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ModCon doesn't play well with Indirect using ODR, need new theory.

mikemac52 Member Posts: 44
Currently running CH at SWT of 140F max RWT 130F max with ODR, Indirect water tank set to SWT 180F max. The smallest zone of the 4 zones is baseboard and at these parameters it cycles 20 min on 10 min off. The other three zones are cast-iron. I would like to drop the CH SWT down to 130-135F to get the mod con to run continuously...
But I can't.

The mod con has 7 presets for various types of emitters. 6 are fixed and the 7th is custom. The presets jump from a max SWT of 140F down to 120F with nothing in-between. The 120 preset has a min RWT way too low for the cast-iron radiators anyway.

The custom preset, if used, makes the machine default to using what ever the Max water temp setting for SWT while using ODR. Unfortunately I need 180F as the max for the indirect and using that as the max for ODR gives much different and not nearly continuous run times. This behavior is as stated in the manual and borne out in practice.

With ODR off then CH and the DHW temps become independent of each other. Which is sort of silly, because with ODR off, the presets become redundant as you can set CH and DHW temps directly and independently of each other just under their own settings. With ODR on, directly set CH temps are ignored and it responds to the presets.

So essentially, the first 6 presets set the parameters (high/low) for the ODR but the 7th effectively excludes using the ODR.

This Modcon has 2 control modes, SWT and RWT control. Though I don't know if ODR works with RWT control. For What reason would you ever want to use RWT mode?

Are there ways to trick the boiler? Would reducing the apparent size of the boiler do anything? Do I need to give up ODR? Or give up on the idea of the boiler running constantly.

With the ODR using 180F max SWT it would lead to longer burn time for the big zone, shorter burn time for the baseboard but also long periods off between cycles. One thought was to do a 1 degree setback in the wee hours before people woke up, then stagger the zones back on to produce a long burn first thing in the day, and then see where things fell the rest of the day. It would be a 40min, 2- 20min, and a 13 min. So it would burn a good 90min for the first cycle of the day. The 40min zone (my big zone), typically was off 45-60 min between on cycles. But I just don't know what is more efficient. Let it go like gang busters with lots of down time, or the constant low simmer espoused by many here. But It seems like I can only get so close, so am I still getting any savings by getting close?

In either case, the mod con condenses most of the time.

Any ideas?



  • JohnNY
    JohnNY Member Posts: 2,650
    I've read this post 3x now and I'm still not sure what your complaint is. You're not short cycling if that's what you're thinking, but if you want to run longer cycles then look at the option to reduce your maximum fan speed on the boiler, which you haven't identified in the post.
    Contact John "JohnNY" Cataneo, Master Plumber
    in New York
    in New Jersey
    for Consulting Work
    or take his class.
  • Zman
    Zman Member Posts: 6,537
    Normally you would set the system up for DHW priority so the heating and indirect can run at different temps.
    I don't understand the rest of your post. What make and model boiler do you have?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • sunlight33
    sunlight33 Member Posts: 301
    Normally in heating for DHW the boiler ignores ODR because outside temperature should play no role in heating hot water.
  • mikemac52
    mikemac52 Member Posts: 44
    I would like to drop the CH SWT down to 130-135F to try to get the mod con to run continuously...
    But I can't.

    While using ODR I can only set the CH max temps to the 6 presets for the type of emitters giving me choices of Max SWT 180, 170, 140, 120. (the other two have differing RWT) So I can't set anything between 140 and 120.

    When using the 7th preset (custom) and ODR, the machine instead uses the global settings for the boiler max and min temps.

    From the installation manual;
    "Heat loads 1-6 show the preset temperature ranges based on
    the load type selected, while heat load 7 provides a custom temperature range. When the custom temperature range is in use, the boiler operates based on the user–defined “Absolute Min” and Absolute Max” temperature settings."

    Unfortunately I need the global absolute max SWT to be 180 for the indirect. If ODR is off I can change Max CH to my hearts content.

    It seems I can't have both, ODR and the Max CH temperature of my choice.

    Using a navien NHB150. Didn't want to seem like I wash bashing a product...
  • flat_twin
    flat_twin Member Posts: 285
    Can you adjust at what outside temp your max boiler supply temp will occur? This would stretch the curve and lower the SWT slightly overall.
  • mattmia2
    mattmia2 Member Posts: 2,863
    It seems like if you set it up so when you have a DHW call, if you make that call close the DHW TT terminals, set DHW priority to on, it will heat to the DHW temperature, not the central heating absolute min and max based on the curve.
  • HomerJSmith
    HomerJSmith Member Posts: 1,286
    Do you have the SWT and the RWT sensors hooked up? Did you go thru the complete programing? Sounds as if you are not programed correctly.

    Your heat emitters will determine the boiler operation. If your heat emitters are putting the BTU's into the living space so that the RWT is lower than the boiler differential, the boiler will run forever as long as there is a call for heat. The ODR will modulate the SWT based upon outdoor temps. The SWT and RWT sensors will modulate the boiler operating temp based upon the programing. I think that's the way it works.
  • Gman66
    Gman66 Member Posts: 40
    edited February 25
    You need to read the manual again; the 6 presets are not just CH max temps, but rather distinct curves meant for various types of emitters. The curves are described by a "min - max" temperature description which describes the supply temp at the the "outdoor high temp" and "outdoor low temp" respectively. You need to pick one of the curves that goes at least as hot as you need for design day (cold) and at or below the lowest supply temperature you would want on a warm/shoulder season day. You can then tweak that chosen curve by judiciously adjusting the "outdoor high temp" which is defaulted to 70F and the "outdoor low temp" which is defaulted to 14F.
  • mikemac52
    mikemac52 Member Posts: 44

    @flat_twin That might be just the ticket. Change the design day temp. Do you think it would be a 1 to 1 adjustment?

    Hi @mattmia2. Yes, the DHW is in priority mode and has no problem ramping up to 180 irregardless of the HC. The main issue is the inability to set HC to 130 with ODR on without affecting the global max temp as using the custom preset would require and DHW limits.

    @HomerJSmith I was so excited to hook up the external temp sensors to the secondary supply and return, and so, eh, disappointed to find that they had no discernible impact on the running of the system. What they have been great for is seeing how the zones are fairing and how absolutely frigid the RWT is on the big zone in the house. (Everything is in series.)

    @Gman66 Exactly what I need to do as @flat_twin suggested also. Thank you. It is a shame though that it is not possible to set the custom preset to Max SWT 130 and Min RWT 104 say, and have it work with ODR, which it won't. Rather than take the rigid presets and manipulate them hoping to get close to the desired results as the RWT temps are a little too low for what is at hand. So while I may dial in the SWT anything done to effect the other side seems like it would be counter productive, at least with the large change I would need to do on the RWT side. Perhaps changing the preset in the spring would be part of the answer.
    As to curves, they're linear, point to point, min/max which makes it seem that that Navien could make the custom preset work with the ODR as the other presets do.

    For now, I'm going to go play with the design day temp before we run out of winter. I should have asked this a month ago...

    Thanks for all the input and sorry for the slow response. Could someone please tell my wife that this is important too. :/


  • GW
    GW Member Posts: 4,083
     If you’re extra clever with control wiring, you could treat the small zone and the DHW together. Problem solved
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • mattmia2
    mattmia2 Member Posts: 2,863
    Did you test that the "global max temp" overrides the dhw setpoint? It sounds like those are only for describing a dh reset curve, not for dhw.

    As far as setting rwt temps, you can't really have it both ways, you can target a rwt or a swt, but one is a function of the other, there is nothing the boiler can do to make it meet both at the same time, you have to pick one and swt is the one you normally would engineer the system for. To change the rwt you would need to change the flow or the emitters.
  • mikemac52
    mikemac52 Member Posts: 44
    edited February 27
    @GW can't do. The temp differential is too great between DHW and CH. That's what has had me chasing my tail.

    @mattmia2 yes, I found out about it when setting the custom preset for the CH and lowering the Max temp. Later when my DHW wasn't recovering because the priority was timing out, is when I discovered it was a global setting. DHW temp readout matched the absolute Max temp and it could not be set any higher.

    Your absolutely right on the rwt, and I just realized that I've been speaking/thinking incorrectly on the rwt /wrong terminology and don't want to tell you what I thought I was talking about... :D (just think min/max and forget the rest) And that helped confuse the rest of the folks on what the heck I was saying. I should have been talking strictly about SWT.

    Sorry to drive everyone crazy.

    I'm happy to say that is seems I can get down to Max SWT 130 by adjusting the design day temp. According to the charts anyway.
    Thanks again @flat_twin and @Gman66 .

    Everything else talked about was relevant.

    It looks like I'll get there eventually, creeping in now on the right settings. Still think they should make it easier and just make the custom setting behave like the others. Then when you want your design day to be at 14F, there it is. Instead you need pretend the design day is -1F and the warm max is 87... ? I mean how many times would you need to call in a pro to adjust it till it is right? More than a few which most people won't do. Then it's just wasted equipment. wasted money. I am impressed with the navian and all the variables and the ability to dial the system in just so but it's not a one shot deal.
    Lucky I'm stupid enough to just go in and... then come here and ask for help. B)

    Thanks again for all the help.

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