LAARS NeoTherm NTV (Sizes 150–285 MBTU/h) - Service Manual - Manual (Page 20)

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Service Manual NeoTherm 80 - NeoTherm 285
Short-Cycling - Fault #61
There are several conditions that can cause short-cycling, and some of
them can be caused by problems with the installation.
Start by checking the control settings for On Hysteresis and Off Hyster-
esis. If these are set too low, the system can short-cycle.
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 produc-
ing 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 cannot 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 run-
ning 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.
On systems using anti-freeze –
This can occur if the concentration of anti-freeze is too high. At concen-
trations above 35%, localized boiling can occur, and this can produce
noise.
A9
Unit is Noisy
A10