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Primary/Secondary with Mod-Cons
Comments
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monoflo
As long as you have enough flow and head left according to the graph on page 9 of the Vitodens TDM I would avoid using the LLH. In a monoflo system, you could conceivably use the internal pump if the flow requirements were 4.5gpm-5.0gpm due to the lower head loss. Obviously the system curve must be known before attempting this. With the 44/60 Vitodens you might never need the LLH with a monoflo system with a properly sized boiler pump.
-Andrew0 -
Since you are the design engineer on this piping plan, reverse return would be a better selection than direct return with balance valves, don't you agree?
I'm trying to make the case that balance valves in this particular situation are NOT indicated. Not needed. Extra baggage.
In being this stubborn, I think that we both made the case that they are unnessessary, unless you feel COMPELLED to put them in and adjust them.
Noel
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primary/secondary
Gentlemen........Great thread butfrom trying to figure out the different slants there seems to me to still be some confission as to which is primary and which is secondary.
I know the BOILER circuit to be secondary what say yee that it be or i am the only one confused
Brendan0 -
The Primary loop
is the loop that carries the FULL LOAD. In multiple boiler setups, the boilers would probably not be IN the primary loop, they would be individual secondary loops, or more likely, manifolded together into a single secondary pair of closely spaced tees on the primary loop.
A single boiler might be a secondary loop, or it might be IN the primary loop.
It's about the pumps, more than the boilers.
Noel0 -
The only
American made mod/con I know of is the Q95 and it doesn't require P/S.
Mark H
To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"0 -
I'm not really sure that any mod-con requires primary/secondary. Many manuals allude to this but only give examples where primary/secondary is used.
If you don't use primary/secondary however you have to design carefully with flow rates at the forefront. Delta-p will matter MUCH more than delta-t...0 -
I may be guilty of causing confusion in that regard.
In my mind, and with mod-cons especialy, the primary is what flows through the boiler(s) as a whole and the secondary is what flows through the emitter(s).
In the event of multiple mod-cons in the same system you can end up with something like a secondary (individual boiler)/primary (complete boiler loop)/secondary (emitters) system.
Really MUCH easier to forget about multiple boiler applications for this discussion.
I believe that most everyone (including manufacturers) will agree that the mod/con is the primary and the emitters are the secondary.0 -
p/s
>I believe that most everyone (including manufacturers)
>will agree that the mod/con is the primary and the
>emitters are the secondary.
That's what I was thinking, but I can see the possibility for confusion.
-Andrew0 -
Reverse Return
I love reverse return and in fact have it in my own house. Generally speaking, yes, I agree.
But in this case, it is still not what you would hope unless each circuit path has the same pressure drop.
By this it means if each is exactly the same size, number of fittings *and* has the same flow rate, they will have the same pressure drops. If any has a greater flow rate or a different size, all bets are off. There becomes at some point a path of lesser resistance. Once that is taken the rest finds a different peace.
As a practical matter, if these are identical bridges or "close enough", the pressure difference across the mains at their point of connection, point-to-point, will be very small. Enough difference to make a flow difference, for what that is worth.
Having seen, tested, done troubleshooting upon and balanced so-called reverse return systems, I can tell you from personal experience that 90 percent of the time you will need some kind of adjustable resistance to meet design flow rates.
I don't think you are being stubborn, Noel. That would never occur to me.
Cheers!
Brad0 -
To Noel & Brad with reference AND deference to Dan Holohan:
As I understand how Dan explained traditional primary-secondary it's really about keeping return temperature UP for conventional boilers. It's not about keeping return temperature DOWN for mod-cons.
On paper I will agree that traditional primary-secondary piping won't allow the return temperature to rise as long as you accept that counter-flow through the "bridge" occurs exactly as you want.
The low-loss header does not rely on this counter-flow. I will guarantee that the boiler (primary) sees the same return temp as the emitters (secondary) in a completely logical manner.
Siggy saw the light when he suggested that low-loss header type devices should be used vs. traditional primary-secondary.
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Define \"need\"
In my decades of experience, I find far fewer that need valves, when the ladder rungs are short and the same.
There aren't many Pri/Sec systems out there with the crossovers that you describe. That's now becoming more common. That's what my decades have shown me. More often, it's a regular reverse return system with a bunch of identical convectors conected to the mains through wide open and dripping balance valves.
Once I even saw an engineer design a 4" primary loop, entirely in the boiler room, with a circulator the size of a Saint Bernard on it, and a 4" BRASS balancing valve in the loop.
That's how it got built!
I later asked the engineer how I'd know if it was adjusted wrong. His answer was almost as long as yours.
Noel0 -
That wraps it up, Mike!
Exactly what I have been saying over several posts
P/S has much greater application in conventional boilers and defeats condensing boilers in some ways without special controls and variable pumping. Simplicity is the first casualty.
A LLH rocks. Amazing, simple device.0 -
Bad engineering
is not what I am about. So, you found one. That anecdote pertains to me how?0 -
I didn't imply bad engineering.
I've asked you since the beginning if there aren't a lot of systems that don't "need" balance valves on them, that get them anyways.
You keep inferring that they are only installed when they are needed.
I've still not satisfied myself that you have described the difference between needing, and not needing them. Help me out, here. I've given you very blunt examples of situations that I've seen them in, and I've found that the room thermostats seem to be doing most of the balancing.
I feel that you've been evasive, though you've not been short for words.
If you are going to be involved with most of the posts on here, it's fair to question your views, as I would any one else.
Keep your cute quips for the bears, I'm not at all about being belittled in the process of discussing heating. I thought you'd been here long enough to know that THAT tactic brings nothing to the discussion.
Noel0 -
No arguement there, Mike and Brad.
Noel0 -
It's still about the pumping, Mike
If you have a mod con that feeds 15 GPM into a primary loop with a different flow rate than the boiler (more, usually), it can be a secondary loop.
It's not about the boiler, any more than "pumping away" is.
You described one of my original senarios, though, but didn't define primary secondary on a larger picture, where we don't have to redefine the terms.
The pump with the whole load on it is the primary pump, in the primary loop. The closely spaced tees are all on this loop. The secondary loops all can be identified by these tees. They're off the branches of these tees.
The boiler can be anywhere in this picture.
If we stick to some same set of terms, whatever terms, it becomes clearer.
Noel0 -
Well
I have seen a few cases where Munchkin boilers were installed without P/S and it wasn't pretty. Once the Munchkin ramped up it's firing rate, the boiler started making a great deal of noise. I only saw this on the larger units 140k and up.
I think this has more to do with the low water content of the boilers than whether they are mod/con. Same thing will happen on any low mass/low content boiler.
As it stands now, the boiler only sees a call to make something hot. It has no idea what the load is and reacts to the delta T. The larger the delta T, the higher the boiler fires. If there isn't enough flow through the HX to pull the BTU's being generated out, the water in the HX will try to become a gas.
If we could modulate the boiler according to the load, the P/S issue would disappear.
I agree though, the boiler control should be looking at supply water temp not boiler water temp.
Mark H
To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"0 -
American Modcons
I get confused as to what is an "American" modcon. As we all know, most of the American modcons are European technology and many use European parts but as far as I'm concerned if it's made in the US, by American workmen, it's American.
That said, it is interesting that the Knight comes standard with a system sensor. The unit works standard with built-in sensors that operate based on water temp sensed at the boiler but can be easily reset to operate on the furnished system sensor, if the contractor installs it.0 -
If we could modulate the boiler according to the load, the P/S issue would disappear.
The Vitodens DOES JUST THAT and I believe that it is currently unique in this ability.
No thermostat connections... Why? Because IT DOESN'T NEED A THERMOSTAT!!!! In fact a thermostat makes this goal IMPOSSIBLE! It does modulate the boiler to the load (actual heat loss) on a nearly instant basis. To achieve this, variable primary (boiler) flow is an ABSOLUTE requirement!!!
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That's why I put American in quotes. NONE are truly of this country. Think Munchkin, Ultra, Knight, etc. for "American".0 -
not fixed primary
I thought your boiler has a variable pump, so your flow rates vary?
so everything is varying, you just have to insure it gets its minimum flow?0 -
but mike?
how does the vito know your "actual heat loss", this is quite curious????????0 -
Hey Mike...
I was under the impression that only Viessmann worries about excessive flow and all the rest are worried about getting a high enough minimum flow through the HX.0 -
It only knows
what you tell it. It responds according to the programming parameters that you set. It does not know what the actual heat loss is.
I have a Vito system with 14 zones on it. We programmed the Vito to keep the highest temp zones satisfied on design day. If the garage slab zone calls, the Vito can't tell the difference and works according to the programming we set.
Mark H
To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"0 -
Read versus Need
Noel, you may want to re-read my posts. The answers are all there. I am the last person anyone would call evasive, far from it. I am just not sure what you are asking. You understand your own questions; I am at a loss. To my thinking, I addressed your questions directly and thoroughly, -not to mention ursine diligence.
For example: The term "need" you seem to be stuck on in a few posts. By "need" I did state that it meant "to meet system flow requirements". Not "need" as in, "without it, life would cease". Asked and answered.
Not short for words but that is a function of being thorough and writing for a broader audience. Others read here too. I do and learn every single day, including from you.
I never mind a good question. All I ask is that you read the answer(s).0 -
decades
you have worked on modulating condensing boilers for decades? really? or do you mean that your decades of experience earned you the right to speak any way you want to the good people that post here regularly? or is it that your decades of experience trump the laws of physics? you are an angry rude person.0 -
I'm sorry you feel that way
I'm passionate, entheusiastic, hungry to understand, but not angry.
The condensing boiler in my house was put in in 1990. The condenser in my previous house was put in in the mid 80's. It vented with PVC, even then. I was being sarcastic when I said decades, because as we all know, they didn't catch on until recently. Nobody wanted one. I see how that could be taken different than I intended it.
I believe I'll explain what I see that is contrary to what is being described, and you can decide. I'm not just going to go away until I understand.
I want to break this system down into segments. There is the segment that contains the boiler, the primary pump, and the resistance that this pump sees, which relates to it's flow rate, and therefore the Delta-T across the boiler. The boiler may vary the temperature, and the input, therefore the Delta-T, but not the flow rate. As with any heat exchanger, if you decrease the flow rate, you decrease the efficiency; the amount of heat transfered. That's basic physics.
The next segment is the primary loop. That can be built with all of the loads in sequence on a single main, to achieve a high delta T. That can be built as a ladder style Two pipe main, reverse return to keep the supply temperature low. Both have advantages. Neither piping system will decrease the heat loss of the structure, therefore the load. The load remains constant, at a given supply temperature, chosen by the controller.
The secondary loops aren't really an issue, here, because we haven't suggested adding head loss to those loops. Those heat exchangers would act similarly, decrease the flow through them, and the output decreases.
The whole issue here, is how decreasing the flow through a mod-con heat exchanger can increase the efficiency of that heat exchanger. In my mind, the opposite will occur. The factors that actually determine system efficiency in this case are the flow rates and the amount of radiation. Decreasing the flow rates through a voluminous heat exchanger isn't the same thing as sizing the supply and return TO the radiation and adding enough radiation to give you the Delta-T that you desire WITHOUT loading down a pump with valves. Keep the Delta-P low and the Delta-T high.
Therefore, I want to know (passionately), "How do you know when you have decreased the flow through the primary loop, and the modcon boiler enough to improve the efficiency."
"Only as much as you need to," isn't really an explaination on why or how much to restrict the primary loop.
I'm sorry for upsetting anyone.
Noel0 -
Great thread all!!!!!
Very interesting reading, good general banter that was informative. Thanks.0 -
Easy...
Noel's neither angry, nor rude. He's one of the best and smartest regular posters on this board (as is Brad), and he has valid questions.
I, for one, am happy to take in this exchange between Brad and Noel, who both have my respect. It will likely help alot of people who are reading it.0 -
Let me try and take a stab at it...
The boiler may vary the temperature, and the input, therefore the Delta-T, but not the flow rate. As with any heat exchanger, if you decrease the flow rate, you decrease the efficiency; the amount of heat transfered. That's basic physics.
There is at least one mod-con that can vary the flow-speed across the HX as a function of firing rate. I presume that the aim of this (higher-cost) approach is to present the lowest-temperature water to the HX as possible by slowing the flow through the primary circuit to achieve the desired ΔT. This mod-con also eschews the use of P/S, opting for a "direct" connection to emitters or an LLH approach.
Therefore, I want to know (passionately), "How do you know when you have decreased the flow through the primary loop, and the modcon boiler enough to improve the efficiency."
I think we can all agree that the greater the ΔT between the HX and the flame/flue gases, the higher the heat transfer, all other things being equal. Flow velocity plays a critical role in determining that ΔT and being able to modulate it hence plays a critical role in the HX efficiency.
For example, if I have a system whose primary loop has a fixed-speed circulator and a supply setting of 160°F but only has one out of 6 zones calling, then the ΔT on the primary is going to be very slim - the boiler will maintain that 160°F supply temperature via low-fire but the return temperature will rise because most of the BTUs are not "used" by just the one zone that is calling. Thus, the return temperature will likely rise above the optimal temperature to maintain condensation.
On the other hand, if a boiler can modulate its output and circulator speed, it can reduce its firing rate and circulator speed such that the primary loop flow slows, a greater ΔT is developed and condensation can occur across a wider range of system conditions. I think this is what Brad was getting at. If I understand his explanations correctly, Brads approach of using setters and the like is an attempt to mechanically achieve what variable-speed circulators can do electronically.
The extent to which a boiler benefits from a variable-speed pump or circuit setter may thus depend in good part on the system it is attached to. A system whose emitters never go above 110°F will likely condense 100% of the time, because even if the ΔT was zero and the return temp 110°F, the boiler could still condense very efficiently. On the other hand, systems with a need for a lot of "hot" water (think IDWH, high-temp baseboard or staple-up, etc) could benefit quite a bit.0 -
You nailed it
Constantin. Exactly. Balancing valves are my second choice to good primary modulation. Another posting is coming to mind...
Thanks
Brad0 -
I repeat, then,
"How do you know when you have decreased the flow through the primary loop, and the modcon boiler enough to improve the efficiency."
ie: Tell me how to set the valves for maximum efficiency.
I say, "Wide open, on a non-variable speed pump."
What say you? (show your work)
Noel0 -
I already showed my work, Noel
If the return temp goes too high, a mod-con cannot condense efficiently. Thus, the aim would be to maintain a ΔT in the primary loop in systems that have a need for high-temp water.
There are many ways to achieve that ΔT though my preference would be a motorized mixing valve or variable-speed circulator since either option can be responsive in far more ways than a fixed-temp, fixed-flow, etc. valve.
I would appreciate if you could show how your setting suggestion is superior.0 -
Ok, but there are a few other things nagging me here. I may be wrong, but this is where I"m at at the moment:
First, emitter temps are average temps, taking into account the dT across the emitter. If flow is increased, your supply temperature can be lower and your return temperature higher, if it's decreased, higher and lower respectively. In "low flow requirement situations", the output required is obviously lower than normal and your supply temperature, regardless of dT, should also be lower in the vast majority of the cases because it's not that cold out and we're of course using reset of some kind here. So worrying that your dT may not be that 20 or 30 you want to see when it's warmer out is less of a concern because the average fluid temp is still dropping.. I would speculate that in most cases, you can maintain a similar or lower return temp simply due to this effect.
That is to say, design your "design dT" for a wider spread, to reduce flow and drop that return temp, great. But when we are not at design, the average temperature of the fluid should still be dropping and condensation should still be roughly achievable because the SUPPLY TEMP is dropping, even if dT rising to some degree.
But this must have practically no effect on the BTUs released into the water by the boiler. Likewise, flow through the boiler must be of fairly minimal concern in terms of managing a dT for optimal transfer. Now this is where I'm shakiest, but this is my reasoning:
dT across the HX supply and return is basically meaningless in terms of extracting heat from the boiler, though in conjuction with flow it is a measure of how much heat you are extracting.
Extraction speed is driven by the difference in temperature between the HX itself and the fluid temperature.
The HX is very hot. Yes?
The fluid average temperature varying 10 or even 20 degrees due to high or low dTs should thus be a very small effect on the overall dT between HX and water. The only real concern is that you have enough flow to transfer the heat away from the boiler fast enough that you do not jack the water temperatures up too fast and short cycle the boiler. So a 10 degree return to supply rise vs a 20 degree (5 degree average temp shift) should be something very small like a 2% or 3% reduction in output. Which is further reduced in importance by the fact that you are obviously far under the output requirements of the boiler (so volume of output is not the concern) and your load is obviously very low at the moment (so percentage efficiency reduction may be significant, but is being applied on a very small load).
And, ultimately, the question is not just "what is the drop across the emitter" but much more importantly "am I dissapating the heat from the boiler". changing your flow rate through the emitter, unless your current flow rate is inadequate, will not dissapate the heat from the boiler faster, so either its own "unused heat" will heat itself (negating your dT increase across the emitter by your reduced flow) or it won't because you are using it all; in which case you're either at design, or in a modulated state with a lower supply temp.
So.
Am I all wet? Cause all this LLH talk and part load dT manipulation and variable speed pumping is starting to sound a lot like a fad to me, and I'd like to see, specifically, where the hole in my thinking is if it's not. Especially since the cost increase on a system for dP pumping and a LLH is quite significant, I am currently very skeptical indeed that it's at all worthwhile.0 -
easy one...
Since I have yet to see an efficiency gain (in numbers) from installing and adjusting balance valves on every crossover branch on a 2 pipe primary secondary system with a fixed speed pump and no mixing valves,
I'd not install and adjust those valves. Cost to buy, and cost to adjust are saved. Future leaks at the packing are avoided. Electric consumption will be slightly less. Combustion efficiency will be slightly MORE, due to the higher flow rate and the controller seeing a need to reduce (shift) slightly the target supply temperature.
Noel0 -
Rob,
It's a matter of degrees.. :-P
I maintain that ΔP circs and the like have to prove themselves on the basis of electrical savings unless you can quantify their impact on the efficiency of the boiler. Some boiler manufacturers publish their HX efficiency as a function of return temp and firing rate.
I'd like to think that a sophisticated design-house like yours could then look at the emitters in the home, the temp-bins predicted by ASHRAE, etc. and establish what percentage of time the performance of the mod-con will be impaired by high return-temps. Remember, this is not just a matter of efficiency, but DOE output as well.
Thus, I strongly disagree with your wording re: water temps, HX efficiency, etc. in mod-con boilers. Return temp can have a huge impact, reducing the DOE output of a mod-con by as much as 10%, if it's too high.0 -
You're not answering the question
How does a high return temp benefit a mod-con boiler? A fixed-speed circulator on the primary loop can cause it. A motorized mixing valve or variable-speed circulator (among other things) can avoid it.
Your contention re: energy savings is also disputed by "real-life" electrical energy monitoring. The ACEEE published test results showing a mod-con with a fixed speed circulator consuming 8x more power than a mod-con with a variable-speed circulator. Since circulators are among the highest energy users in mod-cons, I assume that the variable-speed circulator was the deciding factor.0 -
You are talking about a different system.
I'm not. I'm still refering to the system in THIS post.
Read through it again, please.
When you zone a Mod-Con with circulators and primary/secondary (or low-loss headers) do you use flow limiters in each zone such that secondary flow when ALL zones are calling CANNOT exceed primary flow by more than 30% or so?
Noel0 -
NRT Bob
"Ok, but there are a few other things nagging me here. I may be wrong, but this is where I"m at at the moment:"
I had to re-post to keep it from going vertical on us. Hope you do not mind. You bring up the core paradox.
Your post illustrates just what we have been discussing and what my point was thus far, that primary/secondary with a ModCon is not the best way to go. There is, as you posted, a "Chasing of one's Tail" in an effort to both
a) Extract maximum heat from radiation (low flow there) and
b) Have your boiler outlet temperature and certainly the return temperature below the flue gas dewpoint (high flow rate there to narrow the delta-T) yet maintain efficiency.
This is why P/S has an opportunity to defeat itself in some ways, unless modulation of primary (boiler side) flow can be tracked to boiler fuel firing rate (output).
Series piping makes more sense provided that they can track load.
Parallel P/S makes sense when you want to create potentially different temperatures or zone the place a certain way.
The LLH principle is really something to behold. It took me a couple of times studying it to "get it". The modulation of primary (boiler) flow is key to this however.
Forgive me if the following is too basic; it is to illustrate my understanding and is based on the Viessmann diagram (p. 35 of the manual) but also applies in principle to Calleffi:
Supply water enters the top left, drops to the heating circuit exit, top right.
Return water from the heating circuit enters bottom right and returns to the boiler bottom left.
What happens inside: Return water entering from the heating circuit either rises to mix with the incoming boiler supply or it falls and exits to the boiler. The boiler sees ONLY the coldest return water.
The entering boiler hot water supply can only exit to the radiation circuit. It does not "fall" to mix and elevate the boiler return temperature. One way in, one way out and they are both near the top.
If the boiler circuit were constant flow, there would indeed be times when there would be a T-Mix on the boiler return, especially as the radiation circuit flow rate changed to suit zone calls.0 -
First, let me remind you I am not advocating for high return temps and flow AT DESIGN. You design for higher dTs and lower return temps.. great. No problem. That's ideal and relatively easily achieved.
When you NOT at design, however, I don't give two cents for the DOE output of the boiler being reduced.. we're already under the max output. If my 76kBTU output boiler is now only capable of 65kBTUs.. well, who cares? My load is lower than that; if it wasn't, the zones would be calling and this "problem" would not exist.
All I care about in that case is efficiency, and I care about that less and less as the load drops (though, being me, I still CARE, but its importance is lessened as the volume load you are servicing drops).
My case here is EFFICIENCY is barely moved by flow rate or dTs across the heat exchanger.
Return temp is important. But you can reduce it two ways:
1: high dT with a higher supply water temperature.
2. low dTs with a lower supply water temperature
I am simply saying if you are in a part-load condition (lower than design flow due to multiple zones being turned off) you should also be in a lower average temperature condition (using outdoor reset) because obviously your load is dropping by some mechanism... most likely outdoor temperature.
So your return temp is more or less maintained or lowered anyway in these part load conditions, whether you are using fixed pump and bypass, no bypass, or variable speed pumping.. Ultimately any BTU injected into the system must either be emitted or returned to the boiler.
Changing flow through the boiler will not affect this.
Changing flow through the emitters will not affect this, ultimately.
The only way to truly affect this is either to emit more heat (Buffer tank storing excess), or put less heat in the system (modulation). Any funky tricks with flow may technically preserve some output and a small fraction on efficiency, but IMHO at this time, not nearly enough to make it worthwhile to worry about, at least not upon my cursory overview which may be batty, I'm not sure yet
0
This discussion has been closed.
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