PRV Discharging a little water

camiarrobino

Hi all, just want to make sure I'm not missing something here (this seems to be normal / make sense to me but LMK if you think otherwise).

Due to a washing machine situation (don't want to bog down with details), I have been turning off main shut off valve to whole house before sleep, and before leave for work. This morning, when I went to turn it back on, I noticed a small puddle beneath the PRV discharge pipe on my 2 month old IBC tankless WH. I did a little testing / trouble shooting, and found that pretty consistently, it will spit out a little water when I do the following:

1. turn off water main
2. open one of the hot faucets somewhere in the house to make sure water is in fact off, then close faucet once the little remaining water stops coming out
3. turn the water main back on

once i turn the water main back on, you can hear the water hammer (apologies if my terminology is off) for a split second or 2, as the emptiness in the hot water piping (due to opening a faucet before) fills itself back up. as that happens, the WH spits out a little bit from the PRV, but that's it

to me as an unprofessional, this seems to make sense, just the PRV doing what it's supposed to? When you let the pressure back into the system especially when there is empty air somewhere in the hot water pipes, there is that brief moment of much higher pressure that causes the PRV to open?

just trying to make sure this isn't indicative of something else.....i have never seen any signs of the PRV releasing anything prior to this. thanks!

Jamie Hall

Is there an expansion tank on your hot water? When you turn off the main, you trap water in the system and if the water gets heated the pressure rises and that could be part of the problem (the water hammer doesn't help either).

I would suggest turning off power to the water heater when the water main is shut off.

camiarrobino

I don't believe there is an expansion tank. As I mentioned, this is a tankless on-demand unit, so nothing is being heated while the water is turned off

Zman

Are we talking about a domestic T&P relief?
What is the rating?
Do you know what the actual pressure is?
One of those hose pressure gauges used for irrigation works well to check pressure. Are you on city water or well?
You more than likely need a domestic expansion tank.

HomerJSmith

What's happening is that as the water is heated up, the water expands and when the heater is open to the street main the expanded water in the heater is at a higher pressure than the pressure in the street main and the water in the heater moves from hi pressure to low pressure, to the street. When you shut the house main off the expanded water has no place to go except out the PRV when the pressure exceeds the PRV rating. OR...you have a weak PRV.

Put a THERM-X-TROL ST-5 expansion tank on the sys and set the air pressure in the Ex tank to the street pressure.

camiarrobino

I’m attaching a picture I took of the little tag on the valve. Hoping it may help 

Homer, this can’t be the case, my unit is a tankless on-demand, not a storage tank. There is no stored hot water at all

HomerJSmith

See your second post on the same issue. That's a T&P valve. It opens at a pressure above 150psi or a temp above 210 degs. Put a hose bib pressure gauge on the drain valve, open the drain valve and go thru your routine and watch the gauge. If the heater has its own pressure gauge use that. What is the spike in pressure?

Larry Weingarten

Hi, The valve is set to discharge at 150 psi, which it seems to be doing. The valve is not designed for this sort of operation and will fail if it continues to see pressure spikes. Can you shut off water just to the washing machine? That would be nice for the rest of the system. A pressure gauge as has been suggested... one that has a little red pointer to show how high pressure has been, would verify that the pressure has been getting up around 150 psi. Do the washing machine shut offs work?

Yours, Larry

camiarrobino

yes, sorry about that. it was a mistake on my end to post it to oil heat and couldn't figure out how to delete that one. what you said on the other post, makes sense

camiarrobino

Larry, isn't this what the valve is designed to do? Are you saying that it can only discharge a certain amount of times before it gives out and is defective?

re washer, the problem, is that i learned that the laundry valve doesn't work (not consistently at least), and i learned this as i was giving away my broke washing machine to someone for repair. the new machine is a week and a half away. to get the water to the house turned back on, i capped the laundry hoses which is working fine, and i secured the hoses down in the slop sink just in case the cap sprung a leak somehow. i had been shutting the water to the house off when i leave during day just to be extra safe but i now realize this is probably too cautious. at a minimum, if i must turn the water main off, i will now turn it back on much slower, and also open a hot faucet to (hopefully) give the water pressure spike / water hammer somewhere to "exhaust" to, rather than confined within the enclosed system

HomerJSmith

Good, report back.

Larry Weingarten

Hi, The T&P is thought of as a safety valve rather than an operating or control valve. There may be other words for it, but essentially, it is only designed to operate to prevent something catastrophic. Beyond that it's "good luck!".

Yours, Larry

hot_rod

Its a pressure only valve on tankless, usually 

What is you incoming pressure? Should be below 80 psi

They have tankless specific expansion tanks, about the size of a softball

camiarrobino

hot_rod said:

Its a pressure only valve on tankless, usually 

What is you incoming pressure? Should be below 80 psi

They have tankless specific expansion tanks, about the size of a softball

Attaching a picture, is that an exp tank? If so, not seeing anything that could be it

What you say about only pressure and no temp for a tankless makes sense seeing as there’s no stored water, are you able to tell from the pic I posted if that’s true of the valve I have?

not sure on water pressure, should test soon and I will, willing to be it’s above 80 PSI

Zman

If the relief valve is working correctly and you have pressures anywhere near 150 PSI, you should correct that problem before you start seeing issues with appliances. I am guessing you are on city water? Is there a pressure reducing valve where the water comes into the house? It sounds like there should be.

camiarrobino

Zman, yes on city water, no on reducing valve (not yet at least). Obviously pressure is not always at 150 PSI or else the PRV would always be open. The tankless unit is right next to the water main and again, it was just this 1 time that I quickly opened the water main too suddenly and there was that sudden burst / surge of pressure that the PRV opened

going to test pressure this weekend, am i able to use the port on those check valves on the tankless unit?

Zman

That wold be a great place to check. The gauges with the “telltale’” needle that records the spikes are really nice for what you are checking.

ChrisJ

A T&P uses a wax motor to open under temperature.
It uses pressure to open under pressure. I don't know if tankless heaters have a T&P. I'm assuming they're pressure only.

It may do it many times just fine.

Or it may not. No one knows.

It's intended to be a safety device not a regulating device so chances are that it will fail.


Since it's happening when you open the water main I doubt it has to do with water being heated.
What is your incoming water pressure? Do you have a pressure reducing valve where the water comes into the building? If you do, is it functioning?

hot_rod

The probe of a T&P valve is supposed to be into the vessel, so I don’t know where a T&P would install?

Watts does have a model with a 2” probe, however

dko

Leave a tub spout open (preferably highest fixture) while you open the main.

camiarrobino

All - I finally tested the incoming pressure, and it's 120 PSI. I tested using the drain valve on the tankless water heater. I went outside and threw it on my sill cock as well just for **** and grins, same exact reading. Seems very high. Will be installing an expansion tank soon with my plumber friend

Larry Weingarten

Hi, I'd be thinking about installing a pressure reducing valve on the incoming water supply too. You don't want to expose anything inside to more than 80 psi.

Yours, Larry

EBEBRATT-Ed

@camiarrobino

I would put a pressure reducing valve and an expansion tank in if your pressure is that high. get it down to 50psi or so.

HomerJSmith

That pressure is waaay too high. Install a Pressure Reducing Valve on the supply line to the house and set it to 55 psi. You won't need an Ex tank as an arrester. Incoming water has momentum and when it suddenly stops that momentum is converted to a pressure spike. At 120 psi that pressure spike pushes you over the top (150) and you get the drip out of the PRV. A Pressure Reducing Valve will save you a lot of water when you shower and it's easier on the valves in the house.
https://www.supplyhouse.com/Caleffi-535351HA-3-4-NPTF-Pressure-Reducing-Valve-w-Gauge-Low-Lead-Pre-adjustable

Jamie Hall

You may not need an expansion tank as an arrestor -- but you will need it to handle expansion as the water is heated for a shower or something.