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Troubleshooting Headaches – The Return Pipe That Was A Supply

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The heating business has a way of humbling you. After 25+ years of running service calls, I managed to tackle the toughest of service problems, and thought I knew enough. However, after helping my crew with trouble calls in the fall, I realize I still have a lot to learn. What I have learned is that getting complacent and jumping to conclusions will generate callbacks.

A couple of years ago, a client called with a problem job. A well-intentioned contractor had installed a tankless water heater to provide both space heat and DHW. While he at least isolated the DHW from the space heat with a heat exchanger, this mis-applied product did not work to the client’s expectations and suffered numerous breakdowns.

The client accepted my recommendation to install a Triangle Tube Excellence condensing gas boiler with an integral DHW tank. This system was installed in a 1920’s vintage DC row house with an English basement apartment. Since space was limited, the Excellence was the logical choice.

The system worked well the first winter with no complaints or service calls. This fall, however, the client called to complain that the main floor and second floor radiators were hot and the space temperature was overshooting the set point. The basement is on a separate zone, independent of the top two floors.

I arrived and checked the main floor thermostat. The radiators were warm, the thermostat was set to 70* with an actual temperature of 72*. Old gravity conversion systems with cast iron radiators can store a lot of energy. A couple degree overshoot did not seem too excessive. I was hoping this was the reason for the complaint. This was not the case.

I went to the basement and noticed it was very warm. The thermostat was set to 75* and it was in fact 75*. The radiators were still hot from the last cycle, but the boiler was off and both zone valves were closed. I checked out the main floor thermostat and the Taco ZVC zone valve control and both checked out. I also checked both zone valves.

Everything checked OK and I was scratching my head. The only thing I could surmise was that the zone valve was not closing all the way or that a bit of grit or solder was stuck on the valve seat not allowing it to close all the way. I replaced both zone valves, proclaimed the system “fixed” and went on my way.

The next morning the irritated client called to let me know that not only was it not fixed, but also it was worse than ever. It was one of the first cold nights with temperatures down in the 20’s so I knew the boiler ran most of the night. Callbacks are the worst, because you cannot put them off for a day or two. You were just there and the problem still exists! You have to go there immediately.

This time the main floor was 74* with the thermostat set at 70*. The radiators were very warm, but not hot. Again, the basement was a sauna with the thermostat set to 75* and the radiators very hot to the touch. The basement zone was calling and the zone valve was open. The main floor zone was off and I confirmed that the zone valve was closed and that the piping directly downstream of the zone valve was cool. The valve was definitely not allowing flow when it was off. I was at a loss as to what was causing the phantom flow through the main floor radiators.

I decided to slow down, analyze the situation, and ask some questions.

First, what changed? The client told me the basement apartment was not rented last winter so the thermostat was set to 60*. It was only this fall that the apartment was rented and this new tenant liked to keep it hot.

Another clue: Some of the piping was exposed in the basement. This was an old gravity system conversion and the big 2” steel mains were accessible below some of the basement ceiling. The basement radiators were added later and piped with ¾” copper. The main floor zone piping was cool at the boiler yet the mains were very warm ten feet away. This defied logic.

I traced out and marked all of the piping. As I was marking the returns from the main floor, I noticed that they were very hot. I tracked the supply and return from a basement radiator. These were smoking hot as this zone was still calling. The basement return teed into the returns from the main floor. This tee was about 12 feet from the boiler.

When the flow reached this tee, it back fed through the tee and up through the main floor radiators. As these were piped with 2” black steel, there was very little resistance to flow. The tee tying the two supply mains together acted as a crossover feeding the remaining radiators. Mystery solved. While this backflow was not much, it was enough to warm the radiators and overheat the space. After we cut in two check valves eliminating the backflow the problem was solved This backflow through the return main existed since we installed the boiler but was only evident when the basement tenant cranked up the heat.

It always pays to slow down and think logically rather than grasping at problems or guessing at the cause. The clues are there and the causes are evident if you take the time to analyze the facts. I didn’t on my first trip and it cost me a callback and an irritated client.

Dan Foley

Posted In: Hydronics

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