Ideal piping for mod/cons

Mark_46

Steve,

You make a good point on a high level. I guess mod/cons are square pegs in round holes here in the US, so to speak. But nonetheless, I presume you agree they are still advantageous over single fire boilers?

Yes, the flow through the HX seems pretty key based on manufacturers specs and recommendations and thats where P/S comes in. My lesson learned from this thread is that P/S is a bit of a compromise for a mod/con, but necessary.

Mark_46

Near-boiler piping design

A question about primary/secondary flow rates was posed in what turned out to be a interesting and lengthy discussion thread on this site. Within that thread, it became evident (as a matter of fact, or a matter of opinion I'm not sure) that primary/secondary near-boiler piping may not be ideal for a mod/con boiler type.

So I ask:

1)what is the ideal method for mod/con near-boiler piping?

2) Similarly, does the ideal near-boiler piping depend upon or change with the type of heat emitters being used? Or should a mod/con be piped one way, regardless, and design water temp is independent of near-boiler piping?

Brad White_9

Oh, Lordy....

In my opinion (and founded on facts lest anyone disagree), the highest efficiencies from ModCon boilers are dependent on having the coldest possible return water. Primary/Secondary fights this, it tends to raise the return water temperature.

A German Engineer once said to me, 'send water out just as warm as required to heat the space; take it back as close to room temperature as possible.'

To your question, Mark, "It Depends". The greatest dependencies I see are the rates of flow between the two circuits and their ability to vary, to modulate.

With constant flow, Primary/Secondary (P/S) piping has a nasty but unavoidable habit of passing "unused HWS" back to the boiler return when the secondary or radiation circuit is not drawing the full amount. If the primary flow cannot modulate, then it simply has to bypass, thus raising the HWR temperature to the boiler, possibly quenching condensing but at least reducing efficiency. Even with modulation, the boiler has to have time to respond. Even if it does, the temperature is raised uneccessarily.

(Plug in an on-line/off-line buffer tank in the return here, is one solution.)

The previous legion thread essentially asked if one would use balancing valves on the returns in a P/S with ModCon setup. I voted "yes" as being the best thing to do for a less than ideal situation, not to endorse P/S as a default design approach.

The only reason I can see to use P/S piping with a ModCon is to deliver multiple temperatures of water to various emitters. Radiant floor matched to HydroAir is the widest example. Cast iron rads and radiant floor is another, closer but with room for variation to suit floor coverings. Lots of combinations you can imagine, with the highest temperature requirements setting the pace.

(Clever and careful "hierarchy" can help here, though, by using the high temperature circuit's return to ease the mix-down to the lower temperature circuit's supply.)

The goal regardless is to lower the boiler's HWR temperature.

What is the ideal? To me, One Temperature Required. From there, the circulation of boiler and emitters would be one and the same, modulating flow and firing rate to meet the objectives of "just warm enough" water and "as close to room temperature" return water.

Second choice is, if flow modulation is not possible or in the cards, fuel modulation absolutely should be. Higher flow rates in the boiler beget narrower delta-T's which promote cooler water and lower temperature rises. So long as that temperature to the emitters can do the work, this makes a very attractive situation to my mind. In some ways, I think it is better because of higher boiler-side flow rates, but you lose the ability to optimize temperature at low firing rates. (You may want warmer water at times that a few emitters are calling for example.)

After that, if multiple temperatures are required, a simplified P/S setup with cascading in-series branches, the return from one helping to mix down the others.

More to it, but one has to be reasonable

My $0.02

Brad

ALH_4

ideally

in my mind, avoiding primary/secondary piping lowers the return water temperature in more situations.

Ideal to me:

1. One zone, preferrably a larger one with a relatively higher heat load is constant circulation with no valves. This zone is controlled by indoor temperature feedback to the boiler. Ideally this could be nearly an entire floor level depending on the uniformity of heat load.

2. The rest of the zones are controlled by thermostatic valves instead of zone valves or zone pumps.

3. If the flow through the constant circulation zone is sufficient, a dP bypass may not be needed to maintain minimum flow through the boiler.

The best strategy is to always have the boiler connected to a load.

This strategy is generally more suited to a smaller home, but it ensures the coolest water possible returns to the boiler. Each system really needs to be looked at individually and the appropriate control/piping strategy must be determined on a one by one basis.

-Andrew

hr

Some good reading

try the January issue of www.pmmag.com.

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Mark_46

OK...lots to digest.

A few questions:

1) What does your ideal One-temp required arrangement you describe look like in terms of near-boiler piping? Assume...2 zones of space heating. Then, how should an indirect DHW tank be properly factored in?

2) There is something I don't understand in the buffer tank solution you describe. If primary return water was lowered via the tank, isnt that an 'artificial' way in which to do it? In other words, the return water temp is lowered via the tank rather than as a result of actual space heating, no?

Brad White_9

One Temperature Ideal

1) My ideal model would be one with a single zone and TRV's on all of the radiators, (but you will not let me off that easily, will you?).

My single-zone will have constant circulation, TRV's trimming to suit and boiler firing to an outdoor reset schedule with room feedback.

If you have two zones, I would keep the primary circulator and use zone valves, but then my primary (only) circulator would want to be variable speed responding to constant delta-P. As valves open up, so does pump speed. Without this, the circulator rides it's curve (which can work with the right selection). If the circulator is in an unstable range with one valve open, I would needed a DP valve. I would pipe it from the discharge of the circulator back to the suction side of the circulator so that the hotter bypassed water does not see the boiler prematurely.

For DHW, I would use 100% diversion to an indirect. Get it over with, then go back to heating the house. Tried and true. I would not make it in series with the house heat, but parallel diversion.

2) The buffer tank I described I offered in terms of P/S piping. This is the way I have it set up in my own house:

When the return water exceeds a certain temperature (120F), an aquastat turns on a charging circulator and sends HWR to the buffer tank, off the main via closely spaced tees. Thus the buffer tank is now "on-line". The outflow of the tank, at initial startup at least, is at say, room temperature. This charge of cold water continues to the boiler to prolong condensing.

When the boiler cycles off (condensing non-modulating in my case but the principles still apply), the system draws from this tank until it is depleted. At that point, the circulator shuts off and stops charging the tank. The tank is then off-line. I term this the pole-vault system, passing load to the side and bringing it back in a wide arc after the charging period.

I had cobbled together this arrangement a couple of years ago in too much haste perhaps. If I were doing this today, I would have a simple 2-way valve in the main, with tees on either side. To charge the tank, close the valve. Open the valve and the tank is bypassed.

Now, the buffer tank could be used with any ModCon on the return to the boiler. It is only needed during light load days when the load is below the boilers minimum output. It buys longer run times and longer off-times as it drinks from the tank.

Does that make sense?

Brad

Bob Sweet

Of the manuf.

of mod/cons that Ive installed, not one endorses anything other than pri/sec piping other than on small load systems. (And some don't even give that option)

Seems to me that what is being suggested is the way to go. But what about liability of straying from the manuf. specs. The first thing that will be looked at when a system doesn't perform will be the installers " not following the manuf. installation manual",

That's alot of liablity to assume. Somewhere along the line the manuf. will have to give their blessing to piping variations or we'll end up taking a seriuos hit in the pocket book.

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Mark_46

I see

1) You say "If you have two zones, I would keep the primary circulator and use zone valves, but then my primary (only) circulator would want to be variable speed...".

Im not clear. Is that lone variable speed circulator now within P/S piping because of the multi-zone? Or is it within a single loop with in-line zone valves...plus the diff. press. valve to complete the package?

2) OK, DHW would use something like a parallel primary circuit with its own circulator, right?

3) I understand the buffer tank now. Didnt realize it would have a switch/valve.

jp_2

somewhat agree brad,

BUT.....

buffers:

aren't you trading energy for more efficiency?

whats the added energy cost to raise room temp water to supply instead of return temp to supply temp? certainly a trade off here, maybe?

the return looses heat to the room then, thus keeping 'it' in the envelope but not really where we want it???

Hmmmmmm questions to ponder???

added:

so the real trick is to dump as much heat into the living space as possbile before returning to the boiler, maybe a DHW preheat(buffer) tank would make everything more efficent, and better use of high temp return water.

Mark_46

Hi Bob

Excellent point. I know as far as HTP goes, P/S is the recommended method.

Its all very confusing.

Bob Sweet

Hey Mark,

it sure can be confusing. Until there is a method of designing a system as a system these discrepencies will exist.

imho the boiler manuf. biggest concern is their hx, keeping it satisfied thus pri/sec is "suggested". Not only are they "suggesting" pri/sec piping but they are giving their req. for circ. pumps to be used. Can you say CYA.

Until we have design standards for the system (what the boiler is attached to) manuf. are going to have the final say in how their boiler is piped and pri/sec piping is almost a guarantee of proper flow through the boiler.

Who can blame them they don't have a clue as to how the system piping is piped. They would be foolish to require anything but pri/sec piping.

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Ray M

The root of the problem is the initial design for the mod-con. I have a Buderus GB 142 with panel rads with trv's on all but 2 towel warmers, constant cirulation with a outdoor starter (my wife hates it no t-stat just a dummy stat showing room temp}. I did a study and figured out the balance point for the house and the starter is set at 44 OAT. The design temp is 140@14 OAT so condensing is constant except for DHW production. The manufactures piping diagrams should be followed to a "T" unless you know more than them. I have mod- cons in the field connected to fin tube on reset that performs great. Is it the best application for the boiler? Good question. My answer is the savings during 80% of the season off sets the design days of hi-temp demand. So why get all balled up in your shorts on piping the boiler, system design is more important to me.jmho

Thanks

Ray M

Comfort Tech Corp

Brad White_149

Bob and Mark

That is exactly it, some manufacturers, those with high internal pressure drops and defined minimum flow requirements, require a primary circulator. Even the Viessmann Vitodens, but because it is internal, one does not talk about it as you would with say, a Munchkin. With the Muchkin and others, it becomes an identifiable and some say parasitic expense. It is necessary for the reasons stated, regardless.

Now, when I say "primary-secondary", I mean the traditional and deliberate decoupling of two flow paths, drawing flow from one as needed to the other but passing "un-used" boiler flow back to the boiler. By comparison, a Low Loss Header (LLH) is, to me, a form of "Series Flow" because properly set up, only pure system return water goes back to the boiler. The boiler flow INTO the LLH only leaves to the system. The system flow into the LLH can go back to the boiler or rise to mix with the hotter boiler flow entering there.

Ray is spot on in saying that this "oversizing" with a properly sized modulating boiler, occurs only so much of the year, 20% of the time is as good a guess as any. I would agree.

Brad White_149

Good discussion!

Buffers- I do not see a trade-off of energy for efficiency (if you are spending energy you lose efficiency so to me that is a conflict of concept).

Rather, the buffer tanks stores excess heat from too warm of a return water temperature. This would occur when the boiler is at low load, especially when it is non-modulating (my Monitor MZ) or when you are at the low end of a ModCon's modulation range).

The advantage of the buffer tank is to prolong boiler operation to save short-cycling by storing that water in parallel. The boiler will eventually shut down because the setpoint is satisfied (operating limit on a non-modulating boiler and setpoint on a reset-based boiler of any stripe).

This stored water, a form of thermal flywheel, will be transported to the space while the boiler is resting, the assumption being that the space is still running to meet setpoint.

I would not be heating room temperature water at that point but a diluted buffer tank temperature by the time the boiler got back to work.

Just to be more clear, a buffer tank for me is a "fix" for a boiler/system's inability to modulate output to heating load. This occurs in any boiler where a minimum output exceeds the spaces minimal needs in mild weather.

When close to design temperatures outside, the system should, if properly sized, hum along within it's modulation range. The buffer tank would not be necessary, ideally.

Does that make sense?

Bob Sweet

With the LLH's

Ive installed their biggest advantage seems to be the assured decoupling of hydraulics.

When I've installed LLH I've installed gauges at both supply taps and return taps (system side & boiler side) and I agree what I've seen is the return temps from the system match the return to the boiler as well as the supply temps matching.

But as system load decreases the benefits of low return temps will also decrease, seems to me a buffer tank to create a constant load might be an even better solution

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Mark_46

Hmm

Brad,

As our discussion is building, you have touched on one constraint I have deliberately held off mentioning so far. That is, the minimum flow required through the boiler. For a HTP 80M, it is 1 GPM per 10,000 BTU. I held off on this because I wanted to isolate the ideal piping concept from other constraints that exist in 'reality'.

Now, in my particular case, I actually need 4 different flow rates. 1 GPM per 10,000 BTU through the boiler, 8GPM during DHW production, 4 GPM through zone 1 and 2 GPM through zone 2.

So, with that said, I presume this by default negates the ideal piping concept we have been kicking around earlier in this thread, right? In that ideal you described one zone, one temp running on outdoor reset schedule, etc (of course you know better than I what you described as ideal). But when we introduce the constraints of a real system the ideal can no longer stand, correct?

In my actual case what the 80M, the DHW and each heating zone require actually makes a P/S layout the only way to satisfy everything, correct?

[Deleted User]

Seasoned Buffer Tanks...

On my system, my DHW tank (reverse indirect) also serves part time duty as a buffer tank. When the OSA temperature is between 65 and 40 degrees F, the buffer is on line (paralleld, but none the less, on line). When the OSA drops below 40, or if there is a call for DHW the buffer parallelism is taken out of the picture, and the modulating capabilites of the appliance take over.

Siggy wrote about this some time ago, and the good folks at Amtrol introduced it to me. Works great, avoids short cycling for micro loads, and gives extended high efficiency operation.

Also, as it pertainst to P/S piping, put your self into the appliance manufacurers shoes. If all systems were designed the same, ideally, they would be single low temperature applications, with constant circ and ODR. Unfortunatley, we Americans don't do things that way. We have to have a bajillion different temperatures, and everything is herky jerky stop and go traffic. Tough conditions to run through a mod con with a high pressure drop HXer. With the intorduction of the Triangle Tube Solo, things are changing some what. THe boiler has its own internal mass characteristics and a reasonable low pressure drop to boot. Times they are a changing, and so are the appliances. This is a GREAT time to be in this industry...

ME

Bob Sweet

That looks like

a GREAT way to pipe a condenser. I like the WWSD feature you incorporated into bringing the indirect in and out of play for shoulder temps.

Very nice Mr E.

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Brad White_149

Minimum Flows

Interesting facet to explore, Mark.
This is more "thinking out loud" than anything definitive. Let's see where this leads..


Is the "1 GPM per 10 MBH of boiler" based on total boiler output or the output at a given time? IOW, does the flow follow the firing rate? I suspect not for if not variable, you will be using at least 7 to 7.5 GPM or call it 8.0 because of DHW and because it is at least what you need and also to protect the boiler from making steam...

With a constant flow rate (based on maximum firing rate), the delta-T will narrow as firing rate goes down. Starts at a 20-degree rise and as the firing rate goes down to 25% your delta-T becomes 5 degrees.

Say at design the system runs 140 degree water, returning at 120 degrees. At shoulder-season time the supply temperature need only be, say, 90 degrees and with return water coming back at 85 degrees. Interesting to see how a 87.5 degree AWT heats the space. Makes the case for return water reset? Another topic... but if it heats the space, delta-T is irrelevant, it is incidental.

That said, then the system flow variable can be dealt with.

This is where the buffer tank comes in, banking excess heat not sent to the non-calling zones.

But let's see what the worst thing happens if the smaller zones do not call (I am assuming control valves but no matter). If the smaller zones are not calling, say that there is 8.0 GPM primary flow to be consumed. If the larger zone, the 4.0 GPM is the only one calling, the secondary circulator may "ride it's curve", hitting a balance point where flow and available pressure capability settle. Perhaps this is 6.0 GPM.

Worst thing that happens is that 2.0 GPM rat-races back to the boiler return and the "calling zone" takes 6.0 GPM resulting in a Delta-T of 15 instead of 20, a modest increase in average water temperature.

Here is where the buffer tank comes in handy, to absorb that bypassed 2.0 GPM plus the residually higher return water temperature from the calling zone.

Rambling, sorry.. just had to think that out...

Off to eat turkey and my Susan's scratch-made chocolate cream pie (ok and some vegetables)....

I hope you enjoy yours too!

Cheers-

Brad

Mark_46

Heres what HTP says

Brad,

Thanks, hope your TG day was great.

I visited my cousin who owns a 1926 Tudor. Lovely house. I knew they had steam heat but when I was in the basement (after being offered some hand-me-down toys for my 17 month old son) I found myself looking for the boiler! It's oil fired by the way. What does this mean? Hydronic heating isnt my line of work but I think I have become some sort of heating hobbiest. Strange. By the way, I presume that scratch-made cake is Boston Cream Pie given your neck of the woods? ).

1) To answer your question...straight out of the HTP install manual: "The Munchkin Heat Exchanger does have pressure drop which must be considered in your system
design. Refer to the graph below for pressure drop through the Munchkin Heat Exchanger. The recommended circulators are based on 1 GPM per 10,000 BTU/HR W/20".

2) I may be confused, again ). You mention the secondary circulator may ride its curve...suggesting a system that needs 4 different flow rates at different times and points must use P/S? Have I interpreted that correctly?

S Ebels

If I can jump in here.........

Regarding P/S piping for mod con boilers, the main reason many manufacturers recommend this arrangement is to ensure adequate flow through the boiler under all circumstances. You have to remember that the water content on these little jewels is very low. Some makes have less than others and some HX designs are also much less forgiving than others when it comes to tolerating excessive overshoot of boiler temp.

I agree with Brad that the most energy efficient design is to avoid P/S piping and let the boiler directly see the return water temp. Heat exchanger design and the multitude of system types pretty much dictate that P/S must be used in nearly all cases sad to say.

It was mentioned that the best "system" for a mod con boiler would be a single zone, controlled by TRV's with outdoor reset and indoor feedback. Couldn't agree more! You have to remember that most of the MC technology comes from Europe where that type of system is the norm instead of the exception. Hence the boilers they use are designed around that premise which is the polar opposite of the traditional American system of running to limit with bang bang controls.

For a classic example of what a MC boiler can do in its intended environment, look up Mike T's (Swampeast) synopsis on his Vitodens installation. I think it's in Tech Topics.

[Deleted User]

Thanks Bob...

but I have to give credit to Siggy for coming up with this idea, and the folks at Amtrol for introducing it to me.

ME

S Ebels

Square pegs vs round holes

I would go as far as to submit that it is our system design here that is the square peg. Mod con boilers are definitely the way to go. Although some of our systems here require a compromise of their capabilities, they are still head and shoulders above a traditional boiler in almost any conceivable usage.

Brad White_149

Mark- re: \"Heres what HTP says\"

Hey Mark- To your questions-

"Oil fired" just refers to the fuel used, that's all. Not strange at all if you think of this is a hobby! A lot of folks have taken an interest in their energy use, not just how much but how does it work.. This is a good thing, a sign of take-charge responsibility.

The chocolate creme pie- Boston Cream Pie has a yellow custard with chocolate pudding top and whipped cream. If my Susan made that, it would have some other name. (She is from Hunterdon Co. NJ)

Your more specific questions:

1) The "1 GPM to 10,000 BTUH rule" is pretty common, indicating a traditional 20-degree drop. As Steve stated, the flow rate is to protect the boiler from flashing into steam and to overcome the integral pressure drop; it takes this necessary function out of the equation. Given that the flow is most often constant through the boiler and because the firing rate modulates, the temperature out of the boiler will have to change. It must.

The Vitodens for one, with a variable speed internal circulator, changes speed with firing rate. I cannot say for certain what exact variable it tracks but I see it as keeping a constant outlet temperature because it's nature is to meet an outdoor reset temperature at any given time.

If flow is constant and the temperature modulates, whatever the outlet temperature is, you get. Of course, you hope it is warm enough to heat the house at that time....

2) "Riding the Curve"- Take a look at a pump curve. That sloping line running from high left to lower right, is a plot along which the circulator will perform. It is based on a constant speed and given impeller diameter. Any combination of head (the vertical scale on the left) and flow (the horizontal scale along the bottom) will fall on the curve for the pump depicted.

Say you throttle a balancing valve (increase head). The flow point will decrease to the left but your finger on the curve will go towards the upper left. Head increases, flow decreases. Open the valve, your finger will track toward the lower right.

For "riding the curve" as I am describing, this is a function of flow which is automatically changing, such as by opening zone valves.

Your 4 GPM and 2 GPM zones give you possible flow of 6, 4 and 2 GPM (both, one or the other on respectively). With both zones running, your flow/head point is at the intersection of the curve and the 6 GPM line which corresponds to a given head loss. (Act of faith, that flow rate unless you can measure pressure across your circulator but let's use that faith here for discussion).

Say that flow point is 6 GPM at 10 feet of head (Grundfos 15-58 on speed 2 to make it real.) The smaller zone valve closes so you are down to a theoretical 4 GPM. Your head will rise because there is less pathway for the 6 GPM to flow. So your finger rises up the curve to the left until it hits 4 GPM, which corresponds to about 12.5 feet of head on this curve.

Here is a twist- head increases or decreases as a function of the square of flow. (John Siegenthaler says it is a power of 1.75, I mean to ask him why.). Because the flow has dropped by 33%, pressure drop in the common parts of the piping drop to 56% so the actual reduced flow rate will be somewhat higher than 4 GPM. The actual flow rate will impose a disproportionate effect on the head loss of the system and will settle at a balance point which will occur along that curve.
But this is OK- not enough to make a difference at the emitter level.

Close all but the smaller (2 GPM) zone valve. Head would, if linear, rise to about 15 feet. The reality is, the flow would settle somewhere in between, maybe 3 GPM. Big deal? I think not.

All of this illustrates natural changes in pump operating point on the same curve. I am not suggesting extreme disparity in flow applied to a constant/fixed speed circulator, but you have to know each potential operating point within reason.

Cheers!

Brad

hr

ME is that Amtrol

being used backwards? I though the coil was inended to be the boiler loop and the tank capacity the DHW? Does Amtrol allow it to be used either way? Clever design.

hot rod

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Mark_46

Oh

Brad, Sorry. When I typed 'what could this mean' I was referring to looking for boilers in other peoples basements-lol. I think I've become some sort of heating and cooling addict ).

Oh from NJ? Me too. Very Northeast. Bergen County, sort of outside of Manhattan.

Anyway.

Sorry again, when I asked about the secondary circulator, I was focusing on 'secondary circulator' not your reference to 'riding the curve'...that reference was understood. I was driving at the reference to the secondary circulator in that P/S piping was being assumed?

In summary, as I wrote a few posts up, what I learned from this discussion thread is that P/S is a compromise for a mod/con, but is necessary. But, there are ways to minimize the effects of P/S piping on a mod/con (balancing valves, buffer tank, etc). If you agree, I have the information I was looking for, thanks to you and the others who have offered information in this thread.

[Deleted User]

Yupp....

Hence the term reverse indirect.

The tank is stainless steel, and the coil is copper, so everything is copasetic.

Works like a champ.

ME

Mike T., Swampeast MO

Hey ME,

The "buffer interface solenoid" is exactly what I've considered in my system.

If I were to use a buffer it would have to drop in just when needed...

However, I've found that the Vitodens is designed to be an extremely efficient batch heater when necessary. Just think about efficient "batch heater" for a moment...

Instead of the "drop-in" buffer, what I want is "drop-in" solar without the complexities of the Viessmann designs based on their earlier (not Vitodens) boilers.