Pressure Reducing Valve

[alex14]

The temperature/pressure relief valve on my new water heater (40 gallon) opens after a shower or doing laundry. General Electric recommends installing a pressure reducing valve on the cold-water side of the water heater.

Here’s their logic: The water heater and pressure relief valve are designed for a working pressure of 150psi. The pressure into the water heater is at 90psi. That leaves 60psi for expansion in the tank. Water expands times for every degree it is heated. I either have the temperature set too high on the water heater or the water is too cold coming into the house, but the net change in water temperature is obviously causing it to expand when heated more than the 60psi I have to work with.

I have a few questions based on this…

Is GE’s answer to my problem reasonable?

Since the water pressure coming into the house is 90psi, does it make more sense to install the pressure reducing valve on the main water supply, thus reducing the water pressure for the whole house?

I know, from reading responses to other questions, that having water pressure too high can cause damage to my faucets and the valves in my washing machine. How high is too high?

You guys have been a great help on other questions I’ve had. I appreciate it.

[robalaParticipant]

You should consider installing a PRV on the water main, but more importantly is to install a thermal expansion tank on the water heater. This will compensate for the water expansion as it increases in temperature.

[SylvanLMPParticipant]

Personally I would reduce the pressure coming into the house.
The reasons are as follows.

1- Excessive pressure causes excessive velocity and thus possible erosion and NOISE

2- The excessive pressure can destroy piping because of hydraulic shock from quick closing valves like solenoids dish washers and washing machines.

3- Isolating a pressure vessel from a system is always a bad thing to do in case of a run away firing condition.

4- Expansion tanks are NOT normally required on a hot water heater properly piped in with the right supply pressure

All you need is enough pressure to over come friction losses and then some to have the required pressure to activate the fixture properly.

To find the pressure you really need take the incoming pressure you have and times it by 2.31 this will tell you how high this water will be elevated.

I never let any more then 85 PSI into any structure and this will elevate water to a height of 196 ft
Figuring if you needed even 25 PSI on the upper most floor for a fixture to operate at its peek demand all you do is take 25 times and convert it to feet by using the following formula

25 2.31 = 57.75 ft then take the height of the highest fixture from the water main and times that by . 433 to give you a footage reading

Here look at this 57.75 ft. Is how height 25 PSI will take you.

to prove take this 57.75 and times it by . 433 = 25.00

EVEN adding another 10 PSI for friction losses etc. you only need 35 PSI incoming pressure so give it a fudge factor of 20 PSI your still at 55 PSI INCOMING and yet your should be well under the CDA (copper development associations) recommendations of 8 FPS Velocity

A pressure of 55 PSI would give you a velocity exceeding 25 FPS and flowing through a 1″ pipe your flow rate would be 62.5 GPM THINK about it?

Today with the low flow shower heads and faucets where are you actually going to get this kind of flow conditions other then possibly an outside hydrant.
Most journeymen plumbers today think like car mechanics wanting to get SPEED through piping systems as they love the roar of water shooting past washers and the loud sounds of hydraulic shock (water hammering)

What good is having a car with 300 HP in a 35 mile an hour zone?

Do you think you really need water pressure coming into your home that can reach the moon?

Think of the results when your washing machine hoses give up the ghost ad here you have a zillion pounds of pressure WHY?

Just have a Licensed Master Plumber come over and Properly size the pressure required for YOUR home.

DO NOT let a NON licensed Master do it as most journeymen cant be bothered with facts as one size fits all.

I know I have “journeymen” working under my license and NO WAY would I trust them to properly size a piping system for peek efficiency.

Good luck

[robalaParticipant]

Excellent point, Sylvan. Let me stand corrected: don’t “consider” a PRV, definitely install one.
Alan

[GuestParticipant]

the valve is doing exactly what its supposed to do, release and build up of pressure over the rated limit of the pressure vessel

[SylvanLMPParticipant]

Quote:

Originally posted by robala:

Excellent point, Sylvan. Let me stand corrected: don’t “consider” a PRV, definitely install one.
Alan

Thank you Allen, Thankfully you are a journeyman and NOT a ditch digger so you can understand exactly where IM coming from. ITS really a please dealing with a professional instead of the ditch digging stumblebum mentality so common on here.

.

[SylvanLMPParticipant]

quote:

---

Originally posted by bungie1:
the valve is doing exactly what its supposed to do, release and build up of pressure over the rated limit of the pressure vessel

---

The thermo stress that causes the T&P to relief is putting a lot of strain on this tank and will make it prone to failure much faster.

If just lowering the incoming pressure can stop excessive velocity and not forcing the piping to work at Max pressures think of the possibility of this valve not working JUST ONE TIME.

Having these SAFETY devices acting as operating controls is not in the best interest of the clients.

This isn’t just digging a hole and planting a box for sewerage this is a serious trade we are in and pressure uncontrolled can kill not to mention the very real possibilities of someone being scalded

Unlike others. WE ARE professionals and safety and health is our main priority.
[Edited by Moderator on 27 March 2001]

[fourth yearParticipant]

Old House. Your logic is good up to a point. But unless you already have a pressure reducing valve or a check valve in the water supply to the house or heater, the expansion that takes place will dissapate into the water service mains and not change the pressure in the water heater or house. You should test the water pressure with a gauge when the heater has finished heating the water and while it is heating. If there is a difference then you have some type of backflow preventer in the system and you need an expansion tank.

[ThisOldHouse1Participant]

Thanks for the responses.

The explanation from GE seems more valid as I think about it.

I have a 4.4-gallon expansion tank installed, with an initial setting of 40psi. I just checked it and it’s at ~86psi now. It makes sense that it would seek a balance with the rest of the system.

It seems to me that in order for the expansion tank to be effective, it must have an initial pressure higher than that of the system as a whole. Is that true? If so, how much higher?

I’m a little nervous about charging that tank up to 100psi to compensate for a system that’s overloaded to begin with. It’s rated for 150psi, and I can almost hear the explosion now!

So I bought a few PRVs and plan to install one of them. They’re both made by Wilkins; one’s a model 70 and the other is a 600. The difference is that the 600 has a separate integral strainer cavity and costs $13 more. The price difference is nominal and I want what makes sense for my system.

If I have a water filter installed before the PRV, I don’t think I need the separate strainer. Does it make more sense to install the PRV before the filter, which case I can definitely see the value in the separate strainer?

Thanks again!

[ThisOldHouse1Participant]

Old House. Your logic is good up to a point. But unless you already have a pressure reducing valve or a check valve in the water supply to the house or heater, the expansion that takes place will dissapate into the water service mains and not change the pressure in the water heater or house. You should test the water pressure with a gauge when the heater has finished heating the water and while it is heating. If there is a difference then you have some type of backflow preventer in the system and you need an expansion tank.

fourth year, I got your response after I sent the last one…

You describe how I THOUGHT the system should work, which is why I questioned the response from GE.

Before I installed the expansion tank, the cold water line to the water heater would get warm while the water was heating. This and a visual inspection (Yes, I took it apart) tell me I don’t have a check valve at the water heater. I know I don’t have a check valve or PRV anywhere else in the house, UNLESS it’s at the water meter out by the street.

Based on this, I didn’t think I needed an expansion tank. When the water expands, the extra pressure is supposed to be absorbed by the water supply out to the street. Right? I installed the expansion tank anyway, though I’m still not sure it’s serving a purpose other than to have something else to hit my head against.

Right now, the pressure on both the hot and cold is 90psi. The water heater hasn’t been heating in the last few hours. I want to test the pressure while the pressure relief valve is open, but it hasn’t happened again since I bought the gauge. I think this will tell me a lot.

Either way, based on earlier responses, it sounds like I still need the PRV. I wonder how much I can get for a slightly used expansion tank on ebay.

As always, I appreciate the responses!

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