Primary/Secondary with Mod-Cons

Leo G_99

one thing amiss in your answer, I think. LOL!

"As load drops off, so presumably does your HWS temperature, maybe as low as, say, 110F."

Thinking in terms of a Mod/Con equipped with OR, I always thought that as the load drops off, the boiler modulated its' firing rate down to keep the target temp where it wants it. If the firing rate stayed high, it would overshoot the target, and start to cycle. To my way of thinking, the modulating portion of the boiler is a way to keep the boiler firing for as long as possible at the target temp. Does this make sense?

tekmar, on a mid boiler, tries to emulate this by controlling the firing of the boiler and controlling the mixing of the output to the emitter. By allowing the boiler to both over and undershoot the target temp, then mixing to an average, the tekmar control tries to cycle as little as possible as the load decreases. To me the mod part is just a more refined way of accomplishing this. That is maybe why a VS circ would help to bring the efficiency of the unit up. Less heat made, less water volume needed to extract said heat?

Leo G





Leo G

Mike T., Swampeast MO

AKA Zoning Mod-Cons with Circulators

Gentlemen:

Please accept this in the spirit intended. I both want to learn and perhaps provide topics for consideration of others. If I mention "Viessmann" or "Vitodens" frequently it's not on account of personal prejudice--it's because of close and careful observation of a Vitodens and CAREFUL study of ALL literature I can find NO MATTER HOW OLD OR HOW NEW.

That said and to BEGIN the discussion:

-----------------------------

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?

Andy_14

My VERY LIMITED knowledge says add up all the secondary zone flows and use whatever size pipe it takes to get that flow for the primary loop.

I'm sure you knew that, but since that's all I know....I said it. Now the real Pro's can answer!

Brad White_144

Yes

I make judicious use of balancing valves, the good ones I always write about and balance them to as close to my actual required flow rate as possible. 30 percent is way too wide, in my opinion.

My rationale is that I do not want excess return to cause a narrower Delta-T and also any "unused supply" water quenching the condensing process.

Wayco Wayne_2

No actually

I only use circuit setters when I have a modulating secondary circuit. On straight forward single loop secondary circuits I use a 3 speed circulator and adjust to the closest temp to the design delta tee. WW

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Guy_6

Flow

I will have to think about this further, as I haven't yet had any coffee, but here's my initial thought:

All of the Mod/Cons that I have dealt with are operating on the concept of a certain Delta-T. If the primary loop flow is based on , say , 1 GPM per 10K BTUs for a DT of 20, then wouldn't any "excess" flow from all zones calling would actually be a product of the system being sized too large for the boiler, creating insufficient heat issues? In other words, exceeding flow would equate to exceeding thermal capacity.
Also, the primary loop will restrict itself due to it's sizing, thus protecting the HX to some degree.

Unknown

Brad, could you explain that more?

Slowing the flow in a SECONDARY circuit will drop off FEWER btus, therefore DECREASING the load on the system.

Since this doesn't change the flow rate in the PRIMARY loop, how does it lower the return temp to the boiler?

Noel

ALH_4

> Slowing the flow in a SECONDARY circuit will drop

> off FEWER btus, therefore DECREASING the load on

> the system.

>

> Since this doesn't change the flow

> rate in the PRIMARY loop, how does it lower the

> return temp to the boiler?

>

> Noel

ALH_4

p << s = ?

If the secondary flow is much greater than the primary flow, the boiler is forced to fire hotter to maintain the same supply temperature <i>initially. Once the system has come to equilibrium, as a constant circulation system should, the main effect of excessively high secondary flow rates is a tighter dT. With a dP bypass and zone valves, the pump is circulating through the bypass to varying degrees all the time which also elevates the return temperature to the LLH. I dont see how to guarantee you are within a certain percentage of flow with an internal variable speed pump though as far as I know the Vitodens 24/32 is the only boiler with this feature.

In my opinion, primary/secondary pumping is a real drawback to high head loss condensing boilers. Only under very specific circumstances can you get the coldest return water possible back to the boiler. The Vitodens 6-24 and 11-44 can be used without the LLH with only a dP bypass in many applications. This is as good as it gets efficiency-wise in my opinion. The Prestige literature recommends P/S piping, but the head loss is very low. Why is this? Is it for CYA reasons? As I see it, a boiler design that lets you avoid P/S piping is a valuable feature for a modulating condensing boiler.

-Andrew

Brad White_9

P/S at the Bridge

Noel, I was referring to the flow rate across the bridge, with the secondary circuit drawing off what it will. The primary loop flow is not affected but the temperature certainly is. It raises the HWR temperature more if left unrestricted.

Say a supply flow enters a bridge which has closely spaced tees at 5.0 GPM at 150 degrees. The secondary circulator draws off 3.0 GPM of 150 degree water at a given time (takes it back cooler of course! Say it comes back at 130F). But that excess 2.0 GPM goes back to the return, mixing with the 3.0 at 130F to make 138 degrees. Above condensing and warmer than if the secondary extracted more of the heat.

The balancing valve I speak of is at the return side of the bridge. Without the restriction of a balancing valve there, the only limit to flow is what the primary circulator can move at the differntial pressure created at those points.

I am not advocated P/S piping for ModCons, mind you and it is because of this principle. Just explaining that if this is what I am working with, how will I limit the return water temperature increase.

Unknown

could you simplify?

How will you drop the return temperature to the boiler?

Where does this occur? If the flow rate through the BOILER stays the same, and you don't INCREASE the load, how does the return temperature to the boiler drop?

Noel

jp_2

why not?

why not design the 'zones' for at least minimum boiler flow and forget P/S piping ? of course a safety factor above minimum flow.

D107

Andrew, what's your opinion of P/S for GB-142 or Munchkin T-80s?

Thanks,

David

Brad White_9

That is the key, Noel

and why I like variable primary and variable secondary flow in an ideal world. Constant flow circulators fight the temperature drop especially in P/S piping. What is not used gets returned, simply. Absent load and an even lower firing rate, nothing else one can do with this setup.

Honestly, my best solution especially where close-coupled, is a buffer tank, something to store/bank the higher return water to give the ModCon time to modulate. Low mass is a liability during higher firing rates and is an asset only on start-ups it seems.

Mind you, as another poster stated, it does indicate an over-sized boiler.

My best heating control ever was a Burnham 204 with Centra 4-way mixing valve, TRV's and constant circulation. Flat-line comfort with night-time parallel shift. Enough mass in the boiler to keep cycling down. Sipped water on mild days, larger sips on colder days. So decoupling makes the most sense and allowed my over-size Burnham some rest between firings.

Mike T., Swampeast MO

VERY interesting gentlemen!

Before I make a fool of myself I need to know something about mod-cons other than the Vitodens.

You're using outdoor reset as is very typical.

You've set a heating curve.

Where is the temperature for that curve sensed? Inside the boiler itself? At the primary (boiler) supply? Or is it the blended temperature at the secondary (or LLH) supply to the emitters?

I've studied the literature for a number of other mod-cons and for some reason I think that the curve temperature is either sensed inside the boiler or in the primary loop in most cases. Please let me know. It makes a BIG difference in what will follow. Thanks!

Mike T., Swampeast MO

That's right along the lines of my reasoning as well. Remember though that minimum flow can be guaranteed using differential pressure bypass. It's MAXIMUM flow that really matters as literature for nearly every mod-con I've seen clearly states a MAXIMUM flow through the boiler.

Brad White_9

The GB and the Prestige


sense inside the boiler.

I have one GB in the field that is controlled by the Tekmar 263. It senses the supply water in the secondary loop and modulates the GB accordingly.

Mark H

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Unknown

Still looking for the short answer...

I can't think of ANYTIME that restricting flow through a circuit will INCREASE the load on the boiler.

I don't want to outright disagree with you, unless I understand your POINT.

Why do you restrict flow through any circuit, other than to INCREASE flow through a different LOAD circuit?

Please relate your answer to the boiler load.

Noel

ALH_4

temps

I never understood exactly why these boilers don't sense the boiler temperature as well as the system supply temperature. The temperature in the LLH can vary depending on a number of factors. Wouldn't putting the supply sensor in the supply pipe give a more accurate supply temperature? Shouldnt the boiler temperature be based on the secondary supply temperature and not necessarily on the outdoor temperature because of this? Just thinking out loud.

-Andrew

ALH_4

opinion

In my opinion, both the GB and the Munchkins must be piped using primary/secondary.

-Andrew

Brad White_9

Short Answer? We agree.

That *was* short!

I think you either missed my point or I was not being clear- probably a little of both, Noel.

Restricting the flow in a circuit (a parallel circuit such as a bridge) will not increase the load to the boiler but will pass more unused HWS to the boiler return. It reduces the load on the boiler which can lead to short cycling and raising the HWR temperature above the dewpoint, thus decreasing efficiency in any event.

"Why do you restrict flow through any circuit, other than to INCREASE flow through a different LOAD circuit?"

In primary/secondary piping and as it pertains to this situation (the return side of the bridge), this statement does not apply. Simply, this is because flow in one circuit will not affect flow in the other (which you know).

Throttling my return valve will at best force my secondary (constant-flow) circuit to rat-race or reverse flow between the tees while limiting the pass-through HWS flow to a minimum.

No effect on the secondary circuit flow at all, but does allow a T-Mix which will eventually reduce output to that secondary circuit by diluting the temperature supplied to it.

jp_2

say that again?

Isn't it somewhat impossible to have secondary flow higher than primary?

i can see secondary taking all of the primary flow but not more? what am I missing here?

Mike T., Swampeast MO

I kind of figured that for the GB.

What about "American" mod-cons?

Unknown

We're getting farther away from your statement

"My rationale is that I do not want excess return to cause a narrower Delta-T and also any "unused supply" water quenching the condensing process."

I don't understand how you can avoid "unused supply water" from circulating through the boiler in the primary loop with a fixed flow rate.

If you were to decrease the flow rate in the primary loop to lower the return temperature, you also reduce the load on the boiler. Not apples to apples anymore.

Noel

Brad White_9

Not at all impossible, JP

Whatever goes into a tee has to come out of a tee. Nothing says that flow cannot go backwards between closely spaced tees. If the secondary circuit has a higher flow rate than the primary, that is exactly what happens.

Think of a higher temperature injection system serving a radiant system. You fire a 180 degree boiler to serve a radiant manifold.

Your radiant manifold has a load of say, 40,000 BTUH and a 10 degree delta-T. So you are flowing 8 GPM at say, 110 degrees.

You have to inject how much flow from the 180 degree source into that to obtain 110 degrees? 1.0 GPM.

So your secondary circuit has 8.0 GPM, borrows and returns 1.0 GPM of hotter water. The flows are disparate and so are the temperatures.

Of course in low temperature systems the flow rates begin to converge...until they match. The wider the difference in temperature between the heat source and the emitter requirement, the greater the difference in flow rate.

Does that make sense?

Mike T., Swampeast MO

Of course secondary flow can be higher than primary. That's exactly what primary/secondary (or the low-loss header) allows.

12 gpm secondary - 8 gpm primary = 4 gpm that's bypassing the primary but still circulating through the secondary.

Brad White_9

Exactly my point, Noel

You cannot avoid having "unused supply water" from circulating through the boiler. This is why, at best, I can at least restrict it in a constant circulation system.

ALH_4

flow

The flow can "reverse" between the closely spaced tees, though what I'm specifically thinking of when I say this is the diagram on page 35 of the Viessmann TDM.

-Andrew

Unknown

Good, we narrowed it down...

The reason that you restrict it is?

To slow the flow through the boiler?

That increases efficiency over using a smaller circulator, how?

Noel

Mike T., Swampeast MO

A VERY TROUBLING THOUGHT

I'm fairly certain I can prove that even with exactly equal but fixed flow in any form of primary secondary, that it is impossible to modulate the boiler such that net boiler output = gross heat lost by the structure in any but one and only one set of conditions and regardless of any form of reset or control no matter how well-adjusted.

If so, I'm positive I can prove that to achieve a goal of modulating net boiler output = gross heat loss that variable flow is required somewhere either on the primary side or on the secondary side (or throughout if no form of primary/secondary is used).

Will be quite busy for the next few days, so it may well be next week before I can post as I really need to set up some hypothetical systems and provide drawings to make things easier to understand.

Brad White_9

The reasons

To restrict? Primarily to avoid passing more hot water directly to the return than is necessary at full load. It also apportions flow to the other bridges which have their own balancing valves. Does this slow the flow through the boiler? Yes, but that is secondary (no pun intended).

How does this increase efficiency over using a smaller circulator?
It doesn't. That is a new element in the equation. Ideally we agree the circulators should not have any excess. The assumption is that you have the smallest fixed circulator but still need to trim the flow across the HWS and HWR mains.

Balancing valves fine tune the discrepancies between available circulator selections and the realities of the system in which they are applied.

This is why variable speed pumping is so darn cool!

Unknown

trim the flow to achieve WHAT?

If you don't change the load or the input, what do you gain by "trimming"?

I propose that you aren't doing anything at all for efficiency, you are only selling unessessary balance valves, and unessessary labor to adjust them, and an oversized pump, and increasing the electricity consumption in the process. What effect on consumption will closing the balance valves cause?

I also propose that if you select the fin tube diameter and primary loop diameter properly for the Delta T that you design for, and correctly size the pump, the balance valves that you speak of will NOT change the fuel costs one bit, where ever you adjust them to be set?

What am I not seeing that would cause you to increase the Delta P on any loop?

Noel

jp_2

Ok,

guess I was thinking more along the lines of a DC boiler

so we are talking high flow rates with little dT, can't think off hand of a device that would require that? and the need for P/S in that situation, seems the boiler could handle it directly? since we are talking only about mod/cons here.

Uni R_2

What about a small Vito plus Monoflo?

Andrew,

Would you ever try and set the minimum speed on the circ to the minimum velocity needed for a monoflo system's heat dustribution and run it without a LLH (provided the total head from the heating circuit was low enough of course)?

Mike T., Swampeast MO

so we are talking high flow rates with little dT, can't think off hand of a device that would require that

Remember that many (if not most) mod-cons base the recommended primary flow rate on good old 20°F delta-t or 1 gpm per 10 mbh of boiler "size".

There are certainly times where higher flow (and thus lower delta-t) in a secondary zone is desired. Say a kitchen with lots of cabinets and relatively high loss per square foot of heated floor or limited space for a radiator. Move more water and you raise the average water temperature and raise the output without having to produce a higher supply temp. Similar case in say a north-facing room with high glass to floor area. Say in some places that you prefer a 15° or even lower delta-t (perhaps for more even RFH temps). If the primary circulator is based on 20° delta-t then you're moving relatively more water...

There are also unavoidable situations. After all, if you find a circulator that moves exactly the amount of water called for in your design at exactly your computed head loss, you can consider yourself rather lucky... What happens when you can't find such a perfect match? You generally select the next larger circulator. What if the customer wants relatively small rooms like baths individually zoned? Could well be hard to find a circulator small enough and after all, "Hey! It's a bath. They'll probably want "quick heat" and the increased flow will only help."

Then there are pre-existing challenges as well. Say it's an older home with a monoflow system and there have been significant insulation/weatherization improvements and/or the system was quite generously sized to begin with... To achieve balance you'll have to move close to the original design flow even if such produces a delta-t significantly lower than 20°F. Say it's a converted gravity system with those huge but "backwards sized" pipes. The typical and recommended B&G 100 or Taco 007 is likely to move 20+ gpm.

Say you're trying to save a buck on some sort of HX coil--like in an air handler or an indirect. Instead of using a larger HX, you merely increase the flow...

ALL SORTS OF WAYS THIS HAPPENS AND FOR MANY DIFFERENT REASONS--SOME OF WHICH ARE EVEN "GOOD".

Unknown

I think I'm with you Noel. Energy is conserved. Either flow fast through the load circuits, or flow slow with more bypass.. a btu is a btu is a btu, and the net result on water temperatures in the system, supply and return, should be equivalent. The only thing changing flow rates in load circuits does is perhaps emit heat slightly faster or slower based on the average temp of the loop driving heat transfer out of the pipe. You don't get to "pretend" you are emitting more heat by bleeding pump energy off with balancing valves to get your return water temperature lower. In that case you simply return more boiler water into its own return circuit directly, raising the temperature there instead of through a bypass or via the "natural" return temperature of the load circuits.

As in the other thread, I can see variable speed pumping as useful for maintaining a set delta-T across a boiler heat exchanger or something, but I don't see that as a particularly critical issue. Flow and delta T together draw heat from a boiler, a btu is a btu, and if it's not emitted then it's returned to the boiler.. period.

Now the buffer tank.. whole different ballgame.

Brad White_9

It is not about

increasing or decreasing the load on the boiler, but about herding the water to do your bidding and avoiding those situations of short-circuiting which defeat efficiency.

All of this goes to illustrate what John Siegenthaler recently posted about changing his thinking about parallel secondary circuits in favor of a Calleffi or other low loss header arrangement.

The balancing valves (none of which I sell by the way; I sell nothing in terms of a product) are a tool to achieve proper distribution.

As stated, this is what you do to make the best of an imperfect installation (one you may find yourself dealing with as an existing situation) but one which I would not design new in conjunction with a ModCon boiler.

Unknown

Bear with me, Brad

Apparently, I have trouble focusing my question.

"The assumption is that you have the smallest fixed circulator but still need to trim the flow across the HWS and HWR mains."

Trim until what happens?

Be specific, and brief, please.

Noel

jp_2

BUT WAIT!!!

i gotta think more about this, but if your flow is much higher through the secondary, to put 'much' heat into the circuit the boiler has to run much hotter water temps cause its going to get tempered so much. I thought thats what we are trying to do, reduce boiler temps reduce fuel usage!

as you said 12gpm secondary, 4 gpm primary, as an example.

I understand your points but isn't that the wrong direction?

Brad White_9

Look out Noel! There is a bear with you!

In the case we are discussing (P/S bridge returns back to the primary HWR), flow rate is trimmed until it meets the requirements of the secondary flow circuit off of that bridge. Period.

If the flow is more, you waste HWS flow back to the return. If the flow is less, you could starve your secondary circuit.

It is as simple as that. Not perfect but as good as it gets.

Take out the balancing valves and your "closer to the circulator" bridges will pass what they please without regard to system demand and the ones further away will be starved themselves. It is all about balance and apportionment.

The ideal circulator is variable speed but even so, absent equal pressure drops across all bridges, you need to restrict the less resistive ones until all are perceived as equal.

Mike T., Swampeast MO

jp:

Yes, that's just what happens!

Now try to imagine (or draw) what happens when boiler output drops to 50% and boiler delta-t drops in half as well because of the fixed primary flow rate. The problem grows just as you should be increasing efficiency due greater condensation...