Laars MASCOT LX, Service Manual

Page: 36 / 94

30
Mascot LX Service Manual
There are several conditions that can cause short-cycling, and some of them can be caused by prob -
lems with the installation. The Mascot LX is a modulating boiler, so its input will decrease when there
is a reduction in the heating load. This means that short-cycling is greatly reduced. If the heating load
drops below the minimum input of the boiler for an extended period, the boiler will have a tendency to
short cycle. This can be a symptom of improper control or piping layout. When in DHW mode under
low load conditions, the boiler will normally cycle more often, but should not cycle frequently during call
for central heating.
Short-cycling can occur if the piping in the hydronic loop is too small for the capacity of the boiler. To
put this another way, the boiler is producing enough heat, but the surrounding system is too small to
move the heat out of the boiler. The minimum firing rate on this unit is 20%. If the hydronic loop can -
not accept at least that amount of heat, the unit may short-cycle. The solution to this problem is to use
larger pipes in the primary (boiler) loop, or to install a buffer tank.
A similar situation can occur if the boiler is connected to an indirect DHW tank. If the output of the
boiler is too large in relation to the size of the tank, the system may short-cycle. You may be able to
correct this without changing the tank or piping. Be sure to use the aquastat on the indirect tank to
provide the “call for heat.” You can also try lowering the DHW setpoint. If this does not work, you may
have to change the DHW tank. The tank should be large enough to accept the output of the boiler
when it is running at 20% of the maximum firing rate. The piping between the tank and the boiler must
also be large enough to allow for adequate flow. Here’s the rule – The pipe diameter for the loop
running to the tank must always be larger than the fittings on the tank. For example, if the fittings
on the tank are ½” dia., you must use ¾” dia. or larger piping for the loop.
To check the control setting for “Anti-Cycling:”
• Press and hold the OK/Select and Down buttons for 5 seconds to go to Installer mode.
• Use the Up and Down buttons to go to P23 - Anti-Cycling Time. The default setting is 30 seconds.
• To make a change, press the OK/Select button. Use the Up and Down buttons to change the
value. Press and hold the OK/Select button to save the new setting and leave Installer mode.
A5
Short-Cycling