Zr-Domestic Hot Water (Dhw) Request; Thermal Purge Operation; Frost Protection; Thermal Purge Settings - Honeywell L7224A Installation Instructions Manual

Oil and electric boiler
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NOTE: This feature is blocked if a Domestic Hot
Water demand occurs. Domestic Hot Water
demands are serviced immediately, without
any delay.
NOTE: When the Aquastat is connected to the oil
primary and/or thermostat via the EnviraCOM
bus, the Anti Short-Cycle time does not apply
to recycle events such as loss of airflow or
flame. It applies only to loss of demand.
ZR-Domestic Hot Water (DHW)
The ZR terminal can be selected to service an indirect
water heater heat request. This parameter is set via the 3
digit display (see Adjusting Settings section of this
document). A heat request via the ZR terminal will have
priority over all other features such as the Anti Short-
Cycle feature or those enabled by the Outdoor Reset
Module (See form #69-2335 for more information).

Frost Protection

The Frost Protection function protects the boiler and
potentially the boiler plumbing from possible damage
which may occur should the water in the system begin to
freeze. The Frost Protection feature is enabled only in the
L7248 (cold start models). The feature default setting is
enabled (ON), but can be disabled using the 3-digit
display. See "Adjusting Settings" on page 5.
When the Boiler temperature drops to the Frost Protection
Limit temperature (fixed 40 °F), the burner is forced on.
The Circulator and ZC outputs remain in the same state
as before the Frost Protection function was enabled.
They can be either ON or OFF during the Frost Protection
Parameter name
Boiler temperature drop
Thermal purge temperature limit
Thermal purge temperature rate of drop
Thermal purge time delay
Thermal Purge and Domestic Hot
The thermal purge feature is automatically disabled
whenever there is a call for domestic hot water either from
the Zr terminal or from the DHW Module to insure hot
water is available in a timely manner. See "ZR-Domestic
Hot Water (DHW) Request" on page 7.
Pump Cycling
The Pump Cycling feature exercises the system pump for
15 seconds after a non-adjustable five day period of no
boiler activity. The Pump Cycling feature default setting is
enabled (ON) but can be disabled (OFF) using the 3-digit
display. See "Adjusting Settings" on page 5.
Table 5. Thermal Purge Settings.
Minimum value
Not adjustable
120 °F (or OFF)
Not adjustable
1 minute
When the Boiler temperature reaches the Frost Protection
Limit temperature plus a fixed 10 °F differential, the burner
is turned off unless requested to stay on due to a different

Thermal Purge Operation

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. Thermal purge
is only applied to cold start Aquastat models (L7248) and
will not interfere with domestic hot water call for heat or
boilers equipped with Honeywell outdoor reset
accessories. For warm start models where a Low Limit
must be set, thermal purge is not applied.
In addition to the thermal purge temperature and thermal
purge time delay parameters, two other conditions release
the Aquastat from thermal purge in order to maintain
comfort in the space:
• The boiler temperature has dropped 10 °F from the
beginning of the thermal purge.
• Boiler temperature is cooling at a rate greater than
5 °F/minute while the circulator is running.
The thermal purge feature applies to single zone as well
as multi-zone applications.

Thermal Purge Settings

Some thermal purge parameters are configurable using
the 3 digit display. See "Adjusting Settings" on page 5.
Thermal purge may be disabled by setting the thermal
purge temperature limit to "OFF".
Maximum Value
Not adjustable
160 °F
Not adjustable
10 minutes
Put the system into operation and observe at least one
complete cycle to make sure that the controller operates
properly. See TROUBLESHOOTING section to use LED
to assist in determining system operation.


When attempting to diagnose system performance,
reference to the LED display can help to identify specific
areas not working properly. The LED display will scroll
Err, followed by a digit (1-8). Refer to Table 10 and 11 for
a description of each error and suggested actions.
10 °F
140 °F (L7248)
5 °F/minute
2 minutes

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