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Monitor MZ25C Problems

Perry_3 Member Posts: 498
I am another homeowner who installed a new boiler this year; but have a background in industrial boilers and controls.

Even if your boiler is properly sized to the house - it then needs:

1) to have a properly configured system. Are you doing primary/secondary circulation (a low loss header or Heat exchnager) - or just running the sytem directly through the boiler. If the latter - do you have appropriate water flows for your system.

2) to have proper controls for your heating system and configuration system. You may need to buy an additional control box once you get the right system configuration in order to operate it efficiently.

I have been critized by some for my support for the Viessmann Vitodens boiler because it comes with all the necessary accessories and has the right built in controls for almost any application out there. Some people tell me it is overengineered and that it is better to have a knowlegable person mix and match components - from differernt companies - for each application.

Sounds like you do not have the right mix and match for your house. The boiler will probably do very well when properely connected to the system and properely controlled.

I wish you the best with this. You may have to hire someone who really knows what to do to make your boiler work right for your application.



  • Barbara Schroyer
    MZ25C Problems

    We recently had our local heating contractors install a Monitor MZ25C using Slant/Fin 15 baseboard. We feel it is not functioning properly, as it contantly cycles, takes 4 hours to get the house from 59 degrees to 67, and is using 40 gallons of propane every week....which, I'm sorry, I don't consider to be "efficient". Monitor is of no help, they say the unit is functioning properly. The same story from their representatives. No one can explain why the water temperature out of the furnace gets up to 203 degrees (and shuts off because of a high temperature limit), when the unit is not supposed to deliver over 176! We cannot seem to get an average of 170 degrees for the heat delivery temperature to the baseboards...it fluctuates from 161 to 203! We are very frustrated that no one can give us simple answers but keep telling us everything works fine. Does anyone know of any qualified, experienced technician who might be able to help us get this thing working (we will pay an hourly fee)...else we are tearing it off the wall and selling it on ebay and replacing it with a Trinity. I noticed there were several entries in the forum for people who were very happy with their units, but the dates were not recent. I would like to be able to share that good feeling! Any suggestion would be welcome. Thank you very much for your time.
  • Brad White
    Brad White Member Posts: 2,398
    Ich bin ein MZ Owner...


    The first thing I would check is your calculated building heat loss and the amount of radiation connected to it. No better place to start for the long term. But while you are at it, make sure the internal circulator is on high speed (speed 3).

    Without sufficient radiation to dissipate the heat created by the boiler, the return water will come back much closer to the leaving water, the heat does not go to the space (the objective!) and the unit cycles. I suspect this may be at the root of it. Was the entire system installed at once or was this a retrofit to existing radiation? Does it have a pre-MZ track record?

    I own an MZ, the same as you have, with separate DHW. (EDIT: At first I thought the "C" was for "Combi" but we do have the same unit.)

    One thing I always found annoying was the lack of modulation and propensity to short-cycle. The other thing is that if my DWH tank is set too high (much above 60 degrees C or 140F) the unit shuts down on high limit and has to be re-set manually. It seems you do not have THIS issue...

    Your issue may be one of the system match-up and not technical, so let's start there. Did the contractor or vendor perform a calculated heat loss and what did it say? How much radiation is there and is it apportioned to each room's heat loss?

    How is the boiler piped? In series direct to the house? Or Primary-Secondary?

    Your issue could also be one of not having a secondary circulator and relying on the internal circulator. The MZ has a relatively high internal pressure drop hence may not have sufficient "head" or pressure available to serve your radiation. Hence, flow will go down and the temperatures shoot up. The internal circulator is 3-speed and should be set to the highest speed. Check that while you dig for other paperwork (if any).

    If I had to do it over again, I would have gone with a modulating condensing boiler, not a condensing on-off boiler. With some work (buffer tank and Outdoor Reset) this has been overcome to some extent. The MZ is a rugged unit with some design flaws -beyond not modulating- and has to be properly applied. I might use it as a first stage in a building with a much higher heat loss but connected to a building with a heat loss much below 60,000 BTUH it will cycle a good deal of the time.

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Monitor MZ25C Problems - add't info

    The MZ25C replaced an Burnham oil fired boiler (don't know model or size). It's chimney was an 8" round steel (except where it went through the basement ceiling and through the roof) that went up 16 feet to the roof in an open cathederal ceiling setup). There were two zones - basement - 61 feet slant/fin 15 and first floor 44 feet slant/fin 15. The contractor never did a heat loss. I had one done by a firm that has been building domes since 1972 - they came up with 49,100 (24,814 for upper floor and the rest for the basement). Both my contractor and Monitor feel that this is low and should be somewhere near 80,000. My heat loss calculations have been forward to slant/fin by Monitor for their opinion. We went all the way with MPI - MZ25C, BS40 DHW and their hot water kit. The latest setting that Monitor wanted me to try: manually open both zone valves, set circulator to 1. The only thing that this accomplished was to prevent the unit tripping with return water temps of 200+. My average supply water temp is in the upper 150s and return is the mid 140s. Thanks for your suggestions and comments.
  • Brad White
    Brad White Member Posts: 2,398
    Of Radiation and Output

    **** and Barbara-
    The Slant-Fin literature I have on-line is not clear if rated at "entering" or "average" temperatures. I will assume "average" at 170F (180 in/160 out); I get a total output potential of 480 BTUH per LF of 50,400 BTUH, so this is "on the money" if your 49,100 total heat loss is credible.

    That the basement has a loss roughly equal to the above-ground loss is a little puzzling; it is intended to be living space? Concrete is extremely conductive but if insulated on the exterior can be an effective heat-sink. I would expect the upstairs to be about double the downstairs, absent direct knowledge.

    Is your perceived deficit all over or more noticeable above-grade? That would help nail down the disparity.

    Barbara wrote me off-line and said that the dome is 2,400 SF. I will assume this means both levels. Also it is 2x6 construction. I do not know if there is continuous insulation over the framing. Geodesic domes as you know have a very high framing percentage (40% or more overall) which degrades the wall insulation's purported R-value.

    Still, the call it 50,000 BTUH heat loss works out to under 21 BTUH per SF. I consider that low for Maine. The 80,000 would definitely seem more realistic, a resulting 33.3 BTUH/SF check figure.

    A "gut check" absent calculations tells me that the upper portion, assuming a 5/8 sphere, would be about 67% of your heat loss and the basement 33%. Not knowing what glass area you have and how much of the basement wall is above grade, insulation levels there, etc. it is all I can do to guess.

    I agree with your contractor and Monitor that the calculated heat loss is low, but it raises the question, did they check the radiation to see if it met their projections? No point in installing much more boiler than the radiation can emit, except for the domestic water feature.

    Bottom line is, I think you need much more radiation, even exceeding the boiler rated output. To do this allows operation at lower water temperatures and that is to get you down into the condensing range. By condensing range, you need to get your return water down below 120F for extended periods. Personally, I would select the total radiation output on the coldest day to work with 140 degree output from the boiler. Thus you will be condensing practically year-round with outdoor reset. Mild weather may use 90 to 100 degree supply water if you select the radiation generously enough.

    To add radiation is much easier said than done. Might I suggest steel panel radiators such as Buderus, Myson or Veha? Runtal is another brand but given your investment to date, may be a shade "high end" for what you need for a solution. Nice units though. Point is, more heat in less space. Consider your baseboard as the base-line heating to get some convection up the walls and the panels to fill in cold spots is one suggestion. TRV's may help to keep the panels in-line and the first brands I mentioned have these built-in as an option.

    Monitor may still offer an outdoor reset control. It is/was made by Tekmar and I am sure that Tekmar direct will be able to provide one suitable to your needs.

    The circulator setting is puzzling to me- that reducing the flow would stop tripping on high limit. Less flow means more temperature rise. The MZ literature I have says "speed 3".

    The minimum flow on the MZ25C is 3.52 GPM which means an almost 50 degree rise across the heat exchanger. Thus if you are returning 145 degree water you will get to over 190 quickly.

    The fact that you are struggling to meet space setpoint is telling, especially in that we are not yet in the depths of winter. Aroostook County is getting there but even there, this is nothing near design anywhere in Maine. Ayuh.

    So, to recap: I strongly suspect you are short on radiation relative to your heat loss. Your heat loss should be double-checked and the radiation on each level be verified as proportional to the heat loss of the space it serves. Technically, you cannot have too much radiation. Any surplus goes to lower water temperatures.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
This discussion has been closed.