Control wiring for backup/assist boiler

[Nathan Smith]

I'm trying to determine how to set up controls for a two boiler setup. We have a Weil McLain ultra 310 boiler as the primary boiler. We have a older Weil McLain cast iron boiler as well. I'm trying to determine how to best do the controls so the CI boiler will act as an "assist" boiler when the demand is to great for the condensing boiler to keep up as well as have it revert to a back up boiler in the event the condenser should go down completely. Someone previously had the CI boiler as an assist only so if the condenser went down the CI wouldn't run either. I am an electrician so I'm very familiar with control wiring circuitry, just not sure what controls would most easily achieve my goal. Thought about using an aquastat on the return line to tell the CI to fire but with the modulating condenser that seems like it would fluctuate to much with the outdoor temp swings so the CI could be tricked into firing when it's just mild outdoor temps causing the lower water temp. Unless I monitor the outdoor temp in conjuction with the return water temp. Ideas??

 

[Dana]

I'm wondering what loads you have that would possibly outrun an Ultra 310. It that monster sized boiler in a house? That thing has enough boiler output to theoretically keep my house at 70F indoors when the outdoor temperature is within a few 10s of degrees of absolute zero (-459F).

Knowing a bit more about the application and how it's currently plumbed & controlled might be useful.

 

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[Nathan Smith]

I'm wondering what loads you have that would possibly outrun an Ultra 310. It that monster sized boiler in a house? That thing has enough boiler output to theoretically keep my house at 70F indoors when the outdoor temperature is within a few 10s of degrees of absolute zero (-459F).

Knowing a bit more about the application and how it's currently plumbed & controlled might be useful.

 

[Nathan Smith]

Thanks for the reply. These boilers are installed in a large building that houses a child care center, church, and gymnasium.

 

[Nathan Smith]

As far as controls, there are currently 7 zones each with it's own circulator. There is an outdoor thermostat wired into a control circuit between the ultra and the CI boiler. If the outdoor temp stays above the set point (I think it's 40°) the CI boiler stays off. I haven't dug into the control wiring to much other than that. I just no from past experience that the CI won't run if the ultra goes down. I see on the ultra wiring schematic there are two sets of auxiliary limits. One has a soft lock out and the other has a hard lock out. I suspect the existing control wiring between boilers is coming off of one of these which is why the CI won't run when .The ultra is down.

 

[Dana]

What zone controller is managing all those pumps?

 

[Dana]

How the boilers are plumbed into the system also makes a difference in how they are controlled. The usual configuration for double boiler systems put the boilers into the heating loop on closely spaced tees as hydraulic separators, each with their own local pump:

Paralleling a cast iron beast with a condensing boiler it's more efficient if the condensing boiler is first in line, getting the coolest entering water temperature, with the cast iron being downstream from there. (In the above diagram boiler #3 would be the condensing boiler.) This produces the least amount of additional pumping head, and keeps the inactive boiler from being just another radiator on the system. If it's plumbed in series the secondary boiler is taking the full flow of the heating system even when it isn't firing.

Is that the way it's plumbed?

Without knowing how it's plumbed or what the existing controls are there's no way to come up with a relevant recommendation.

 

[Nathan Smith]

Sorry for the long delay. Hard to find the time to get on here. The boilers are plumbed parallel. I will try to attach a couple pics that I labeled. In the pic
"CIR" is Cast Iron Return
"CIS" is Cast Iron supply

"UR" is Ultra return
"US" is UltraUsupply


Since my last post I have determined that the control circuit for the cast iron currently runs through a relay that has it's coil energized by the 120 volt supply to the ultra boiler circ pump. It also runs through the outdoor t stat. So ultra pump must be energized and outdoor temp must be below 30° for cast iron to fire.