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Thread: aquastat setting and boiler question

  1. #1
    DIY Senior Member bbillcee's Avatar
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    Default aquastat setting and boiler question

    hey guys

    my boiler has a coil in it, in the summer i turn off the boiler and go to the hot water heater

    the coil is garbage so this year i will probably leave the boiler and hot water heater and bypass the coil

    so my question, is the aquastat mainly for the coil or is it to keep the boiler semi hot so it doesn;t have to fire up cold or both?

    anyway what temp should i keep the boiler at? 160?

    do u think it will be much more costly keeping them both on or roughly the same?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    It's a little bit of both. The coil needs the boiler to be some minimum temperature, or you'll not get the water passing through the coil hot enough as it flows through that water bath. But, some boilers don't work well if they cool off too much, so there is some minimum required on some. Many of the newest don't need to maintain a minimum temp and only fire it up when the system calls for heat. Most of the older ones can be damaged if the return water is too cold (a result of the supply being allowed to cool off too much). So, without knowing the make and model, it's hard to tell.

    Most boiler aquastats have a high/low setting. You might be able to set the low to say something like 140 and the high to whatever is required when things need heat (maybe 180, or somewhere near there). They also often have a differential setting that gives it some flexibility about when it turns on. If yours is set up for an internal coil, it may not have that high/low/difference adjustment. You might be able to retrofit one to save some energy.
    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 Senior Member bbillcee's Avatar
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    its a 4 year weil mclein eg 55

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    Turn the low limit as far down as you can get it. Better yet disable it altogether.

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    DIY Senior Member bbillcee's Avatar
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    so basically we dont want to keep it 'pre heated' then

    leave the the aquastat on lowest setting?

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    No you don't. Look into a temperature modulating control for your system Tekmar, Taco, Beckett etc. What this control does is sense the outside air temperature and your boiler temperature. If it's really cold outside the boiler will operate near or at its high limit temp. If it's warmer outside the boiler temp will drop. these controls will save you a minimum of 15% and I have seen close to 30% on some systems. Payback avg. about 2 1/2 years.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    As noted, some boilers will die prematurely if you let them cool off too much, and that will dictate how low you can go. The new, modulating, low-mass boilers can cool off to ambient, and only turn on when needed. The older ones can't handle that. An outside reset is common on the newest, highest efficiency units. Keep in mind that constantly running boiler fired at the proper level makes the highest comfort levels, and you have less waste energy that results when you short cycle things.

    This should be discussed in the installation manual.
    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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    Nope, just not so. Well unless you are talking something 40 years old or so. cooling off should not have any effect on the boiler at all unless the seals are not tight or the press nipples are rotting. Where cooling off can be a problem is when the boiler is under fire and return water comes back too cold. Then you can get condensation within the boiler and that in turn will cool off the fire and that in turn leads to incomplete combustion and that finally turns into a gummy mess in the boiler and acidic hydrocarbons in the flue.

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