Pressure Reducing Valves Closed Hydronic loops, accurate set-up

aircooled81

How do you set your PRV's accurately? looking for rational tips on preventing overshoot when you walk away.

What's the most proffered process to eliminate overshoot on PRV settings?
I have been a proponent of downstream Iso Valve and gauge to set just the leaving side of the PRV before opening it to system, but if you let it sit long enough isolated from the loop (couple minutes) it can still overshoot - may be related to ambient temp on the small volume of water downstream of PRV and upstream of Shut off valve though?
I have also seen brand new PRV's adjusted and set to specific pressure but later find they no longer feed. Not sure if method above is the general cause of this occurrence. Do PRV's need to have a substantial flow through them while setting the pressure?

When it comes to expansion tanks, Rule of thumb has always been set the expansion tank to the PRV pressure when piped together or within reasonable distance. If you were to take into account the elevation difference between the PRV and the expansion tank, would you measure the tank at the inlet or the center? Sounds trivial but an expansion tank located 3 feet above the PRV will in theory employ slightly less than 1 psi and if it's a tall model expansion tank mounted on the floor with the inlet at the bottom and PRV 4-1/2 feet above would affectively see 2psig more than the prv's outlet pressure.

aircooled81

Great Question, I love it! No expert here, just learning as I go.

Because the 'why' has always made me enjoy this trade, there has to be a reason no-one suggests "about" in the installation manuals, not all expansion tanks have enough room for decreased acceptance volume, and I'm trying to learn the things I just don't know yet.

Closed loop static pressure setting off by 1 or 2 psi is not detrimental IMHO.

Couple, let's call that 2psi.
When the expansion tank initial fill pressure is higher than the make-up water pressure, tank stays empty until system pressure exceeds. I wonder how this affects the Point of no pressure change?
When the expansion tank initial fill pressure is lower than the make-up water pressure, tank fills with water until it compresses the air inside diaphragm/bladder to match the static fill pressure.

If I'm using P1P2=V1V2 appropriately here...
Example, 20gallon expansion tank, desired PRV to 12 psi,
- but it's actually outputting 14psi.
- expansion tank set to 12psi pressure.
the tank volume at 2 psi increase is reduced 7%. That's telling me my 20 gallon expansion tank is now a 18.6 gallon tank now. This decrease in tank volume is 9% at 15psi Make-up water, 13% at 16psi make-up water, 16% at 17psi make-up water 12psi expansion tank and so on. They typically loose a little air, gotta hope there's room for that too.
As long as your expansion tank has additional room you wont exceed it's limitation to expansion.

EdTheHeaterMan

Ok. The slight rise in the pressure between the brand spanking new auto-feed valve (I like calling them that in lieu of PRV) is most likely the temperature change of the small volume of water in the small container between the outlet of the auto-feed and the shut-off valve.

Try this experiment, Get a ziplock bag, and fill it with ice. Place the ice bag on the pipe between the auto-feed and the iso valve. now watch the pressure drop back to the auto-feed pressure. As the water cools the water will contract, but the auto-feed will maintain pressure so the pressure will not drop below the Auto-feeder pressure setting.

Now that the experiment is done, remove the ice and watch the pressure rise again. It will be higher than before the experiment. because more water was added to the cold section of the pipe.

The proper way to set the auto-feed valve is with it connected to the system and use a screwdriver as a stethoscope to listen to the water flow stop at the selected pressure. if the water pressure is too low then increase the pressure adjustment on the feeder. If the pressure goes above the desired setting then turn the adjustment lower. Not rocket science.

As far as the 1 pound difference between the gauge, the expansion tank, and the auto-feed, AGAIN, it's not rocket science

aircooled81

point taken, agreed and thanks for the reply.

EdTheHeaterMan

@aircooled81 Good math problem for the classroom. Let's look at a real-life situation.

If you have 100 gallons of water at 65°F and you increase it to 240°F the water expands 5% or in this case you have 105 gallons. Now that is a pretty big system for it to hold 100 to 105 gallons of water.

Most residential cast iron boilers hold between 5 to 20 gallons of water, (the ModCon boilers hold less than 1 gallon) If the house was built after 1950 the radiators are probably copper tube baseboard or copper tube connectors so there is not a lot of water in those. Now you might have 50 gallons of water in there... and that is still a lot. Your high limit is set at 180 or 190 so the most the average water temperature will be is say 200° at any one time. and that's high too.

Maybe you will expand 2 gallons, 3 at the most. The # 30 Extrol will hold that expansion with no problem even if the air pressure drops to 9 PSI empty. Again... Not Rocket Science.

If you do Air conditioning (as I did) You have a bottle of nitrogen hanging around your truck for refrigerant line work. Make up a fitting to go from the Nitrogen regulator to the tire valve on the Extrol Tank (some adaptors needed) and carry an accurate digital tire gauge. (if you don't use Dry Nitrogen, then get yourself a bicycle pump). Check the air pressure on the Extrol tank during maintenance and charge as needed. it can be included to check and extra to add air, or you can include the pump in your maintenance fee. Either way, you have added value to your service that the competition does not offer. You might even find a bad tank from time to time. Now that is an opportunity to increase sales and save your customer the bigger bill for a service call later on.

This is why we do maintenance in the first place. To prevent the emergency call later on. The more things we check the more things that will need attention.

Keep thinking, keep asking questions, and keep learning. you will become more valuable as you get more experience

Good luck to you both @aircooled81 and @Youngplumber. May you live long and prosper.

Mr. Ed

EdTheHeaterMan

Follow up. It has been said that the membrane between the water and the air will lose about 1 PSI of air pressure thru the membrane much like oxygen can penetrate PEX tubing that is not equipped with the oxygen barrier. I believe it is called osmosis but I may be wrong. (you have heard the saying "He forgot more than you will ever know." That is one of the things not important enough to forget)

After several years of pressure dropping thru the membrane, adding a little air back into the tank will prolong the life of that component.

hot_rod

Another thing to think about with expansion tank sizing, say you have a fixed operating temperature boiler, running to 180F setpoint, maybe a tankless coil job. The distribution is all radiant mixed down to 110F.
You could in fact down size the expansion tank, knowing that a large portion of the system does not go through a 60-180 °F temperature change.

Same concept with solar thermal, only the fluid in the collector goes to 300° under stagnation, not the entire volume.
the formula is quite complex however.