Multi-speed circulator pump

Bluchip

Homeowner here.
Have a hot water heating system consisting of a boiler, 4 zones, and fin-tube. 

Was spending quality time nursing the boiler and staring at the system components.  I realized the 4 Grundfos circulator pumps are 3 speed units. All are set to Hi. 

What is the purpose of three speeds and what conditions call for what speed? 

Some may have read my ordeal with the propane fueled boiler being set up and running for a decade without the propane orifice. I say that because it is clear the original installer wasn’t very careful. Did they simply set the pumps to HI without any concern to efficiency, longevity, or overall boiler performance?
So, I am curious about the when where what why of the 3 speed pump. 

Thanks in advance!

pecmsg

Measure the temperature difference. 20 - 30°F is fine.

(You do want to maintain a minimum of 140° return water)

Bluchip

@pecmsg You may have to dumb that down for me! Do you mean the temp differential between supply and return at the boiler? Perhaps I should note that this is a modulating boiler (Weil McLain Ultra 155) and as such it has a return and supply sensor.
Thinking out-loud to myself kind of between the lines of your response, I would guess if you have too great of differential (return falling below 140) then a greater flow rate will help slow the heat loss of the circulating water and thus raise the return temperature?

hot_rod

Increasing pump speed, increases the gpm, which in turn increases the output of the heat emitters a bit.

Depending on your radiators, fin tube designs around a 20 ∆, panel radiators 30- 40, radiant floors 10- 15∆. So by being able to adjust the pump speed you can get the system to flow at the design condition.
On a design day. The delta moves a bit from cold start up, wide delta, and closes as the load gets taken care of.

pecmsg

The difference between the outlet of the pump and the return to the boiler. 

1 circulator at a time.