When to circulate?

[Rory]

I have a Siebel controller on my system, set to circulate when 1 sensor is 8 degrees more than another. I have 3 sensors, 1 at the boiler, 1 at the top of my storage tank, and 1 at the bottom. I was under the impression that the temperature difference being monitored was between the boiler and the top of the tank, which makes sense to me to ship heat and increase the temps at the top of the tank without encouraging thermal mixing. However, the instructions from my dealer seem to indicate that it is comparing the boiler to the bottom of the tank. It would seem that this would mix the tank and actually cool my water off by circulating to the boiler when it's not running. My observations haven't nailed it one way or the other. The manual says it is set for DT0/DT10/DT20, but I haven't seen the terms defined. My guess is "delta temperature" between and 2 elements from boiler to top to bottom, but I'm not sure.

What is the recommended method?

 

[stee6043]

My understanding is that you should continue to circulate the water until your boiler output is less than the return from your tanks. I think about it this way - there is still "value" to the supply water since it is hotter than the bottom of your tanks. By continuing to circulate you should be raising the temp's at the bottom of your tank, eventually, even though the water may be cooler than the very top of the tank.

 

[steam man]

Use the bottom of the tank sensor. You have the "room" to store more heat with a cooler indication. Depending on the height of the tank you can have a big difference due to stratification. Next, you can fine tune the controller for the on/off differential. 8 deg sounds low. I would probably want to set it maybe 15 deg or so on and 4 deg or so off. Watching the process would give a better indication as long as the temps are rising in the tank.

Mike

 

[chuck172]

The tekmar controller instructions say to put the sensor at the bottom of the storage tank. Makes sense to me, as long as your replacing cooler water with warmer water, that's a good thing.

 

[Rory]

Yes, it makes sense to me now, as well. I think that's what's going on, as circulation had stopped when I came home at noon and the bottom of the tank was almost the same as the boiler.

 

[timberr]

I am using a Tekmar 156 to control my storage temp, my tank sensor is about 1/3 up from the bottom (same level as my return to boiler) and above the return from my secondary loop. I have the source temp set to 170*, when the boiler hits 170* the circulator turns one, the storage temp is set to 180*, when that is reach the circulator turns off.