Troubleshooting Boiler and Hydronic Control Problems
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Troubleshooting Boiler and Hydronic Control Problems

How to determine is you have problems with your hydronic heating system piping, valves, or zones.

As I begin to start up my 1950’s era hydronic heating system I am reminded at how exquisitely efficient and temperamental they can be. Though not as peculiar as older steam heating systems, they have a personality of their own depending on how they were installed, what repairs have been made in the past, and how they have been used and maintained. Hopefully this will help you solve some typical control problems, temperature, and noise. Here is a review of some common hydronic control problems and give troubleshooting tips for each.

Let’s begin by looking at some general problems and solutions, then move on to room temperature and noise problems. This article will focus on the problems with the hydronic controls and not necessarily the electrical or plumbing repairs in detail.

General Problems

Zone Valve end switch does not make connection — This could be caused by an insufficient supply of voltage to the valve due to a faulty transformer or to an excessive drop in line voltage supply. Change the faulty transformer, if that is the case. On older systems the transformer may be installed in a junction box near the zone valves, similar to a doorbell transformer.

• If too many valves are powered by the same transformer, add a transformer.

• If the overcurrent is causing an overheated end-switch blade or burnt-out contacts, ensure that the actual current draw is within the specification of the end switch.

• If the valve motor is burned out or worn out, replace it. You can usually purchase the motor and leave the valve body in place without shutting down or draining the system.

• If the actuator gear or motor gear is worn out, replace the actuator.

• If the valve is installed backwards with respect to flow direction, reinstall it correctly. Usually only happens on new systems, but if the valve had been replaced, it could have happened then. A reversed valve is usually discovered during the first heating season.

• If end switches are damaged by silicone from lubricating sprays, replace the actuator. Avoid using any lubricating sprays again.

Premature valve failure — This frequently is due to O-ring or plug shaft failures. The problem begins when oxygen enters the closed system and damages the internal parts of the valve. Hydronic valves that are sold for closed-loop hydronic heating system applications cannot handle an environment with oxygen, which is why they are made for closed-loops.

This problem is common in older solar heating applications. The solar panels and tubing used may not have an oxygen barrier; although watertight, they are oxygen permeable and the oxygen corrodes the inside of the valve.

If you’re working with an open-air hydronic system or an oxygenated water application, replace your valves with ones designed for fresh water applications such as Honeywell’s VC valve.

Honeywell VC Series Zone Valve

Premature valve motor failure — The lubricant inside the motor can dry out under a combination of over-voltage, over-temperature, and high duty-cycle conditions, causing the valve motor to fail prematurely.

If the valve motor is constantly energized for more than a few hours per day, reduce the duty cycle of the valve. If the motor is energized for months during a system-off period, turn the valve off, too.

Definition: Duty Cycle - 100% duty cycle is defined as continuous operation without any damage occurring. For intermittent duty cycle (<100%), alternate energized and de-energized state at regular intervals to allow the valve to completely cool down to room temperature.

Insufficient hot water in combination heating-hot water systems — Combination systems are designed to both heat the house and supply hot water for showers, baths, and other domestic use.

If you notice insufficient hot water you may need to install a zoning panel with the domestic hot water as the priority. When there is demand for domestic hot water, the zoning panel turns off the home’s heating system and redirects all the energy into heating the water for the hot water tank.

Room temperature: too hot, too cold

Here are some tips for tackling room temperature control problems.

Temperature swings in the spring and fall — Overheating in the spring and fall indicates an opportunity for outdoor temperature reset (OTR). The OTR controls work by varying the hydronic loop supply temperature in proportion to the outdoor air temperature, and solve the problem of excess heating on mild weather days. See this article about on how to install a hot water temperature reset:

Thermostat location — Make sure the thermostat is mounted on an inside wall. A thermostat mounted on an outside wall will be overcooled by the wall, needlessly calling for heat and resulting in too much warmth.

Thermostats should not be in the sun or where dead air will give a false reading. An exception to this is when you have separately zoned sun rooms; in this case the thermostat should be located in the sun.

Thermostatic radiator valves (TRVs) — TRVs can be used to rezone a room or space without having to repipe. Most contractors tend to use a centrally mounted or room-mounted thermostat versus one mounted on the radiator. Keep TRVs in mind as a potential solution where you have one thermostat controlling a large area or number of rooms.

Zoning — A zoning (or zoned temperature control) system helps maximize comfort in any type of home. Zoning can help eliminate problem areas that are too hot or too cold by delivering heating when and where it is needed.

Bedrooms, for example, may be kept at an energy-saving setting when vacant or used for sleeping, while the main living areas are kept warmer or cooler when they are in use.

Zoning can compensate for dynamic load changes caused by solar gain on the south and west sides of a home.


Use good plumbing practices to help eliminate noise problems. Here are some fundamentals and a few tips to help quiet the system.

Pumps and valves — With modern high-head pumps, noise can occur in larger installations with long pipe runs and multiple zones. Here are some ways to reduce noise:

• Make sure that the long pipe runs are secured properly.

• If you have more than three zone valves supplied by one pump, it should have a bypass loop with a throttling valve or a differential pressure regulator. This allows the water to bypass when only one zone valve in the group is open. Without this the pump will be pushing water against closed valves and all of the water will be forced through one line.

• Your system may need to be rezoned using more than one pump.

Water hammer — Water hammer noise can be reduced with some suggestions here:

• Make sure zone valves are installed correctly, not backwards.

• Move the valve from the supply to the return side of the loop. This way there will be more water to absorb the shock of the closing valve.

• Slow down the closing speed of a motorized valve, or use a TRV.

• The differential pressure across valves may be too high so it is important to make sure the pump is properly sized. It may be possible to slow the pump speed while providing adequate flow if you do not want to replace it.

• A water hammer arrester installed next to the inlet of the valve is also effective in reducing water hammer noise.

Air in system — Make sure you’ve got proper air removal with an air separator in the common supply piping and with air vents on the boiler and radiators.

It is preferred to have the expansion tank on the high-temperature supply side of the boiler. This arrangement increases the pressure in nearly all parts of the system when the circulator operates, and prevents problems with air in the system.

Locate the air separator on the supply side too, because that’s where most of the air is generated. It comes out of the water at its highest temperature.

Pumping away from the boiler, air separator, and expansion tank. Air separates best at lowest pressure, suction side of the pump, and highest temperature, discharge side of the boiler.

Boiler Piping Schematic Diagram


Emerson Climate Technologies


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Comments (3)

An excellent introduction to Troubleshooting Boiler and Hydronic Control systems.

Duke Lord

I am having circulation problem with my upstairs heating loop. I have purged the system and when using the pump the loop circulates fine. The zone valves ( 3 ) are on the return side of the boiler. When I switch off the boiler/ unplug it, the house side of the zone valve heats up instantly. This tells me that the zone valve opens instead of closes when deenrgized. I do not believe that the valve was installed backwards as the system is old and has been functioning fine. Perhaps the ball in the valve has just failed and needs to be replaced. Any HELP?

Thanks Duke

It sounds like the valve is "normally open" (NO) so that when there is no voltage going to the valve, the port rotates to the open position. You can see if you can purchase a similar model that is NC or "normally closed". The other issue with having zone valves on the return side is that when loop is open it can back-feed the other loops, especially with NO valves. It is possible that the seat on the inside of the valve have failed and is passing, especially if the water has not been treated, the water is hard, or the valve is older than 15 years. Also check to see that it is not a 3-way valvve. Here is a link for wiring diagrams for NO and NC zone valves- Good luck Duke.