Effect on GPM from PSI

Tom_133

Gentlemen,

I am curious if there is a formula and if so would someone post it. I am trying to calculate the effect on GPM from PSI, what I mean is the flow rate through 1/2" copper pipe at 10psi is less than at 100psi or am I confusing something? I have a Kohler K2976 and it is a 3/4" shower valve with two heads and I am trying to size the correct water heater for it. I will be reducing the incoming water from 110 at the street to 60psi. I am planning using the Hydro Versa PHE130-? was hoping to use the 80 but don't want to come up short


Kohler's directions state

Determine the correct water heater size and capacity for your installation. A typical shower installation uses an approximate mix of 75% hot water and 25% cold. A custom shower application using three 2-1/2 gal/min (9.5 l/min) showerheads can use about 45 gallons (170 liters) of hot water in 8 minutes. Choose a water heater large enough for your installation.
The valve is calibrated to 104°F (40°C) at the first stop position. The maximum temperature limit stop is positioned so the outlet water temperature does not exceed 120°F (49°C).
The factory calibrated pressure for hot and cold water inlets is 43-1/2 psi (3 kg/cm squared). The factory calibrated hot water supply temperature is 149°F (65°C) and the cold water supply temperature is 59°F (15°C).

hot_rod

The pressure drop is related to how many gallons/ minute GPM you are trying to move thru the pipe. Determine the required flow rate, then calculate the piping distance and all the fittings in that path.

Kind of old school, but I still have this slide rule sizer from the PHCC. Put in GPM demand, pipe size and it tells you pressure loss and velocity.

No doubt there are online versions now.

Rich_49

Tom

Are you doing space heating also ? Where are you located ? Smallest Versa Hydro is the 130 , this is why I ask about space heating also .

Tom_133

My space heating load is minimal, I have a radiant load of 22K. The heating module on the 130 will drop to around 10K with variable speed pump. I am hoping that the tank itself will provide enough thermal buffering so I won't short cycle. I do have that shower and another one in a different bathroom.

I have been curious about the effect on gpm that psi has and if there is a way to calculate it.

EBEBRATT-Ed

3 shower heads @ 2.5 gpm = 7.5 gpm x 8 min=60 gpm total=45 gpm hot water and 15 gpm cold water. That's Kohlers Example. For your 2 shower heads it would be 30 gpm hot and10 gpm cold.

Tom_133

I am with you on the math. Heres another scenario: lets say my water pressure is 40 psi and I always maintained good hot water through my on demand, but then the city came and put in a new line and now my psi is 80 and my on demand doesn't keep up how much more water is the on demand seeing because of the higher psi? I had this very scenario and was curious if there was a formula for GPM through a 3/4" pipe at 40psi or 50psi or 60psi etc?

Larry_52

I thought on demand used a volumetric flow meter.

Snowmelt

Tom what kind of on demand do you have, the reason I ask this is because there's a lot of tankless out there that shows gpm. If you can read the gpm then we can or you can answer them questions easier.

icesailor

If this is the shower valve you are going to use:

http://www.us.kohler.com/onlinecatalog/pdf/1145569_4.pdf

It MUST be plumbed full size 3/4" copper or PEX tubing to let it operate as designed. That valve is a high flow valve. You really need to pipe back to a minimum of 3/4" ID supply for the thermostatic and pressure balance part to work properly. You also must be careful that you can supply hot water at an equal pressure to the cold, or the valve will not work properly, They also, because they are thermostatic, need hot water inlet temperatures at higher than 125. There is a limit to how low the flow can get on the cold and how low and cold the hot can be and not be overwhelmed by the cold.

I've installed them. They do NOT like to be fed with 1/2". If the valve is installed on a second floor, and the pressure reducing valve is in a basement, you might want to try 65#. That may make first floor showers uncomfortable. You picked that valve because you want something nice. Be nice. Pipe it BIG. You won't regret it.

That means don't get skimpy on a on demand water heater. Make sure that the GPM flow at 125 degrees outlet is adequate for the valve.

Tom_133

To hatterasguy, and snowmelt- That was just a scenario to illustrate my question. I put a pressure reducer on that on demand and it's been great.

@ice
That is the shower valve the plumber on the job installed he did so with 3/4" lines, this new valve has thrown my spec'd unit out and I want to make sure I can supply enough hot water to the unit to at least run two 10 minute showers back to back but only two shower heads max, the math from Kohler says that should be about 60 gallons. So I should be okay with the 80 gallon Hydro Versa.

I really want to know this: a 3/4" copper pipe at 40psi flow= X GPM, a 3/4" copper pipe at 80psi flow = X GPM or are they the same?

icesailor

Its my experience that the tables and math are all published and provable. Doing the math can give someone a serious headache.

If your building is piped to acceptable standards, and you have 60+#PSI of building water service pressure, and the valve is piped 3/4" ID, you will have more than adequate pressure for the valve to operate satisfactorily. The hotter the supply water is, the better it will work. Read the PDF I posted. I don't think that Kohler recommends 80# PSI to the valve, and most codes require a maximum of 75#. 80# PSIG will put a serious hurt on a body. Especially on a first floor.

remodel

Bernoulli's equation, you write the equation from the outlet point to the point where you assume the static pressure is X psi. You calc the the friction loss in the pipe and it is an iterative equation. The zero velocity side of the equation has a static pressure inserted = Xpsi. Don't forget to get the correct pipe material friction loss numbers, you are essentially converting everything into head and thereby getting your velocity which you convert to flow. Flow = velocity*area. But to simplify, think about putting a pipe at the bottom of a tall bucket of water. Fill the bucket to lets say 92 feet = 40 psi you are going to get some good flow through that 3/4 inch pipe you just stabbed into the bottom of that bucket, now add 46 feet =20psi of water to get 60 psi, you will get more flow etc...

icesailor

That explanation makes my head hurt.

Pipe it in 3/4" IPS pipe, have plenty of hot, hot water to back it up, have under 75# PSIG at the service entrance, and no one will ever know that there was ever an issue.

Jamie Hall

There is a fundamental flaw in part of the reasoning: the flow is not dependent on the inlet pressure, but on the total pressure drop. Now part of that drop is going to be in the piping. Part of the drop is going to be in the fittings. Part of it -- a good sizable part of it -- is going to be in the shower head or whatever on the end.

So bottom line -- to ask what the flow is at 40 psi inlet pressure or 80 or whatever, you absolutely have to know what the pressure drops in the various bits are in relation to flow through them, and then you can work and figure the flow. Fittings like shower heads are particularly notorious at having widely varying pressure drops at only slightly varying flows -- it's deliberate.

Tom_133

Ok, let's forget my shower. I was just curious about the possible formula and with two shower heads if the hydro versa 80 gallon seemed big enough, I think that has been answered thanks guys.

Rich_49

Tom the 55 gallon would be big enough . I install these regularly and can say with confidence that from a cold middle of winter fill that the unit can heat that 55 gallons to 158* in 11 minutes flat . The heat exchanger is like no other available on the market .
Keep in mind also that since you will be using it for space heating as intended you will probably store at 150* - 160* . It has a Taco 5000 Series mixing valve included for the domestic so the storage volume of the installed unit is already increased . Don't know if you considered that but it certainly is applicable to your sizing .
I have not had one customer call and tell me they hadn't enough hot water with the 55 installed in homes with 6 occupants and 4+ baths , one of those being a master bath with high flow fixtures such as and exceeding your requirements .

Abracadabra

I think what @Tom is trying to ask is this: will a 3/4" inch pipe supply less GPM at a lower PSI than at a higher PSI. To put it in extreme terms, will a 3/4" pipe at 1PSI provide less GPM than a 3/4" pipe at 100PSI, everything else left the same. The answer is yes.

Here's some math for you: http://water.me.vccs.edu/courses/CIV240/lesson3b_math.htm

remodel

Jamie is correct about pressure/head drop. Types of fittings, type of pipe, length of pipe all important. Ice...yes make my head hurt writing about it but it's a sweet equation and also defines PONPC. But every simple answer I read is correct as well.