Primary/Secondary with Mod-Cons

Unknown

How is it relevant?

I asked him originally why he put balancing valves in his primary loop at the crossovers.

I gave a similar example of a single balance valve in a primary loop, and he called the example of ANOTHER designer throttling the primary loop, "Bad engineering".

What do you want me to do, run my examples by someone for approval?

I'm sorry you can't make the connection. It's not THAT subtle.

Noel

[Deleted User]

Remember, I live in a scientific laboratory...

And I have a Ameropean mod con in my basment (Munchkin T-50) in my basment with flow meter on the hydronic side, gas meter on the fire side, and logging capabilities.

I have played with the flow rates before, and my system is piped as a one pipe primary, not P/S like the manufacturer would LIKE to see. Hey, it's MY laboratory...

Scenarios? Questions? Results may not be applicable to other non Honeywell European modcon controls, but glad to share what I can.

Great discussion BTW.

ME

hr

Sure

a dual coil would work. Who manufactures that tank for you? I only been able to find dual coil (solar) tanks in the 80 and 120 gallon size.

hot rod

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Paul Rohrs_4

Mark,

Question:

Since you have your system piped that way, you can answer a question about flow. Let's say you change out your zone/boiler pumps and they double the flow rate. This will in turn narrow the Delta T through the boiler.

My feeble mind wants to know:

A. Will the boiler ramp up the firing rate to accommodate a higher delta T? Let's go have coffee.

B. Will the boiler ramp down the firing rate because the boiler percieves the narrower delta T as a reduction in the secondary load it's heating? Let's go have coffee.

C. The boiler firing rate (modulation) stays the same. Let's go have coffee.


I'm aware that changing the flow rate in a zone doesn't increase the ability of the zone to dissipate the heat, but I'm curious how the boiler would react. At any rate, a uniform theme is that we should all get together for coffee.

A CoffeeStock for ALL Hydronicians. Let's solve the world's hydronic problems.

Regards,

PR

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Unknown

Are you monitoring combustion exhaust temperature and/or analyzing its content under different conditions?

I think the big question here is, under part-load and/or full modulation, whether a slower flow rate, with higher dT and cooler return, makes a real noticeable difference in efficiency. If so, then a case could be made for variable speed pumping, as long as the whole system is variable speed pumping (as yours is, or both on the primary and secondary circuits).

I would also be curious about the effect on cycling.

Unknown

And, do you know how hot the HX in the boiler gets under various firing conditions? Just curious

ALH_4

hx

That seems to make sense to me. This thread has a lot in it. I just have a few points/questions. They may be valid or they may not be.

1) Is average hx temperature what is important? We always use average velocity in pipes, but the actual velocity is very different depending on where it is measured in the pipe.

2) I do think the gains from VS boiler pumping are marginal. Most likely this is why most manufacturers are not bothering with it.

3) Are wider supply/return dT's good/bad/unimportant in the boiler heat exchanger? To a limited extent, it seems to me that a wider dT might be good if it is achieved by a lower return temperature rather than a higher supply temperature. It seems like this could happen if the system flow rate is low.

When you ask how hot the HX gets, do you mean the dT across the heat exchanger wall?

The more I think about all of this, the more I am starting to confuse myself.

-Andrew

Unknown

I realize I posted that with a certainty I don't have at all.

it seems to me average temperature across the HX could be quite significant.. IF the HX temperature ranges are relatively low, then a shift of 10 degrees could result in a significant difference in heat transfer to the fluid.. less "constipation".

But if the HX itself is very hot, then a 10 degree shift could be quite insignificant and I would think dT would be nearly irrelevant as long as your flow is transferring the BTUs out of the boiler fast enough to slow its temperature gain to avoid a short cycle.

I have to think the HX gets pretty hot and thus dT is relatively unimportant, again as long as flow is appropriate to transfer the BTUs out, and my tendency to think of conservation of energy as the overriding principle here would also seem to indicate to me that all this focus on flow rates through the boiler and emitter circuit is very, very secondary to the actual heat emittance vs output of the boiler, both of which are nearly unaffected by typical flow rate ranges.

But, I could be totally wrong. I'm confused too andrew

jp_2

So,

thats why you constantly 'fiddle' with the reset curve

sounds like an indoor feedback would be a benefit, but i think we talking that one out last heating season.....

Constantin

Hmmm...

Wouldn't the importance of temperature depend in on where you are in the HX, the design of the HX, and the burner itself?

For example, I imagine that most heat exchangers try to present the coldest water to the sections of flue gases that have already been scrubbed of most sensible heat. ΔT can only help... and the HXs inside the Monitor FCX/Geminox are a good example of that... steel chamber for the hot gases, a stainless one for the condensation stage.

Many EU oil boiler designs like Ryll incorporate plastic secondary heat exchangers, though stainless is an option also. Getting a plastic HX past ASME would be quite the firefight though!

I seem to recall the French HX inside Munchkins, Trinities, and the like to have several sections (or loops coming off common manifolds), but only one combustion/condensation chamber. For those designs, perhaps an average HX temperature is more representative of overall heat transfer efficiency. But IIRC, that design also has a "cooler" loop at the far end of the chamber.

As for the benefits of VS pumping, I think the main benefit is the ability to significantly reduce electrical consumption on top of the usually "nice to have" efficiency increase as a larger ΔT is maintained across the HX. Some simple closed-loop control via the temp sensors can then keep the HX safe from overheating.

Deviation outside parameters on the other hand can be flagged and the boiler shut down for safety reasons. I.e. for a given change in input, a boiler with a constant return water temp should have a corresponding output change. If that change is not realized, the boiler knows that something is wonky (airflow obstructions, LPG "mouse turds", etc.)

HX's are usually designed to allow flex and thus should show little to no fatique due to expansion and contraction cycles over time. However, a very-low-turndown mod-con in a constant-circ system could theoretically maintain an almost constant temperature across the HX all winter, causing little to no flex. It would just heat and cool slowly as the supply and return temps changed.

Leo G_99

Local guys

try this addy:

http://www.alliedboilers.com/frame_main.htm

Leo G

Unknown

You write very clearly

I wish I had that talent.

I tend to agree with your post.

Brad opened my eyes when he compared btuh transfer/pounds of water at different flow rates. There does seem to be some return to minorly balancing some systems, though I'm still trying to discover how large a system would be to make all of those valves pay back the cost to install and adjust.

I agree with Constantin; I wouldn't design a system this way, if return temperature was this important and flow rate was not. I'd build a straight reverse return system so that the return water wasn't diluted. I'd reset it aggressively to keep all zones open as much as possible, thereby lowering the return temperature as much as possible. No return water temp dilution, marginal need for balancing.

Noel

Mike T., Swampeast MO

No, I do not "fiddle" with the reset curve. The first season I used the curve I'd calculated and used for the old boiler--never changed it once.

The second season I optimized the curve--it took 6-8 weeks with at most six incremental changes with days in between. Since then, the curve has not been changed and I see no reason to ever do so. Why would I want to change the curve when it's nearly perfect? After all, the sun dial and moon dial settings are nearly dead-on accurate with the maximum temp I can maintain in any room of the house--regardless of how far I open TRV(s).

I did however change the sun dial from the 61° I used most of last year to 66° when I took the boiler off standby a couple of weeks ago.

One of the worst things you can do with a Vitodens is "fiddle" with the reset curve. Coded changes to the heating curve force the boiler to dump its operational data store and re-learn.

ALH_4

well

This is getting over my head. I like to try to understand these things, but I'm no boiler designer.

I agree that the individual heat exchanger design makes a difference. I talk about the Vitodens a lot because I have some experience with it. I am only familiar with the operation of other condensing boilers through the study of their literature. The boiler literature never includes the really interesting information about the internal workings and design rationale.

I agree that parallel coils will not behave the same as one coil. A cylindrical burner is certainly not the same as a hemispherical one. Even the gap between the coils in the Vitodens is calculated. The Giananni(sp?) hx seems fairly random.

I think there are efficiency gains to be had by controlling the boiler dT with certain hx designs. How much? Is it worth the expense? I do not know.

I still think P/S is something of a necessary evil with low mass, flow restrictive condensing boilers. How evil? Probably not that evil.

-Andrew

Brad White_9

Not exactly, Leo

When I said the load drops off, the HWS temperature of course should drop off with OD reset. If the flow rate also drops with it, the temperature rises proportionately.

What I was getting to is, if the primary or boiler flow rate remains high, the modulating fire of the boiler should have the best chance at efficiency. By keeping the water flow rate high, the temperatures remain low. It is just that the outlet temperature to the radiation has to be sufficient to heat the building. That is the paradox and the balancing of boiler and radiation as a system.

Say I can heat 5 GPM by 20 degrees to deliver 50,000 BTUH, say from 120 to 140F.

If take the same number of BTUH and us it to heat, oh, say 40 GPM (exaggerated for effect), it would only rise by 2.5 degrees. I deliver the same BTU's per hour on paper, but would water at 122.5 degrees heat a house that needs 140 F at the radiators? Nope. But I would have a darned efficient boiler!

Mike T., Swampeast MO

Andrew:

You said, "I never understood exactly why these boilers don't sense the boiler temperature as well as the system supply temperature"

(This thread is getting a bit difficult to navigate.)

Remember--I'm ONLY referring to MODULATING boilers here...

If outdoor temp is used to produce a "target" and if ANY form of primary/secondary is used, the primary (boiler) supply temp is equal to the secondary (emitter) supply temp ONLY WHEN PRIMARY FLOW IS EXACTLY EQUAL TO SECONDARY FLOW.

If this were true (primary flow = secondary flow) and such were within the acceptable range of flow through the boiler than there is NO NEED for any form of primary/secondary.

If secondary (emitter) flow cannot be GUARANTEED in the acceptable range then and only then is primary/secondary in some form required.

Once you're using primary/secondary you are ASSUMING that primary flow is NOT EQUAL to secondary flow.

With unequal flow between the primary and secondary it is IMPOSSIBLE to make primary supply temp = secondary supply temp! It just won't happen--EVER!!!!

SO, to maintain the outside temperature related target, the burner MUST modulate to maintain such at the SECONDARY (emitter) supply--NOT THE PRIMARY (BOILER) SUPPLY!!!

Why? Because depending on how the flow rates between primary and secondary vary, the secondary supply temp will be either higher or lower than the primary.

Again, where do "American" mod cons with a target temp based on outside temp sense their target? If not at the secondary (emitter) supply, then outdoor reset is complete BS!!!

jp_2

reading system supply. temps

not possible! as the way i understand these boiler controls.

remember not all systems are just radiators.

if you had a mix of rads here, bb's over there, some infloor up there, then you could only monitor supplies if it were a multiplexing system, which I understand these controls not capable(numerous temp setting). then you'd have to know all 'zone' flows too.

so for keeping a 'boiler' flexible, it only reads its own output. you design the rest

edit:

rethink what you said about unequal flow rates P to S being unable to be at same temp? I think they can be. got to concider mass too. if the secondary is much smaller than primary you can have equal temps with unequal flow rates.

hr

I agree, Andrew

at some point we start triping over dollars to pick up pen nies. How far to chase the coldest return temperature under many and all conditions possible.

I fired an Ultra today and thought about all these questions.

Yes, it must be piped P/S due to the HX design. And yes the mixing at the closely spaced tees does raise the return to the boiler somewhat.

It's a single temperature slab on grade cabinet shop application. Supply at design is 108F. I'm not sure a lot of delta T and or P paraphernalia would show that much improvement in performance or operating cost.

About 5 years ago I did a near idenical cabinet shop, size wise. I used a Polaris, a spirovent, expansion tank, pump and thermostat..in that order. Just doesn't get any simpler or better than that. To this day the owner tracks LP consumption and scratches his head in disbelief. I've not been back to even replace the ignitor. Wish my modcon installs were that trouble free

Keeping in mind someone will need to troubleshoot and maintain all that technology.

I am attracted to mod cons that are ported out enough to not need P/S or a low loss "devise" For this job today it sure would be the simpliest, and probably most efficient way to getting the simple job done. Which is basically raise the concrete temperature.

hot rod

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ALH_4

temps

I apologize for my very poorly worded post.

What I was getting at was: Why is the Vitodens supply sensor in the LLH instead of in the supply pipe?

-Andrew

[Deleted User]

Paul et al...

As with any hydronic heating question, the answer is IT DEPENDS!

I can't raise the GPM flow rate without having to pipe another pump in, and am willing to do that , but have other issues to deal with at present (read MAJOR BASEMENT remodeling project).

The Honywell European controller that seems to be prevelant on these boilers is a prgrammable controller. It puts out "blocking codes" based on parameters that were programmed into the boilers control chip by the end OEM manufacturer. My understanding of the control is that it will shut down the burners if not enough differential is seen (lack of load), or if the rate of rise is too quick (cessation of load) or if the end dictated temperature is acheived (set point acheived).

The important thing to understand is, that even though many manufacturers use this control (Knight, Munchkin, Triangle tube, and many others) it is up to THEM as to what the controller will do as it pertains to modulation and burner shut down.

The programmable control has functions that we mortal human beings can not see and do not have access to. It's a secret, and I could tell you, but then I'd have to snuff you out...

Viessmann on the other hand has their own programmable controller, and it does things completely differently because it is not limited to the 1/5th step stages, so it can respond differently to changing loads on a more proportional basis.

What I have seen, at design conditions, is my little Munchkin boiler running at minimum burner input, with a 7 degree differential, and an actual thermal performance of around 98%.

Conversely, when the burner ramps up, the thermal efficiency drops off fairly substantially, like down to around 86%.

So, in response to yours and everyone elses questions, "It Depends". Not only on the appliance, but on the makers programming, the burner type (Viessmann Matrix versus everyone elses stainless steel tube, some with a cloth cover), flow rate, operating temperature and flame operating characteristics.

The equipment we are dealing with here is borderline artificial intelligence. It "sees" things we do not, and responds accordingly. There is nothing more amazing than seeing one of these beauties operating at part load conditions, maintaining a perfect delta T, all while running at or near an idle condition, and hearing a zone valve in the system open up and hearing the boiler blower RPM respond immediately, then hearing the zone valve close, and again hearing the burner fan speed respond accordingly.

Bottom line, I don't think that variable speed through the heat source is really going to make that much difference in overall thermal performance. But that is just my humble gut experience back by field observations.

My home system is a perfect example. The T-50 should have 5 GPM or there abouts running through it. I'm running at half that amount, approximatley 2.5 GPM, and I don't hear or see it short cycling like you would expect. I suspect that it changes its on/off differential based on what it is "seeing" as it pertains to loads.

THe boiler manufacturers set these things up P/S so that the heat source has a stable flow over which to monitor demand and thermal performance. THe controls are smart, but they too can be confused by variable flow rates, and in order to keep their operating parameters stable, they want to see a constant flow. The load can change the delta T, but they need a stable flow in order to judge those conditions. Unless of course, they are a Viessmann, and in some cases, they have total control over not only the boiler flow, but also the system delta T.

And I think it is important to note that the flow rate through the boiler (most mod cons) is not as critical as one would think (look at the Knights delta T table) as it is important that it remain a constant for the purpose of load profile evaluation.

Primary/secondary piping WAS introduced as a means of boiler protection from long term condensation production, but in the case of mod cons, it is being done for stability in control logic.

When I find some spare time, I will modify the piping on my system so I can run 2 pumps in parallel, thereby doubling the flow rate and do some evaluations in the field. I also need to wait for better heating demands. It's supposed to get up to the mid 60's today.

Hopefully this will satisfy eveyones lust for information for the time being. If it only stands to raise more questions, then so be it:-)

ME

EDIT, CORRECTION; The controls on my Munchy and the Knight are made by SIT from Italy. The earlier version on the Munchy were Honeywell Europeam. This company (SIT) is very diverse. Check them out at http://www.sitgroup.it/

Sorry for any confusion...

ME

Mark_46

What is ideal

Hi Brad,

You say "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."

May I ask what, in your opinion, is the ideal way to pipe a mod/con? Bear with me if you already answered this, this thread is a mile long ).