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Weil Mclean temperature rise too quickly error

bill794 Member Posts: 10
I've seen some discussion about Weil Mclean boilers and the occurrence of the temperature rise too quickly error over the past several years. This error causes the boiler to shut down for 5 minutes and then it attempts to restart. I have an Ultra 155 and a Gold Plus indirect water heater and I had this problem. I am operating this boiler with a Tekmar 402 house controller. I called both Weil Mclean and Tekmar technical support and they both suspected that water flow through the boiler stopped. I have to say that this system works great except of this one annoying error that occurs about once a week or so. Because TRTQ is an auto reset error, all that happens is that a lot of errors accumulate in the U-Control diagnostics.
In my case, the cause of this problem HAS NOTHING TO DO WITH PUMPS OR WATER FLOW THROUGH THE BOILER STOPPING. What caused the problem is my case is that I set the DHW calls on priority in the Tekmar controller.
The Tekmar controller manages heating in 20 minute intervals. At the beginning of each interval, all the zones that need heat come on. When they all come on at once (some have been off for a while) a lot of cool water gets dumped back into the boiler loop. The water circulating in the boiler loop is often below 100 F at this point, and the temperature can drop by 15 or 20 degrees at the beginning of each heating cycle. As a result, the Tekmar controller calls for the boiler to start because the boiler loop temperature is far below target. If a DHW call happens to come in shortly after the Tekmar calls to start the boiler, the Tekmar starts the DHW pump and shuts down the injection pump. Because the indirect water heater has 6 gallons of water that is directly in contact with the DHW (which is usually set at 130 F or above), the water that is pumped out of the indirect water heater is also around 130 F. In a typical Weil Mclain boiler loop, there is 6 gallons of water in the indirect water heater and only a gallon in the boiler. This water from the water heater quickly raises the temperature of the water in the boiler loop and that triggers the TRTQ error. When the boiler times out for 5 minutes, the problem is gone and it restarts and operates normally. This problem is made worse by using a high head pressure pump on the water heater, because that pump moves a lot of water quickly.
The things that were weird about the error messages from my boiler (I cataloged about 10 of these) were that the supply and return water temperatures were low (~110 F or less) and about equal to each other, the flue temperature was ~10 degrees BELOW the supply and return water temperatures, and the pump RPM's were also low. All this data suggested that the boiler was just starting up when the TRTQ error occurred, and that rapid boiler heating was not responsible for raising the water temperature.
The system I installed in my house is instrumented with Honeywll thermometers and I can see the rapid temperature rise. I fixed this problem in my system by switching the house controller to not prioritize DHW over heating. In this case, when a DHW call comes in, water continues to be returned from the heating zones and that cooler water reduces the rate of temperature rise seen at the boiler output becasue this cooler water is mixing with the water out of the water heater.
I hope that this post helps people who have struggled with the TRTQ error!