2 Staging for 2 sets of Twin Furnaces

JUGHNE

I have 4 Armstrong GUK125D20-3-A NG furnaces. Each have 5 ton AC on them. Installed in 2000.
The control wiring is a real knuckle head job, done by "pros" from at least a 100 miles away.

This is a community center and the last wedding people had one AC T-stat down to 45 degrees; this ran for the next 2 days with 45 amb overnight. To thaw out coils, I put the system switch off and the fan to on.......find out that 2 AC are still running. Never seen that much ice on compressors.....looked like SR buckets.

This is 2 sets of twins with 1 T-stat each.
I would like to use 2 stage stats for each.
One set of twins have Heatcraft 70k98 boards.
The other set has the 70k98 in one and a HW ST 9120C 2028 in the other.
Will this last mis match work?

The "master" furnace has an added relay in the blower compartment.....I assume this is to isolate the heating controls.
The 2 Z connections are not connected.

This is a 12,000 sq ft building with 6 other AC units totaling 22 tons plus the 20 tons that the twins deliver.
The double twins handle the dance/basketball floor only.
The need for staging seems obvious to me. I have been stressing this for 16 years and finally was given the go ahead to change T-stats to simple HW 5000 with min temp locked to 70 and max heat to 70 also. People really love to play those things.
Have already changed one AC because of low amb operation.

Any ideas appreciated....Thank You.

ratio

IDK about those particular twinning kits, but all the aftermarket kits I've seen have been nothing more elaborate than relay boards—no interlock more fancy than a power loss relay to kill the R wire if the power to the lag unit gets cut off. You should be able to do it with a few relays to isolate the second furnace. Might need a relay on the second condenser too if the lead furnace can't handle the low voltage load of another contactor.

I don't know why anyone would install a twinned furnace & not stage them. You'd have to get pretty esoteric to not benefit from the turndown. It'd be even more fun with two stage gas valves…

John Mills_5

The twins with the same boards will work if they have twinning terminals. Otherwise get creative or get a twin board. The ones with the Heatcraft in one and HW in the other won't work with an external kit.

I've used: https://jacksonsystems.com/products/jackson-systems-universal-twinningparalleling-kit.html

To stage, hook W2 for furnace 2 to its W terminal then when W2 calls on the thermostat, the 2nd furnace fires. But true, no lead/lag. Just have to reverse the wiring each year. We have a twinned zoned system like that. The tech that does their clean changes it each time.

JUGHNE

The plan now is to replace the mis matched boards with HW ST 9120U's. Add a single 50 to 75VA transformer as needed to run both furnaces. (now the "bad" twins have 1.26 volt difference on the reds....the good ones have only millivolts)
Connect 2 stage t-stat with 7 conductors. R-G-common to both boards. W1 & Y1 to first unit.....W2 & Y2 to second unit. Boards say 2 more wires between...."twin" and "C".

As these are 16 years old I will not attempt any lead-lag. Just wear one AC/gas train out first, otherwise end up 2 equally worn out systems that may both fail at the worst time. (We are over heated & cooled in this building so there is redundancy.)

Does anyone see any pitfalls with this approach?

ratio

The gas company might send someone out to investigate the reduced consumption?