ASHRAE Journal - March 2022 - 43

TECHNICAL FEATURE
cycling is a problem of system overdesign. The discussion
on modulation will discuss this further.
Modulation of boilers is investigated next to see its
effect on short cycling. Modulation ratios in boilers are
now commonly as good as 10:1, but for Sites 2 and 3, the
ratios were reported as 5:1 by the manufacturer.
Modulation can behave differently depending on how
the manufacturer programmed the controls. When the
boiler can be modulated down to the heat loss rate, with
appropriate programming the temperature in the climate-controlled
space can remain constant. That takes
advanced programming and sensors because a client
wouldn't like the temperature to drop to the minimum
value and then stay there. See Case D starting at 68°F
(20°C) in Figure 8, which has a 5:1 modulation.
In Figure 8, Case E also has a 5:1 modulation, but with
a 55°F (13°C) thermostat setting. In Figure 7, we saw both
thermostat settings had some cycling, but in Figure 8,
the cycling is much reduced. The heat production rate
can't match the low heat loss rate, but modulation still
reduced cycling. With modulation, neither cycling could
be called short.
The effective modulation is less than advertised.
Data shows that boilers normally operate in the lower
part of their capacity (as shown in Figures 2 through 4).
Therefore, if the range of normal operation is half of
the capacity, the effective modulation is half of 10:1 over
the normal range. For the boilers at Sites 2 and 3, with
a 5:1 modulation, the boilers could only match the heat
loss during a quarter of the heating season. However, as
shown in Figure 8, it still has a positive effect on reducing
cycling in all situations.
Next, these results will be applied to the preceding
data, which suggested no seasonal loss of effi ciency.
Previously, three types of heat loss related to cycling
were listed. Those will be applied to the fi ndings on
short cycling. First, purging happens after every cycle.
Therefore, purging loss is most severe with overdesigned
boilers in winter. Purging is less common other times
and with modulated operation. However, this might
only be a couple percentages of loss. The loss is directly
related to the number of purges. Therefore, this should
be worse in winter, but not much worse.
Second, radiation from a cooling boiler is loss. It is
assumed that boilers are located in spaces that are not
climate-controlled so any heat that they release is waste.
Boiler space has to be ventilated to provide makeup
FIGURE 8 Model results for modulated systems, 68°F and 55°F (20°C and 13°C).
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68
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60
58
56
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20
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Time (Minutes)
combustion air, so any heat in the boiler room is likely to
vent out of the stack. However, boilers are often located
with climate-controlled space around three sides and
above, so radiant heat warming those surfaces isn't necessarily
loss. Since overdesigned boilers in winter have
the greatest cycling, again they have the greatest losses.
The loss from radiant cooling of a boiler is not necessarily
the same in every heating cycle because in a short
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M A R C H 2 0 2 2 ashrae.org ASHRAE JOURNAL
43
Air Temperature (°F)
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ASHRAE Journal - March 2022

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