primary/secondary

yellowdog

Am I the only one that thinks a primary loop is always the boiler or production loop and the secondary loop is always the distribution loop regardless of where the expansion tank is placed? I know that technically the expansion tank can go on either, but it shouldn't change the designation of the loops. I even found @DanHolohan referring to this about his own book in a 2014 thread.

A few of Dans quotes from his thread are below:

"With primary-secondary, you have to set the tees that lead to the secondary circuit no more than 12 inches apart. As the primary flow enters the first of the two tees, it "looks" ahead, and then makes a choice. It can either go straight for 12 inches and be past these two tees leading to the branch circuit, or it can divert through the first tee's branch and flow through the entire secondary circuit."
"When the secondary circulator is off, no water will flow through the secondary circuit because the 12-inch "gap" between the tees in the primary main is the path of least resistance."
"The compression tank belongs in the primary main, as does the air separator and the fill valve. Make sure you install your primary circulator so that it pumps away from the compression tank."

I figure its time for a spirited debate about this, apologizing if its already been debated multiple times. It just bothers me that you could have two systems piped identically and different people would label the two different loops differently based on where a single component is installed.

DanHolohan

My advice comes from Gil Carlson. He invented primary/secondary pumping. 

GGross

I think most would assume that the primary loop is the one with the boiler, I think it only gets weird when someone puts the expansion tank on the loop where the boiler is not located, because in my opinion the expansion tank should always be treated as part of the "primary" since it can't be valved off from the heat source, and you would think anything considered as "secondary" could safely be valved off from the "primary" I'm not sure if it came up much in early P/S discussions since the application was different and "secondary" loops were truly secondary.

In modern residential applications (think standard mod/con piping) both loops are mandatory to get heat to any portion of the building, so generally when discussing the 2 sides of my low loss header, or closely spaced tees, I would refer to them as the "boiler loop" and "system loop". I always consider whichever loop has the expansion vessel to be the "primary" though.

yellowdog

Most mod-con boilers come with or sell a "primary loop" piping kit (aka low loss header). These usually do not have provisions for an expansion tank.

hot_rod

Yet my recollection in Gils opinion through Dan was whatever loop the expansion tank is installed into, that becomes both the PONPC and the primary loop?

This dedicated series loop pic, fig 4-8 is an example of the boiler being a secondary, as are the loads. In all 4 P/S options the tanks best location is not in the boiler loop, specifically.

But also important are the specific hydraulics of the various loops. With high pressure drop mod cons, the wrong placement can cause pressures to drop below static fill, examples below.

In these other two schematics, the ∆P is plotted with the tank, PONPC, in two different locations.

With the pressure drop across the boiler and coils in the indirect, putting the tank in what I consider the indirect tank loop assures positive ∆P across the circuit.

And the link to the article that better explains those two drawings.

https://www.pmmag.com/articles/83943-picturing-p-br-john-siegenthaler-pe

GGross

yellowdog said:

Most mod-con boilers come with or sell a "primary loop" piping kit (aka low loss header). These usually do not have provisions for an expansion tank.

Generally the primary/secondary kits that manufacturers sell are close spaced tees, not a low loss header. with a low loss header you can definitely locate the expansion tank on the boiler loop. The primary/secondary kits I've seen for reference look like this (see first pic below), and pipe the boiler to the other loop in the fashion that the original p/s systems had their secondary loops piped to the primary loop. So if a secondary loop is piped to the primary with close spaced tees, and the primary loop is the loop with the boiler, then which loop is the secondary when the boiler is piped into the system loop with close tees and the expansion tank is on the system loop as well? Do you call the boiler loop a primary and secondary loop, and the system loop is also secondary? I think it is entirely possible that when discussing P/S in early iterations that language was often used that was not meant to be all encompassing, and more than likely the thought that boilers would be commonly piped into a heating system in the same way that "secondary" loops were piped into the primary originally just wasn't a thought.

GroundUp

My theory on this is that "primary/secondary piping" was never meant to have a designated "primary loop" or "secondary loop". The phrase was simply to iterate that they are indeed separate loops. "Boiler/system piping" sounds goofy and could be taken several other ways, so I feel P/S was only meant to differentiate. I refer to the boiler loop as the boiler loop and the system loop as the system loop because that's what they are and I don't feel either one should technically be "primary" or "secondary" since they're both required for the system to operate. Think of it like the whole "Kleenex" or "Sawzall" thing. We all use those words as a general term, despite knowing that the snot rag in your pocket is a Great Value tissue. That's how many people are with primary/secondary IMO.

dko

I first learned reading B&G articles
Primary, secondary, and common piping were non-changing positions and taught you how to place things like pumps/tanks within that convention. I strictly remember reading "do not pump towards the common piping."

So if you happen to pipe and place pumps/tanks in a way that doesn't conform to the original P/S instructions, does that change the P/S locations? Or would you say you put it in the wrong place? But if it still proper yet non-conforming, do we just say p/s is swapped?

I think it's just semantics.

HomerJSmith

@yellowdog, that's been my beef for years. I agree with you, but you are fighting an anachronistic idea held by many, and I feel without justification. I refer to the boiler loop as the supply loop and the load loop as the distribution loop. Wordwise primary and secondary are a conservation of words over supply loop and distribution loop and less ambiguous than primary and secondary which jumps all over the sys depending upon the location of the Ex tank. But,alas, you being yellowdog and me being the low dog on the totem pole, not much I can do to change the way the world works. Fighting anachronistic ideas is a lonely job and exhausting.

Teemok

I'm glad this isn't a settled thing. I just talked with a 30 year vet tech from TT about this. He said he's old school and calls the boiler loop the primary. I would never call the system loop the primary, though it might technically be. I learned P/S with the boiler loop being the primary loop. I now recognize, with certain boiler/system piping arrangements, the boiler loop has no business being called the primary loop. When talking about the concept of hydraulic separation I use the terms P/S but when identifying parts of a system I don't anymore. I'm not sure why the PONPC needs to be called the primary loop but I'll do it if it makes communication clear. If the fill and expansion are on the boiler loop, I'd use both terms, Boiler/primary.

hot_rod

To me the definition of Primary Secondary is "closely spaced tees". Without the tees you have no primary secondary piping. Regardless of which, either or neither you want to call primary.

A low loss header or hydraulic separator is primary secondary in one convenient package. It is actually two sets of closely spaced tees in one pipe or box.

I suppose a two tee monoflo piping is primary secondary piping also? Two tees with a common pipe between.

DanHolohan

I sat with Gill in an office in New York City and he told me that the primary loop was the one with the compression tank. Just saying.

pedmec

i think the confusion comes from the fact that the original application of a primary/secondary system was not for tiny boiler rooms and boilers. It was to be able to add a sub circuit to an original piping system without having to affect the system circulator. hence, creating a secondary circuit out of the primary circuit. i think when the mod cos came out the piping looks just like P/S and it was easy to apply the term that we could relate too. i have an old B&G application manual explaining all the fine details on primary secondary piping. good read.

dko

This is the B&G P/S tech manual I've read
Never knew names, but looking up Gil Carlson now- he was at B&G during the conception of this so assuming this was his work? Very cool






Interesting to view the common piping as a compression tank "NPCP." When did we change to the longer PONPC

That was the only mention of a compression tank in the entire manual and that got me thinking...
I just learned the first pre-pressurized expansion tank was invented by Amtrol the same year the P/S was invented, 1954.

Teemok

Defining primary secondary as a piping concept of hydraulic separation is one thing. I don't think that's a source of confusion. Layering the critical concept of PONPC on top of it and stipulating that the loop that has the fill and expansion correctly connected be called the "primary" is the point of much confusion. Hot rods diagram above with the PONPC on the indirect loop is a good example. It seems best to just use two labels. That loop would be correctly be called the indirect loop and it would also be the primary loop. Referring to it as the indirect-primary makes it clear. "Primary loop" alone doesn't give enough info given the layering of concepts and the variety of piping configurations. It may have been very clear to Gill but to many learning it's not. They are hearing the term "primary" being used with mixed meanings by bosses, sales and tech support people.
This term confusion reminds me of the word decimated. It's pointless to try and convince the US population that the term "decimated" doesn't really mean almost completely destroyed. It means, as is indicated in the spelling, the loss of ten percent. If used as it should, you just confuse people and clear communication is the point, not being correct.

hot_rod

I remember sitting in one of Mark Eatherton's classes at Red Rocks, many years ago. He spoke of primary, secondary and tertiary piping. I had never heard the word tertiary, (third in order or level) much less how it applied to hydronics.

An example could be an injection mixing, variable speed circulator systems from a primary series loop, via a secondary loop, to a 3rd loop.

These were common when cast iron or copper tube boilers were the go to for low temperature radiant.

HomerJSmith

I mis-typed in my earlier post as @Teemok reminded me. I should have said "Boiler Loop" instead of "Supply Loop". Boiler loop/distribution loop! There is no other way to interpret this. I don't have to go looking to find where the Ex-tank is in the sys.
@hot_rod is right (he's usually right, and I give great credence to his opinions) that P/S is meaningless unless there is a means of separating the flows in a sys with closely spaced tees or a hydro-sep. The fact that this discussion is ripe with controversy is indicative of an unsettled concept.
The Common piping in the B&G tech manual must have some form of restriction to create flow thru the Secondary circuit Fig 2. A Monoflo tee in the circuit as indicated.

hot_rod

HomerJSmith said:

I mis-typed in my earlier post as @Teemok reminded me. I should have said "Boiler Loop" instead of "Supply Loop". Boiler loop/distribution loop! There is no other way to interpret this. I don't have to go looking to find where the Ex-tank is in the sys.
@hot_rod is right (he's usually right, and I give great credence to his opinions) that P/S is meaningless unless there is a means of separating the flows in a sys with closely spaced tees or a hydro-sep. The fact that this discussion is ripe with controversy is indicative of an unsettled concept.
The Common piping in the B&G tech manual must have some form of restriction to create flow thru the Secondary circuit Fig 2. A Monoflo tee in the circuit as indicated.

This is going to be a similar discussion as the fill valve question.
Leave it turned on or off. There are going to be strong opinions either way.

As a trainer I need to come up with a way to explain the concept so everyone in the class is on the same page as we look at piping options throughout the class.

For me to make it simple and understandable the expansion tank provides two functions in hydronic systems.
It establishes the PONPC and determines, on multi loop system which could be called the primary.

I'd like to think we all, or most are on the same page with the PONPC concept, and its features and benefits, so the P/S determination is not much of a stretch from there.

If Gil is to be considered the grandfather / inventor of P/S then going with his determination , or opinion as Dan recalls keeps the message consistent. It would be nice to think the "Red School House" is on the same page

Teemok

Use additional labels that clarify but always in accordance with Gills original intention. The primary is the denotation for the loop with the expansion tank and fill valve. "primary" becomes a descriptor. That loop could also be a distribution, system, indirect or boiler loop. When someone asks "is it on the primary loop" you must then define "primary" with them to answer correctly. Wrong as it is, many people mean the boiler loop when they ask that question. When I'm working closely with someone who knows the concepts involved and we just want to id a location, primary or secondary are not terms used. Asking someone "which loop is your primary loop?" might be an ego trip lead in but it isn't an efficient way to find out where the expansion tank and fill valve are connected.

GGross

Forgive me but I need to throw a wrench in the works. I originally learned piping basics from my late father (Bob you actually came up to Petoskey to meet with him one time, at least I'm pretty sure that was you!) We sell primarily Viessmann boilers so we use a lot of low loss headers. He always taught me the low loss header was a point of no pressure change (didn't know what that really meant back then) and it was an excellent place to pipe the expansion tank, if you had a placement for it. We would regularly pipe the expansion tank off a side tap on those LLH, in a circumstance like that, which loop would you consider the "primary loop"?

hot_rod

I do remember that trip to Petoskey, with Jim Eardley I believe? A great spot to live and work up in that area. Some $$ in that bay area also.

I suppose a low loss header could in itself be considered a "loop". Water just circulates around through it really. Also a mini buffer tank.

If the expansion tank is connected onto it, I'd consider it a primary "loop".

Truth be known with our Sep 4, connecting the tank to any of the 4 connections makes the entire sep the PONPC.

If the proportions are correct, virtually no flow happens between top and bottom connections, so it goes a little further than closely spaced tees in that respect. The math work out to the diameter needing to be about 3 diameters larger than the branches.
Primary secondary rule of thumb is one pipe size larger for the trunk. So the tees need to be closely spaced. With a sep and the large diameter we have more spacing available between connections.

In this example of a 2" separator, the 40 gpm flow across it at 4 fps velocity produces a mere .44 fps velocity top to bottom within the chamber. That is the key to dirt and air removal also, a low velocity zone.

Not all seps or LLH follow this rule and some that I have tested are not doing much "separation" Flow on either side induces flow in the other, sad

HomerJSmith

What information does the primary/secondary terms convey other than where the Ex-tank is located? Other than the Ex-tank is the PONPC, when most question asked on this forum, I think, have to do with the boiler and distribution piping arrangements. One has to then think is this the boiler loop or the distribution loop with primary/secondary reference? Oh, you have to look for the Ex-tank and hope the usage is consistent with Gil.
I'm for the conservation of words and ideas. That's the way conversation has evolved. One does not say, "A female birthing person." One says "WOMAN". Well, least not lately. Whatever conveys the greatest information in the fewest words, I'm for. I'm also for Mocha Almond Fudge ice cream. Yum, Yum! And also, Women's rights, they should have the right to Vote.

Teemok

The terms primary / secondary don't tell you which loop is which. It only tells you there is hydraulic separation between two loops and that one should to have an expansion tank and fill connection. Not much info density there.
A hydro separator or buffer tank is a loop unto itself ? What? It's common piping not a loop.

hot_rod

Multiple boilers would be another example of confusion if the boiler and expansion tank were considered primary . Piped parallel like this or connected with close tees in series into a “loop”

hot_rod

Teemok said:

The terms primary / secondary don't tell you which loop is which. It only tells you there is hydraulic separation between two loops and that one should to have an expansion tank and fill connection. Not much info density there. A hydro separator or buffer tank is a loop unto itself ? What? It's common piping not a loop.

Agreed, common pipe or header is a better term, since it depends on circulation from a circ on either side to operate