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Community and Q&A

Hydronic Heating System Aquastat Setting

cheapskate | Posted in General Questions on

I have a three level 2800 sq foot  house with a four zone hydronic heating system. The boiler is a New Yorker FR-122-W with a Honeywell LB124A aquastat. I have a summer/winter hookup.
The house is a converted barn structure. When we built the house we were unable to  find vintage cast iron radiators on our budge, but  recently I acquired some for free and so I changed the main living zone from fin tube to radiator heat. Everything went OK and my wife and I love the change.
My question is can I or should I lower the aquastat settings? Last heating season I had them set at 180/160. Is there any saving to be had by lowering the set points?
One other zone in the lower level (finished at a later time) is also cast iron radiators. The two other zones are much less frequently used and are fin tube.

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  1. greenright | | #1

    You owe it to yourself to install a Honeywell outdoor reset and experiment with different temperature curves. Most Slantfin- style modern baseboard are designed to run at 180 water, but you may find out that you don’t need 180 even on design day and most of the time 140-150 will be plenty- especially with the cast iron radiators. Here is where the outdoor reset comes into play. With some trial and error you can probably save around 20-25% lowering the water temps depending on outdoor temps.

    1. cheapskate | | #6

      I am a pretty handy guy but generally ignorant about HVAC.
      I am a retired electrician and I am pretty tooled up. The boiler tech is the only person that comes to my house to fix or service anything. I actually serviced it the first few years but decided that I was being penny wise.
      Could you elaborate on the outdoor reset? Sounds intriguing. I will google up.
      Thanks for the help.

  2. paul_wiedefeld | | #2

    You can lower it, it'll cut energy usage a tad. That's a huge boiler, how much oil did you use last year? Even fin tube doesn't necessarily need 180 degrees, that's a myth.

  3. cheapskate | | #3

    My house is very well insulated with stress skin panels (R 26 wall 45 roof) and we are cheap. The programable thermostat is set at 64/59. My oil usage is around 440 gallons per year for heat and domestic hot water.
    I think the heating contractor was unfamiliar with stress skin panels but I really did not know the boiler was over sized. It was installed in 1993 and may be due for replacement, thought the coil was replaced few years ago.
    I have it serviced each year and the tech says it is running at 84-85% efficiency. He says its fine now but if I do replace it what size would you recommend?

  4. Expert Member
    Akos | | #4

    The thing you have to watch with non-condensing boilers is return water temperature. I'm not familiar with oil but with gas the limit is 140F. This generally means a supply temp around 160F/150F. If you start dipping bellow that, you can corrode the heat exchanger in the boiler.

    If you do get an outdoor reset controller, lot of them have programmable RWT limits specifically for this reason.

    Going from 180 to 160 won't save much but it is something. The best way to save cost is transition to heat pumps. Air to water heat pump is a hard retrofit unless you have a low loads or large radiator areas. A ducted heat pump tends to be the best option but it involves running ducts which can be disruptive. Even a single wall mount mini split in the main living space can make a significant reduction in run time plus give you cooling in the summer. You can keep the oil boiler for heating rooms around the perimeter to even the temperature.

    P.S. Depending on your local fuel and utility costs a resistance boiler might be cheaper to operate if you electrical service can support it, this is the case around me.

    1. cheapskate | | #7

      I will not be switching to a heat pump or changing my heating system. It is completely impractical plus I doubt I'll be here a lot longer.
      The outdoor reset controller sounds promising. I need to research.
      Thank you.

    2. paul_wiedefeld | | #10

      I think that boiler can handle low return temps based on the manual.

  5. paul_wiedefeld | | #5

    It’s probably 3x-5x oversized, but oil boilers come in larger sizes so you probably can’t do much better. Propane, gas, and electric come in smaller sizes. If you have enough radiation, you could even think about an air to water heat pump upon replacement.

    This would be the best way to size it.

  6. cheapskate | | #8

    Reading up on the outdoor reset controller, it looks like it replaces the Honeywell L8124 aquastat is that correct?
    I am concerned about my flue temperature and condensation with low water return temps. How can I mitigate this concern?

  7. cheapskate | | #9

    Rereading Akos Toth's post, I see the RWT is programable. I would guess that would keep condensation from forming in the flue. Like I said my understanding of HVAC is limited plus I guess I don't read thoroughly either.

  8. cheapskate | | #11

    I am intrigued with the idea of installing an outdoor reset controller but don't understand how to integrate it with my summer/winter hookup. Any details about that would be appreciated.

  9. cheapskate | | #12

    Well. I actually purchased an ODRC(the Tekmar 256) only to find out it can't handle a tankless coil.
    From my research, I discovered that a controller that can handle both functions is prohibitively expensive and the fuel savings minimal with a tankless coil.
    I think a better solution would have been to have installed an indirect hot water heater but space constraints won't allow for a retrofit. So I will work with what I have. As I noted I only use 400-440 gallons on oil a year so I can live with that.

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