Page 29 - Oil & Energy - March 2013
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March 2013 • 29
When it comes to heat delivery, we have to ensure we are sending the right amount of BTUs at any given point in time.
Since BTU delivery is a function of flow rate, then we must deliver the correct flow rate at any given point in time.
representing 100 percent water. We design
a heating system to heat the building to a
certain temperature under the worst case
outside conditions. You’ll notice that there
is no Delta P to be found in the above equa-
tion. This is because Delta P has absolutely
nothing to do with heat delivery.
ACCURATE DELIVERY
When it comes to heat delivery, we have
to ensure we are sending the right amount
of BTUs at any given point in time. Since
BTU delivery is a function of flow rate, then
we must deliver the correct flow rate at any
given point in time.
In a baseboard system with zone valves,
as the valves close down the BTU require-
ment goes down as well as the required
flow rate. With a fixed speed circulator,
the speed can’t change so the only variable
left that can change is the system DeltaT,
which will get smaller. A system designed
for a 20° Delta T may never actually see
a 20° Delta T and could run as low as 7°
or less.
The Delta T circulator will slow its
speed to maintain the programmed tem-
perature difference between the supply
and the return. The result will be the Delta
T will
always
be 20° even as the heating
load changes due to zone valves closing or
changes in the outside temperature.
Remember the system was designed to
heat the building under worst-case condi-
tions, which only occur between 5 percent
to 7 percent of the time in heating season.
That heating load fluctuates every day!
A Delta P circulator installed in a zoned
system will vary its speed to maintain
a pressure differential in the system.
It doesn’t know anything about how much
heat is required in each zone so it becomes
a “however many zones are open at any
given point in time” speed pump.
CONSIDER CAST IRON
Today with the high cost of energy,
the industry and the consumer are both
focused on high-efficiency boilers, and that
is a good thing. But perhaps we should also
consider the existing oil- or gas-fired cast
iron boiler. These boilers run best when
they can operate at steady state.
During the dead of winter, the
burn cycles will be longer with lower
return water temperatures. But when
the weather is milder the boiler tends
to short cycle because of a fixed boiler
high limit and a fixed speed circulator.
Short cycling greatly reduces efficiency.
If you combine a Delta T circulator with
an outdoor reset control that will vary the
boiler’s fixed high limit, you have created a
cast iron boiler that will be about as efficient
as it can possibly be.
THE BIG PICTURE
It’s important to look at the system as
a whole and not just the sum of its parts.
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Remember that the heating system is a big
complex machine designed to keep people
comfortable while at the same time mini-
mizing energy usage. The system’s success
or failure is directly related to how well
these components work together. When it
comes to comfort, it’s all about the BTUs
and getting them to the right rooms at
the right time and that function is directly
related to Delta T.
