Gas/Propane Basics
Figure 1:
Retries continue until ignition is successful or the
call for heat is removed.
It is important to remember that only pilot gas
flows during trials for ignition in an intermittent pilot
system. Main gas never flows until after the pilot is
lit. Our studies have shown that even in the case of
LP gas, there is not enough accumulation of pilot gas
to present an explosion hazard under even the most
severe conditions.
Furnace and boiler manufacturers are implementing
continuous retry modules like the S8610U because
they have a significant advantage over controls with
lockout timing. Without lockout timing, shutdowns due
to temporary problems like short-term utility interruption
and temporary drafts are virtually eliminated.
With a continuous retry controller, the system resumes
normal operation as soon as the temporary problem
disappears.
At the same time, the five-minute delay between
trials allows unburned pilot gas to dissipate, which
provides a safer, quieter light-off. The end result is less
service time spent troubleshooting temporary, often
uncontrollable, problems.
There is a standards difference between furnaces and
boilers using intermittent pilot ignition. The furnace
standard calls for 100% shutoff on both natural and LP.
The boiler standard calls for 100% shutoff on LP only.
However, even though 100% shutoff is not required on
natural gas boilers, many boiler manufacturers have
historically used 100% lockout models on both natural
and LP gas. Many of these same manufacturers are
now switching to the continuous retry concept on new
boilers. The recycle time inherent in continuous retry
makes the S8610U a suitable replacement for lockout
controls on either natural or LP gas and in either furnaces
or boilers when as the guidelines in the installation
instructions are followed.
A second feature of the S8610U that makes it a
universal replacement module is its ability to handle
both single-rod and dual-rod ignition/sensing hardware
(see figures above). The module is shipped for use with
single rod systems. To convert it for use in dual-rod
systems, remove the jumper from the sense terminal
and cut it close to the circuit board as shown in Figure
1 to the left.
We will pick up with the S8610U Universal Replacement
control in the next article, titled “When to Use the
S8610U.” Keep in mind there have been two modifications
made to the S8610U that we will cover in the next
series of articles. The original modules came in all gray
plastic cases; however, the two later versions came in
black plastic cases. We will cover the two S8610U modifications
in our next article.
I invite you to visit our new Facebook page “Timmies
Tips on Gas.” It is located at www.facebook.com/
groups/timmiesgastips/?ref. I look forward to seeing
you there. ICM
Timmie M. McElwain is President of Gas Appliance Service, which
provides training for those servicing gas combustion equipment. He is
a certified instructor and test proctor for the Propane Gas Association
in their CETP program.
20 ICM/July/August 2019