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Question from home-owner about a new, Rinnai M090C boiler

Chris_NHChris_NH Member Posts: 12
In current generation, high-efficiency condensing boilers, how much should the supply water temperature vary from its target or set point temperature (T-set)? I ask this because our brand-new, Rinnai M090C (propane) boiler shows a variation of almost 50 degrees F, which I think may be the reason it does not effectively heat our house. Today, for example, with an outside temp of 50 degrees F, our boiler displayed a set point temp of 165 degrees F. However, over a period of several hours, while our first-floor thermostat was set to 70 degrees, and continuously called for heat without getting satisfied, the boiler’s supply side water temperature cycled between a low of about 125 degrees and a high of 174 degrees, as measured by the boiler’s internal sensors. The total time of each “cycle” was about 6.5 minutes, with the boiler firing (“C.Heating Active” mode) during each cycle for about 1.5 minutes and idling (“CH Pump Overrun” mode) for about 5 minutes. Is this normal for the M090C, and could this be the reason it is not effectively heating our house?


  • Jamie HallJamie Hall Member Posts: 12,706
    Outdoor reset active? And if so, what is the curve set for?
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Chris_NHChris_NH Member Posts: 12
    Yes. Outdoor reset is active. There is a probe (temperature sensitive resistor) on the side of my house connected by a pair of wires to the Rinnai. The 165 deg. F value I mentioned in my post must be the point on the curve (i.e. T-Set value) corresponding to a 50 degree outside temp. Currently, outside temp has dropped to 46 deg. F and the displayed T-Set value (on the home screen of the boiler UI) has moved up to 172 F. So the boiler seems to be calculating a T-set value based on measured outside temp. The curve shape is determined from parameters entered by my installer (min temp = 130, slope = 2, max temp = 185, offset = 0.) My major question is should the boiler allow supply water temp to wander over a 50 degree range around the current T-set value. Based on some training slides I found on the Rinnai website I think that M-series boilers are designed to hold supply temp with a range of only 14 degrees (T-Set value - 5 F to T-Set value + 9 F). Then, to prevent short cycling, it has a CH Pump Overrun delay time parameter that can be set from 0 to 15 minutes. My installer set this parameter to 1 min in an attempt to minimize supply temp variation. But since my boiler spends about 5 minutes in CH Pump Overrun after the max temp is reached (and flames go off) a large variation occurs anyway. This does not seem right to me. But is my understanding correct? Thanks for responding by the way.
  • lchmblchmb Member Posts: 2,984
    edited March 2019
    with standard baseboard (or do you have something else?) I'd have your installer set the minimum temp to 140. Is the boiler piped primary/secondary? Have your installer contact Rinnai directly and they can walk him through changing the settings to assist in a longer run cycle on the boiler. If they cant resolve it over the phone a rep can do an onsite visit.. Once the CH pump overrun activates it has a hold off of 2 minutes minimum I believe.. It's trying to create a long on cycle for the boiler. If your zone is to small (or has flow issues) it may cause this issue..
  • Chris_NHChris_NH Member Posts: 12
    OK. I'll have this conversation with my installer on Monday. Thanks for the feedback.
  • lchmblchmb Member Posts: 2,984
    if your comfortable doing it, you could change your heat curve to a higher set point (2.5) to see if that helps raise the house temperature for now
  • nibsnibs Member Posts: 462
    Am by no means an expert, but just reset the Parameters on my
    Rinnai E50, I believe your mode should be similar.
    My situation is a very high thermal mass in floor radiant but will tell you what I found to be significant.
    Para 1..............176F mAX.................. i USE 122 Depends on heat emitter type.
    PARA 5..............3.5 .............................I use .05 (heating curve)
    PARA 7.... out door reset, default is 14.......... I did my calcs at O so that is what I use, was a big change in the operation of the boiler.
    PARA 14 gradient speed.. default is 10 F/minute, I use 1F/min,
    Sounds like you could use a higher value up to 28F/min.
    PARA 27 Min T set default is 100F I lowered mine to 90F max is 158F This also was a big game changer,
    PARA 73 altitude adjustment......... you need to calculate.
    Your numbers will all be higher than mine so do not take mine as an example.
  • lchmblchmb Member Posts: 2,984
    Chris how did you make out with this? Did they find a solution?
  • Chris_NHChris_NH Member Posts: 12
    No not yet. I have not called my contractor back since their last visit two weeks ago when they raised the minimum temp of my outdoor reset curve from 68 to 130 degrees (which helped) and put in a larger circulator (which made my system a lot noisier). I frankly wanted a break from contractor visits and more experience using the system before I called back. But I plan to email them on Monday with these two questions. Is the variation that I'm seeing in the supply temperature of my M-series boiler normal, and is the new circulator really necessary?
  • Chris_NHChris_NH Member Posts: 12
    By the way, to answer an earlier question, yes the new boiler was installed with a primary/secondary piping layout. This looks great. Regarding CH Pump Overrun time, my boiler enters the overrun mode for about 5 minutes each time the supply temp reaches the current outdoor reset temp in spite of the fact that my contractor set the CH Pump Overrun parameter to 1 minute. The menu says this parameter can be set from 0 to 16 minutes. I'm going to ask my contractor to have a Rinnai M-Series expert either come to my house (or talk to us over the phone) to explain what's going on.
  • Chris_NHChris_NH Member Posts: 12
    One other thing. Is there a way for me to set a preference on this forum so that I get an email notification each time a new comment has been added to this thread?
  • nibsnibs Member Posts: 462
    Your home and the way you use it is unique, Your contractor may or may not know the ins and outs of your boiler. Either way you live with both of them 24/7.
    To change the parameters on your boiler is quite simple, I suggest that you go through the parameter set up page and learn what each item does, and what effect each has. Then you can either carefully and with knowledge make your own adjustments, or at minimum suggest changes to your contractor.
    I caution you - unless you know well what a parameter does, do not change it, you can do damage.
  • Intplm.Intplm. Member Posts: 1,132
    Chris_NH said:

    One other thing. Is there a way for me to set a preference on this forum so that I get an email notification each time a new comment has been added to this thread?

    @Chris_NH Click on your settings profile section. There's a place in there to set you preferences.
  • rick in Alaskarick in Alaska Member Posts: 1,073
    On the title to the thread at the top, there is a star off to the right. Click it, and it will send you an email every time someone sends a reply. Assuming you have an email address setup in you profile.
    Does the boiler run up to the high temp, 170 or so, each time and then go in to the pump idle mode? I think I understood you to say the max temperature varies, but not sure. I am just wondering if it does go up to the high temp and then just idles, if the pump isn't able to move the flow properly. I am also wondering if you might just have some kind of a bad sensor or control board issue.
    Get you service guy over there and get Rinnai tech support on the phone to walk you through these issues. Those guys are great at helping you out and will stay with you until you do.
  • lchmblchmb Member Posts: 2,984
    edited April 2019
    what size pump did they install? As far as Nibs suggestion, I would caution against this til the issues are at least addressed. If you start changing settings on the contractor they may say your causing the issues.. let tech support walk the installer through it..
  • nibsnibs Member Posts: 462
    @Ichmb, I agree with you if the contractor is going to work with tech support, do not make changes, but do be part of the conversation, and learn what the settings they change do, and why. Then monitor the system closely.
  • lchmblchmb Member Posts: 2,984
    cant argue that logic!!
  • Chris_NHChris_NH Member Posts: 12
    My installer visited today with a Rinnai technical rep. The first thing my installer did was reinstall the original circulator. They agreed that the bigger circulator did not fix the short cycling problem. The core issue, it turns out, is that the Rinnai M090C boiler cannot throttle down (modulate) enough to match the 25 kBTU/Hour emitting capacity of my first-floor heating zone. The M090C has a limited (1.2:1) turn-down ratio, if I'm reading the spec sheet properly. So, with its 75 kBTU/Hour (approx.) output capacity, the M090C can modulate down to only about 62.5 kBTU/Hour. When paired with the 25 kBTU/Hour capacity of my first-floor heating zone, this results in a high cycle rate (6 or more per hour), which in turn results in an average supply water temperature that is 10 to 15 degrees BELOW T-Set, the target (outdoor reset) temperature. The near-term work-around is to raise the outdoor reset curve to aggressive levels. This produces a better heating experience at the expense of reduced efficiency. Both my installer and Rinnai are optimistic they can come up with a better long-term fix, which may involve a different model boiler.
  • Chris_NHChris_NH Member Posts: 12
    edited April 2019
    I should clarify. It is my understanding that increasing the outdoor reset curve by 10 or 15 degrees in my case will raise the average (over time) supply temperature to the value that results in the desired heat output for a given outdoor temperature. But it does not solve the short-cycling problem, which reduces efficiency and adds wear to the system. It also does not keep the return temperature in the condensing temperature range (< 130 F) as much as would be possible if there was less temperature variation, which again reduces efficiency below the ideal. And counter-intuitively, because my boiler cannot modulate down enough, the average supply temperature will always be lower than T-Set by 10 or 15 degrees, which will prevent my heating system from reaching the heat output capacity of its baseboards and radiators (25 kBTU/hour downstairs, and 15 kBTU/hour upstairs), which should occur when T-Set is at its maximum value of 185 degrees.
  • lchmblchmb Member Posts: 2,984
    edited April 2019
    The M series brochure shows the boiler with a 4 to 1 turn down. Which I learned the hard way is for natural gas only. For some reason the LP version has a smaller turn down. I look forward to hearing what their solution to your problem is. Sounds like they wish to make it right though..
    P.S. I like the Rinnai products, they stand behind them..
  • Chris_NHChris_NH Member Posts: 12
    edited April 2019
    @ rick in Alaska: thankyou for the tip. I'm now receiving emails when a new comment is added to this thread.

    @Ichmb: I agree with your thoughts regarding Rinnai. I have high hopes they'll come up with a fix. And I'm also convinced my installer is trying to do the right thing.
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