I think I figured out why the boiler keeps rusting, and have corrected it. I'm posting what I did below, and would love to hear any feedback on anything else I should do.
If you don't feel like reading my story below, I believe I fixed the rust problem by moving the hot-water-loop temperature sensor from the side of the loop right AFTER the boiler to the side of the loop right BEFORE the boiler.
I think I fixed the odd impossible to get the temperature right problem by putting the outside air sensor OUTSIDE instead of in the air intake.
The rest of the story, below -
I'm not an HVAC person, but the company's network administrator. Two HVAC companies and a plumbing outfit (who installed the boiler TWICE) have looked at this and couldn't fix it. One of them insisted on a mixing valve being installed that did absolutely nothing (Which makes sense, the Tekmar just sped up the variable speed pump to overcome the mixing valve).
Anyway some background -
- This is the second boiler for this location. The first one rusted out in 2002, at 7 years old.
- Is probably the third chimney - they keep rusting off
- The system is a radiant floor heater, with the floor hot water loop and a boiler hot water loop. A Tekmar mixing controller and variable speed pump seperate the two loops.
- Because the boiler rusts so badly, one of our maintenance staff cleans all the rust off at the beginning of the heating season.
- Adjusting the thermostat up or down takes up to three full days to see any changes in actual room temperature, and once the floor has heated up, a warm winter day can see temperatures above 85 in the room
- There were complaints that the floor got so hot on a warm winter day that nobody could stand on it
So, knowing absolutely nothing about this, I started reviewing the settings in the Tekmar - First problem I found, the system was set up as a radiator system instead of a high mass radiant floor system. It was trying to heat the water flowing through the concrete floor to 150 degrees! I changed this setting to High-Mass Radiant, and the Target went to 90. I *thought* was done - it was a pretty obvious screw up in the configuration. Don't worry, the boiler loop is set to a minimum of 150, and it never gets below 145 when the system is running.
The next part gets really screwy - The system still didn't really work. The floor no longer got hot enough to turn the room into a sauna, but it didn't get hot enough to heat the room, either. At the same time, the boiler was almost always running. So, I checked the controls again. At the time, it was 32 outside. According to the Tekmar, it was 65 F. What the heck? So, I started tracing wires until I found the "outdoor" temperature sensor -- Inside the air intake duct for the boiler's combustion air. It was reading 65 degrees because the boiler had shut off (after the room thermostat was satisfied) and then it shut the outside air damper, and then went into WWSD mode.
I took the outside air sensor and put it OUTSIDE in a shadowed area. Then I watched the Tekmar to see what it was doing - it wanted the water at 88 degrees and was running the boiler. The boiler came on, the injection pump started, and the water temperature was heated to 94 degrees in about 2 minutes. The boiler shut off, and the water temprature almost instantly cools back off to 87 degrees. Boiler starts back up, water is back at 94 in under two minutes. It just didn't seem right, so I went back to tracing wires to see where the sensors were.
Boiler loop sensor - in the middle of the boiler loop, opposite the side with the in and out connections.
Room loop sensor - Attached to the loop right after the hot water pipe from the boiler. Soooo....as soon as the boiler would start injecting hot water in to the loop, it would think the loop was satisfied and the boiler would turn off.
I moved the sensor to the "cool" side of the loop (where it returns from the room), and watched the whole system for a full cycle of on/off/on
This time, the boiler stayed on for a good 20-30 minutes, the loop temperature gradually rose to 94 degrees, then stayed off for 45 minutes as the loop temperature gradually dropped back to 87 degrees, when it started back up again.
And for the first time ever - the room temperature went to 68 degrees, and varied between 67 and 68 degrees the whole day. No more wild 10 degree swings or the heat shutting off because the sun came out...
I'm not sure if I fixed it in time to save the boiler from rusting out, but at least when we replace the boiler again, the new one will have a chance at lasting for 20 years.