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Thread: Outdoor Reset Control

  1. #1

    Default Outdoor Reset Control

    I recently replaced an older boiler in an apartment building i own with a Utica MGB-150 non-condensing boiler in hope that i would save some money on gas bills at the house. I also had 4 zone valves put in to replace the existing 4 pump system so that water would only circulate in each zone when it was calling for heat (with the pumps, water seemed to circulate even when the pump wasnt on, probably because of natural convective movement).

    When I got my first gas bill, i was shocked that it was over 15% higher than with the old coal coversion boiler (some of the difference was due to higher gas costs). According to my gas statements, the new boiler seems to consistently use about 10% more therms per heating degree day than the older system.

    The old system had an outdoor reset control (Johnson Controls A350R), and that was left in place by the heating contractor, but not used. My thought was to use the old reset control on the new boiler to keep supply temps lower when full blast is not needed.

    I understand how to hook up the control, and how to adjust it, but my concern is about return water temperature and the flue gas condensing. Most websites i visited dont seem to make anything of condensation on cast iron boilers, but do if its a steel firetube type boiler. But i contacted the manufacturer by email and the reply says to "maintain return water temp of no less than 140F to prevent condensing and premature failure of the heat exchanger." If I set the minimum supply temp so that the return is always 140F, im probably not going to realize significant fuel savings and probably wouldnt waste my time hooking it up.

    Another thought was to adjust the heat anticipators on the thermostats in the apartments to increase boiler run time. If only one zone calls for heat, the boiler's run time is fairly short (around a minute or two). I just dont want there to be too much overshoot because tenants are likely to turn thermostats up if they are uncomfortable because of large temperature swings.

    Any input is appreciated -- thanks.

  2. #2
    DIY Member zimmee66's Avatar
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    Feb 2005
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    Des Moines, Iowa
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    Default re: Outdoor reset control high, bills/condensing

    First and foremost, I am no expert (though I do have a boiler license), but had to learn some painful lessons on my own after having an apartment boiler replaced by a disreputable company. I replaced oil with gas, so computing fuel use comparisons was serious math, but like you I was dissapointed with the "improvement".

    Anyhow--YES, hot water cast iron boilers DO need return temps over about 140F, but this gets fairly complicated because of boiler sizing, radiation type (baseboard, etc), and many other factors. The first question may be sizing...

    If your boiler has a bypass pipe (and it probably should) then supply and return water are mixed to raise the temp of returning water to the needed 140 +

    I wouldnt advise the anticipator approach, it isnt really a good solution.

    Generally speaking, it will be PIPING that needs to be changed if you dont have a bypass. Resets don't fix the root problem.

    I suggest a visit to:

    www.heatinghelp.com/homeowners.cfm

    These guys are SERIOUSLY obsessed with heating, and very knowledgeable and helpful. The get a little tired of the same old Q's though, so read the archives and then ask questions.

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