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Thread: Need help with cycling issue- Monitor MZ25S Boiler

  1. #1
    DIY Junior Member craigw78's Avatar
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    Default Need help with cycling issue- Monitor MZ25S Boiler

    I just finished installing a Monitor MZ25S Boiler and am having trouble with excessive cycling. Itís a NON-MODULATING combi boiler and the DHW side operates fine. However, when warmed up, and operating in heating mode, the burner fires for less than a minute and shuts down. The pump continues running after the burner goes out and the boiler fires again after about 3 to 5 minutes. The burner continues to fire for another 30 seconds to a minute and then stop again. This cycle continues as long as the boiler is turned on. I assumed I was having a flow problem so added a Grundfos UPS15-58. The MZ25S has an integrated Grundfos UPS15-42 so I am now running both pumps. Even with both pumps my flow meters are only showing about a combined flow of 2 gpm.

    My house is small so I only have one zone servicing the main floor. The main floor is 750 square feet and I have about 950 feet of pex ran in four loops which average just under 250 per loop. The loops are attached to my subfloor with aluminum heat transfer plates. Also integrated into the boiler is about a 2 gallon expansion tank and actuator to switch between the DHW and heating modes. I have added a y-strainer and Taco Vortex air separator. As far as I can tell nothing is plugged.

    The boiler is a 95000 BTU 95% efficient unit- with an output BTU rating of about 90,000. My plumber ran a combustion analysis which he said was normal and the CO and CO2 were within manufacturers specs. I know the boiler appears oversized, but itís not as much as it appears. I have a full unfinished basement with uninsulated concrete walls, single pane windows and an exterior access uninsulated door. I am planning on trying to heat the basement with heat loss from the main floor loops and heat loss from my boiler and manifolds. I may add a supplemental hydronic heater if necessary. My main floor has significant north facing windows with poorly insulated 4 inch walls. The ceiling has about R-40 insulation. As such, I am guessing my heat loss would be in the 50k to 60k range. Because of space limitations with my house, I wanted a combi boiler and the Monitor was the smallest I could find.

    I canít figure out why my boiler is cycling so much. I am assuming its overheating, but that doesnít seem to make much sense. I have been careful to bleed the system thoroughly. My vent is somewhat long for the boiler at about 45 feet including additions for 90ís because I had to go out my roof from the basement, but I doubt that is the cause. Monitor only officially rates their exhaust venting for up to 30 feet, but unofficially one of their techs told me previously he does not see problems until they exceed at least 60 feet. The 60 foot range seems more reasonable as the vent is 3 inch pvc and all other manufactures seems to permit at least 60 to 70 feet with 3 inch pvc on similar sized units. Any thoughts would be much appreciated. Luckily its been a warm fall, but my luck is running out. Thank you.

  2. #2
    In the Trades Tom Sawyer's Avatar
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    It's cycling because the boiler is massively over sized for the amount of radiated floor area. I suspect the boiler may not be properly piped either. The only way around this is going to be to install a buffer tank sized to the load. I see this all the time with mod-con boilers. The staple up radiant can't shed heat fast enough so the delta T is way too narrow. In short, the burner heats up way faster than the floor can shed it.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  3. #3
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    A new respect for you Huck.

    Every good radiant floor heating sysytem starts with a proper heat load, performed by an experienced designer, using dedicated software.

    DIY radiant floor heating; always the first mistake.

  4. #4
    DIY Junior Member craigw78's Avatar
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    Default Thanks for the input Tom

    Thank you for your suggestion Tom. After posting my thread I noticed the earlier posts about buffer tanks and it looks like that will be a good place to start.

    Badger, in researching my project I have read a number of your comments on other memberís posts and basically I just feel sorry for you. While clearly knowing about radiant heat design, you feel it is more important to belittle those attempting such a project on their own than offer constructive advice or feedback. If the most constructive advice you can give is your mantra of ďDIY radiant floor heating; always the first mistakeĒ then I suggest you find another website to attempt to reign over. Donít complain about people doing DIY projects on a DIY forum. Let me remind you, the name of this site is ďTerry Loveís DIY Forum.Ē Your persistence in interjecting your opinion here suggests you may have an antisocial personality disorder and/or the possibility of some more significant mental health issues.

    I looked at your profile and you have 0 blog entries, 0 articles posted and 0 started threads. Of your 41 posts, most contain the same condescending crap you left here. It appears your primary source of amusement is to withdraw from the real world and criticize and bully people in what should be a non-confrontational forum on the internet. Rather than wasting your time leaving disparaging remarks here, I suggest you work on your mental health issues, find a more constructive use of your time, and work on developing and maintaining your client base.

    Lastly, donít assume I am not doing things correctly and cutting corners. I have received a permit from my local building office; I am using the largest, most reputable, local plumbing supply business in my town and am working with a single sales person at the business; I am working with a local master plumber who has over twenty years boiler experience; and, tonight I have an appointment for an HVAC engineer to come troubleshoot my system and confirm whether a buffer tank is necessary for my system. So donít give me your BS about DYIíers just messing stuff up and not have the necessary commitment to safely and professionally complete a job.

  5. #5
    In the trades Dana's Avatar
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    Sensitive, eh?

    You may be doing it legally, but that's different from "doing things correctly" from a design & implementation point of view. I can't really speak for Morgan, (BadgerMountainMN) but his words are mostly true:

    Many (possibly MOST) DIY radiant systems have fundamental design flaws that are much cheaper to avoid in a first-round analysis than to fix once the system is up and not-so-much running. There's a wealth of internet dis-information about how radiant is done, but most DIYers haven't done so much as the back of the napkin math on it. Hydronic heating system design is way more than a plumbing problem.

    In this instance, beyond the mere (and dead-obvious) low thermal mass relative to boiler output issue, getting at your low estimate of 50KBTU out of 750 square feet of radiant means you need to get 67BTU per square foot out of the floor, about 6x what's "reasonable" to design in for a thin-plate radiant solution, and maybe 3x what would be reasonable to do with heavy aluminum extrusions for the heat transfer plates. But without a better heat loss estimate who knows? With a modulating combi (there are at least a couple out there, Navien & Rinnai both have offerings) you might have stood a chance of not short-cycling (or not- I don't recall offhand just how low those units can modulate) but the BTU limitations of the floor might only be discovered once you get to the really cold weather (or maybe not, if your actual heat load is only 15K, but I suspect it's more.)

    Even if you could pump 50KBTUthrough 750 square foot of floor with your current heat transfer plates the floor temp would be between 90-100F, and damned-uncomfortable to walk on. I think you'd need boiler output north of 220F (40F+ over the operating-temp rating of the PEX) to get there with sheet metal transfer plates through 3/4" of subfloor plus ~1/2-3/4" of finished wood floor. To get any condensing benefit out of the boiler you'd need outgoing water temps under 130F and at least a 15F delta-T. You're not even close, if that's your true heat load- you'd have been better off with a right-sized cast-iron beastie + indirect (and more radiation.) But I'm hoping your heat load is actually much lower.

    Start with the heat loss calc. If you have annual fuel use of some prior system and the system's specs it's pretty easy to put a fairly accurate stake in the ground as to the whole-house load at any given design temp. In Billings the outdoor design temp is about -7F. We can rough it out based on average annual heating degree days against fuel use to come up with BTU/hr numbers.

    If that small bit of back of the napkin analysis tells us you're at least close we might be able to direct you to a cheaper way out of this short of ripping it up and starting over. In barely-insulated leaky houses it's often cheaper & easier to fix the heat leaks to bring the peak load down (particularly the air leaks, which are probably a large fraction of the heat loss in your case), than say, swapping out thin-plate aluminum for heavy extrusions or (gulp!) chucking the boiler for a 50-80K mod-con and adding an indirect. In some scenario adding flat-panel radiators to get both more radiation and more thermal mass into the system MIGHT be a solution, but only if the numbers work out would that be the "right" way out. From a comfort point of view tightening up the place and adding insulation where -reasonable is far superior to increasing the radiation output to better match the boiler's output.

    A non-modulating 90K is a lot of boiler for a tiny house, low insulation levels & air-tightness notwithstanding. Tom is right- you'll almost certainly need more thermal mass to stop the short cycling of this non-modulating boiler, but that's really just the beginning.

    FWIW: I'm not a professional heating system designer, but I have designed and debugged more than a couple heating systems (including my own- it's an unconventional kludge, but it works as-designed.) It's not only DIYers that skip the math- having "Plumming an heeting" spelled correctly on the side of the truck is no guarantee of system design competence, eh?

  6. #6
    DIY Junior Member craigw78's Avatar
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    Hi Dana, thank you for weighing in. Before I started my project I received bids from 2 plumbers. I requested bids on combi-boilers because of the marketing hype. Both plumbers happily told me they would install combi-boilers and charge me between five and six grand for the work. When asked how much they would charge to install the tubing and perform the necessary prep work including removing my old furnace and ductwork, and a chimney (or at least subbing that stuff) their bids jumped to over 9 grand. Their bids included basically the same components I chose to install except with modulating boilers. I recently purchased my house for under 100 grand and at nearly $10k I couldn't justify hiring out the whole job. Ergo my decision in making this a DIY project with input as needed from pros. By the way, the 50k to 60k heating loss estimate came from one the plumbers (it does include both my upstairs and basement).

    My point in telling you this is that had I chosen to hire either plumber I would quite likely be exactly where I am now, except with about $6,500 more in out of pocket expenses. I probably would have had a modulating boiler, but as you said, that may or may not have made a significant enough difference. The Monitor boiler I purchased however was basically a very lightly used open box item I bought from a retailer for $500. That is how I arrived where I am currently. Your comments on heat loss are very helpful. Neither plumber I spoke with indicated my floors or general plan would have any issues. However, I should also note supplementing with hydronic kickspace heaters or wall heaters as necessary have always been part of my plans.

    As such, you and Tom's thoughts and comments are greatly appreciated and I am sure will be utilized in my system. As for my comments about Morgan's input, I think my comments are justified. Considering this is specifically classified as a DIY forum his comments are elitist and inappropriate. If this was a “licensed plumbers' only” site his comments would have been spot on, but clearly it isn't. Morgan should do everyone a favor and find another forum if the only advise he has to offer is "DIY radiant floor heating; always the first mistake."

    Again, Thank you very much Tom and Dana.

  7. #7
    In the Trades Tom Sawyer's Avatar
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    craig, you make an interesting observation in that there are a whole lot of plumbers and heat guys that are more than willing to sell you top dollar equipment and not have a clue how to install it properly. I have straightened out more bad radiant systems installed by professionals than I care to remember. Radiant, and particularly under floor is always tricky because heat transfer depends so much on the installation as well as floor thickness and coverings as well as how well the boxes are insulated. Top that off with guys trying to control the temperature with a standard wall thermostat instead of a floor sensor and you usually have a recipe for disaster. Here's the number one key though to any hydronic system. Water flows to the path of least resistance. Unfortunately there are a whole lot of ideas out there about how to maximize radiant performance. If you are really interested you can find articles by John Seigenthaler ( probably spelled that wrong ) that will fill in the blanks for you. Another great source is www.heatinghelp.com (Dan Holohan's site) that is chock full of advice from seasoned professionals.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member craigw78's Avatar
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    Yeah this whole process has been rather eye opening. I have learned an incredible amount in a short time, most of it pleasant, but then again, I am a ways from having a viable, efficient system. I had a hvac engineer check out my system last night and he indicated its not nearly as bad off as I originally feared. Unfortunately, I did not know him when I started planning out my project as it has been in the works for some time. Had I known him earlier, I would have did things completely different- I probably would have ended up going with an open tank style water heater system. Anyway, that’s all water under the bridge at this point.

    The engineer confirmed your thoughts that a buffer tank would be a great place to start in reducing the short cycle issue. He also confirmed Dana's recommendations about tightening things up. As such, those are my next projects.

    Thanks again guys & have a great day.

  9. #9
    In the trades Dana's Avatar
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    So, how much fuel did the place use with the old furnace?

    Old school plumbing & heating guys will typically walk into a place and make this sort of heat load calculation"

    "Lessee, that's about 750 feet, 1500 if you count the basement, times 25-30BTU a foot, but hey this place is pretty leaky, better make that 35-40BTU, call it 50-60K."

    And that approach reliably oversizes it EVERY TIME, in many instances by more than a factor of two, sometimes even more than 3x.

    Fuel use per heating degree-day times equipment efficiency doesn't lie- it's a measurement, and it doesn't require dissecting the house to determine it's construction and running a calibrated blower door test to be able to calculate the infiltration rate.

    To his credit, Morgan measures the tightness of houses with blower-door testing as part of the heat loss calc. I'm gonna go out on a limb here and say that none of your pros who have looked at it took it that far. ;-) I've yet to find a heating pro in my area anywhere near that thorough.

  10. #10
    DIY Junior Member craigw78's Avatar
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    Unfortunately my past usage would probably be less accurate than a size based room by room calc. For the last two winters I haven't spent much time at home and kept the heat 60 degrees or lower the majority of the time without really heating my basement at all. In addition, I recently replaced my windows and entry door as well as insulated my attic. I think I will talk to the heating engineer I had over and see about getting a heat loss calc. I think that will provide the most reliable data. Using the BT = t *(Qh - qload)/(500*dT) formula it looks like my buffer tank should be 80 gallons or more. I currently have a 40 gallon tank I am thinking of adding to see if it improves my situation and if it does I will either daisy chain in another tank if that is feasible or replace it with an 80 gallon tank. Pumping shouldn’t be too much of an issue since my system currently has a Grudfos 15-42 and a 15-58.

  11. #11
    DIY Member Buffalobillpatrick's Avatar
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    craigw78,

    I'm a DIY guy & live very remote. I have designed & installed 3 systems & am designing 4th now. Still learning.

    You got a good deal on the boiler, if it can be controlled & lasts for 5 or more years.

    I agree that you need a buffer tank. bigger tank will allow your boiler to run fewer times per hour. Put a 3-way thermostatic mixing valve on its output will allow a wider delta-T & fewer boiler cycles.

    Good info here:
    http://harscopk.qa.jplhosting.net/up...e962d0ef7d.pdf

    boiler on time formula:

    Volume = [ t x INPUT ] / [ 500 x ∆T],

    where

    t = run time in minutes, INPUT = boiler energy input, ∆T = the allowable tank temperature drop.

    ie: 120 gal = 13.3 min X 90,000 / (500 x 20)

    The off time depends upon current load. Could be hours or days?

    I bought a big Rheem 120 gal. commercial storage tank for my next project.

    I agree with what you said about Badger, his posts have felt like a thumb in my eye, & ended a couple of my threads on various topics & sites.

    BBP

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    In the Trades Tom Sawyer's Avatar
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    I find humor in the whole buffer tank thing because 20 years ago everyone was jumping on the mod con low mass boiler theory of heating a small amount of water as quickly as possible. It was supposed to be the most efficient way and truthfully in some cases it can be but now, twenty years down the road, everyone is jumping on the high mass boiler theory. goes to show that everything comes full circle sooner or later. The best advice is to move to Florida and to hell with boilers and heating altogether
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Retired Defense Industry Engineer jadnashua's Avatar
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    IF, and that is a BIG if, the modulating boiler can go low enough to exactly match the load, you get the best of both worlds. But, especially in a multi-zone system and a mild day, that is highly unlikely. Outdoor reset helps, but a buffer tank can often help as much or more and may be the only way when the boiler is considerably oversized from cycling it to death early.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Buffalobillpatrick's Avatar
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    Read the 1st line of OP

    His boiler don't modulate.

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by Buffalobillpatrick View Post
    Read the 1st line of OP

    His boiler don't modulate.
    It matters not! To keep the boiler from short cycling, it needs a good load - it's just easier to match things up with a mod/con, but the same principle applies to any boiler.

    His is both way oversized, and has a very small load.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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