How much does water expand in a vacuum ?
If you rely on advice from a big box store, you'll have many things in your home that may work most of the time, but will never pass code and won't work at all under some circumstances.
A PRV is essentially a one-way valve...this makes your house plumbing 'closed'. Any expansion caused by heating isn't going to expand your pipes like a balloon...the pressure rises and something has to give. This is usually the T&P valve on the WH. It could be the washing machine hose ballooning out (not good for it!), or a faucet supply hose, or often, the weak link is the toilet fill valve or a dripping faucet. With a closed system and a large use of hot water (where you're reheating a significant volume of it), the pressure will rise high enough to open a T&P valve UNLESS something is either leaking out the required volume, OR, there's a place for it to go. The goal is to keep the pressure steady at a safe value for plumbing, and not rely on the T&P safety (fault) valve to do it. Nothing in your house likes the water pressure that high. The only way to keep it steady that's approved and doesn't waste water is an expansion tank...
I worked with programmers for over 25-years...while a generalization, they all seem to think they're always right...code says, you need an expansion tank, regardless of what the guy at HD says! The PRV will regulate the supply, but inside the house, when the water expands, without somewhere for it to go, the pressure WILL rise, and often significantly. Note, many water suppliers install a check valve on your water inlet that does the same thing - makes your house a closed system, and that requires an expansion tank, even if the local water pressure isn't too high as well. If your house doesn't have one, it may when they ever do some maintenance on the supply. Having a closed system makes the whole water supply safer since if for some reason, yours got polluted (you left the hose sitting in a puddle and the dog just shit there), it can't work its way back into the system and create problems. Same reason why a special valve is required on sprinklers, you wouldn't want a hiccup in the supply to draw fertilizer, insecticides, etc., into the water. A hiccup might occur when say the firemen open up a few fire hydrants to fight a fire...the pressure drops, and water from your house gets sucked back out into the system, or they shut things down for maintenance, instead of having to drain hundreds or thousands of houses, the water stops sooner since what's in your house, can't get back to the supply.
You still don't understand how a retrofit recirculation system works...there's a thermostatically controlled crossover valve in the system between (normally) the furthest fixture's hot and cold. When the hot water is below the thermostat's setting (at the crossover, not in the WH), the valve opens, and pushes water back into the cold water supply line. Once the water at that valve reaches its setting (often 100-105 degrees or so), the valve closes. So, and since it may not have to push that hotter water all the way back through the supply system to achieve the desired temp in the hot line, the cold water line would never get above the thermostat's setting in the crossover.
The system I have is a self-contained box and has the pump, crossover, and thermostatically controlled valve all in one neat package (RedyTemp). It is the only one I know of where you can adjust the aquastat's value. I have mine set to just make the water warm at the sink (near body temp). The shower supply takeoff is closer to the WH, so it's hot almost immediately. At the sink, it gets hot quickly, since it's already warm. If I flush the toilet, by the time I get to wash my hands, the cold is actually cold since it has flushed out enough warm on the cold side. Everywhere else in the house, the cold is cold because I stop it earlier than with most of the other retrofit recirculation systems, and this approaches the convenience of a dedicated return line (not easily done in many places on retrofit).
All this other stuff just confuses me.
Oh ok, I see what you mean now. BUT, the problem seems to be heating the water at 90 PSI instead of at 70 PSI, so it's opening the TP valve on the water heater. I was thinking that by regulating the whole house's pressure at 70 PSI, it might be fine, right?
The higher the incoming static pressure is to start with, the closer it is to the point where the T&P valve will open. It takes very little volume increase to create vast amounts of pressure in a closed system. Lowering the incoming pressure will help the health of ALL of your plumbing fixtures. But, it will still expand and increase over the incoming supply if there's no place for the expanded water to go. Just put in an expansion tank with the PRV, and it should solve your problem. Note that a T&P valve is designed to be tested once a year; not operate daily. It is a safety, emergency relief valve to protect the WH from literally blowing up and taking your house with it. If yours has been operated a lot, it's a good idea to replace it when you put in the PRV and expansion tank. Then, all things being right with the world, it should only open during an emergency, fault condition, or you manually open it to test that it's still capable of opening during your annual check.
Seriously? Blow up THAT bad?Quote:
Note that a T&P valve is designed to be tested once a year; not operate daily. It is a safety, emergency relief valve to protect the WH from literally blowing up and taking your house with it.
Hmm, OK. I just replaced the valve on the water heater, maybe 18 months ago, and now it's dripping. Tired of this...Quote:
If yours has been operated a lot, it's a good idea to replace it when you put in the PRV and expansion tank. Then, all things being right with the world, it should only open during an emergency, fault condition, or you manually open it to test that it's still capable of opening during your annual check.
The T&P valve and a PRV are two different, separate, items. A PRV goes on the house's inlet water and is what adjusts the street pressure to the desired pressure in the house (most people recommend somewhere around 50psi - the new gauge you bought will help you to adjust this, should you need to on a new valve). Then, the expansion tank goes somewhere after the PRV, but before the cold inlet to the WH. Because the T&P is not designed to open and drain regularly (only in an emergency situation or fault), when it has been draining regularly, it's not a bad idea to replace it. The PRV keeps the street pressure from coming in too high; the expansion tank keeps it from rising because the heated water is bigger in volume.
Yes, a bad or non-existent T&P and a runaway WH can literally blow a house down. Do a computer search, and you'll find pictures of some that have happened in the past...you don't want to mess with one of those, and you really want one that is working properly.
If a WH keeps heating and there's no place for the water to be relieved, water expands over 500x its size when it makes steam...think about it, when it blows the seam because it isn't working, it almost instantly becomes over 500x bigger. It's not pretty. Luckily, the safety features usually work, and they don't fail often.
Thanks for the patient explanation Jim. Gotta go look into all this.
I just went and found a video at http://www.youtube.com/watch?v=o3jeShUBDUo
I'm wondering if we have one somewhere already and I just don't know where? It's not in the water heater area though.
I also just found this: http://community.homedepot.com/t5/im...v=mpbl-1&px=-1
This entire thing is getting out of hand and it is too laborious to try to sort through all the "solutions". IF you do not have a pressure regulating valve, OR a check valve in the system then you have an "open" system so you do NOT need an expansion tank because the pressure buildup is absorbed by the city system. However, in that case the pressure should ALWAYS be the same as the city pressure in the street so it should NOT increase, or change, unless the city pressure changes and that seldom occurs to any great extent, assuming the main lines are adequately sized. If your pressure fluctuates, or slowly rises without using hot water, then you need a plumber to check out your system because that is an indication of a failing pressure regulator.
Ja, there are a whole lot of ifs with solutions based on conjecture. The first bit of conjecture was that the temperature stacking was the cause of the TPR relieving. I don't think there was any anecdotal evidence presented to indicate how hot the water got due to stacking. The boiling point of water at that altitude is nothing more than a distraction since the water in the tank is not at atmospheric pressure and so not affected by altitude.
The second bit of conjecture was that high city pressure was the cause. That I highly doubt.
There is/was also misunderstanding of how a PRV (pressure regulating valve) works in so far that it doesn't "regulate" the expansion pressure on a closed system. There was also conjecture that the absence of a PRV meant it was an open system and not subject to expansion caused pressure. There can be a back-flow preventer without a PRV present causing a system to be closed.
As in many cases, a prognosis is only as good as the evidence provided by the OP preferably without preconceived notions.
Just leave the faucet dripping and be done with it.
As a side note to this thread. I purchased two watts pressure gauges from the Home depot. Hooked first one up and it immediately rose to 100psi. OK...pressures a bit too high. Checked on it the next day and it still read 100psi. Deadman hadn't moved a bit. Removed gauge and yep...needle stuck at 100psi. Go get new gauge and hook it up...whoa...now I've got 180psi. Return gauge & head to supply house for "quality" gauge...damn...they are closed early for the holiday weekend. Hopefully they will have something better than the HD offering. Any suggestions?