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Why pipe hot water right back into my modulating boiler?

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New homeowner here; I’ve got a Munchkin 140M (140,000 btu) heating a 2,700 sq ft 1895 house in climate zone 4 (almost 5) via cast iron radiators.

My understanding is that this boiler senses the Delta-T and modulates accordingly. My understanding of how this would work on practice is that (assuming it hasn’t fired in a while and the delta is large) it will start firing full-blast and then scale back as the radiators heat up and the Delta T decreases.

if this is the case, i’m wondering why you would pipe the hot water out right back into the furnace, as mine is piped.

Seems like it would prevent the boiler from firing at full-blast, thus making it take a lot longer for the system to come up to temp. Maybe this is done to prolong the life of the furnace (maybe it’s a little under powered)?

Thanks







Comments

  • hot_rod
    hot_rod Member Posts: 22,251
    edited January 2023
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    That boiler really prefers to have a primary secondary piping. In that case the boiler would have a dedicated circulator to assure adequate flow. It looks like a bypass valve is in there to always allow a % of flow through the boiler. Without that, and the adjustment it has, the boiler could run up to high limit and shut down, or worse go into high temperature lockout. So it looks like a work around to a zoned direct piped boiler :)

    The boiler must be pushing 20 years, so while not ideal, it has worked properly??

    The best efficiency of a condensing boiler would be with the lowest return temperature. Yours always blends some hot supply with the return. Good for the boiler not good for best efficiency.

    If you need a manual

    https://htproducts.com/munchkin-dis.html
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Ironman
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
    edited January 2023
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    I understand everything except primary secondary piping. Would it be difficult do pipe it properly and would new piping work with a new furnace, when this one goes?

    So, for now, I've got to leave it open, or risk breaking my furnace. Is there any way to find a sweet-spot of efficiency vs. boiler life? Seems like it's just guesswork.
  • hot_rod
    hot_rod Member Posts: 22,251
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    When you do replace the boiler, the piping method depends on the type of boiler you decide on. Some must be piped with a dedicated loop or hydraulic separator. The fire tube brands can be piped direct, if you follow the manual and assure minimum flow rates.

    Cast iron radiators do take some time to get up to full output, regardless of the boiler.

    If you go with a mod con type boiler again, two things to balance.

    The supply temperature to get adequate heat output. But you also want the coldest return to keep the efficiency up.

    Sooo to make a long story long. A room by room load calc, then an assessment of the radiator size and output, and then you could nail down the required supply temperature.

    Of course any improvements to the home will lower the load, allow lower supply, and provide the best boiler efficiencies.

    The home and the radiators dictate the boilers operating condition.

    Some good info here about maximizing your hydronic system.

    https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    MikeAmannSENorthEast1895er
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,607
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    That bypass valve looks like it was an add on to me.

    @SENorthEast1895er

    Primary/secondary is used to make sure the boiler has the proper flow no matter how many zones are calling. There are other reasons as well.

    There is a book on this site available "primary/secondary pumping made easy"
    SENorthEast1895er
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
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    Any books or tips on this site for doing load calcs? I've done some before using online calculators, but I get huge variances between different calculators.
  • hot_rod
    hot_rod Member Posts: 22,251
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    This one seems to be a crowd pleaser

    https://slantfin.com/slantfin-heat-loss-calculator/
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mattmia2
    mattmia2 Member Posts: 9,734
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    That converted gravity system should have so little resistance it doesn't need the bypass but the indirect might need it. It might be more efficient to have it on the indirect zone only but your money would probably be better saved for the new boiler which probably isnt that far off rather than trying to change it at this point.
    SENorthEast1895er
  • mattmia2
    mattmia2 Member Posts: 9,734
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    Or the zone valves themselves have too much resistance.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
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    @mattmia2 what's the give-away that this was a gravity-fed system?

    One zone valve is completely open (and corroded); I and the previous owner use the other to try to balance the zones. However, the radiators aren't balanced (no lock shield), so that makes everything pretty difficult.
  • mattmia2
    mattmia2 Member Posts: 9,734
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    If the zone valves are not working, how does it switch between domestic hot water and domestic space heating?

    It was originally gravity because the mains are very large. Do you know if this boiler replaced the coal boiler or was there another boiler in between?
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
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    I don't know if there was a coal boiler; there was likely a coal furnace at some point. There is also an oil tank in the basement.

    Zone valves work; I was thinking of the manual valves I need to adjust to heat the front and back of house.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
    edited January 2023
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    @hot_rod

    from the Munchin 925 controller manual:
    The Munchkin is designed to function in a closed loop 15 PSI System. To assure you that you have adequate pressure in the system, we have installed in the outlet manifold, a pressure switch that will not let the Munchkin operate without a minimum of 10 PSI water pressure. This assures you that if the system does have leak, the Munchkin will lock out (PRO on the display) before it damages the Stainless Steel Heat Exchanger.


    My understanding of this would be that the boiler will protect itself if not getting enough flow. So to get the coldest inlet temps, I should shut the bypass valve and just open it until I don’t get a lockout (assuming this is a low-pressure protective lockout, not a high-temp lockout that you mention).
  • hot_rod
    hot_rod Member Posts: 22,251
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    That particular switch is more of a shutoff if the system looses water. It keeps the burner from
    firing with low or no water

    Been a while since I tinkered with a 925. It could also be watching inlet and outlet temperature. That would assure adequate flow, or lock out. Most all mod cons have that function. The Munchkins were “pioneer” boilers, teaching us what was possibly and what improvements we see on todays mod cons 

    Seeing that bypass has been added makes me think it may have had inadequate flow issues. It is a work around.

    Following the piping in the manual would avoid all that piping nonsense
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mattmia2
    mattmia2 Member Posts: 9,734
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    i would not mess with the bypass unless you want to hasten your need to replace that boiler. there is a reason very few munchkins are around anymore.
  • Jersey2
    Jersey2 Member Posts: 165
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    It was really called the Munchkin? Very interesting.
    I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.
  • Jersey2
    Jersey2 Member Posts: 165
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    I guess it is durable for being 20 years old. I guess they were ahead of their time with efficiency, those Munchkins.
    I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
    edited January 2023
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    @hot_rod, yeah, definitely frustrating that the previous owners shelled out for a really nice furnace, but then cheaper out on the install (piping).

    I’m in the process now if trying to figure out if I should get it re-piped in anticipation of replacing it with another mod con, or leave it be (in anticipation of replacing it with a heat pump and conventional boiler, per the idronics low-temp article i think you shared)). Already did the manual j, might do some field tests to see how the house does with 130 output temp.

    At the very least, it only runs the circulator doesn’t fire the furnace until inlet is below a set point (output - 30 degrees is default)
  • hot_rod
    hot_rod Member Posts: 22,251
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    A new system will involve a major repipe. I would run with what you have. Depending on how far out the rework is?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    mattmia2
  • HomerJSmith
    HomerJSmith Member Posts: 2,467
    edited January 2023
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    When that boiler was installed, the installer just didn't understand mod/con boilers by the fact as to how it was piped. The purpose of a bypass on the old cast iron boilers was to make sure that the return water to the boiler was above 130 deg. and the use of a ball valve was a poor choice. Evidently, the installer didn't read the installation instructions. Of course, why bother if you know it all.

    I would look at the programing.
    SENorthEast1895er
  • JUGHNE
    JUGHNE Member Posts: 11,074
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    I service 2 older Munchkins, the oldest installed in 2003.

    The install manual shows options for piping.
    The traditional piping (zoning with valves) is not primary/secondary.

    The entire system water is pushed thru the boiler. Only 1 pump. There is a flow bypass valve shown.

    But, "preferred" piping diagrams show closely spaced tees (P/S) with 2 pumps.

    This I&O book is dated 4/1/2003.

    IIUC, later versions did show only P/S piping with 2 pumps.
  • HomerJSmith
    HomerJSmith Member Posts: 2,467
    edited January 2023
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    Yes, the installation manual did show that. But, the Vision 1 manual, circa 2002, only shows P/S installations, which I always followed and learned in training at that time. The problem with that single pump for ZVs was that the heat exchanger needed a flow that would prevent flashing, which is why the bypass was thought necessary, much like the old GlowCore boilers. A differential bypass shown on the installation diagram provided the same effect as a straight bypass as shown on the pic above, but without the protection in terms of gpm in the ZV sys. The pump that was needed to provide flow thru the HX with the pressure loss on the distribution sys had to be substantial which created a problem of excessive flow when only one ZV was operating. I'm not aware of anyone doing a pressure loss on these systems to determine pumping requirements and went with the recommended Taco 0010 pump for a 140M.

    I have only come across one Munchkin installation with that configuration which incidentally was installed by the homeowner and his friend in 2004, without the straight bypass or differential bypass. Still working, how well, I don't know. I did a cleaning after 8 yrs and fixed a couple of installation errors. He did install it with a Taco 0010 single pump. I suspect that the input and output sensors modulated the boiler down. Oh, well. Just because HTP shows it in the manual doesn't make it the best installation.

  • JUGHNE
    JUGHNE Member Posts: 11,074
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    The 2003 Munchkin is set up P/S with 2 circs.

    The other was installed in 2010 (by others BTW), with only one circ which is an Astro 50.
    Others here said that is seriously undersized. Owners can not see why to change it.
    I set the max temp down to prevent cycling on high limit.
    No zone valves on this system nor by pass. Plenty of convectors.

    The heat exchanger was first cleaned in 2021. The installer assured me that no one had opened it before.
    The coils were surprisingly clean, the original ignitor and flame sensor were changed at that time.

    Both of these are 199,000 btuh.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
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    So now i’ve got a concern regarding the setup here; i think i’m going to be pretty restricted in how low i can set my boiler temps due to the hot water setup here.

    i fiddled with the hot water heater’s temp which seemed fine until i also fiddled with the output temp of the boiler (although my memory of the sequence of events may be off, which would affect my assumptions about how the system works)

    It looks to me that i have a simple mechanical thermostat on the water heater that asks the boiler for hot water. However, I’m fairly certain that that is simply an on-off call for hot water; i have a hard time imagining that calling for a specific temperature.

    So my concern is that, if I set my outdoor reset so that it, at some point, is only putting out 110 degrees, and my water heater wants to get up to 120 degrees, my furnace will fire indefinitely.

    i know that the munchkin/s vision 1 system (which may or may. or have been included in the install of this system) avoids this problem when properly programmed, piped, and paired with HTP’s own water tank.

    However, i’m not sure if it’s able to do that with any water tank, or if i am piped correctly (ok, one thing i can be fairly certain of on this install is that it’s piped incorrectly). Can i still program the vision 1 system to send out higher temps when the water heater calls?

    this is a bit of a problem as the house has 2 units (1 rental unit), and the previous owner (not the installer) told me our tank is undersized. his solution was to run the water into the tank at scaling levels, then put a thermostatic valve to mix in. cold water on the tank output.

    vision 1 manual

    Programming the unit: function 3, temp 185: Maximum Domestic Delivery Water Temperature Setpoint the Installer will program for SuperStor Indirect Fired Water Heater (Note: Does not apply if used with Mechanical Control) Range: 95oF to 185oF

    (i can’t find the exact quote, but i’m pretty sure it says i need a different circulator for the water heater for a proper setup)




    water heater valve




    house thermostat valve




  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
    edited January 2023
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    wrote a longer post, but seem to have deleted it. short version:

    1. Can I use the Vision 1’s ability to output a higher temp water to the water heater? (i don’t think so)

    2. do i run the risk of having the tank call for heat (let’s say 120 degrees) while the furnace is only outputting 110 (due to outdoor reset) and running the furnace indefinitely? (i think so. problematic because i’ve been told the tank is undersized for this 2 unit house. it’d be 5 ppl max. currently set up to “overheat” water in the tank, then mix it with a thermostatic valve on tank output to control temps. this drastically limits how low i can set my temps.)







  • hot_rod
    hot_rod Member Posts: 22,251
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    Is the DHW setting accessible in your control? If so you must have the Vision upgrade. Did you try setting it to 180? What happens?
    I doubt you will get much recovery with 110 supply to the tank. Looks like 3/4 piping also to the tank coil?
    As far as sizing the tank, calculate the load for the two “typical” households.

    A realistic water demand calculator at www.iampo.org/water-dem
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mattmia2
    mattmia2 Member Posts: 9,734
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    Most modern mod con boilers have separate terminals for the domestic heat and domestic hot water calls. Sounds like from what @hot_rod said different revisions of the munchkin did or did not have this feature. If it has this feature then you can set a fixed setpoint for a DHW call and use outdoor reset for a DH call.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 80
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    ok, i can access the vision 1 programing functions on the 925 control. almost everything is at the default settings. The two exceptions are:

    Design Supply Water Temp: 190 (default 201).
    (i don’t see an attached outdoor sensor in the wires poking out, although i haven’t popped the cover off to see what’s inside yet, so i don’t know)
    Indirect Boiler Set Point (flow): 194 (default: 180).

    i’m going to try running an informal test tomorrow; turn the water temp down for the day, crank it up in the evening, and see if the water temp gets up to a higher temp.

    Per the Vision 1 Manual;
    Maximum Domestic Delivery Water Temperature Setpoint the Installer will program for SuperStor Indirect Fired Water Heater (Note: Does not apply if used with Mechanical Control) default value: 185


    i assume it doesn’t have to be their specific SuperStor indirect fired water heater. And i’m not quite sure what a mechanical control would be, so… i’ll just keep an eye on output temps and see if they get up to 185.