Proper Aquastat Settings

Discussion in 'HVAC Heating & Cooling' started by Backglass, Nov 8, 2007.

  1. Backglass

    Backglass New Member

    Messages:
    75
    Location:
    New York
    Hey all...back with more questions from the money pit.

    Six months ago we moved into an old house with an oil fired boiler system. For reasons unknown, the previous owner had the DHW portion of the boiler disconnected, capped off and a propane water heater installed. Since the DHW portion of the boiler is no longer in use, I wanted to make sure that the temperatures settings are correctly set since winter is rapidly approaching and we have already learned that the previous owner did many things ass-backwards.

    It is a Honeywell Triple Aquastat (either an L8124 or L8151). Currently the "HIGH" dial is on 160, the "LOW" is on 140 and the "Differential" is 10. Does this sound like a standard setting to you for a two-zone, heating-only boiler in a 150 year old drafty New York farmhouse?

    On another note...We have also been told that the boiler must be left on year round because even in the summer it must heat & circulate from time to time to keep corrosion/deposits/general ugliness to a minimum. Is this still true with the DHW disconnected?

    Thanks!
    Last edited: Nov 8, 2007
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,059
    Location:
    New England
    I think that those settings would cause the boiler to cycle a lot and be inefficient. I don't have enough experience to tell you what it should be, but while that may have been reasonable if making DHW, I don't think it is best for space heating. Some boilers cannot tolerate the return water being much colder than the source. I have no idea if yours is one of them. On the new, high efficiency ones, they'll cool off to ambient, and only turn on to prevent freezing if the system isn't calling for heat in some manner. Older ones cannot handle that. their heat exchangers are not designed for the thermal stresses of that. Most radiator type systems are designed for 180-degree water to provide their full output. Obviously, you don't need that all of the time. Your boiler probably isn't able to adjust the output dynamically, so if a lower temperature would work, it could run that longer and you'd be more comfortable. A constant even heat rather than on again off again is more comfortable, if your system can provide it. Retrofitting that capability to an older system may not be easy or ecconomical.

    Hopefully, someone who has more practical experience will answer.
  3. Backglass

    Backglass New Member

    Messages:
    75
    Location:
    New York
    So you are thinking it should more like 140-180? 160-180?

    I am new to the boiler world. :confused:
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,059
    Location:
    New England
    Without knowing the deisgn of the boiler, I'm not sure...this isn't an area of expertise for me. If the boiler could stand it, you could let it cool way down when not calling for heat, then rise up to the design temperature, if it ever gets there, based on the needed heat. But, some boilers can't handle that. Mine will. It will drop to room temperature, and only heats to where it needs to based on the outside temperature and how much of a drop it sees between the output temperature and the returning water. On a cool day, 100-degree water may be sufficient to keep the house warm, while it might need to be 180 on a super cold night.
  5. CHH

    CHH New Member

    Messages:
    225
    Location:
    Denver, CO
    I'd leave the darn thing alone until I had some measure of its performance. Maybe the radiators are oversized and 160 degree water is fine. As Jim notes, cooling off may be bad for the boiler - it may develop leaks. Then again, maybe cooling off is fine. Of course letting the boiler cool down will slow response time on a call for heat.

    One thing you might consider is having a tech service the boiler for the heating season. Ask the tech about the aquastat settings and whether or not the boiler can cool down between cycles. If the tech doesn't know or sounds "iffy" then note the brand and model here and somebody will probably be able to tell you about it.

    If it turns out that the boiler has to stay hot then maybe start thinking about a replacement. From a simplicity point of view, the old boilers are great and anybody can repair/replace the controls and circulators. But if heating costs are on the high side of a couple thousand a year then a high efficency boiler will use about half to two-thirds the fuel and replacement makes sense. A propane fired high efficiency unit might actually pay out in reasonable time if propane isn't too expensive in your area.

    As for runing the system in the summer to keep corrosion/nastiness down, well, that sounds silly to me. I used to shut down my old cast iron boiler system for summer and it didn't cause any grief. That said, cooling a boiler off could allow leaks to develop if it has any seals.
  6. Backglass

    Backglass New Member

    Messages:
    75
    Location:
    New York
    Thanks for your insight CHH. I DO need a replacement, this I know. I estimate the boiler is at least 20-30 (or more) years old. I know we could save on efficiency alone and plan to do this next season (carrying two mortgages now, can't seem to sell my old house in this market, but thats another long ugly story :().

    I just want to make sure that I am getting the best out of this old girl while she's still around. The fact that it is a triple aquastat, yet the DHW is capped off led me to wonder if the settings were ever adjusted properly to compensate for the absence of hot water demand. The more I read it seems the DHW works off the low limits side anyway so it may not be a concern.

    My thoughts exactly but being new to the boiler world, I wasn't sure if this was "Hydronic 101". We DID have it off all summer and had no leaks.
    Last edited: Nov 11, 2007
  7. BigLou

    BigLou New Member

    Messages:
    138
    get an inderect fired hot water heater . hook it up to the boiler and leave it on year round to make DHW. it will be more efficient then a propane hot water tank and keep your boiler happily running year round

    Lou
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