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  1. #1

    Question re: Honeywell L7248L aquastat features / settings

    I hope the Pros monitoring this forum can help me, where Honeywell unfortunately has declined to provide information. Our oil company installed the aquastat along with a Solaia boiler & Riello burner, but they don't seem up-to-date on the L7248L features.

    The aquastat's full model number is "L7248L 1070 1." The L7248L is connected to a Taco SR506, one of whose zones is indirect DHW. The SR506 connects only to the TT terminals on the 7248; right now the aquastat's ZR is not used. In the interest of saving energy and money I am investigating Honeywell Outdoor Reset modules and also the (apparently) built-in Thermal Purge.

    Question 1: When I step thru the aquastat settings the only ones listed are HL_, F-C, and ELL. Could you please explain why duu, tPL, and tPt are unavailable? I do understand that the Reset settings (such as OtL, BtL) should be shown only when an approved reset module is detected on the Enviracom bus.

    Question 2: Are Thermal Purge and Outdoor Reset mutually exclusive?

    Thank you very much for your help!

  2. #2
    Join Date
    Nov 2011
    Location
    Long Island, NY
    Posts
    279
    The thermal purge would only work if a circulator were controlled by the aquastat. Your circulators are controlled b the SR506.

  3. #3
    Quote Originally Posted by R Mannino View Post
    The thermal purge would only work if a circulator were controlled by the aquastat. Your circulators are controlled b the SR506.
    I'm a bit confused by your comment. The L7248L install manual says:

    "The intent of thermal purge is to insure usable residual
    heat in the boiler is circulated until it is sufficiently
    depleted from the system before the burner is allowed to
    fire. To that end, on a call for heat, the burner is held off
    while the circulator runs until the boiler temperature drops
    to the thermal purge temperature or a time delay is
    exceeded. Both of these parameters are adjustable.
    When the boiler temperature reaches the thermal purge
    temperature, the burner is allowed to fire."

    I don't read this as requiring the [single] circulator to be connected to the aquastat; if Honeywell's language is accurate it suggests that a call for heat starts the purge, and a drop in Bt ends the purge and fires the burner. Maybe I'm missing something.

  4. #4
    Join Date
    Nov 2011
    Location
    Long Island, NY
    Posts
    279
    Ask yourself this: how does the L7248L communicate with the SR506. In other words how would the 7248 make the circulator controlled by the SR506 stay on?

  5. #5
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,826
    A thermal purge is just that, a time or temperature related cool down after the call for heat has been satisfied. An outdoor temperature reset control is another method of energy management but comes at the situation from an entirely different direction. That is, the OTR control should be set to respond to both a call for heat and reference the outdoor temperature. This then, on a call for heat, limits the boiler temperature to that indicated by the heat curve as set-up by the installing company. OTR controls work best with radiant systems, those being floors, walls, ceilings, cast iron products and radiant steel panels. They also should be connected to a condensing boiler so that lowest temperatures can be used to heat the home. This is important difference in the two approaches.

    The post purge allows a standard boiler (non-condensing materials) to be used as it treats each heat cycle as a 'cold start'. That is, the boiler fairly quickly passes from condensing temperatures to higher temperatures above condensing. This protects the non-condensing boiler from acid condensate. However, a condensing boiler with an outdoor temperature reset can remain at condensing temperatures continuously without damage. Thus, the specific goal when using an OTR control should be to set the heat curve such that the circulator never shuts off, the call for heat never ends and the Btu loss from the structure is exactly matched to the Btu input from the heating system. That is the most energy efficient, as well as the most comfortable.

    An OTR can be used on a non-condensing boiler but the controls must be set such that the minimum boiler temperature is above condensing temperatures at all times. That's roughly 150F.
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  6. #6
    Well, now I can answer my own question. Sometimes a letter to the CEO gets you the cooperation you need - I spoke to a Honeywell product manager in their Environmental & Combustion Controls division.

    The reason I don't see the settings I expected, is that my aquastat (date code 0829), while only 4 years old, has neither of the features that interest me. Although this basic (electronic) design was introduced in 2004, it was not until 2010 that Honeywell added Outdoor Reset capability, in spite of the fact that residential O.R. has been available for over half a century. Go figure.

    As for Thermal Purge, it made its appearance only this year (2012) and not voluntarily. Federal energy efficiency regulations mandated this feature. [BTW Thermal Purge as defined by Honeywell is different from skippedover's definition.]

    So, for my purposes, unfortunately my aquastat has all the features of Honeywell's analog design from 25 years ago, and if I want Reset I will have to buy a current-production L7248L.

  7. #7
    Join Date
    Sep 2005
    Location
    Atlanta GA area
    Posts
    20,251
    Wonder if the aquastat came from Ebay???
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  8. #8
    Join Date
    Nov 2011
    Location
    Long Island, NY
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    279
    The outdoor reset would help your installation, but the purge feature wouldn't work unless a circulator was wired to the aquastat.

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