The Glitch
Overview: An old New England home has
been heated with a 2-pipe steam system serving plenty of sturdy cast-iron
radiators. As part of an extensive renovation and insulation upgrade, the owner
decides to upgrade the heat and convert the system to hot water.
The piping system, installed by the local plumber, is shown below. It leaves as
much of the 2-pipe distribution in place as possible. A new gas-fired boiler is
installed and operated on outdoor reset control.
Exercise: What are some design details that you think are
incorrect or missing?
The Fix
Over decades of use, a lot of “mud” can build up in a steam
heating system with cast-iron radiators. It’s really a shame to push that mud
into new portions of the system.
I’d recommend a high-quality dirt
separator on the receiving end of the flowing mud, and installed at a low point
in the piping whenever possible. Be sure to leave room for a blow down valve at
the bottom because you’ll surely be using it. Also, be sure the flow velocity
into the dirt separator is at or less than 4 feet per second.
Another
concern with these systems is low return water temperature to a conventional
boiler. Those steam radiators represent a lot of surface area. They may not
require water temperature above, say, 130 degrees F even on a design load day.
This is even more likely when the building’s thermal envelope has been upgraded
and thus its heating load substantially reduced.
Just as with low-temperature floor heating, there should be a mixing
device between the boiler and distribution system. The controller for that
mixing device should monitor boiler return temperature and respond by
decreasing heat flow to the distribution system (by adjusting the mixing valve)
as necessary to prevent flue gas condensation in the boiler.
Other details that are incorrect:
1. If you expect hydraulic separation between circuits,
DO NOT install closely spaced tees as shown in the glitch drawing. Instead, provide
a minimum of 6 pipe diameter of straight pipe both upstream and downstream of
the closely spaced tees.
2. There should always be a good microbubble air
separator on the outlet of the boiler.
3. The circulator is shown at the low point of the distribution
system ― the most probable location for sediment to collect.
4. Install an air vent on each
radiator.
