Modulating Boiler, Zones and Mixing Valves -- oh boy!

josephny

I should first start by saying that I know extremely little about plumbing and heating (but that will become evident below). But, I'm working hard at learning about radiant heating so that I can install a system in a home that I own. I'm getting lots of help here on this forum and from a local supply house.

I'm hoping someone can help me comprehend the following:

Setup: Modulating boiler (IBC SFC-199, for example), no mixing valve (which I believe is the right way with a modulating boiler), 2 zones.

Temp setpoint for both zones is 70*f. Zone 1 is 60 degrees and zone 2 is 65 degrees, so both zones are calling for heat (and the pumps are going).

How does the boiler know how much modulation to perform (firing rate)? That is (to try to use the proper terms), how does it determine demand and then modulate accordingly?

Thank you!

Jamie Hall

Measuring the supply and return temperatures and applying a control curve to that.

josephny

Maybe I'm not doing so bad: You identified 3 concepts (supply temp, return temp and control curve) and I'm completely ignorant of only 1 of those concepts (I'll bet you know which one).

I found this:

http://tech-controllers.com/blog/heating-curve---what-is-it-and-how-to-set-it

But it might take a me a couple of years to grasp, and then a couple more to use it to help me understand how it answers my question.

I truly am grateful for pointing me in the right direction.

hot_rod

You set the boiler operating temperature on the control. You can set a fixed temperature or use an outdoor reset sensor which will automatically adjust the boiler output based on outdoor temperature.
The boiler burner firing rate will vary around the setpoint, as Jamie mentioned. It does that by varying the speed or the burner fan, which then controls the amount of gas going to the burner.

Most of the boiler manuals have a section explaining the control operation and adjustable functions.

Really no need for a mixing valve unless you need multiple temperatures 10° or more apart, for example.