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Our diagnostic process in our service department often bleeds into the rescue process used by our rescue team, and they often inform each other.
Every morning before the teams go out to their service jobs, we have “nugget time” or “why time.” These nuggets are centered around interesting service jobs or rescue projects that we are working on—studying them in a group setting for 30 minutes every day as an amazing way to spread knowledge and experience sideways across our departments.
Understanding “nuggets of wisdom” and “why” is so critical to a successful service visit and rescue project and it promotes independent thinking by the techs; they see how the laws of physics tell the molecules precisely what to do and give us clues on what is happening. These laws of physics also help us determine when to zoom out to see things globally.
Global vs. Local
A simple example of this change of perspective can be seen in with multi-head mini split systems.
One of our service techs recently went out to look at a three head system. The homeowner said there was a chronic problem with no blower operation from one of the inside heads. The homeowner was insistent that YouTube told him it was a bad board or blower motor in the inside head. Our tech patiently explained that there may be a bigger issue at hand. He then turned off the other two inside heads and now, magically, the troublesome inside head was working perfectly!
But if it wasn’t a bad board, then what was the culprit? Our tech explained to the homeowner that the robot brain in the outside unit was heating up the inside coils with refrigerant before turning on the blowers. So, if the outside unit didn’t detect that the inside coils were warm, the blowers would not turn on.
The tech noted that the outside unit had already ramped up the compressor in an attempt to send more refrigerant to the inside coils to warm them up. The tech added some refrigerant and the blower problems disappeared.
A change from a local view to a global view let us realize that the boards were fine, and the robot brain in the outside unit was doing what it could, but there wasn’t enough refrigerant to make all three inside heads work correctly.
The tech proposed a leak test on entire system as the next step.
No-Heat Situtation Looks Like a Local Problem (at First)
This concept of expanding a local perception to a global view was perfectly embodied in a recent series of service visits.
We received a call for a caretaker about a no heat issue from a zone in a large residence where he had just begun caretaking. Our tech noted that it had two boilers, more than 10 fan coils, and constant circulation chilled and hot water loops.
The hot water loop was served by two Taco circulator pumps controlled by disconnects with Hand Off Auto switches and called by a tekmar pump controller.
This tekmar pump controller was designed to turn on one pump and run it for a certain amount of time and then shut it off and start up the other pump. This rotation scheme would ensure equal run time of the two pumps for the hot water loop.
Additionally, there was a differential pressure switch that was designed to sense if one of the pumps had failed—this would tell the tekmar to switch over to the other pump.
Each fan coil had Belimo motorized ball valves on the chilled and hot water coils and constant air circulation from belt drive blowers.
The caretaker mentioned that, historically there had been no complaints of any issues in the heating season. He also mentioned that during his walk-through of the mechanical room, he noticed that both heating circulator pumps were in HAND (and they were both spinning) so he switched them to AUTO.
However, as soon as the weather got cold, he started to have issues with inadequate heating in one zone.
Collect Information - Keep an Open Mind
The caretaker told our service tech that the heating circulator was running in the mech room. At the troublesome fan coil our tech confirmed that the supply pipe to the hot water coil was hot but there was a huge delta T between the supply and return pipes.
The water temp entering the coil was 140°F but leaving the coil it was 65°F. The air temperature in that zone was around 60°F. The blower was moving air nicely through the fan coil so there was either very low water flow thru the heating coil, or perhaps the cooling actuator was stuck open?
The tech confirmed that the cooling motorized valve was closed, and the chilled water pipes were room temperature. The tech then checked the heating circulator pumps and confirmed that the supply side pressure gauge was reading pressure above return side pressure so there was flow through that heating pump.
Our tech had had a recent experience with replacing failed motorized ball valve actuators and valve bodies—was this possibly another broken actuator or stem?
It looked like the actuator and stem were in open position but perhaps the stem had broken free from the internal ball inside the valve body?
This would explain the situation at hand and was not unrealistic given the age of the installation….
The tech suggested replacement of the valve body as a possible next step.
New Information and a Change of Perspective
The next day the caretaker reported that now he was experiencing the same no heat issue in two more zones. This changed our local problem into a global mystery.
That morning during our “nuggets” or “why” session, we discussed what could possibly explain a lack of heat in additional zones—we had to expand our view.
All of a sudden, why did we now have a huge delta between supply and return water temperature on THREE fan coils?
It would be very unlikely to have THREE failed motorized ball valve stems. What had changed from the heat working well during previous winters and now not working well as soon as the weather got cold?
The caretaker then called and told us that they remembered that they had noticed that both heating circulator pumps had been in HAND and that they switched them both to AUTO.
A global issue with hot water delivery to the fan coils would fit nicely with the poor heating performance in multiple fan coils…
A Closer Look at Heating Loop Circulators
Now we could deduce what had happened and how we had arrived at our no heat situation.
The caretaker dug up a picture of the original installation of the heating loop circulator pumps and noted that there were no check valves installed.
When the circulator pumps were re-piped later, no check valves were installed.
Our guess is that when the system was first started up years ago, the tekmar turned on one heating loop pump and had the other pump off. Because there were no check valves, some heating water flowed backwards through the inactive pump and some water flowed out to the hot water coils in all of the fan coil units. When it got cold enough out, the heating could not keep up as there was not enough hot water flowing, so the original caretaker simply put both heating loop circulator pumps in HAND and heating problems went away.
When our tech returned, he noticed that the Tekmar pump controller was not in fault. This presumably meant that the differential pressure switch was correctly alternating between the two heating loop pumps and was confirming correct flow. This Tekmar control would also be able to detect if one pump wasn’t working and signal a fault and turn on the other pump.
Upon closer examination, it appeared that both pumps were spinning (both were in AUTO mode) even though the Tekmar was only supposed to power one pump at a time?!?
Our tech noted that in HAND both circs are spinning in correct direction… in AUTO, both circs are spinning, but one is spinning backwards!
Ah-ha! This meant that there were no check valves on these two pumps!
Conclusion: No Check Vales, Differential Pressure Switch Set Too Low
As soon as the new caretaker put the two heating loop circulators back into AUTO, the tekmar took over and ran one pump and turned off the other. Because the differential pressure switch was set incorrectly, it didn’t detect the fact that there were no check valves and did not send out a fault signal. That meant that the caretaker had no idea that there was inadequate flow of hot water to all of the fan coils.
During the original installation, if the differential pressure switch had been set correctly (set for a higher differential pressure) it would have faulted out the tekmar control the moment the first heating loop pump turned on (as so much of the flow was simply bypassing backwards through the other heating loop pump). Then the tekmar pump control would have switched to the other heating loop pump and then it would have faulted out as its flow would flow backwards through the first heating loop pump. At that point, the tekmar control would have shut down both heating loop pumps.
To resolve this issue, instead of adjusting the setting on the differential pressure switch (or insisting that check valves be installed on each circulator pump) the original caretaker (or installing contractor) simply put both heating loop pumps into HAND. Ha! Problem solved! Until the new caretaker put the pumps back into AUTO and indirectly revealed that check valves were missing and the differential pressure switch was set too low!
We have since proposed the installation of check valves on the heating circulator pumps and correct adjustment of the differential pressure switch.
And to think that all of this was uncovered from a no heat call at one fan coil! This was a great lesson in being flexible and being able to move from Local to Global thinking and viewpoints.
Brian Nelson
Brian Nelson is the co-owner of Nelson Mechanical Design, a “green” mechanical contractor serving the energy efficiency and home comfort needs of Martha's Vineyard, MA since 2004. The company designs, installs, and services just about anything to do with heating, cooling, domestic hot water, water treatment, geothermal, heat pumps, and radiant. NMD is committed to preserving the fabric of the Island it calls home. To learn more visit nmdgreen.com.