Lochinvar KHB110N - Cascade setup

michaelhepp

I have just replaced 2 oil fired Burham boilers with 2 NG Lochinvar boilers, as I finally got NG in our street! They are connected in paralel to a heating loop that supports a 15 zone radiant heating system and 1 in-direct DHW zone.

They are controlled by a Tekmar 2 stage boiler & DHW control, Model 252, installed in 1997 when we built the house. This controller determines if we need 1 or 2 boilers to provide heat as needed. The controller doesn't support 0-10V DC for boiler control, so we just connect the output control from the Tekmar to the thermostat connection on each respective boiler. The Tekmar control also handles alternating which boiler is primary every 40 hours of runtime so they wear evenly. When the Tekmar 252 gets a heat or DHW call and the water in the heating loop isn't warm enough it fires the appropriate boiler. The Locinvar auto modulates it's firing rate based on the incoming/outgoing water temperature built into each unit.

The system is working fine but i'm wondering since the Lochinvar's support a cascade mode if we should do some minor reconfiguring to support this. My understanding is that this would be more efficient since the Lochinvar's would talk to each other and when #1 gets to 100% it dials down to 50% and then fires up #2 and they both then run at 50%. From what I've read, 2 condensing boilers running at 50% is supposed to be more efficient than 1 running at 100%.

First question: Is this 2 @ 50% efficency claim really the case? i do see that when both boilers are running that they pretty much match the other boilers firing rate just by them independantly monitoring the built into incoming/outgoing water temps.

Second question: If I do this, I know i need to wire the 2 boilers together, install 2 system supply sensors on the heat loop feed (1 for each boiler) and make the appropriate configuration changes on each of the boiler interface, making 1 of them the Leader and the other the Member. The Lochinar's would then be configured such that if #2, Member loses communication with the Leader, #1, it takes over as the Leader until communication with the Leader is re--established.

I can then reconfigure the Tekmar 252 to think it only has 1 boiler, the Leader, and the Leader would then decide if and when #2, the Member runs.

My question is that if #1, Leader goes offline, how will Member, #2, know that there is a heat call since the Tekmar output is connected to the thermostat inputs on #1, the Leader. Lochinvar documentation says that you can connect 0 - 10V BMS to both boilers but can you do the same with the thermostat wire?

I've attached some pics that might help.

The Tekmar 252 is in the upper right hand corner, the remaining 5 Tekmar controls are the electronics controlling the zones.

Thanks in advance for any help and guidence.

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