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Thread: Can I do this to my modcon?

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    DIY Junior Member ToddinMaine's Avatar
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    Default Can I do this to my modcon?

    I'm a homeowner trying to get a handle on my newly installed Utica SSC-100 modcon boiler. It cycles incessantly with short burn cycles.
    I don't know how to post the diagram, but if anyone cares to look, my system is quite well represented in fig 5.5 on page 14 of the Utica IOM manual here: http://www.ecrinternational.com/secu...ument/2563.pdf.
    Referring to fig 5.5, I tried closing the "heat exchanger ball valve", thus adding the entire volume of the CH loop to the heat exchanger loop, allowing for extended burns, in contrast to the constant heating and cooling of the small internal loop. In fact, in the system of fig 5.7A on the next page, the ball valve is shown to be shut, as noted.
    My question is this: Can I simply leave the heat exchanger ball valve closed on my system and enjoy the benefits of longer burn cycles?

    Thank you,

    Todd in Maine

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    Retired Defense Industry Engineer jadnashua's Avatar
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    It depends on how your system is piped...it is designed for a primary/secondary loop. That heat exchanger loop is built-in as a primary loop. Unless the flow to the external load is not plumbed properly, that local (internal?) primary loop should work. There are some very specific requirements for the distance between the takeoffs on the primary loop that must be followed or it doesn't work properly. WHen there's a call for heat, most of the heated water should travel through the heating loop, and not just return to the boiler. If yours does, it is not plumbed properly.

    The thing would probably tell you why it is shutting down. Is there an error code? If your external heat loops are not done well, the flow may short circuit back to the return, and it could reach max temp and shut off quickly. Otherwise, most of the heat will exit that primary loop and not reach the limit which would shut off the boiler.
    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 Junior Member ToddinMaine's Avatar
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    Quote Originally Posted by jadnashua View Post
    The thing would probably tell you why it is shutting down. Is there an error code?
    No, it's not faulting, but simply reaching temp. setpoint, sometimes within 30 seconds and shutting down.

    I can't judge whether or not the piping is correct (though the house IS warm today - a 12 deg. day), but I do know when I closed that valve, essentially making the entire house the primary loop, the boiler brought the zone up to temp then throttled back and cruised for fifteen minutes until the demand was satisfied. I didn't observe any ill effects, and I wonder why I shouldn't be able to leave it set up that way.

    Thanks - Todd

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    Retired Defense Industry Engineer jadnashua's Avatar
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    That boiler is designed to have a primary/secondary loop. THey provide a primary loop internally that is enabled when the valve is open. When you close it, it will work properly IF there's a primary loop installed external to the unit. All of the heating loops come off that primary loop and are called secondary loops. A primary loop allows the boiler to operate with less stress - bypassing it could shorten its life. If you study the diagrams in the installation manual you provided, you'll see that all of the alternative configurations provide that primary loop externally to the boiler. It may be that your boiler is way oversized for your house. This is a common (unfortunate) situation if a proper heat load analysis was not done prior to its selection. Replacing a boiler with the same size as was originally there is rarely the right size - often 2-4x oversized which affects efficiency, costs more to buy, and makes short-cycling more common. A mod-con can help since it can throttle down from max, but even 20% of what you have may be too much most of the year.
    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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    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    A condensing boiler wants to be loaded while maintaining nominal flow through the heat exchanger. The distribution piping and radiation determine flow if the by-pass is closed.

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    DIY Junior Member ToddinMaine's Avatar
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    Got it. It's all about insuring the heat exchanger has adaquate flow through it regardless of what's going on in the secondary loop. So then, is this a classic example of the need for a buffer tank in the primary loop?

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    In the trades Dana's Avatar
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    Quote Originally Posted by ToddinMaine View Post
    Got it. It's all about insuring the heat exchanger has adaquate flow through it regardless of what's going on in the secondary loop. So then, is this a classic example of the need for a buffer tank in the primary loop?
    How much radiation (and type) do you have? What is the total plumbing length & diameter (including that embedded in the radiation), approximate number of ells, and the pump used for the CH loop?

    What is your 99% outside design temp and heat load at that temp?


    If yours is a single-zone system (as in figure 5-5) using low-mass heat emitters (like fin-tube baseboard), as long as you have sufficient flow and a sufficient amount of baseboard it should be tweakable to a balancing point where it modulates with long burns. But if you don't have enough baseboard to get the heat out (at any flow rate) at the output temperature being called by the outdoor reset, it'll short cycle. I didn't see a programmable hysteresis in a quick glance through the manual. Increasing the output temperature/curve increases the BTU output of the radiation, which may help, but increasing the hysteresis (the difference between the low & high temp at any ODR setpoint) would also help, if that's an adjustable number.

    If the CH pump is undersized creating a low-flow condition in the radiation it may be contributing to the problem too, and when you close the primary bypass valve completely you essentially have both pumps driving the radiation flow, which may "kinda work", but it's not exactly legit (it would be OK in some systems, not so much in others).

    If your heat load at a design temp of -10F or so is less than 40KBTU/hr and the system has only 50-60' of fin-tube you will likely have cycling issues with a boiler that big unless you add either mass or radiation, (or run it fixed-temp at 160-180F, losing all condensing efficiency.) At 130-135F average water temp (the condensing threshold on most systems) 50' of fin tube can only deliver ~13,000-14000 BTU/hr, which is below the 17-19,000BTU/hr minimum-fire output of the boiler. But if you have 150' of fin-tube on a single zone you can probably run in condensing mode 100% of the time and it would never short-cycle if the flows are set up properly.

    Whether a buffer tank or a buffering hydraulic separator is needed depends on the system particulars, so let's have 'em! In most single-zoned residential systems in central ME the 20KBTU/hr min-mod input for this boiler wouldn't have a short cycling issue when it's +12F outside, so rather than throwing a thermal mass band-aid at it, lets run at least a napkin-math analysis on the system to see if there's a simpler/better way to address the issue.

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