Gas/Propane Basics...
Timmie McElwain
President, Gas Appliance Service
gastc@cox.net
Hydronic Controls
On Gas Systems
There are not a lot of differences between gas and
oil hydronics. The obvious one is the burners and
fuels are different. The controls, however, are
typically the same with some exceptions. I want to, in
this series of articles, address the differences and how
to work with them. In addition to the diagnosis and
solving of problems on the two different fuel systems,
we will briefly address some steam related facts.
However, our emphasis will be on Forced Hot Water
systems used on gas both Natural and Propane gas.
We will be emphasizing the older systems and not in
this series get into Modulating/Condensing equipment.
I want to address controls that are unique to gas. Below
is an example of a typical gas package boiler of the
1950s still used in the present.
Pumping Away
Systems work better when the compression tank
is connected on the suction side of the pump. This
is called the “Point of No Pressure Change.” This
is the way commercial systems have been installed
for years. This typically means that the circulator is
installed pumping away from the boiler and toward
the systems.
The idea has in recent years caught on with residential
and small commercial piping. It was probably
not done in the past because no one ever gave people a
good reason to change, but now there is a good reason.
Just look at all the changes that circulators have gone
through over the past few years. These new circulators
are smaller and run at higher speeds and higher
heads.
That makes a difference in the way the system operates.
We have seen so much of a difference, in fact,
that we see a brand-new opportunity for you.
When a circulator pumps away from a compression
tank (expansion tank), all the circulator’s pressure appears
as an increase out in the system. This sudden increase
in pressure helps move air out of the radiators.
Start-up becomes much easier, and, usually, there are
fewer air-related problems from that day on.
On the other hand, when you pump toward a
compression tank (typically, when the pump is on the
return side of the boiler), the circulator’s pressure
appears as a drop in pressure on the circulator’s inlet
side.
If you’ve ever piped a feed valve into the inlet side of
a circulator on the return side of a boiler, you’ve seen
this drop in pressure. The feed valve opens every
time the circulator comes on. It can be a real
problem.
This sudden drop in system pressure also
makes it harder to get the air out of the radiators.
System start-up is tougher (especially on one-pipe
systems with venturi-type fittings) and, in some
cases, air can actually be sucked into the system
through the air vents.
No one really noticed this problem with the
older circulators, but nowadays, many residential
systems are using small, high-speed circulators.
These circulators, because of the higher heads
they produce, can help you remove system air—if
you install them pumping away from the compression
tank and toward the system, or they can
work against you.
When you install them pumping toward the
compression tank, they will drop the system pressure
on their inlet side (as much as six psig) every
time they come on. That sudden drop in pressure
will expand the trapped air bubbles up in the radiators,
making it even more difficult to get air out of the
system.
Have you ever noticed how it always seems to be
those last couple of convectors, the ones closest to the
return, which give you the biggest problem on startup?
Now you know why.
Why not use this pressure phenomenon to your advantage?
Pump the supply side. The system will work
a lot better and you may be amazed at the results. We
say this even as we recognize that pumps installed on
the return side have worked for years. They’ve even
become a tradition. Most of the drawings published
over the years show the pumps on the return side and
with the low-head pumps, this usually is not a problem.
However in light of today’s high-head circulators,
we’ve become convinced that your systems will work
20 ICM/November/December 2017