# Primary/Secondary muli-temp



## CTs2p2 (Dec 6, 2011)

So what I'm trying to figure out is on the secondary piping of (any Mod/Con boiler really, but what I'm installing now and got me questioning this) is a Viessmann Vitodens 200, zoned with circulators, 2 low temp, and a few high temp.. I noticed in ALL the piping schematics in the Viessmann book, with multiple temps Viessmann always draws the low temp zone/s first, off the low loss header before the high temp zones.. Viesmann also shows in all these drawings an actuated mixing valve (Viessmann draws in the wire controlling the mixing valve too, from their heating control) is this their reason, for drawing simplicity..? I would think that it wouldn't matter which was first but thought that the hotter water would go first then the radiant, but after looking at the book a little closer while piping today I switched it up and fed low temp stuff first (maybe a gpm thing).. I've done a number of the Viessmann 100 wall hungs and a few of the Triangle Tube Solo 150 and 250's.. I don't know for sure that I don't have a few piped the opposite way already.. Any thoughts as to why it would make more sense to supply the low temp zones befor the high temp zones?


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## Hydronictech1 (Jan 13, 2012)

I always feed high temp zones first, as the return will reduce temp in the boiler loop.


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## OldSchool (Jan 30, 2010)

I agree ... high temps first


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## CTs2p2 (Dec 6, 2011)

That's what I thought and how I've always done it, high temp first..

The answer I got from Viessmann was that the largest load should be piped first.. And that the typical European install is mostly radiant and high temp zones are usually smaller and/or more of a back-up. I don't know how factual this answer is, most people can't just say "I don't know" 

This system has been fired up and is working flawlessly so far..


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## Hydronictech1 (Jan 13, 2012)

CTs2p2 said:


> That's what I thought and how I've always done it, high temp first..
> 
> The answer I got from Viessmann was that the largest load should be piped first.. And that the typical European install is mostly radiant and high temp zones are usually smaller and/or more of a back-up. I don't know how factual this answer is, most people can't just say "I don't know"
> 
> This system has been fired up and is working flawlessly so far..


Viessman's feeding you a line of bull. The boiler has to be sized for both, with adequate flow in the boiler loop to do both. Primary secondary hydronically uncouples the loops, so flow rate in one has no effect on the other. Is it going to work? of course, but your heating capacity in the high temp loop is reduced whenever the low temp loop is running, because you'll have reduced water temps in that loop. 

Say you need 160 degree water in the high temp loop, but the low temp loop is reducing the temp in the boiler loop by 5 degrees. You have to run the boiler at 165. doesn't sound like much of a deference, but modcon's efficiency decreases as temperature/firing rate goes up.

I would pipe it the other way next time.


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## CTs2p2 (Dec 6, 2011)

Wasn't really a sizing question. Yes the primary and secondary circ's don't effect one another because either closely spaced tees or low loss header hydraulically separates them. The boiler is sized for the entire load of the house, not just the radiant.

The radiant is tempered with mixing valves, and boiler is running off outdoor reset, so any zones calling isn't going to change the target supply temp... Only a change in temp outside makes boiler go from 160 to 165. The water in the slab when running stays at @110-115*f no matter the outside temp or if the boiler is sending 180* to baseboard.

Viessmanns answer was not my job specific but more to why their drawings are that way, and that answer was the largest load should get taken care of first.

Here's a better description of the exact job in question..
Okay secondary piping only off the low loss header
Supply is 1 1/4 x 3/4, 5 tap manifold. Return is the same.
3 feeds get circs to do baseboard.. Returning as 3
2 feeds get piped to remote mixing stations.. Returning as 2

So here were my options (remember all water leaving headers is same temp based on outdoor temp mixed further away)
1. feed 3 baseboard zones and then the 2 radiant manifolds
2. feed 2 radiant manifolds and then the 3 baseboard zones


Having thought about this while piping it and off and on ever since. I've began to think that having the mixed loops first makes sense (to me now) in this application anyways.


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## Hydronictech1 (Jan 13, 2012)

CTs2p2 said:


> Wasn't really a sizing question. Yes the primary and secondary circ's don't effect one another because either closely spaced tees or low loss header hydraulically separates them. The boiler is sized for the entire load of the house, not just the radiant.
> 
> The radiant is tempered with mixing valves, and boiler is running off outdoor reset, so any zones calling isn't going to change the target supply temp... Only a change in temp outside makes boiler go from 160 to 165. The water in the slab when running stays at @110-115*f no matter the outside temp or if the boiler is sending 180* to baseboard.
> 
> ...


What matters is weather you are returning the mix fluid to the boiler/primary loop before it tees off to the high temp zone. This is what I thought you were saying, but when you talk about manifolds, it sounds like you are feeding the high and low temp zones in parallel. With remote mixing, and parallel supply, it's irrelevant which gets fed first. I'm not exactly sure which you are talking about now. If it is true primary secondary piping, with a high temp loop, and closely spaced tees off of that loop for the different temps, then there is no reason to ever feed the low temp first. The size of the load is unimportant, the temp of the fluid is.

When I talk about increased temps and decreased efficiency, I am referring to the fluid temps required to meet the heat load, which you have to program into the boiler when you set the curve. If you are cooling the primary loop with the return from the low temp zone before it gets directed to the high temp zone, you will have to increase the highest allowable temp on the boiler to meet the demand at the design temperature, thereby decreasing the boiler's efficiency throughout the entire heating season by offsetting the heating curve higher for no reason.


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## CTs2p2 (Dec 6, 2011)

Original post said low loss header application.. therefore not a single loop with multiple sets of closely spaced tees. I'm sorry that you were confused. 
If that was the application it would be pretty obvious to pipe high temp first.
I appreciate that your enthusiastic and want to share your knowledge as you seem knowledgeable. But its only help if it pertains to the actual question.

BTW the loop with multiple sets of closely spaced tees (true p/s as you said) You won't find that drawing in any of viessmann's diagrams, triangle tube either....


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## Hydronictech1 (Jan 13, 2012)

I must have missed that, my mistake.


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## CTs2p2 (Dec 6, 2011)

It's all good.. 

In your defense I should have wrote that first post a little more reader friendly. Everyone skim reads including me at times, it was an easy miss.. I've never heard or thought to call it parallel piping (which is exactly what it is) and that would have cleared things up. That was my mistake.



I also thought it would be completely irrelevant with parallel piping, I was going to pipe the BB zones first then the radiant... But it caught my attn while skimming through the book all 7-8 pictures with both temps, showed radiant zones before BB.. 
Maybe they figure if all or many BB zones are running, and are first they will steal all the flow, and remote pumps won't get anything if they call for heat.. Where if they are first the flow will be going through the mixed tee that may want flow too, allowing it to get what little it needs. 
Idk just guessing?


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