Pressure Reducing Valve help


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Old 01-12-11, 08:10 AM
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Pressure Reducing Valve help

I'm a DIY'r and was trying to adjust the pressure on my pressure reducing valve into my boiler. The pressure on the boiler has been running a bit high (around 20 psi).

The PRV is a Watts 1156F (http://media.wattswater.com/1910265.pdf)

Anyway, like an idiot, when I went to adjust the screw to reduce the pressure, I accidentally loosened it too much and the entire screw came out. It's a little strange, because even with the screw out, the psi is still at 20 (might be because it is constantly running this time of year and the temp is constantly at 165 - 185).

Anyway, my question is how much should I tighten the screw when I put it back? The range in the PRV is 10-25, so is it fair to assume if I tighten it slightly less then half it will be set around 15 (which I need for 3 stories)?

There's no way to cool the boiler all the way down right now while I need heat to the house, so I'm just nervous about setting it correctly.

Any help would be greatly appreciated
 
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Old 01-12-11, 09:34 AM
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Well by taking the screw out you lowered the pressure. Put the screw back in and just snug it until you feel resistance. Leave it like that until you can set it correctly on a cold boiler. You should be alittle higher then the cold 12 psi with 3 floors.

Now why did you touch it???? What was wrong with 20 psi. As long as it dont get up to 30 psi your ok. The boiler starts at 12 psi cold then when the water heats up it will rise, and this is totally normal. Now if it gets up to around 30 psi then you may have an issue there or your expansion tank.

Mike NJ
 
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Old 01-12-11, 09:56 AM
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Pressure Reducing Valve Screw Fell Out ... what now depends on your skills...

I'll summarize what you can do to achieve the goal you originally set for yourself, and then explain why you probably shouldn't have started down this path, although now that you have...

1) Turn off the electricity to the boiler. You may wish to allow it to cool, but it isn't necessarily required.

2) Unless you have a 1/4 turn ball valve boiler drain, these rarely used valve frequently fail to seat drip-free when they are needed.
You should have a remedy on hand before opening the boiler drain. A spare boiler drain valve, or at least a solid cap with a gasket for the hose threads. If you aren't a confident as a boiler fitter, get the cap. This is step 2 because it allows the boiler to cool while you go to the store. (You also need a length of hose and slip joint pliers for that cap.)

3) Attach a length of hose to to the boiler drain and run it to a convenient drain that can withstand hot water. Don't use a toilet. Hot water will shatter a toilet.

4) Shut off the fill water valve to the boiler. This should be a valve before the fill water pressure regulated valve. If there isn't one, shut off the house water. (You should install a fill water shut off ASAP.)

5) Slowly open the boiler drain. You only want to remove enough water to get a little below your target fill pressure. Let's assume your target is 12psi, and you drain water to reach 10 psi. (drain to 2-3psi below your target). The volume or water removed should be a fraction of the volume of your expansion tank.

6) Close the boiler drain.

7) Open the fill water shut off. With the screw out, fill water should not be flowing and the boiler pressure should not be rising.

8) Begin to thread the regulator screw into the pressure regulated fill valve. Tighten very slowly until water begins to flow. The boiler pressure will rise very slowly to a final fill pressure value. 12 psi is a common choice. Allow 15-30 minutes per 1/4 turn adjustment until you reach your target fill pressure. It helps to have something else to do. Patience is not optional.

9) The boiler pressure now reads 12psi (or whatever your target fill pressure was) and it doesn't rise.

10) Turn the boiler back on.

11) When the boiler fires the water temperature rises, the water expands compressing the air cushion in the expansion tank. The boiler water pressure rises accordingly. A typical hot running pressure is around 20psi. If the pressure doesn't rise, either the boiler stayed plenty hot and the temperature didn't really have to rise or the backflow preventer (between the fill regulator and the boiler) may be defective allowing boiler water to contaminate the water supply lines.

12) If the boiler pressure reaches the safety limit, typically 30psi, the relief valve will open and dump boiler water to reach a safe pressure. (Don't test this valve by flippling the handle, and don't use it to reduce the boiler pressure in step 5 above. Its designed to dump very hot water very fast to prevent a steam explosion. If you're standing close enough to operate the valve you may be scalded.)

If you read the above through step 11, you should understand that the fill water regulator will fill the boiler to maintain a minimum pressure, usually adding water when the boiler is at its coolest or cold. When the boiler fires and the water gets hotter, it expands and the pressure rises. This is normal, and perhaps desireble. In taller homes a 12 psi fill pressure at the boiler is a negative pressure at the highest point in the system. A higher pressure is needed at least temporarily to force air in the system out of the vents so the the circulating pumps can do their job. That's what the handle on the fill water regulator is for.

My own opinion would be that you probably shouldn't have set out to alter the fill water pressure. At this point the fill water pressure regulator does require adjustment though. If the steps above don't make sense or don't seem to work, you can have a heating technician make the adjustment for you, it shouldn't require any parts so it shouldn't be a terribly expensive service call.

If this hasn't soured you on ever approaching the boiler again, you may want to gather addditional general information before your next foray. The installation and service manuals for the boiler and the burner are often a source of valuable information as is the (now venerable) "Guide to Oil Heat" from RW Beckett. These can all (usually) be found on the web for download, although manuals for very old boilers or burners might be difficult to find. Watts, Wirsbo, Taco, and Honeywell also have well written instructions on-line for many of the components of a heating system.
 
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Old 01-12-11, 10:31 AM
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Thank you for your in depth responses, I truly appreciate the time you took to explain the fix. I understand exactly what you are recommending.

One question I do have (and I'm not trying to take shortcuts, just wondering why this wouldn't work) is what would happen if I left it alone for now. I tightened it a little further after I started feeling resistance so I'm assuming its on the low end of the range. Next time it has a chance to cool, then I could check where the psi is. If it's drops below 12, I'll know it's not set correctly and then I could tighten the screw.

The only issue I see in doing that is if I don't catch it on time and the 3rd floor calls for heat. In this case there would not be enough pressure and the circulator would not be able to push the hot water to that floor. Am I missing any other issues with this approach?

BTW, I originally stated messing with it because I noticed the fill cap was loose. I have not idea how that happened, but when I went to fix it, the lock nut was also loose which turned the screw. Dumb....I know.....I should learn to leave things be

Would appreciate your thoughts on the approach above, thanks
 
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Old 01-12-11, 12:19 PM
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York: The thing you're missing is that reducing the pressure setting screw will NOT reduce the system pressure by itself. You could turn it all the way to the lowest psi setpoint, and the system pressure would not change. The PRV will add water if the system pressure (cold) drops below the setpoint, but it won't drain water if the pressure is above the setpoint.

You say the fill cap (on the PRV?) was loose. No sure what you mean - you need to post a photo and point out the "fill cap."
 
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Old 01-12-11, 12:19 PM
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Polychrome... bravo... excellent, and thank you!

York, I see that poly has gone offline for now, and will probably return to answer further (at least I hope so!) but in the meantime, I'll take the liberty of answering your questions-

By the way, the part about cracking the terlet bowl I had never thought of! I wonder if poly knows that from experience! yikes... imagine explaining THAT to the homeowner! I do know that you don't want to put hot water down a terlet because you can melt the wax seal at the bottom... so heed the advice!

I think you are safe to leave it be for now... as long as you have not let any water out of the system in the interim. As long as the pressure stays up where it was/is, you should be OK.
 
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Old 01-12-11, 12:59 PM
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Thanks Guys

Mike - yeah realize that now. Not sure what I was thinking when I did it since the other side of the pressure reducing valve is the potable water supply and the pressure can't "reverse" into it (hence the backflow preventor1). Related to the fillcap question.....click on the link in my original post. On page 2, there is a diagram which shows the "fast fill lever and cap assembly". When you unscrew this, the adjusting screw is underneath. The actual cap had been loosened which it what made me mess with it (in addition to the psi being around 20)

Trooper - thanks, I am hoping that is the easiest option for now. The pressure has remained a constant 20 or so since I "adjusted" the pressure valve last night. I live in CT (been shoveling out of the 2+ ft of snow last night) and the boiler will probably not get to cold start until March. At that point I'll let it get cold and if the lowest it goes is above 15, hopefully I should be good

One additional question...........let's say that psi drops low (way less then 12-15) and the 3rd floor zone calls for heat......is there any safety risk if I don't catch it right away? I would assume that the circulator would just keep spinning because the tstat temp would never be satisfied. Any issues I'm not thinking about there?

Thanks again
 
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Old 01-12-11, 01:27 PM
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That plastic cap with the lever can be loose... it doesn't really serve any function except to hold the fast fill lever, and perhaps to prevent dust from accumulating inside... the locknut only needs to be snug enough to prevent the adjustment screw from turning.

Was there a small metal rod inside the hollow part of the adjustment screw? If not, your fast fill lever won't do a durn thing. That rod is what the lever pushes on to open the valve. It just sits in that hollow screw.

The pressure has remained a constant 20 or so since I "adjusted" the pressure valve last night
If it truly sits at a constant pressure, while the TEMPERATURE varies as the boiler heats and cools, it might be a good idea to verify the gauge operation. If you see the temp at say 130 and 20 PSI ... and the boiler fires up and goes to 180 at 20 PSI it's a good bet that the gauge is pooched.

let's say that psi drops low (way less then 12-15) and the 3rd floor zone calls for heat......is there any safety risk if I don't catch it right away?
No, not a safety problem. What will happen is just as you say, the zone will call for heat and never satisfy. It won't hurt the pump at all, BUT zone valve motors are not intended for continuous operation. If it sits there for hours and hours with the motor energized there is some possibility that the small motor in the zone valve could be damaged... ooops, there I go assuming that everyone uses Honeywell valves! or at least a motorized valve... but still, zone valves in general are not continuous rated devices...

Just make it a routine to check the pressure day to day...

(below assumes you have a 'bladder type' expansion tank)...
One other thing that is within the realm of DIY is checking and adjusting the pressure in the expansion tank... and if that hasn't been done in years, I would recommend that as a spring time maintenance item... stop back then and I'll post the directions, or maybe I'll post it later this evening... print it out and save it for later use... It's easy...
 
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Old 01-12-11, 02:00 PM
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Was there a small metal rod inside the hollow part of the adjustment screw? If not, your fast fill lever won't do a durn thing. That rod is what the lever pushes on to open the valve. It just sits in that hollow screw.
Yes, there was a metal rod that was loose. I put it back so the manual fill should work if needed.

If it truly sits at a constant pressure, while the TEMPERATURE varies as the boiler heats and cools, it might be a good idea to verify the gauge operation. If you see the temp at say 130 and 20 PSI ... and the boiler fires up and goes to 180 at 20 PSI it's a good bet that the gauge is pooched.
Sorry, should have been more specific. It does vary when heated. The high / low temp is set for 165 - 185. When it hits 185, the psi is about 21/22. At 165, the psi goes back down to 19/20. A 2-3 degree rise for 20 - 25 degrees seems normal right?

BUT zone valve motors are not intended for continuous operation. If it sits there for hours and hours with the motor energized there is some possibility that the small motor in the zone valve could be damaged... ooops, there I go assuming that everyone uses Honeywell valves! or at least a motorized valve... but still, zone valves in general are not continuous rated devices...
Hmm, I'm not even sure if I have zone valve motors. I have the circulators, but don't see anything else motorized.

One other thing that is within the realm of DIY is checking and adjusting the pressure in the expansion tank... and if that hasn't been done in years, I would recommend that as a spring time maintenance item... stop back then and I'll post the directions, or maybe I'll post it later this evening... print it out and save it for later use... It's easy...
I do have a bladder expansion tank so would definitely appreciate those instructions

Thanks again for taking the time to answer my questions!
 
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Old 01-12-11, 02:43 PM
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A 2-3 degree rise for 20 - 25 degrees seems normal right?
Yeah, no complaints with that.

I have the circulators, but don't see anything else motorized.
Not a problem. You might have mentioned earlier that you are zoned with a circ for each zone and I just didn't pick up on that. Some systems have just one pump and electric valves that open/close for each zone, some are like yours with a circ for each zone.

I do have a bladder expansion tank
OK, here 'tis... There are other ways to do this, but I think this is about the easiest.

Note: many tanks are installed upside down, with the air valve on top. This procedure assumes the air valve is on the bottom. If you've got a bad bladder, you probably won't get water out the air valve if it's on top... but you might.

Also, since you've got a 3 Story home, you do want the minimum to be at LEAST 15 PSI when cold.

The main point to understand here is that the boiler side of the tank MUST be at ZERO when you are checking/charging the tank or you won't get an accurate reading. That's the reason for the 'rinse and repeat' part.

===========================================

1. Shut off boiler and allow to cool to under 100F.

2. Shut off water supply line to boiler.

3. Drain only enough water from the boiler drain to drop the system pressure to ZERO. REPEAT: DO NOT COMPLETELY DRAIN THE BOILER! ONLY ENOUGH TO DROP THE PRESSURE TO ZERO!

4. With an ACCURATE tire pressure gauge, check the air charge in the tank on the air valve opposite the end of the tank that's connected to the system. If ANY water comes out of the air valve, the bladder inside the tank is shot and the tank needs replaced. If no water comes out the air valve, and the pressure is less than 12-15 PSI, continue to step 5. If the pressure is OK, turn the water supply to the boiler back on and repressurize the system, turn the power back on to the boiler, no service is necessary.

5. Using a bicycle pump, or a small air compressor, add air to the tank until you have 15 PSI air charge.

6. Check the boiler pressure gauge again, and if it has risen off ZERO, drain some more water from the boiler drain until it is again at ZERO.

7. Check the air charge on the tank again. If it is below 15 PSI, add air to the tank until it is at 15 PSI.

8. Repeat steps 6 and 7 until the boiler stays at ZERO and the tank stays at 15 PSI. At this point, the tank is properly recharged and the water supply can be turned on to re-pressurize the system, turn the power on to boiler and return to service.
 
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Old 01-12-11, 03:31 PM
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Excellent, I just bookmarked the page for some spring maintenance.

Again, thank you for all your help. This site is a great resource for us DIYers. I appreciate it
 
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Old 01-13-11, 06:17 AM
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The other thing I forgot to mention (which I think matters) is that my hot water heater is set up as a seperate zone off the boiler. The point being that the temp in the boiler never gets down to very cold levels. Even during the summer when the heating zones are off, the water heater zone will kick on every few hours to heat the water heater. The lowest I've ever noticed the boiler temp go down to is in the 120 - 130 range. At those temps, the psi just doesn't seem to get down low enough to even need the pressure reducing valve to add water (even if it is set too low). Based on that, I would think that even if I completely screwed up the setting on the low side, most likely it would not need to add water anyway. Am I thinking about this correctly?

One other question......when they talk about the 15 PSI minimum for a 3 story building, the only consideration is vertical lift, correct? My 3rd floor zone has long horizontal runs along on the basement floor and then along the attic floor. There doesn't need to be any additional PSI factored in for the long horizontal runs, correct?
 

Last edited by NJT; 01-14-11 at 05:05 PM.
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Old 01-13-11, 10:05 AM
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when they talk about the 15 PSI minimum for a 3 story building, the only consideration is vertical lift, correct?
Correct. The physics is the fact that the pressure required to raise water ONE FOOT is 0.432 PSI. Therefore, to raise water 20 feet - 8.64 PSI ... 30 feet - 12.96 PSI ... PLUS the 'headroom' of 3-4 (I like 4) PSI needed to assure that the system pressure remains positive at all times.

The long horizontal runs don't mean a thing to the static fill pressure.
 
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Old 01-13-11, 12:26 PM
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Trooper - thank you for all your help!

If you don't mind me asking one more question that I've wondered about with the boiler (sorry!). On the 3rd floor zone, I've always heard a bit of air going through the circulator. Not a huge amount (nothing like a "waterfall" noise some people have described in these forums), but enough to notice it's there. I don't hear it up stairs on in the walls, just when I'm standing next to the boiler. And it's only on that zone.

Any issues there or is that pretty normal on the zone that goes to the highest point in the system?

I have automatic air scoops that I hear working sometimes (a small puff of air), but it never seems to eliminate all the air out of the system.

(BTW, I have a hydroair system (not radiators) so there is no bleed at the top of the system on the 3rd floor).
 
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Old 01-13-11, 03:11 PM
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Any issues there or is that pretty normal on the zone that goes to the highest point in the system?
I dunno 'normal'... but probably fairly common.

Could be that your minimum pressure is 'marginal'... because perhaps your gauge isn't spot on accurate...

Could be because your system is piped 'old school', with the pumps on the return side and pumping toward the expansion tank connection... this would have the net effect of SUBTRACTING the 'head pressure' of the pump from the static system pressure... and this is one of the main reasons for adding the extra few PSI to the system static fill pressure.

There is a long and somewhat confusing explanation to the paragraph above, which I won't go into here (again)...

Let's just say that if it's only an occasional air bubble that you hear, and the system is heating fine, and the noise doesn't keep you awake or cause ogeda, then not to worry about it.
 
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Old 01-13-11, 03:13 PM
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(BTW, I have a hydroair system (not radiators) so there is no bleed at the top of the system on the 3rd floor).
I think it would be somewhat unusual for there not to be a bleeder in that air handler. It is probably inside the cabinet... have you given it a real good look over?
 
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Old 01-13-11, 03:37 PM
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I would think that even if I completely screwed up the setting on the low side, most likely it would not need to add water anyway. Am I thinking about this correctly?
If you are saying what I think you are saying, I would say that you are correct.

As long as the system pressure at the minimum temperature that can reasonably be expected is enough above the pressure required to prevent the tippy-top of the system from dropping below zero, you are fine.
 
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Old 01-13-11, 05:05 PM
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Could be that your minimum pressure is 'marginal'... because perhaps your gauge isn't spot on accurate...

Could be because your system is piped 'old school', with the pumps on the return side and pumping toward the expansion tank connection... this would have the net effect of SUBTRACTING the 'head pressure' of the pump from the static system pressure... and this is one of the main reasons for adding the extra few PSI to the system static fill pressure.
Pressure is still showing a steady (around) 20 or so. So unless the gauge is off, it seems okay for 3 stories.

It is piped old school with the pumps on the return. When you say 'adding the extra few PSI to the system static fill pressure'.....you mean that's how the required minimum goes from around 13 (30 feet x 0.43) to 15 or 16 PSI?

I think it would be somewhat unusual for there not to be a bleeder in that air handler. It is probably inside the cabinet... have you given it a real good look over?
Good point, just looked and there are a couple bleeders inside the cabinet.
 
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Old 01-14-11, 05:11 PM
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When you say 'adding the extra few PSI to the system static fill pressure'.....you mean that's how the required minimum goes from around 13 (30 feet x 0.43) to 15 or 16 PSI?
No, not exactly.

When determining the proper 'static' fill pressure, one would find the height of the system from the bottom of the boiler to the top of the highest point, in feet. Multiply that height by 0.432 and then ADD and extra 3-4 PSI of 'headroom' ... that 3-4 PSI extra is what I was talking about.

So 30 feet would be 13 PSI plus 4, or 17 PSI. That should be the MINIMUM pressure that would ever exist in that system (measured at the boiler when cold, or, in the case of a warm start boiler, at the lowest temp it is expected to operate at).

Yes, I would think that 20 PSI would be fine... what do you estimate the height to be?

It's very possible that your gauge is in error...
 
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Old 01-15-11, 07:54 AM
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The height is about 30 feet or so, so I'll keep an eye on the gauge next time it cools down to make sure its at a min 17 psi. If it drops below, I'll adjust the pressure reducing valve to increase the pressure back to at least 17. Sounds like a plan

Next time I'm at the store I'll probably pick up a manual gauge to verify the pressure. The boiler is only a few years old so hopefully its still good, but ya never know.
 
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Old 01-15-11, 02:04 PM
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I think the gauge is probably OK... ya might wanna spend that $10 on a six pack instead!
 
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Old 01-16-11, 06:16 AM
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I think the gauge is probably OK... ya might wanna spend that $10 on a six pack instead!
Ha, good point

Again, thanks for you help with this. This is a great forum for us DIYers and appreciate you spending the time to walk me thru the issue
 
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Old 01-19-11, 08:59 AM
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Alright, so I went to let some of the air out of my 2nd floor zone and noticed a few different valves when I opened the air handler in the attic (I have a Hydroair system). There are 3 or 4 valves that look like a screw that can be loosened with a flathead screwdriver. There is also another valve with a nut on the top. Any ideas on which one should be loosened?

Also, on the top of my boiler there is a Watts air vent (cylinder with a red and black cap - DuoVent High Capacity Air Vents with Manual Vent Feature, Air Vents, Water Safety & Flow Control - Watts). I read on these forums that the black cap should be loose and the red should be snug. On mine it's the opposite (black is snug, red is loose). Did someone set this up incorrectly and that maybe contributing to why air is not escaping the system?
 
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Old 01-19-11, 09:51 AM
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There are 3 or 4 valves that look like a screw that can be loosened with a flathead screwdriver. There is also another valve with a nut on the top. Any ideas on which one should be loosened?
Without seeing them, it's hard to say... can you photo them?

Did someone set this up incorrectly and that maybe contributing to why air is not escaping the system?
Good question... I suppose it's possible that someone took the caps all the way off and mixed them up... I think that if you open the black one (which is snug), you will find that you get water out almost immediately... I think... can't hurt to try...

Is there any marking embossed onto the top cover that might indicate which is which?
 
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Old 01-19-11, 05:51 PM
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So I opened the black one and no water. I've been letting it run and seems like the air in the system is being removed. I'll keep my fingers crossed over the next few days. Hopefully that helps
 
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Old 01-19-11, 06:55 PM
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Something is wrong...

http://media.wattswater.com/ES-DuoVent.pdf

The RED cap is supposed to be a MANUAL VENT... in other words, when you open it, it's open, and water should come out.

The BLACK cap is the one connected to the float mechanism and should only vent air.

If the red one was loose, and nothing was coming out... then it's plugged up with something. If there were no air inside the device, then I wouldn't expect anything out of the black... but still the red one open and NOTHING coming out? something wrong.
 
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Old 01-19-11, 07:04 PM
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I unscrewed both of them and no water out of either (the red one has always been loose). The caps don't look like they were put on backwards because they match what is in the manual. Red on the left and black on the right when the "Watts" wording on top is the right way (ie. not upside down).

When I unscrewed the red one, there was a red valve inside (best way I can describe it). Is it possible that you have to push on the red valve inside with a screwdriver to release the water manually?
 
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Old 01-19-11, 07:10 PM
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Ya know York, I'm not sure now... I've never used one of these myself. I'm wondering if the red thingy is a 'pusher' to sink the float and open the black valve.
 
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Old 01-19-11, 07:16 PM
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It almost looks like there is something under the red cap that you need to push. I'm guessing that would cause the water to start flowing (don't want to try it!).

Loosening the black cap has definitely seemed to cut down on the air in the 3rd floor zone. Still a little when it first starts, but seems to be a lot less when it gets going.
 
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Old 01-19-11, 07:26 PM
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I think you might be right... the cutaway drawing looks like there's a small o-ring at the bottom of a stem under the red cap. If you do try it and it leaks, couldn't you just screw the red cap down tight?

How's the pressure holding up?
 
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Old 01-20-11, 05:44 AM
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I could try it, however at this point I'm more inclined to leave it alone! I just don't want to cause any other issues with leaking or with the automatic vent side.

Pressures still pretty steady (fluctuating in the low 20s, which is normal for the system). I would have expected a slight drop when the air came out, but maybe there wasn't as much as I thought in the system.
 
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Old 01-20-11, 10:47 AM
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So the more I read about this topic, the more questions I have! Specifically on the air in my system....

I have read that having the circs on the return versus the supply could impact air in the system, especially when the water feed is before the circs. This is due to the drop in pressure on the inlet side, which could trick the pressure reducing valve into adding water (new water = new air).

On my system (I have included a link to a rough diagram below), the circs are on the return, but the water feed is on the outlet side of the circs (and not on the same pipe run as the circs). The expansion tank and air vent is on the supply side. Although not ideal, I would think this would not create any issues since the pressure on the outlet side is always the highest in the system. The pressure reducing valve would alway "read" this high pressure and should not add water. Am I thinking about this correctly?

http://i1194.photobucket.com/albums/...ork/boiler.jpg

NOTE: should have noted in the diagram that the third pipe next to the circs is the water heater zone.
 
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Old 01-20-11, 03:14 PM
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I have read that having the circs on the return versus the supply could impact air in the system,
It's not as much the pump's location in relation to the boiler, but rather the pump's location in relation to the expansion tank.

It's more about whether the pump is pumping TOWARD or AWAY from the expansion tank location.

It IS true that the best 'technical' location for the air removal device is on the hot SUPPLY out of the boiler. That would lead to the location of the expansion tank being mounted at that location also. Followed by the pump, pumping AWAY from the expansion tank location.

The reason that all this affects the ability of the system to rid itself of air has to do with the fact that when the pump is running, it develops a PRESSURE DIFFERENTIAL (or PUMP HEAD) between the suction and discharge sides of the pump.

The system pressure at the connection point of the tank NEVER CHANGES. It is known as the "Point Of No Pressure Change" ( PONPC ).

It follows that if the pump is pumping TOWARD the tank, it's HEAD will be SUBTRACTED from the system pressure. If it pumps AWAY from the tank the head will be ADDED to the system pressure.

Air bubbles will COMPRESS with higher pressure, and EXPAND with lower pressure.

By positioning the pump so it pumps away from the ExpTank connection, the bubbles will be kept as small as possible in the rest of the system, making it easier for them to flow back to the boiler and not grow larger and get 'stuck' in the far reaches of the system piping.

As the bubbles come out of the boiler, to the LOWEST PRESSURE point in the system, right at the air scoop and expansion tank, they are the largest they will be... and that coupled with the fact that the HOTTEST water also exists at that point means the efficacy of the air removal device is at it's greatest there because hot water has less capacity to hold air in solution.

All very cornfuzin' ain't it?
 
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Old 01-20-11, 03:31 PM
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Although not ideal, I would think this would not create any issues since the pressure on the outlet side is always the highest in the system. The pressure reducing valve would alway "read" this high pressure and should not add water. Am I thinking about this correctly?
Yes, I think you are correct.

Let's analyze it a bit... since the expansion tank connection point is on the other side of the boiler, and that point will always be say 12 PSI, there WILL be a pressure difference between that point, and the pressure at the pump discharges. The amount of that pressure difference will be equal to the FRICTION in the piping between the pumps and the boiler, through the boiler, and between the boiler and the PONPC.

There probably won't be much difference though... relatively short lengths of large diameter pipe, and not much friction through the boiler. Let's take a WAG and say 2 PSI.

What this means is, as you say, that the reducing valve will see a HIGHER pressure (12 PSI + 2 PSI) when the pumps run, and NOT add water as a result of the pump running.

Were your feed on the SUCTION side of one of the pumps, it would see a LOWER pressure when the pump ran, and would likely feed some water.

Let's say the pump head is 5 PSI in this example. You would subtract the 2 PSI that exists on the discharge side from this in order to determine the pressure at the suction side. 5 - 2 = 3 PSI. This 3 PSI would SUBTRACT from the pressure at the PONPC. There would exist 9 PSI at the suction side of the pump.

If the water feed were at that location, the valve would feed an extra 3 PSI of pressure during pump operation. When the pump shut off, you would see that extra 3 PSI on the gauge. Theoretically, it would now read 15 PSI in the system. It shouldn't continue to feed water though... because the next time the pump ran, the suction side would only drop to 12 ...

Still cornfuzin' ja?
 
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Old 01-22-11, 06:21 AM
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Thanks Trooper....the scaring thing is that I understand what you're saying!

I'll keep an eye on it. Pressure has been steady. There seems to be some air still in the system, but it seems to be less (and only when the zone first starts which makes sense because the air has settled). We'll see....
 
 

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