Ditch the twinning control?

[cheelr]

I'm a chiller guy helping a friend whose twinned furnaces stopped working (in cooling season). Someone (he thinks the landlords handyman) jury rigged the wiring, probably to get the unit running in heating. One furnace's t'stat wires were jumped around the twinning control. When I showed up in cooling nothing came on with a call for cooling from the t'stat. I also found the R t'stat wire lifted on one of the funaces. I powered it down and wired everything properly. When I restarted and the t'stat calls for cooling, the c.u. comes on (it's hitched right to the yellow and common on the t'stat side of the twinning control), but the two y and the g's going to the furnaces on the furnace side of the twinning control stay at 0 volts, so neither of the fans start. Both fans start if I jump 24v right to the g furnace t'stat connection. Looks straight up like the twinning control isn't working. Looked around and since this control is discontinued nobody has one locally. Can find one online, but the things pretty pricey and I'm worried about getting one bad out of the box.

I was wondering what the considerations would be for completely eliminating the twinning control and wiring the t'stat parallel across both units. They are identical, have just single stage heating/cooling, and have fixed single speed fans, are on the same leg and breaker. I realize I'd be paralleling the two xfmr's, what would be the rub?

[cheelr]

The jury is apparently undecided. I'm giving it a go without the twinning control. Can't think of any difference in operation besides when one fan fails the other will continue to run, with some supply air short circuiting, can't see an opportunity to do damage, only low air flow and poor heating/cooling performance if it happens. The occupants will likely notice this.

[54regcab]

I wouldn't wire both transformers up to the red line, phase and voltage would have to be the same or one might burn out. If one transformer didn't have power it would act as a "short" to the other one. Use the power from one of the transformers to run both units. Split W/G/Y and common.

[Chris_Worthington]

The largest obstacle to over come would be the recirculation of the air, meaning should the blower of one not be on, the supply air will be dumped into the second's return, causing issues with heating & cooling..... (assuming there is no back draft dampers installed of course as they are sometimes ever installed on such an application)

Heating= gonna trip a limit and there goes you're second stage/redundancy ("IF" not all heating)...

Cooling= there goes the cold air entering you're evap and the heat you needed to reject...

I have never understood this twining with no forms of redundancy and/or no concerns of one failing...

Why not be able to lead/lag? Why not be able to continue on as a single appliance? Why never any back draft dampers?

[cheelr]

Worked fine in cooling and heating. And I thought of the short circuit problem - but the xfrmrs are hard wired (through a plug) to the incoming hot wires which are wire nutted together in a 4 by 4 on the wall next to the units, so there doesn't seem to be an opportunity to have one powered and the other one not. Didn't go with single transformer because I wasn't sure just one would have a big enough VA rating to power both at once in all conditions.

Cooling of course has no possibility of staging, lead/lag or partial capacity back-up, there's just one c.u. and parallel evap.s. I'd prefer having some heating capacity staging, lead lag, half back up capability and/or backflow prevention dampers too but the original install had none of this as part of the twinning control. The only thing it loses is a full unit shutdown when one fan fails to run. Although that seems like a nice feature and may make the occupants notice something was wrong a little faster, I can't see how it protects anything from damage, and the 2-3 hundred more it would have cost to get a new twin control was something my friend did not want to part with for this small diagnostic advantage.

[54regcab]

How big of a house needs twinning? You can buy a 140k furnace, I can't imagine needing MORE than that w/o REALLY long duct runs.. It seems like more sense to have each area with it's own HVAC unit.

[cheelr]

This isn't a house, it's a store with 20 foot ceilings, I posted in residential because the installing contractor (not me) used twinned residential furnaces and 6 ton c.u.'s. Two systems total (4 furnaces). The place is about 50 x 100 with an open mezzanine over a third of it. that's just a guess-I didn't measure it or size the job, just got one unit going when it broke.

[beenthere]

The Johnson twinning control is still made. I think.

Many landlords in commercial require the tenant to pay for all repairs. Its possible the last tenant has an HVAC friend just wire it for heat in winter if he was moving out, instead of replacing the twinning control.