted regarding pressure and invariably someone will suggest going to Lowes or Home Depot and getting one of the little pressure gages with a hose thread attachment, then measuring your line pressure, as if that would yield any usable information. In truth, those gages only read "Static head pressure" which is the pressure on the system when all valves and faucets are closed and there is no flow, however, the performance of your showers or fixtures is dependent upon "Dynamic Pressure" or what is more commonly known as "Working Pressure" or "Flow Pressure".

In order to effective measure dynamic head pressure we would need to install a Tee at the point where the fixture is attached to the water distribution system and measure the line pressure as the water passes the gage and into the fixture.

IN one of my previous posts I described how the static head pressure on the municipal main can vary by changes in elevation. The same is true of the water in your house distribution system. Let us consider a two story house where the water main is located near the floor in a basement.

While we may measure 45psi static head pressure at the main we must remember that the water will loose 0.434psi per vertical foot of rise. This loss is technically defined as "Vertical Static Head". In addition to vertical static head we must also consider the "Friction head loss" which results from friction between the moving water and the internal wall of the pipe. Thus "Dynamic Head Pressure" can theoretically be estimated by computing the vertical static head loss and the friction head loss, then subtracting the losses from the static head pressure at the main.

let us put this in perspective. Imagine a house where the water enters the basement 1' above the basement floor and the basement has an 8' ceiling. We have a 7' rise to the ceiling, then a 1' rise for the floor joists and flooring material on the 1st floor. Now let us assume the house has 9' ceilings on the first floor and another foot for the floor joists to the second floor. We then have another 6' rise from the 2nd flr floor level to the shower head so our combined vertical head loss is (7'+1"+9+1'+6") x 0.434psi = 24' vertical x 0.434+ 10.416psi vertical static head loss. This means that if you had 45psi static head pressure at the main you would have 45psi - 10.41psi vertical static head loss = 34.59psi at the shower head. From this we must then deduct "Friction Head loss" however, computing friction head loss is not nearly as simple as computing vertical static head loss simply because as the velocity of flow increases on a pipe, the friction head loss will increase proportionally.

now let us imagine that the water main is at the front of the house but the master bath is at the rear corner where we would have a 50' horizontal run of pipe plus the 24' vertical rise for a total of 74' of pipe. If we were to be precise we would need to also compute the fitting insertion loss for each fitting on the run. By example, a 1/2" elbow has a fitting insertion loss equal to .9 feet of pipe. W could compute all the fitting losses or we can rely upon the code approved standard of allowing an addition 20% for the pipe length. Thus our pipe length is now 74' + 20% = 1.2 x 70= 84'.

Now let us imagine the we have a 1/2" copper pipe running directly from the main to the master bath.

Now let us consult the copper pipe friction head loss table.

In a 1/2" copper pipe with a flow rate of 1gpm the friction head loss is 0.008psi per linear foot of pipe thus the friction head loss to the master bathroom at a flow rate of 1gpm would be 84' x 0.008psi= 0.672psi. Not enough to really be concerned with, but let us examine what happens if we turn on the shower at 2.5gpm and the lavatory at .5gpm. The flow is now 3gpm and the friction head loss is 0.161psi per foot x 84' =13.54psi friction head loss.

In this example, even though we had the code minimum 45psi static head pressure at the main, because the line was improperly sized to the master bath the resultant "dynamic head Pressure" at the shower would now be 45psi minus (10.41 vertical static head loss + 13.54psi Friction head loss) or 45psi - 23.95psi loss = 21.05psi.

Even this would not be a big problem for conventional shower mixers because they are rated to perform with a minimum of 8psi and a flow rate of 3gpm, however, we commonly hear complaints from homeowners that they did a remodel on their bathroom where they removed a conventional mixer and replaced it with a pressure balancing mixer. A pressure balancing mixer requires a minimum of 20psi dynamic head pressure at a flow rate of 3gpm. From our example you can see that the new shower mixer requires 20psi while we only have at best 21.05psi available. Needless to say, if anyone were to run water anywhere in the house, the resultant pressure to your pressure balancing mixer would not be sufficient for it to operate properly, and sadly enough, most people get on the phone to the mixer dealer or manufacturers complaining their product is no good.

Please believe me when I say, there is no guesswork in plumbing and although you may feel confident that you will save a pile of money by doing it all yourself, this is a prime example where you really should consult the services of a plumber, if for nothing else but to plan your layout.

This is also why in most jurisdictions you may perform maintenance on a system without pulling a permit, but all new work, whether it is building a house from scratch or simply doing a major rehab requires all the proper computations and pulling a permit before the work may commence.

    Bookmark   May 27, 2008 at 3:46PM
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FYI...I have single-level slab-home with about a 2' rise from street-level.

Lazypup...sure do appreciate your information on this topic (and elsewhere). Things make a lot more sense after I read you. My plumbers and inspectors don't cover this ground -- they just come and go and send me the invoice. Hell, on one visit to change out my under-counter valves they switched hot/cold upon reconnect!

    Bookmark   May 27, 2008 at 4:35PM
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Gents, I appreciate all the posts. Here's another one for ya:

I moved into my place several years ago; it had a history of going through HW heaters at a phenomenal rate. The static head pressure was somewhere upwards of 90 psi so I had a PRV installed, reducing the static head pressure down to 40 psi right at ground level, yet my new, gas fired HW heater was still pushing water out the relief valve at a unnerving rate. By the way if you ever have your HW tank blow or crap out, CHECK THE WARRANTY; lots of them are still covered. Just call the manufacturer with the model number if you don't have the records and you may get a new, FREE one.

So, I heard about the expansion tank and finally caved and got my plumber to install one (all in all they're pretty cheap). The tank's been behaving great with not a drop coming out the relief valve now so problem solved right?

However, the other day I was applying some foam tubing to the HW line leaving the tank when I noticed that-as my washing machine was filling up with cold water-the cold water line supplying the heater was getting hot. Now, I figure, the drop in cold water pressure from the washing machine filling allowed hot water from the tank to flow BACK into the cold water supply line. Does this seem wierd to anyone? Should a HW heater allow this to happen, like doens't it make it sort of inefficient?

I was thinking of cranking up my PRV as it is only at 40 static head pressure at ground level in hopes that this would prevent the hot water from "backing up" into the cold. Does this seem resonable??

As well, if I do crank up my PRV, what's a resonable pressure for it? The HW GAS heater is at the same level and fairly close to the PRV.


    Bookmark   March 2, 2011 at 11:23PM
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MY water heater, the most expensive water appliance in my home has a data tag that states 14PSI inlet is max. If you exceed that all bets are off. The top of the tank is 17" across and I estimate the inside dia. @ 15". The force on the top of the tank is Force=area x pressure @15" dia. the area is 176.715 sq. in. X 14PSI = 2747lbs force on the top alone, or 1.23 ton!
Forget the fancy formulas and read the max inlet pressure rating on the W H and adjust accordingly. If the tank is to have the life of that stated by the manufacturer you need to have the right static head pressure, (inlet pressure) at the tank!

    Bookmark   April 24, 2011 at 1:12PM
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@Lee H.
Your back of the envelope calc is mostly right but you're not accounting for the void in the center of the tank where the hot gases are passing. The actual tank is shaped like a stretched donut so the real top area is more like pi*(7.5^2 - 2^2) or 164 sq inches assuming my guess of 2" of radius for that exhaust void. So the force is about 2300 lbs (~17% lower). Still a lot but really these tanks are built to handle that. Plus I'm sure it can't be 14 inlet psi, how could you even run a hot shower a couple floors up? In my house I calculate a 8 psi loss just from going up in height from my basement heater up to the shower. There's no way you could use hot water if the tank was at so low a pressure. I bet that tag really says 140psi maximum but your house pressure really should be in the 40-60 range

    Bookmark   April 27, 2011 at 2:48PM
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