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Wrong aquastat?

Purchased our home about a year ago and battled the polar vortex and a thirsty oil boiler all last winter. Now I figure before the heating season begins I should get everything in order. I can't find a make or model of the boiler anywhere, just a big blue box. Earliest service tag I can read on it is back in the mid 1990's.

This boiler had a tankless DHW coil, but it has been removed and a plate welded in. There is now a 39 gal indirect tank in the room fed with a zone valve and an aquastat on the tank.

But reading around online it seems like the Honeywell L8124L triple aquastat installed isn't right for a indirect DHW set up.

The Petro service tech last season set the control for 180/160/10 and I later lowered it to 175/140/10 with no loss of comfort or DHW. Summer time making only DHW I've dropped it to 160/120/10 again with no loss of DHW.

But what is the correct boiler control for this set up? The house has 3 zones of baseboard fin tube radiators plus the zone for DHW and a single circulator on the return side of the boiler. I intend to replace the wiring mess of zone valve end switches with a Taco valve controller with DHW priority as well. Hopefully with a post purge, there is seemingly always a lot of heat left in the boiler after any call for heat or DHW.

Any insight would be appreciated, I don't expect this boiler to be around forever but not looking to trash it with the wrong aquastat prematurely either.


  • Zman
    Zman Member Posts: 7,447

    That controller is designed to be able to run at a higher temp when you have a call for heat and a lower temp to keep the DHW coil hot the rest of the time. With an indirect it can be reconfigured to eliminate the "always on" feature.

    You want to be sure that you are not consistently returning water colder than 130 degrees to the boiler. Your present settings sound like you are, although if the tankless feature has been disabled, the boiler may be ignoring the 120 setting.

    Does your boiler maintain a temperature when there is no call?

    I think you have 2 choices;

    Keep the aquastat you have and set it so it turns on around 150 and off around 160 and forget it. Make sure the tankless feature is disabled.

    Buy a controller like a tekmar 256 that will give you some outdoor reset and boiler protection features.I would leave the existing aquastat (just set it high so it only serves as a safety) and wire the tekmar in series.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • icesailor
    icesailor Member Posts: 7,265

    I personally would set the "HI" at 160 to 165 degrees, the "LO" to 140, and leave the "Diff" at to. It has already worked fine for you. The boiler will always maintain 14o degrees, even when you get a call for heat or hot water. The boiler will be shutting off when the "HI" gets to 160 or 165 degrees. All that "LO" does is stop cold water from coming into the boiler and stops the circulators until the boiler catches up.

    There is actually a good chance that the wiring is incorrect because if you look carefully and see that the two terminals "ZC" and "ZR" aren't being used, the boiler isn't wired for optimization. If you have all zone valves and one circulator that feeds ALL the zone valves, it is correct. If it has two circulators, it probably isn't.
  • MrMet112
    MrMet112 Member Posts: 3

    Thanks for the input guys. I've checked the boiler temp a few times over the last couple days. Lowest I've seen is 140 first thing this morning after all night with no heat or hot water use. With a family of 4 we are probably making hot water enough times in a day that it will never really cool much below that.

    The ZR and ZC terminals aren't wired, I have all zone valves and a single circulator.

    I read somewhere you can remove and cap a wire the in aquastat that will stop the keep warm and convert it to a cold start. Good idea or bad? Sounds like that would bring cold return water into play easily unless it holds the circulator back until the low limit temp is reached.
  • icesailor
    icesailor Member Posts: 7,265
    Cold Start:

    I tried to explain to you that the way you have it is the most economical way that you can set it up. It reads 140 degrees because that is what the operating control setting (LO) is running it at. If you make it a cold start, the very little money you MIGHT save will be well be used up in additional repairs, cleaning and early boiler failure you will suffer if you do it. If you turn on the smallest zone and let it run until the boiler stops, the temperature gauge should read what the High Limit "HI" setting is. You don't need to be cutting any wires. I have no idea what wire someone is talking about. If you insist on wrecking your boiler, turn the "LO" down as low as it will go. When you need hot water, the boiler will be cold and will be condensing water into the boiler until the overall temperature of the boiler and piping reaches 140 degrees.

    Some here will suggest that there is no problem running it as a cold start oil boiler. They don't thoroughly clean boilers as part of their living. If they did, they would never recommend turning ANY hot water oil boiler into a "Cold Start" boiler.

    "Cold Start" boilers need to be thoroughly cleaned EVERY year. Warm start boilers can go three years. A dirty cold start boiler will cost you money to run.
  • MrMet112
    MrMet112 Member Posts: 3

    I had a feeling cold starting an old boiler like this wasn't a good idea which is why I asked first. Not looking to wreck the boiler, just trying to get the oil consumption under control at $3.50/gal. I set the stat for 140/160/10, now when we get into winter months, I just raise the high limit towards 180 as the outside temp falls? Last winter I found that the 180 limit was rarely reached but 175 seemed to work where the burner would shut down and the circulator continued to run before the stat would release the call for heat.

    I had the unit serviced in the fall, and I plan to again this fall before the heating season. I can see already it needs a new smoke pipe, I foil taped some pinholes in it last week.

    As for zone controllers, you have a preference? Tekmar, Taco, Honeywell Aquatrol? In the current set up I have no DHW priority or purge it is all just paralleled end switches. I don't think outside reset is going to get me much without quickly getting into low return temps, I don't mind putting one in knowing I'll activate that feature when I replace the unit. Only the Honeywell has a return sensor and I can set the minimum to keep above condensing temps.
  • icesailor
    icesailor Member Posts: 7,265
    Temperature Limits:

    Leave the High Limit at whatever you have it until it gets so cold that it doesn't heat the house. Then raise it 5 or 10 degrees. You may find that it doesn't be changed.

    The "Sweet Spot" is on that coldest day, the burner runs continuously and the temperature is at the high limit setting. If the burner stops but the circulator runs continuously and the system temperature doesn't drop, and the thermostat is just "Almost" satisfied, you have hit perfection. If the house starts to cool, raise the temperature. You probably don't need 180 degrees.