Solutions for flawed hydronic heating systems.

In the June Hydronics Workshop column, “Recent Sightings,” we looked at four hydronic heating systems that were “operationally challenged,” to say the least. Each system suffered at least one design flaw that prevented it from reaching its potential as a silent and efficient conveyor for Btus.

This month I’ll show some solutions for these problems. Keep in mind, in most cases there are other valid ways of correcting the problems. My solutions were developed to show a variety of hardware options.

A Major Makeover: The first piping system described last month is shown again in Figure 1A. Among its multiple afflictions are:

  • A mutation between primary/secondary and two-pipe design that resulted in a very high differential pressure across an open zone valve.
  • Two boilers that were fired as a single heat plant (no staging).
  • And heat migration up the supply riser because of the placement of the zone valves.

The reworked piping design is shown in Figure 1b. To begin with, the revised design places the boilers under the control of a two-stage controller that determines — based on outside temperature and the heat demand of the system — if one or both boilers are fired when there’s a call for heat. Such a control improves the seasonal efficiency of the boilers since there are many times when the demands of the system can be met by a single boiler.

The boiler piping is also modified so that hot water flows only through boilers that are firing. It’s pointless to circulate heated water through an unfired boiler since the latter only acts as a heat dissipator — sort of like a radiator with a jacket.

The boiler loop ties to the distribution headers using closely spaced tees to form a primary/secondary connection. This prevents the boiler circulators from interfering with flow in the distribution circuits and vice versa.

The zone valves in the air handler circuits have been moved to the supply side to prevent upward heat migration. Swing checks prevent heat migration up the return risers. A differential pressure bypass valve has been installed to prevent excessive pressure differential from building up across a single (operating) zone circuit and creating flow noise. The other zone circuits use a flow-check valve on the supply to prevent heat migration and either a swing check or underslung return to prevent the same. Both the supply and return header piping have been generously sized to reduce flow velocity and variations in flow depending on which zone circulators are operating. The expansion tank locates the point of no pressure change close to the inlet of both the boiler circulators and the distribution circulators.

Breathing Easier: System No. 2 (“All Choked Up”) from last month was bottlenecked by a 3/4–inch header supplying seven individual zone circulators. Needless to say this header is much too small. This has been corrected in Figure 2. Again, a sampling of both swing checks and underslung returns have been shown as options for protecting the return risers from heat migration. All in all a pretty straightforward fix for a case of hydronics asphyxiation.

Minor Surgery: Piping system No. 3 — shown again in Figure 3a — had zone valves on the supply risers that flowed into a “bull head” tee supplying the system’s circulator. The expansion tank was also incorrectly located relative to the circulator.

The corrections are shown in Figure 3b. The zone valves have been moved to the supply side of the zone circuits. Swing checks are again shown on the return to prevent heat migration into an inactive zone. The expansion tank has been moved near the inlet of the circulator. At least 10 pipe diameters of straight pipe have been installed upstream of the circulator to minimize turbulence at its inlet.

Silencing the Scream: Last, but not least, is the boiler that screamed like a “bottle rocket” when hit with a slug of glycol returning from an ice cold driveway slab. The system piping, as installed, is shown again in Figure 4a.

The basic problem with this system is that there is no temperature reactive control to limit the release of heat from the boiler loop when that heat is needed to maintain a proper boiler return temperature. The bypass circulator that was thought to be the solution may provide some temperature boost under steady state conditions (after the slab has warmed to normal operating temperature). However, under transient conditions (like a cold slab startup) the large cold thermal mass is in pretty much total control of system fluid temperature. This chilled thermal mass can drink up heat from the circulating fluid faster than a sweaty basketball player can guzzle down Gatorade. Heat is pulled into the slab much faster than the boiler can replace it, even when firing continuously. The boiler temperature will quickly drop and typically remain well below dew point for several hours of continuous operation until the slab finally warms up to normal conditions. Such a scenario will get played out repeatedly each season and can lead to serious internal (fire-side) corrosion or thermal shock, both of which can trash the boiler in short order.

To fix the problem the system needs a control that senses the descending temperature at the boiler return and reacts by limiting heat release from the boiler loop into the distribution system. A variable speed injection mixing control with a return temperature sensor is one way to accomplish this. Such an approach is shown in Figure 4B.

Think of the vertical risers that connect the boiler loop to the distribution system in Figure 4B as a bridge. Control Btu “traffic” across this bridge and you control boiler loop temperature.

In this case the return temperature sensor allows the variable speed injection control to detect when the return temperature approaches its minimum allowable value — typically around 135–140 degrees F for a gas boiler. The control reacts by slowing down the injection circulator in the bridge piping, thus limiting the Btus that get released from the boiler loop. If necessary, the control will even stop the circulator all together to allow the boiler return temperature to quickly recover to a safe value.

By the way, this means of protecting the boiler does not restrict the boiler’s heat output in any way. In fact, the boiler is usually firing continuously during such a cold slab startup mode because even with return temperature protection it is usually operating well below the setting on its limit control. The control action described simply “lifts” the temperature differential the boiler operates to a range where damage to the boiler will not occur. Remember, for any given flow rate through the boiler, its Btu/hour output is the same regardless of whether the temperature rise is from 50–70 degrees F, or 150–170 degrees F. It’s the delta T that counts.

If you’ve sketched your own solution to these piping problems, check to see if they address the key issues we’ve discussed. Remember these anomalies as you run your next newly sketched “virtual hydronic system” through its paces.