Welcome! Here are the website rules, as well as some tips for using this forum.

# Primary Secondary piping

Member Posts: 6
can somebody give me a real answer on primary pump and secondary pump. I was always taught that the primary pump/ primary loop was the loop that comes off the boiler. The secondary loop was either domestic or the area of the house that I was heating. Radiators, baseboard, in-floor etc. Recently reading a book and it shows the old timers piping as secondary is the piping right off the boiler and the primary loop being my system. My grandfather taught me this way too and explained it as my primary concern is the system and heating that, hence the primary loop was the system. (the reason we measure the system space to figure out boiler btu) Can someone clarify the right/wrong way. NTI, Grundfos, other boiler installations manuals show me primary is off the boiler and then secondary is the system. Thanks for the help.

• Member Posts: 807
The primary is always the loop coming from the boiler where the heat is MADE. The secondary is always the 'load' where the heat is needed. I believe this is universal. These principles are important to have clear so that we can TALK about them and understand each other.
• Member Posts: 6
Thanks for the input. How come? the book by Dan Holohan page 28 shows the opposite. we are going over this book with all our service techs at our company and this was the topic of conversation. it got really interesting.
• Member, Moderator, Administrator Posts: 16,490
That drawing from How Come? is suggesting a way to use a modern boiler on an old gravity-return system. In this case, the system is the primary loop (it has the expansion tank) and the boiler is the secondary loop. The primary circuit is always the one with the expansion/compression tank.
Retired and loving it.
• Member Posts: 21,846
edited March 2020
There are a number of different way to pipe a P/S. In some cases the boiler is within the loop, called the primary loop.

In some cases the boiler and loads tie into a continuous loop, called a Series Primary Loop, so the boiler would be considered a secondary loop..

In some cases the boiler injects into a straight section of piping, is that the primary loop?

What is clear in all cases is the need to have closely spaced tees (properly piped) to have a primary secondary piping.

When a 4 port hydraulic separator is used which side is the primary? Maybe the separator itself is the primary loop?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 15,312
In reality, I don't think it makes a bit of difference. To me it will always be a gray area as far as which is which.

1 boiler with primary secondary in a house I would call the boiler the primary and the house the secondary.

But what about a big chilled water loop with multiple large chillers in different buildings each tied into the main campus loop seperately. You wouldn't call the chillers primary and the campus loop secondary then
• Member Posts: 6
Dan thanks for the insight and the rest of you guys. I didn’t even think about the expansion tank part. Great book by the way. Service techs are really into it. Great website and forum. Never stop learning.😃
• Member Posts: 2,120
> @Timmysledge said:
> Dan thanks for the insight and the rest of you guys. I didn’t even think about the expansion tank part. Great book by the way. Service techs are really into it. Great website and forum. Never stop learning.😃

I recommend a fantastic book titled "Primary Secondary Pumping Made Easy" if you like Dan's books. Service techs love that one too.
• Member Posts: 21,846
Maybe anything tied into the branches of P/S piping "tee"is a secondary, regardless if is putting energy in or taking it out. Irregardless where on the loop those connections are, within inches of one another or a thousand feet apart on a district system.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7,347

In the proper sense, I was taught as Dan said: the primary loop is where the expansion tank and PONPC is. Siggy also identifies it that way in his book.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 6
Bob your right in our back door. Any chance you would like to come into our new training room and teach a little.
Dan do you still do seminars? Or travel tithe Midwest?
• Member Posts: 295
i prefer boiler loop and house loop.
or
boiler loop, hot loop (fin/tube BB) and mixed loop (low temp radiant)
still unsure what tertiary is?
but yeah, standard terminology is needed in all industries
• Member, Moderator, Administrator Posts: 16,490
@Timmysledge, thanks. I retired from seminars in 2016. The bit about the compression tank location in the primary circuit comes from my teacher, Gil Carlson. He came up with the concept.
Retired and loving it.
• Member Posts: 21,846
In the drawing I show with the 4 port hydro sep, the expansion tank could be in either of the bottom ports. Would the primary loop change with it?
Actually the entire sep becomes the PONPC
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member, Moderator, Administrator Posts: 16,490
@hot_rod, Gil always talked about the primary circuit being the one with the compression tank. The secondaries use the common piping as their compression tank. In this case, that would be the hydro sep. I think he would have wanted to see the compression tank on the "hot" side of the hydro sep so it would be closer to the circulators that flow to the radiators. But then, he also used to say "a difference to be a difference has to make a difference." ;-)
Retired and loving it.
• Member Posts: 21,846

@hot_rod, Gil always talked about the primary circuit being the one with the compression tank. The secondaries use the common piping as their compression tank. In this case, that would be the hydro sep. I think he would have wanted to see the compression tank on the "hot" side of the hydro sep so it would be closer to the circulators that flow to the radiators. But then, he also used to say "a difference to be a difference has to make a difference." ;-)

It is a tricky call with a hydrosep.

IF you have a mod con that want you to pump into the boiler on the return, as most do, because of the high pressure drop in the tight HX, then the tank may be best at the return port?

Distribution side pressure drop may be much lower, unless it is all small diameter, long pex loops

At the end of the day I think any of the 4 ports would work with a wide open sep, very low pressure drop in that chamber. In a properly designed one, which we feel is a 3:1 ratio pipe size to chamber size.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 21,846
Here is a drawing I remember from a 2006 article Siggy wrote. Used with permission. This was a common drawing showing how the boiler and zones circ can pump away from the PONPC via the closely spaced tees...but
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7,347
Here's one we did a while back. The Sep4 is definitely the PONOPC. Does that make both the boiler and system loops secondaries?

Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 21,846
I'm in the camp of "does it really matter" which side is called which?

I'm more concerned that installers don't understand the concept of closely spaced tees, that is what can make or break a P/S system.

I like your use of that upper "convince" port on the sep.

Some P/S mistakes we see over and over
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7,347
edited March 2020
hot_rod said:

I'm in the camp of "does it really matter" which side is called which?

That was somewhat my point with the above pic. However, if I'm drawing, specifying, instructing, giving advice, etc, I try to be consistent by identifying the primary as the loop with the PONPC for clarity.

hot_rod said:

I'm more concerned that installers don't understand the concept of closely spaced tees, that is what can make or break a P/S system.

I like your use of that upper "convince" port on the sep.

Some P/S mistakes we see over and over

Or the mistake that seems to be made by a lot of "plumbers" who don't understand hydronics or can't think abstractly: that it doesn't matter which port of the Tees is used to connect the loops.

Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 21,846
Ironman said:

hot_rod said:

I'm in the camp of "does it really matter" which side is called which?

That was somewhat my point with the above pic. However, if I'm drawing, specifying, instructing, giving advice, etc, I try to be consistent by identifying the primary as the loop with the PONPC for clarity.

hot_rod said:

I'm more concerned that installers don't understand the concept of closely spaced tees, that is what can make or break a P/S system.

I like your use of that upper "convince" port on the sep.

Some P/S mistakes we see over and over

Or the mistake that seems to be made by a lot of "plumbers" who don't understand hydronics or can't think abstractly: that it doesn't matter which port of the Tees is used to connect the loops.

Plumbers have come back to my jobs and capped off between the two tees, not understanding the concept

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 182
Would a hydraulic separator be better or worse if they were designed with the ports closely spaced?

• Member Posts: 21,846
rsilvers said:
Would a hydraulic separator be better or worse if they were designed with the ports closely spaced?

A hydraulic separator provides 4 or 5 important tasks

First it provides hydraulic separation, it is a super sized “closely spaced tees”

Being a large diameter it becomes an excellent low velocity zone to provide air and particle removal

Lastly, by adding a magnet it becomes a magnetic separation device, protecting all the components especially any ECM circulators

It establishes the PONPC by connecting an expansion tank at any of the 4 ports
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream