Navien Tankless Combi Boiler


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Old 03-28-13, 07:38 AM
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Navien Tankless Combi Boiler

Hello All,

I am sharing my planned system design with the hopes of getting some clarification on things. I've read some people saying that Navien units can't provide the required amount of heat because the internal pump maxes at 5GPM, even less with a smaller diameter pipe--limiting its heat output to 50,000 BTUs and a water temp of ~110* F.

My house requires 121,800 BTUs. I'm planning on using a Navien CH-180-NG which produces 150k @ 91% for an effective output of 136,500 BTUs. I'm planning on using Slant/Fin MultiPak 80 baseboards with the 83A heating element which can produce 730 BTUs / linear foot with 180* water at 1 GPM (more BTUs with a higher flow rate). I plan on using a zone ciruclator setup with Taco 007 pumps or the Bumblebees, plus a Taco SR503-R switching relay. My plumbing layout will be a Series Loop.

I've sized my baseboards such that even at 1GPM through the convectors, the required BTUs will be met. I did this on purpose to "over-calculate" the required baseboard lengths to account for the unknowns in my house, like the condition of the insulation, etc. It is a 60 y.o. brick home with historic value (former home of Aretha Franklin, also where she raised her two sons at, she was alread a Motown star when she and her former husband purchased the home).

As a special note, the primary loop will be 1" to attempt to maximize the flow for the internal pump, but the heating/secondary loops will be 3/4".

I'm confused on the whole issue now because so many people are bashing the Naviens saying they max heat output at 50,000 BTUs. If true, that would seem like grossly misadvertising their products claiming they can output 150,000 or better. That also makes me wonder about other manufacturers like Takagi, Rinnai, and Bosch, which all have similar internal pumps that I'm guessing max at around 5GPM too... I've been on the phone with Rinnai for 2 days trying to get info on the speed of the internal pump and either I sat on hold for unbearably long or nobody could find the info... why would they claim their Q175CN can do 175,000BTUs if the internal pump will max out at 5, heck, I'll give 'em credit and say 6GPM? Same goes for Takagi, Bosch, and Noritz (all well known, well respected companies)?

So if you have info on this, please help. Let me know if I'm doing the right thing or not, heading in the right direction or not, whatever. Please be specific, include references and calculations when possible. I want to put this issue to bed.

I am installing myself because (1) the house needed a total reno and (2) I'm fairly proficient with most all home improvements except electrical. I'm paying cash for everything and can't afford to pay somebody $10k (I've been quoted as high as $30k) when all the materials will only cost $4,500.
 
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Old 03-28-13, 08:11 AM
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121000k btu, I would say this house is 5000 square ft?

Did you do a heat loss calc?

If your going modulating boiler your probably best to add/over radiate the home so you can run the mod con as low temps as possible.

Not liking the combi boilers and your best off with an indirect if the home is indeed that big.

You will get plenty of advice here but IMO your tackling this the wrong way.
 
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Old 03-28-13, 08:37 AM
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@lawrosa

3,500 sq ft... big windows, some parts of the house appear to be insulated, others appear not to be insulated. My plan has been to over-radiate the house all along, but I do plan to run the Navien at 180*. I spoke with tech support on this issue already and they said the unit is just fine operating at that temperature.

Also bear in mind I'm doing a zone heat layout. The house will be split into 3 separate heating zones, basically one for each floor. It will be rare for all three zones will call for heat simultaneously. At best, 2 zones call simultaneously, but most often, one zone will be heated at a time. On the rare occasion like a winter holiday will all 3 zones call for heat at the same time.
 
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Old 03-28-13, 09:52 AM
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but I do plan to run the Navien at 180*
Why??? Thats only at design temps... You may as welol just put a standard boiler in then.

Possibly you need to educate yourself more on how mod/con boilers work at low temps for fuel savings. You want the boiler to condensate as long as you can. And that happens at boiler temps below 120f or so.
 
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Old 03-28-13, 10:43 AM
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I am here to learn

I'm here to learn, thats why I'm asking for input. I want to run the boiler at 180* because that's the design temp of the baseboards:

180* water * linear ft = total room BTU requirement

I could very well run cooler water temps but I'd have to run higher flow rates, which I'm OK with, I just don't know what the ratio on that is. My end goal is to keep the rooms at a nice warm temperature during winter months. If I run it at 160 what will my flow rate have to be? 8 or 9 GPM? If I go with Taco 007 thats sustainable since the max flow rate is 23 GPM, but if I go with the Bumble Bee which tops out at 15GPM I'd be running it near its maximum. Dont flow rates effect head loss?
 
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Old 03-28-13, 11:29 AM
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Flow rate stays the same.

180f water is only needed on the coldest day of the year which is your design temp. Say your design temp is 0f. When its 0f outside the boiler will run at 180f. It will adjust itself temp wise due to the outdoor reset sensor. As it gets warmer out the boiler temps will get lower.

So if it was 40f out the boiler may only run at 100f for example.

Additionally you need to set the boiler up with primary/secondary piping with close space tees. Are you familiar with that?

Last I would like to point out that this combi unit hot water per minute will only be about 3 gallons per minute or so. Thats like one shower. Do not plan on running multiple fixtures. Because of the DHW being combined the boiler is grossly over sized.

Better off with a indirect or stand alone HWH IMO.

based on your square footage you probably only need a 90k boiler. Really need to do a actual heat loss.

Others will chime in.
 
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Old 03-28-13, 02:03 PM
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If I Understand You Correctly

You're saying that the 730 Btuh / lin. ft. of baseboard is not what I want them to operate at all the time?

See I thought that if I want to keep the house at a comfortable, say, 74* in the winter that I'd have to get the maximum output from my baseboards. If you're saying I can get 74* while running the boiler at maybe 150* (I'm just picking a number for illustrative purposes) then that is good news to me. Mind you, that's maintaining a 74* indoor temperature with outdoor temps averaging in the 30s, and hit-or-miss insulation.

As far as insulation goes, I plan to address that next summer with blown-in.
 
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Old 03-28-13, 03:21 PM
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As far as insulation goes, I plan to address that next summer with blown-in.
If there's no IMMEDIATE need to replace the boiler, do the insulation FIRST!

More important than insulation even is INFILTRATION. Air infiltration is by far the biggest factor in heat loss of a building. Seal every crack, every gap, every wall penetration, anyplace that air can leak in or out.

When that is all done, THEN do your heat loss calc and size your boiler properly for the heat loss.

3,500 sq ft
While a 120K BTUH boiler is not HUGELY oversized, I would be willing to bet that your heat loss is less than 90K BTUH.

You haven't told us where you came up with 120K BTUH and whether or not you've done a heat loss estimate.
 
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Old 03-28-13, 03:33 PM
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I want to run the boiler at 180* because that's the design temp of the baseboards:
Yeah, sorta... 180°F is the temp that manf RATE the BTU output of the baseboard. It doesn't mean that you HAVE to run 180 water. Fin tube baseboard will provide useful heat output all the way down to about 100°F.

If you intend to run a mod/con, what you want to do is install enough baseboard so that you CAN heat the home properly with say a maximum of 140°F water. This will mean more cost up front for the extra baseboard you will need to buy, but it is offset by savings year after year after year in the fact that you don't NEED to run 180 water to heat the home!

To do this properly though, you need to do a ROOM BY ROOM heat loss estimate so that you can install the proper amount of heat emitter in each room, and that the extra amount that is installed is PROPORTIONALLY the SAME in each room of the home.

The SlantFin heat loss program that is shown in the 'sticky' at the top of the forum should be downloaded and carefully loaded with data. It will tell you the heat loss of each room so that you can make educated choice of how much BB to install in each room.

Take a look at the SlantFin charts for the BB you intend to use. Use the BTU number at 140°F for the heat output. Divide the room BTU requirement by that number to determine the number of feet you need to install.

By the way, the SlantFin program will allow you to specify the boiler temperature you wish to use. Set it at 140 and it will automatically calculate the footage required. The program has pre-loaded data for all of the SlantFin baseboard offerings.

Do it this way and your mod/con will run in condensing mode ALL WINTER LONG. The home will never NEED 180 water and you will reap the benefit of the extra efficiency of the mod/con all winter.

Remember that once the home requires water above the temp that a mod/con will condense, away go all the benefits of a condensing boiler... you are right back to the mid-80% efficiency of a standard boiler system.

Design properly from the beginning and benefit from the extra 10-15% efficiency all winter.
 
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Old 03-28-13, 03:46 PM
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I thought that if I want to keep the house at a comfortable, say, 74* in the winter that I'd have to get the maximum output from my baseboards.
One more small point just to clarify a bit...

You don't need MAXIMUM output.

In order to keep the home at the setpoint you choose, all you need to do is replace the heat lost to the outdoors.

EXAMPLE:

A hypothetical room requires say 10K BTUH to maintain indoor temperature at the outdoor DESIGN TEMPERATURE in your area. You learn this number by performing the heat loss estimate.

For this room you will need to install heat emitters that are capable of replacing that 10K BTUH.

Let's say the MultiPak 80 ( 730 BTUH / FT @ 180°F entering water temp) has a rating at 140°F EWT of say 500 BTUH / FT. (hypothetical number, I haven't looked at the chart)...

Dividing 10K by 500 tells us that 20' of MultiPak 80 will provide 10K BTUH at 140°F EWT.

If you run 180 water, you could use 14' ... but you would lose the advantage of being able to run the cooler water and save money.
 
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Old 03-28-13, 03:51 PM
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FLOW RATE

In a hydronic system, you can't simply 'jack up' the flow rate to whatever you want to... you need to stay within the VELOCITY guidelines.

These guidelines assure that the water will be moving FAST enough that any AIR in the system is kept moving and can't collect somewhere that it shouldn't... and gurgle through the piping.

They also assure that the water will be moving SLOW enough that you will not have 'velocity noise'... sounding like someone is running a tap somewhere every time the heat turns on in the home.

This guideline is between 2 and 4 Feet Per Second (FPS).

For the typical 3/4" piping that is used to connect baseboards, 4 GPM puts you in this range and is what we generally use as a design factor. Smaller pipe, less GPM, bigger pipe, more GPM. But the VELOCITY stays between 2 and 4 FPS for a nice quiet system.
 
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Old 03-28-13, 03:58 PM
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OK, so we're going to run a 3/4" circuit... and we know that we want to design for 4 GPM...

Enter the "Universal Hydronic Formula".

In a nutshell, what this formula tells us is that for every GPM of flow, we can extract 10K BTUH and we will have the sought after 20°F Delta T ... 'difference in temperature' between the supply to that circuit and the return from that circuit.

So, we're running 4 GPM, right? That means that a typical 3/4" circuit can supply 40K BTUH to the home.

Let's use the hypothetical figure of 500 BTUH / FT from the previous post.

Dividing 40K BTUH by 500 BTUH tells us how many feet of radiating element can be installed on that circuit. In this example it works out to 80' of element. This does NOT include the connecting pipe.

You need to run these numbers using actual data from the charts and heat loss program.
 

Last edited by NJT; 03-28-13 at 06:27 PM.
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Old 03-28-13, 04:06 PM
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Lastly...

Remember that 20° Delta T (DT) ?

As the water travels through the circuit, it will cool a bit at every heating element.

140 in, 120 out.

What does this mean for the rooms at the END of that loop?

It means that if you size the footage of baseboard based on the 140 temp at the beginning of the loop, you may not have enough footage to maintain a balanced heat output all the way through the loop.

Almost certainly you will need to spec the LAST room in the loop for MORE footage due to the fact that the water will be nearly 20° cooler.

Back to the charts... how much BTUH output at 120? That is the figure you need to divide into the rooms heat loss in order to arrive at the correct footage to be installed.

All the rooms in between need to be considered also.

I believe that the SlantFin program uses an AVERAGE number for it's calculations, so the first room on a circuit may have a foot or two too much, the last room a foot or three too little.

Best to always back up the program with actual calculations and install the correct amount in each room so as to balance the system output correctly.

Rooms in the middle of the loop you might want to use 130°, etc...

You wanted ejumication? I hope you got it! -
 
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Old 03-28-13, 04:47 PM
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By the way, recommended reading, all of the pertinent articles here:

Technical Menu
 
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Old 03-28-13, 04:58 PM
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I don't think anyone posted a link to the intended boiler, so here 'tis for anyone wants a look.

http://www.navienamerica.com/__DATA/...SME_120307.pdf
 
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Old 03-28-13, 05:05 PM
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Waitaminnit...

Is this boiler even a MODULATING boiler?

Or only CONDENSING?

If not modulating, I would dismiss it from my short list.

You want a boiler that not only condenses, but also MODULATES.
 
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Old 03-28-13, 05:52 PM
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Here is some really great baseboard for mod/con boilers. Look at the outputs to water temperature.
High output. Very lw water temperature. Good for condensing temps.
http://www.smithsenvironmental.com/E...4cBro_SM_4.pdf
 
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Old 03-28-13, 06:21 PM
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Old 03-29-13, 06:19 PM
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The Navien CH combi is a modulating condensing boiler with 10-1 turndown.
It also has a feature that allows you to cap the maximum fire rate as low as 30% of its maximum fire rate. For that reason, I would get the largest one allowing it to go to a full fire of 199,000 BTU's for domestic hot water production but the heating max fire rate can be as low as 60,000 BTU's (30%).
 
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Old 03-29-13, 07:11 PM
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I looked at the manual, specs, and website but nowhere did I see it stated the minimum fire rate which would have confirmed that it was a modulating boiler... that's why I asked, it just wasn't clear.

I don't see the point in oversizing, even if it is a modulating boiler. 60K BTU is barely below the heat loss of the home (I don't believe the heat loss of that home is 121K).

Why not select a boiler that can go down to 30K or 20K ?
 
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Old 03-29-13, 07:29 PM
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The Navien CH-240 (199,000) has a minimum fire rate of 20,000 (10 to 1 modulation)

The high fire rate on combis is used to produce an acceptable domestic flow (4.5 GPM at a 77 degree rise)

http://www.navienamerica.com/__DATA/...ompressed).pdf
 
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Old 03-30-13, 08:07 AM
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Thanks ZL, that's not the same spec sheet I found...
 
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Old 03-30-13, 03:18 PM
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I got the 121k from doing heat loss calculations on every room and hallway of the house. I slightly increased the numbers for each room to account for the insulation variability t/o the house.

Thanks NJ Trooper, per your advice I recalculated the baseboard need based on a 140* entering temp. But if you're right in thinking my heat loss is over-sized, maybe I can keep it the same! My house is 3,500 sq ft, 3 floors, big windows, open space all around it except on the south face (where its warmest, go figure). I'm in Detroit, MI so it gets really cold for about 5 months. Lowest temps might o is 0* temporarily, but winters seem to average about 20-30*. Lets just assume for now that the house is totally lacking insulation.

LZ700 I was wondering how your primary/secondary calculations factor in with even flow on the two loops. I plan to limit my secondary loop to no more than 4GPM per NJ Trooper. So with the internal pump on the Navien flowing at probably 5GPM, I was wondering how that all works out.

I plan to use 1" copper on the primary loop (thin tube wall) and 3/4" pex on the secondary loops.
 
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Old 03-30-13, 04:15 PM
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I plan to limit my secondary loop to no more than 4GPM per NJ Trooper
Be careful...

Understand that I didn't mean to limit the entire system to 4 GPM!

4 GPM is a 'standard' design for 3/4" tubing...

You are probably going to want 4 GPM going through EACH ZONE.
 
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Old 03-30-13, 04:17 PM
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I got the 121k from doing heat loss calculations on every room and hallway of the house. I slightly increased the numbers for each room to account for the insulation variability t/o the house.
What 'package' did you use to perform the heat loss?

If you were required to enter 'wall factors', then the lack of insulation is already accounted for.
 
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Old 03-30-13, 06:46 PM
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So with the internal pump on the Navien flowing at probably 5GPM
Is this something that you saw in the manual?

I'm certain the boiler pump will flow more than 5 GPM.
 
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Old 03-31-13, 10:46 AM
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Here You Go:

Sorry for not being clear, I did mean to say that each zone will be 4GPM. Can't recall what websites I used but I don't remember entering wall factors. And I got the internal pump speed from Navien Tech Support. It maxes out at 5GPM, goes down from there based on the diameter of the pipe, smaller pipe offering lower flow. A 3/4" pipe will allow for about 4GPM.
 
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Old 03-31-13, 11:30 AM
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Did the websites allow you to do a ROOM BY ROOM estimate?

I still highly recommend that you go to this link:

http://www.doityourself.com/forum/bo...alculator.html

and download the program and re-calculate the estimate.

Web based heat loss estimates I'm very skeptical of, especially if they don't ask for any construction details.

For all we know, they could just be calculating overall square footage and multiplying that by 35 BTU per SQ FT... which is just plain wrong.
 
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Old 03-31-13, 11:34 AM
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I got the internal pump speed from Navien Tech Support. It maxes out at 5GPM,
Tech support or no... let me just say that I'm a bit dubious of this claim, especially in view of their NON-commitment to provide a quality install manual. I read through the whole thing last night and found quite a few ummmm... let's call them inconsistencies.

Does this mean that the boiler must operate with a SIXTY DEGREE DELTA T ? No, I think not.
 
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Old 03-31-13, 06:23 PM
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The CH can have a limitation of around 5 GPM internal boiler circulation along with
limited external piping. That's why loads above 50,000 BTU's or long distances require primary/secondary piping.

But the burner does not fire on a 20 DT, its a modulating burner that will fire at the input BTU required till it gets to about 6 degrees from set point. So at 5 GPM, it could be at 199,000, 35,000, or 19,000 with only 5 degrees to go. Just have to love true boiler modulation. I don't like the traditional primary/secondary piping with this model or other brands like the Viessmann 100.

The easiest way to relate this is if a on/off boiler has a 20 DT control aquastat that is set at 180' what's the boiler out put once it hits 180 and doesn't turn back on till 160? Nothing the burners off. The modulating boilers attempt to stay on in a lower fire continuing to add heat to system till system is heat saturated and return water climbs.

The key here is hydronic mixed stream formulas or proper extraction of heat from the secondary injected into the heating primary.


To the OP, look at the Navien drawings online, way better performance than a loop with closed spaced tees. Besides, 1" primary wont meet the suspected 120,000 BTU's you claim you need. That being said, 3,500 sq ft with light insulation I could see you needing that. That's only 35 BTU's a sq ft.
 
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Old 03-31-13, 06:37 PM
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NJ,
I don't totally disagree with you about their manual, having been to Asia and Europe many times relating to boiler products, I have discovered American installers tend to need everything spelled out for them versus Europe boiler manuals expect the installer to have more knowledge thus no need to print.

Take ACV/Triangles tube boiler manual for example, all they did was insert a 1000 warning messages for the North American installer because he might tend to pay attention to warning messages about install what not to do.
 
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Old 03-31-13, 06:52 PM
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ZL, I do understand what you are saying, but what I'm talking about are downright errors... spelling, grammar, and the install graphics being 'messed up'... one graphic talks about zoning with valves and the diagram shows a Taco SR panel. The FOLLOWING diagram shows zoning with circs, but no panel. Stuff like that... it's just messed up! They should have taken the time to have some native English speakers edit and correct.
 
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Old 04-01-13, 06:59 AM
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To the OP, look at the Navien drawings online, way better performance than a loop with closed spaced tees. Besides, 1" primary wont meet the suspected 120,000 BTU's you claim you need.
Could you link to those Navien drawings. I was planning on using the layout from Page 25 of the manual provided on Navien's site. This is for a baseboard system. Fortunately, they offer the wiring diagram with a Taco SR503 Relay (exactly what I was planning to use) on Page 20.

But I'm curious which design you're advocating I use. Could you please be specific?

You also mentioned that 1" primary won't meet the suspected 120k BTU... what do you mean by that? Are you saying if I use 1" piping on my primary loop I won't get to 120k BTU? What should I do then?

Just BTW, Page 5 of the manual shows the connection size for hot water output and return to be 1" NPT...
 
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Old 04-01-13, 04:13 PM
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Fortunately, they offer the wiring diagram with a Taco SR503 Relay (exactly what I was planning to use) on Page 20.
That's sorta one of the 'messups' I was talking about with the manual.

They show the SR panel on the same diagram with a ZONE VALVE installation.

You also mentioned that 1" primary won't meet the suspected 120k BTU... what do you mean by that?
I wonder the same thing...

If the boiler pump is going to cap at 5 GPM, then why would one need to use bigger piping if that was going to be the max flow? 1" pipe is FINE for 5 GPM flow.

The key here is hydronic mixed stream formulas or proper extraction of heat from the secondary injected into the heating primary.
Nobody gave me the key... cuz I don't understand it.

ZL, you got some 'splainin to do!
 
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Old 04-01-13, 06:34 PM
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There are many types of primary/secondary piping applications but most are usually only familiar with the one first used. This is the one where the boiler is the primary flow looping around from supply side to return side and the secondary flow (heating loops) it closely spaced tees off of this boiler primary loop.

This design was necessary in boilers such as copper tube or steel that needed adequate flow at all times to prevent condensing or flashing (boiling) in the heat exchanger. The primary flow was consistent to protect the boiler and the secondary zone flows could then vary depending on demand.

With the advent of low mass condensing boilers, the flows are reduced to prevent erosion in the smaller heat exchangers, yet the firing rates can be increased often based on the Delta T which when installed right represents what the heating loops are doing and how they are performing.

So you don't always have to have a big primary loop calculating the upstream, downstream and inner tee spacing based on the combined flows.

lawsonrw, because the boiler may flow 5 GPM and is 1" connections doesn't mean the primary loop can be 1". I suppose that could be the case if all zones didn't exceed 3 GPM, because you have to add up the flows and stay under the recommended 4fps velocity. 5 + 3 = 8 which is under 9 GPM that 1" will handle.

However I don't like the boiler primary loop applications for low mass boilers when there is a better way that takes less space. I recommend you make the boiler loop with its internal circulator the secondary loop injecting heat into the heating system primary. Buderus, Viessmann, Navien, Baxi and a few others do this.

attached is a install diagram I grabbed from the web. IT shows the boiler as the secondary loop (lower flow). This will work best as the heat is carried away to the left, and the boiler return always sees the coldest system water.

Navien makes nice manifold for the CH all made up, with 1-1/2" connections and the chamber is 2" acting like a hydraulic separator, with all the service valves need installed.
 
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Old 04-01-13, 06:39 PM
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I found the accessory manifold
 
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Old 04-01-13, 06:43 PM
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2nd page of the manifold sheet
 
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Old 04-01-13, 06:52 PM
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Navien makes nice manifold for the CH all made up, with 1-1/2" connections and the chamber is 2" acting like a hydraulic separator, with all the service valves need installed.
Sure and its probably $500 plus.... Can make yourself for about $50 bucks....

Just saying. Most manufacturers offer this manifold, but rarely plumbers will buy them... In fact I never saw a pre packaged manifold in any installation.
 
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Old 04-01-13, 07:31 PM
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Interesting

I guess they don't specify the construction of this manifold on purpose. It looks like two flow-ball valve on the end of a 2 inch pipe connected thats tee-d off to the cold inlet and hot outlet of the boiler. Can anybody confirm this?
 
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Old 04-01-13, 08:07 PM
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$50?
That might buy the 2 1-1/2" dielectric unions
What about all the ball valves, other unions, purge ports, relief valves and copper and fittings?

I see a lot of projects with these used in the northeast
 
 

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