Understanding Primary/Secondary Piping -ModCon

ced48

I understand how primary secondary boiler piping works and why it is used on Mod Con boilers. However, I do not understand how a system that has a 5 gallon flow rate thru the primary-boiler loop and a secondary flow rate of the typical 2.5 flow thru the heating loop, 20 degree delta t, does not end up with a boiler that is always short cycling. I am in the process of installing a Triangle Tube Challenger boiler in a small home, about 30,0000 BTU heat loss, standard aluminum-copper finned baseboard. I chose the 105 rather than the 85 because of the DHW output. The engineer at T T thought it was a good choice, but I am concerned about short cycling. Any thoughts, advice would be appreciated.

SWEI

might not be necessary

as the boiler will control temperature so a simple series buffer will do the job.



You almost certainly want to opt for the smaller boiler.  Upsize the indirect if your DHW demand dictates.

ced48

Tankless

I do not plan on using an indirect, just want to go with the tankless- My space is limited, so any kind of storage tank probably won't work

SWEI

tankless

without any storage does not always deliver what people expect.



A buffer tank on the heating side will reduce short-cycling.



30k heat loss is a tough match for a mod/con.  I would probably choose a PTS60 with a SMART indirect and a buffer for the heating side.



You might also look at the new 40k Cadet.

ced48

So, No Way?

So, are you saying that there really is no way to install this combi boiler in this small house and have it run efficiently, without heating and storing a large amount of water?

CapeCod

combi boiler

sounds like my post from not too long ago.  I too wanted a combi boiler (PE110) for the small footprint and have a heat loss of only a bit more than 30,000.  It short cycles and I wish I had went with a smaller version + separate indirect tank.

SWEI

not quite

but please understand that proper sizing is critical to efficiency, comfort, and the longevity of your boiler.  Your load is 30k at design conditions.  The other 97% of the time you will need less than that to heat the space.  The CC 105 has a minimum firing rate of 29k.  The CC 85 is 23k.  Either of these would be cycling on and off every few minutes during 90% of the heating season.  A properly sized mod/con will run at high fire under design conditions, but will be able to throttle down 75-80% as demand dictates.  It will run for hours at a time much of the winter.  This produces better comfort (lower temperature swings) higher efficiency (lower fire is usually more efficient on most mod/cons) and longer life (less on/off cycles on the ignitor and controls, less thermal stress on the heat exchanger.)



DHW is a different problem. You need to understand your first hour and continuous demand numbers and make sure the system (tank, tankless, combi, indirect, whatever) will deliver what you need.  Have you lived with a tankless water heater before?  They are great at some things, but not at everything -- despite what the marketers would have you believe.



Primary/secondary piping acts like an automatic transmission, allowing the boiler and the system to run at different RPMs.  It doesn't do what a heating buffer tank does -- allow an oversized boiler to run longer during periods when the system is not able to dissipate the energy it generates.  It's best to properly size the boiler if you can.  I'd probably install a Lochinvar WHN055 (minimum firing rate 11k) or a Cadet CDN040 (minimum firing rate 9k) if it were mine.

pipeking

SO BUFFER TANKS

 r only used for oversized boilers? or boilers that can't modulate down to the load?

Gordan

You're really asking two things

1) Will the boiler short-cycle? Regardless of the piping arrangement, if the boiler is oversized it will cycle. Adding thermal mass (through buffers, high-mass emitters etc.) allows the system to absorb the higher-than-needed output of the boiler without rapidly exceeding its temperature setpoint, and then gradually coast down to the setpoint-minus-differential before the boiler is called upon to fire its burner again. The greater the difference between the minimum boiler output and the minimum heat loss, the more thermal mass you will need to avoid those cycles being overly short.



2) How does hydraulic separation (e.g. closely spaced tees) affect boiler cycles? The answer is, it shouldn't. The secondary loop can't gain more or less heat than the boiler loop gives off. It may have less flow at a greater temperature differential, whereas the boiler loop will have a greater flow at a smaller temperature differential - or the other way around, if system flow exceeds boiler flow. It will, however, affect the boiler return temperature, which has some effect on the efficiency of mod-cons.



I have a Prestige Solo 60. At 15 F outdoor temp and 68 F indoor temp, my heat loss (measured, via boiler output) is 33000 BTU/hr. Yet, at anything over 38 F I'm cycling. A Solo 110 would have been much worse. I was contemplating getting it for the same reason you are: DHW. But I got a 26 gallon reverse indirect (Turbomax) that I keep at 140 F (with a thermostatic mixing valve at the outlet) and I haven't regretted it. There's enough hot water for a continuous shower and enough in general. The household is two adults and two toddlers; no teenagers yet.

ced48

Still Don't get it!

What I still don't understand is what will keep even a smaller boiler from firing on and off, if the secondary system isn't adding enough colder water to knock down the returning water in the primary? And how in the world does a muti- zone system ever function efficiently without all zones running together? The system was sized for the coldest days with all zones functioning together. Won't this type of system short cycle like crazy?

Gordy

Bottom line

A properly sized boiler should run flat out at design temp. Anything les than design once minimum modulation is reached it will cycle to help this a buffer tank can be utilized.



Zoning has its pluses, and minuses. Being able to setback unused portions of a dwelling is a plus. Heat calls on small zones are a minis cycling where a buffer comes in.



I highly doubt at design all zones would be calling at once unless radiation, and heat loss were exactly dialed in zone by zone a rarity. Wind direction, solar influence are variables that play into it.



In the end you try to pick a boiler with an output that matches the load as close as possible, and go with it. Custom sized boilers would be nice at a price

ced48

What do you think of this?

Suppose I go with the smaller CC85, 65K. Get rid of the primary loop, and run 4 gallons per minute thru the boiler, required, and 4 gpm thru the single zone heating loop, same circulator? There is about 65 feet of baseboard, and 140 feet of additional piping, mostly 3/4".

Eastman

How cold does the potable water supply get?

How cold is the cold water?  The details on TT's website specify a max of 2gpm at 70 degree rise.

SWEI

physics

will intervene in that fantasy.  Any thing you do which pulls BTUs out of the boiler will deliver those BTUs someplace.  If there is no buffer tank, they will flow into the space.  The space will overheat.  If a thermostat turns the boiler off, it short-cycles.



Most of the magic of a mod/con boiler comes from modulation.  When you oversize them, they can't modulate very well.

ced48

I still Don't Get It-

I would like to thank everyone who has answered so far-I now know that I will have to go to a smaller boiler, or buffer. I will probably use a Cadet 40 and an indirect DHW. What I still do not understand about primary/ secondary piping is how I will ever be able to get the return water cold enough in the primary, without the flow of the secondary being equal to or greater than that of the primary. Water returning in the secondary at a rate of half of that of the primary will only lower the water entering the boiler by a small percentage. The water in the primary can only lose a very small number of BTU's on it's own. Modulating down will not solve this, or will it? Don't all applications using baseboard heat distribution face this dilemma?

SWEI

flows

The flow of heat must remain equal on both sides.  The flow of water can change based on ∆T in each loop.  So for a load of 32k, the boiler loop might run 2.13 GPM at 30F ∆T while the distribution loop ran 6.4 GPM at a 10F ∆T.

ced48

Boiler loop Must Be 4 GBM

The boiler flow must be maintained at about 4 gpm, that's why the secondary flow will always be slower, thus my problem in understanding this thing-

SWEI

4 GPM

at 32,000 BTU/hr is only 16F ∆T.



I assume we're talking about the Cadet CDN040 here?  p.38 of the install manual shows 2.1 GPM for 35F ∆T, which is what I'd design for.

Zman

BTU

I think you are getting hung up on this flow rate thing. What it really comes down to is BTU's.

Your 65 feet of baseboards will distribute about 600 btu/foot with 180 water and 150 btu/foot with 110 degree water. That's 39,000 BTU/hour to 9,750 BTU/hour depending on how you set up your heat curves.

Now look at the output ratings on the boilers.The 2 SWEI suggested have minimum outputs of about 8,100 and 9,900. That looks like a pretty good match.

Select a boiler then decide how to pipe it. Gavonni(sp) exchangers like the cadet are finicky about flow rates, they must be piped primary /secondary. Fire tube exchangers like the TT and many of the Lochinvars can often be piped direct.

The bigger question for your design is DHW. You will not be happy with a 40k btu instantaneous. A smaller boiler will require some DHW storage. GPM X 500 X delta T = BTU/Hour.

Hope that helps,

Carl

ced48

Yes, I'm Getting Confused-

The Triangle Tube boilers have minimum flow rates, the CC85 requires a minimum of 4 gpm. In taking a closer look at the Cadet, it looks like I can have a much slower flow rate, right, 2.1? If this is true than, it will work. Why do the TT boilers require so much more flow? You guys have helped me a lot. SWEI thanks for hanging in there with me.



I,m thinking I 'll be fine with a little 20 gallon indirect, two people, 2 showers, not much else.

Any better ideas?

SWEI

flow and BTUs

Sorry - got confused as to which boiler you were referring.



TT specifies a 4 GPM minimum flow for the CC85 _at full input_ which means the boiler is delivering 75,000 BTU/hr. That results in a 37.5F ∆T.  As the boiler modulates down, the required flow goes down proportionally.



The Cadet manual shows 2.1 GPM at its maximum output of 32,000 BTU/hr.  Less BTUs = less GPM needed.

CMadatMe

I'm Fighting With an Engineer

Over exactly what this gentleman is trying to understand. Spec'd called for a few Vitodens 200's. The engineer never spec'd the Manifold Distribution Center so the nice guy I am I quoted it to my customers to save tons of labor as well as installation material cost. Now standard boiler pumps are Grundy UPS26-99 as we like a 40 degree boiler delta.



This guy wants the boilers to run on a 20 because that is what he want his system side to run at. I told him YOU ARE HYDRAULICALLY SEPARATED! You can run your system at what ever you want. A BTU IS A BTU IS A BTU. I'll move then there you can take them away how you see fit!



I said, why would you spec boilers that you would operate under the conditions that don't allow them to perform the efficiency they can produce? Like a deer in a headlight..

HDE

Argle Fargle!

Engineers just dont get it very often, oh wait I am a Mechanical Engineer - Oh Snap!

Paul48

Chris

Can you explain why you want to run a 40 degree delta t on the boiler, and how that improves efficiency?

Paul48

Answer

I think I may have found the answer to my own question. Let me know if I'm right. Running a 40* Delta T is running the least gpm through the boiler, which will be the closest to system requirements?

SWEI

increasing ∆T

lowers the returning water temp and raises combustion efficiency. 

Paul48

SWEI

I was refering to DeltaT of the boiler,and its affect on the system.Primary vs. secondary

ced48

Why Not Eliminate Primary/Secondary?

In my case, 2.1 gallons of required flow to the boiler, why not just run a single loop at say 2.5 gpm, and eliminate one circulator? There would always be more than 2.1 gpm flow back to the boiler. There is only one zone, and the indirect.

ced48

Why? Space

I'm working with very limited space-that's why I'm trying to simplify.

CMadatMe

I want the 40

To give me the best opportunity to condense even while producing domestic hot water. GPM is nothing more then a conveyor belt moving btu/hr out to where it is needed. Does it really matter how many gpm's I move out? Remember we are hydraulically separated.

CMadatMe

But

At full fire you have to move a minimum of 4gpm through the HX.. Page 80



http://www.triangletube.com/documents/31/Challenger%20Combi%20Manual%200811.pdf

ced48

I Have Switched Boilers-

I am now considering going with a Cadet 40-My new question on primary secondary deals with flow in the secondary possibly being greater than that of the primary, thus lowering the temperature in the secondary supply. How is this a good thing?

CMadatMe

Optional System Sensor

Use it to maintain the system supply temp based on the outdoor curve rather then the boiler maintaining the outdoor curve on it's side. I prefer to use a low loss header rather then a couple of closely spaced tee's. Have the sensor in the header. The low loss header will help with cooler boiler return water temps and protect the HX from any system debris. Also acts as your air separator. Caleffi's does not come with a well. I use a honeywell 1/2" well. If you want a less expensive LLH you could just use the Viessmann 80/60 LLH.



http://www.viessmann.ca/etc/medialib/internet-ca/pdfs/doc/wb2_acc.Par.0595.File.tmp/Vitodens_200_LLH_Single.pdf

Paul48

GPM

"Does it really matter how many gpm's I move out"?

Maybe mistakenly, but I would think it does matter. The gpm's you don't move out are returned to the boiler,raising the return temps.You are essentially isolating the boiler from the conditions that exist in the system. It is designed to operate based on those conditions.Sorry.........I'm a little dense, sometimes. I need somebody to tell me something that gives me that AH-HAH moment.

CMadatMe

Grasshopper

Don't get caught up in "gpm" think "btu/hr." X amount a btu/hr is leaving the boiler and the system is taking it's x amount of btu/hr.

Eastman

?

I can see that a high delta will improve domestic hot water efficiency.  But if the rest of the system is designed to heat evenly at 20, and the boiler is running at 40, are you not just elevating the average temperature across the hx?  The lowest possible return temperature is dictated by the secondary loop.

ced48

New Smaller Lochinvar

Looks like Lochinvar has a new, smaller, Knight boiler WBN051, 39,000 BTU net, and about 16 inches square by 30 inches. Modulates from 50k down to 10k. I'm thing this is the boiler for this job.