Getting the silent treatment

Hydronic heating systems are known for their ability to control the end user’s comfort levels. These systems also move energy around a building quietly through much smaller distribution piping than traditional forced air systems. They can provide enviable efficiency with the addition of zoning and smart control strategies. When the hydronic system is properly designed and installed, distribution and stand-by losses are also minimized. Consumers very quickly forget all of those benefits when the promise of a quiet, efficient system is not delivered on.
When you can hear a hydronic heating system operate, something is just not right. The following are some of the possible causes that could impact a typically silent heating system.

AIR/DIRT IN THE SYSTEM
When a hydronic heating system is commissioned one of the most important points to consider is the proper removal of air and dirt in the system. Failure to do so can cause whistling and gurgling noises that can be heard anywhere in the distribution piping. Other system components (boilers, pumps valves, etc.) could also be negatively impacted and may contribute to additional noise issues.
The vast majority of air can be removed in the filling process by relieving air at the highest point of the distribution system. Dirt can be removed by properly flushing the system prior to commissioning.
When the system consists of radiant floor heating, each loop should be flushed separately in order to relieve the air in the loops. This can be done at the manifold location if adequate shut offs (loop isolators) and fill and drain valves are available. If not, it can still be done at a common piping set.
Once this is done, there is still the oxygen within the distribution fluid that can result in what we refer to as entrained air. When the fluid is heated, oxygen separates in forms of micro bubbles. When more of those get together, a bigger air bubble forms and eventually, multiples of those can cause the same whistling and gurgling noises and maybe even air locks.
The most effective way to deal with those microbubbles at one central location, is through the proper addition of a dirt and air separator installed near the boiler in the primary piping. When the system is started most air separators can remove dirt and air while taking advantage of the fluid velocity.
Specially designed dirt and air separators can remove both, even at the very low velocities that are becoming more prominent with the increased adoption of variable speed pumping systems.

PUMPS/CIRCULATORS
Most circulators available today are so quiet it is nearly impossible to hear them run, even right at their location and particularly when they are equipped with ECM motor technology. Larger commercial pumps use larger motors that may give off a soft humming noise and it is typically contained to the pump location.
When you can hear them throughout a building, more often than not, it involves the air and dirt in the system. Pumps and circulators can also be heard through the distribution piping if they have been oversized for their duty. This would be referred to as velocity noise, which is the result of too high a velocity through the piping.
It could be due to an oversized component or a result of changing of system dynamics, such as zone valves shutting off that are not addressed sufficiently. The problem is even more noticeable when fast acting zone valves are used.
Self-sensing ECM motor circulators or commercial pumps with self-sensing drive technology can respond to those changes on their own and compensate for changes in the system. Constant volume devices require the installation of a form pressure bypass or balancing valve to address changing system dynamics. When the noise increases right at the pumps, check for cavitation and dead heading, or failing bearings in larger pumps and motors. When using larger split coupled and base mounted pumps, check for motor rotation and alignment.

DISTRIBUTION PIPING
This is a typically silent part of hydronic heating systems. When noise comes from the piping and it is not a result of the issues discussed previously, it could be due to inadequate securing of the piping. If typical mounting guidelines specific to the material used are not followed, the pipe may not be restrained enough and could move as a result of changing system dynamics.
Once again those changes can include the closing of zones and change in fluid velocity. Fixed fluid temperature operation or improper fluid temperature control can also be a cause.
When the fluid temperature is greater than what the building needs, the calls for heat naturally become shorter and off times longer. This allows the fluid to cool before being heated again. The resulting expansion and contraction in the piping can allow the pipe to move and will sometimes result in ticking noises. The addition of a responsive control system can minimize the expansion and contraction greatly, as the fluid temperature is controlled according to the building needs (referred to as outdoor reset and/or outdoor reset with indoor temperature feedback).

BOILERS
While one can hear a boiler firing, it is typically contained to the boiler’s physical location. When the noise goes beyond that of the typical firing noise and comes across as a hissing and gurgling sound there can be a couple of causes.
First, there may be inadequate flow through the boiler, which causes the water to boil internally. Secondly, if the system’s fill pressure is too low and the water temperature is too high, boiling of the water internally can occur. Maintaining proper fill pressure and removing any air remaining in the system can virtually fix noises caused by the boiler. Finally, ensure that the boiler’s high limit and operating limits are set properly.

HEAT EMITTERS
Heat emitters typically operate noise free as well. When you get a noise coming from a heat emitter, it is generally a result of either a lack of fluid temperature control causing expansion and contraction of the heat emitter, or again, air inside of them causing a gurgling noise. Controlling the fluid temperature based on building load to minimize off times eliminates the expansion and contraction greatly.
Although there are some other possible noisemakers you may encounter in a hydronic system, the causes discussed here are the most common contributors. Always ensure that you are servicing mechanical components as per manufacturer recommendations. Your properly designed and professionally installed hydronic heating system will ensure your client’s satisfaction. When our clients work or reside in a comfortable environment and do not even know the hydronic system is running, we have done our job. Silence is golden.

Mike Miller is chair of the Canadian Hydronics Council (CHC) and director of sales, building services with Taco Canada Ltd. He can be reached at hydronicsmike@taco-hvac.com.