The system should meet a basic list of performance goals:
- Correct Control. In response to zone demand, the necessary pumps and valves should turn on, and be off otherwise.
- Heating Performance. The system should deliver Glycol at the appropriate temperature to each of the zones.
- Maintainability. The system should provide a mechanism to determine that it is operating properly, with tools to determine both the desired and actual state of pumps, valves, and system temperatures.
- Heating System Compatibility. This subsystem is part of a larger system. It should operate so as to maximize performance of the larger system.
Taking these goals in sequence, the original implementation did not meet expectations.
- Correct Control - The commercial zone controllers are a very effective solution for straightforward installations, but the installers struggled for months to get all of the pumps and valves working together. For instance, pump P15 would not always come on, and P18 would sometimes run when there was no demand. Despite repeated efforts, the controls never worked properly.
- Heating Performance - The ‘outdoor reset’ function governing the mixing valve was buried in a set of very complex configurations on a propane backup boiler. There was no way to see what temperature it was trying to achieve, and in this application outdoor temperature alone is not a good predictor of desired water temperature as sunlight has much more influence. As a result the mixing valve very often did not provide reasonable temperatures. The mixing valve was often manually over-ridden in order to get enough heat.
- Maintainability - When there were problems, it was very difficult to see and understand what was happening. There’s no practical way to draw an overall control schematic for a system of this type since the manufacturer does not provide internal schematics for their controls. Additionally, the number of iterations of control wiring attempted in the course of this project made documentation impractical. As a result, there were no documents describing the wiring or control of this subsystem.
- Heating System Compatibility - Effective use of heat storage depends on maintaining maximum thermal stratification in the storage tanks. This in turn requires the lowest possible circulation speeds and the highest possible ‘Delta T’ - difference between supply and return temperatures. Since P15 is a fixed-speed circulator, it had to be set to a relatively high speed in order to transfer enough heat at times of peak demand. That means that most of the time it was running too fast and returning water at very high temperatures. This would very quickly destratify the heat storage tanks.
In this chart, P15 came on at about 9:00, injecting very hot water into the bottom of the storage tank. This destroys stratification by dropping the temperature at the top of storage and raising the temperature at the bottom of storage. This creates two problems:
- The drop in temperature at the top means that the heat source (an automatic pellet boiler in this case) must come on-line sooner than it would have.
- The increase in temperature at the bottom means that the heat source is heating warm rather than cool water. That results in a shorter and less efficient cycle.
Shorter and more frequent cycles are a much less efficient way to utilize the heat source. Feeding the boiler with relatively hot water also forces it to reduce its output which reduces efficiency still further.