Point of No Pressure Change

Trying to get a firm grasp of this "Point of No Pressure Change" aka PNPC

Let me see if I understand this correctly.

With the circ on the RETURN, when the circ is ON, there will be somewhat LESS pressure on the suction side, and perhaps a bit MORE on the discharge. I can see this on my system by measurements. It appears that the total differential across the pump is "shared"... most on the suction side, and some on the discharge.

Now, let's say I move the circ to the supply side, just after the air scoop and expansion tank.

In this configuration, as I understand it, I will not see LESS pressure on the suction side because the air cushion in the tank will react to compensate ? I will now see the TOTAL differential on discharge ?

Is this correct ?

How far can the pump be located from the air scoop ? Can it be 3' of pipe and a 90* elbow away ? How about 10' of pipe and two 90* elbows away ?

Is there any reason I can't pick up a pair of pump flanges and a pipe nipple of the correct length, and make a "pump replacement" section so that I don't have to re-pipe the return if I want to move the pump to the supply side ? In other words, can I mate two pump flanges face to face and expect it not to leak ?

Thanks!

 

Pumping away (or rather, not) is the reason I'm here at all....

The principle behind the PONPC is pretty much as you describe. However the effects can be quite profound.

My old system was pumped on the return, with the expansion tank (ET) hung from the spirovent on the supply. In effect, the pump was pumping right into the tank. The result was the pressure differential needed to actually move the water (since we're circulating, not pumping, after all) was developed almost entirely by creating negative pressure upstream of the pump. Talk about air problems! Basically, whenever the circ would come on, the pressure in the 2nd floor loop would drop to near or below atmospheric, at which point all the compressed microbubbles would become macro-bubbles. What a racket! Find my very first thread here. Months of woe.

To fix it, I moved the ET to just upstream of the pump. You don't HAVE to pipe the ET off the air scoop. They make them that way for convenience. The ET just needs to be connected somewhere upstream of the circ inlet. A few inches to a several feet is fine. Just not on the whole other side of the system.

The new system is piped and pumped by the book, on the supply. Whew.

 

I _KNEW_ and hoped that Xiphias would ring in on this one...

Let me say first that I don't have any trouble with the way this system is working in regard to air elimination. The scoop and vent work fine. All is quiet on the home front.

The object here is I would like to get as much of the re-piping done and in place before the new boiler (if I can afford one!) is installed. This way, I figure I can do all the grunty plumbing work, and let a contractor worry about pulling the permits, installing the boiler (by simply connecting the supply and return), and getting the inspections done. I should be able to save a ton of money that way.

Getting back to PNPC:

My understanding is that you want the air scoop to also be at the PNPC, which should be the lowest pressure point in the system if you are pumping away. This would cause the maximum amount of air to seperate from the water and get caught in the scoop, right ? So it seems to me that it's a bit more than just convenience to mount the tank on the scoop , no ?

What about my idea of making a "pump replacement" section out of a nipple and two flanges ? Think that'll work OK ? I could pull the pump from where it is on the return, slip the pipe and flange section in, and bolt it up without having to re-do the near-boiler return piping at this time. Move the circ downstream of the existing tank and scoop, and run the wire. Then when the boiler is installed, that piece could be removed, and pipe run to return on new boiler. Sound feasible ?

Thanks!

 

The flange to flange should work. It would be tough to do threaded when you are dealing with 1/4" of so increments but should be workable in copper.

I would use a 3/4" lateral off a tee for the tank, and then let the tank hang low off a 3/4" by 1/2" reducing elbow. When tanks are off the bottom of scoops I can't help but think that crud is dropping on top of the diaphram.

I would drop straight down from your scoop or spirovent through a tee to a purge valve. The tee would be for the feed.

My new system was supposed to be like that, except the tee for the tank ended up too low for going through the joist... Grrr! It's almost down to notch-ville.

Just remember that you want a long straight pipe leading to the scoop. You also want I think at least a foot of straight pipe before the circ and 18" or more straight afterwards. Many people try to always have the circ on the vertical so air pockets can't mess it up.

I don't think 10' and 2 90s would matter.

Are you going P/S?

 

http://giveme5.honeywell.com/Product/Supervent/SuperVent%20Sell%20Sheet_LoRez.pdf

Jeff, this is what I am using. The bladder tank mounts right beneath the supervent, along with the antibackflow valve and PRV assembly which are TEE'd in at the bottom where the tank mounts. The Supervent/tank is between the boiler supply and the pump. This puts the tank and the supervent at the lowest pressure point, and at the supply where the water is hotest. Everything was packaged as a kit by Honeywell, think I got it from Patriot. You want the bladder tank charge"pushing/assisting" the pump, not the other way around with the pump working "against" or fighting the bladder pressure. The Supervent is very impressive, it takes a LOT of trapped air of my system that I wasn't ableto eliminate in the past when I used to manually bled and had the old pressure tank stuck up in the joists.


Pete

 

I like this:

http://i77.photobucket.com/albums/j50/xiphias1758/supplyheader.jpg

which in close-up looks like this:

http://i77.photobucket.com/albums/j50/xiphias1758/system2.jpg

but I'm somewhat biased, naturally.

Air elimination should be most effective on the supply due to the recent heating of the water, which does a good job of liberating air. All factors being equal, regional/local pressure at the air eliminator really shouldn't make much difference.

There is, however, a big difference in air elimination approach between the standard scoop/hyvent and the "microbubble resorbers" like the Honeywell, Spirovent, and the new Taco 4900 (replacing the Vortech). The standard scoop creates a pressure differential just like an airplane wing, and more or less hopes the air goes up and over to be captured by the vent. Eventually, most of it does. The resorbers use an "increased surface area" approach to collect microbubbles. Over time the microbubbles coalesce to become macrobubbles and float up to the venting point. To collect the microbubbles, the Spirovent uses a piece of their (Spirotherm Co.) baseboard heating element (a circular wire mesh around a piece of copper tubing). The Honeywell literally uses a section of wire brush. The Taco uses fancy-schmancy "PALL rings."

The traditional scoop needs 18" or so of straight approach piping because it's relying on the flow to stabilize a bit to get the air to the top half of the pipe before it enters the scoop/wing. Not laminar flow, but at least somewhat less turbulent. That maximizes the efficacy of the scoop. The resorbers don't need the straight approach (but some straight length probably doesn't hurt) because the method of removing air is totally different. They don't care if the flow is turbulent, they're relying on the bubbles to stick to their presented collection surface.

Of all of them, I'm most impressed with the Taco 4900. Not the "PALL rings" so much as the conical vent head. A rep showed me a cutaway. Compared to an old Spirovent I disassembled, it had much better engineering inside. IMHO.

Can't speak to the flange idea. Sounds like something ripe for being a PITA, though. Would be easier to pre-cut-in a PONPC connection. I thought you were doing a full new layout anyway?

 

Thanks for the suggestions / comments!
I'm storing them in my mental footlocker.

Xiphias sed:

"I thought you were doing a full new layout anyway?"

Basically, yes. I'm not going with the P/S just yet though, but will design the new layout with the ability to add it later if any funds are available _after_ the new septic system, and potable water well ... I'm sure you understand the expenses involved in both of those! So, you can see that I need to keep costs as low as possible. (oh yeah, the lady of the house also wants me to finish the work on the new bathroom upstairs... and have I mentioned the new roof needed on the garage ?)

I'm still in the musing / learning / planning / design stage. Right now, in spite of _wanting_ to go the multiple circ route, I'm pretty sure it will be a ZV system. (In fact, initially, it won't be zoned at all) I'll be building the manifolds with wide enough spacing to use circs later if I so decide. There will be a pair of close-spaced tees installed and plugged so I can easily change-over to P/S when money allows. The first step is going to be building the supply and return manifolds. When those are in place, I'll drain the system and connect the new manifolds. Then, replace the boiler... It's much clearer in my mental footlocker than it is here...

-

 

Have you figured out your zones?

I'm going from 1 to either 2 or 3.

My preference would be small properly sized circs.

 

The flange to flange connection would work but so would removing the old flanges & using a nipple with a coupling on one end & a union on the other. Short term (until you replace the boiler this summer) I'd have no heartache using the flange to flange method. It would likely be a lot easier.
That's a nice looking set up Xiaphias has but I still don't like feeding into the supply. Depending on the location of the low water cut off, you could have a boiler making steam. If you have ever heard the result of water coming into a steam boiler above the water line, you'd know why I don't like it. I much prefer to feed into the bottom of the boiler.

 

Grady, does feeding the bottom increase the risk of shock to the boiler?

 

Actually it would, or should, decrease the potential for boiler shock since the cold water is being tempered by the hot water already in the boiler. All of this is moot if the system is tight & nobody decides to flush out all that "dirty" water.
Even when I filled my system for the first time, I never opened a purge valve. I filled slowly & let the auto vents do their job. In the aprox. 12 years it has been in, except for the addition of another zone, no water has been added to the system.

 

Okay, here is a question!

visualize a primary pump on a P/S system. Pump is located after the bladder tank, pumping away, towards the Tees...

Does moving the pump to the other side of the Tees (to the boiler return side) after the pump? Theory behind P/S with closely spaced tees says both loops don't see each other. Imagine we are only moving the pump one foot, from one side of the Tees to the other... But, how does this affect the PONPC for the pump suction side???????????????

Pete

 

Possibly a somewhat gray area, particularly given p/s and some modcons' pumping directives. But for a standard boiler and standard p/s, I will say that you are no longer pumping away.

Read the book, follow the directions, shouldn't have any problems.

 

Problem comes from the Tekmar sensor placement for the supply temps. I only want to run the primary circulator for the heating cycle. I want the DHW piping direct through the boiler. It is a sticky wicket.

Pete

 

why not tell the tekmar not to worry about dhw? let the boiler fire to limit. that work?

 

Does the Tekmar boiler sensor really need to go on the boiler loop? The TV is doing the boiler protection. Why not let the 260 do full ODR on the heating loop?

 

Xiph.. I think the boiler will cycle off the upper limit if the Tekmar sensor doesn't read the supply temperature to the DHW. Not a bad thing, but on the other hand, it won't be able to sense how long to run a purge cycle to fully recover latent boiler heat. I have a piping arrange figured out that will do what I want. Took a lot of doodling at work!

Pete