# Twinned Furnaces Questions



## MrControls (Dec 11, 2008)

I need a little help making sure my Trane XB90 furnaces are setup correctly. The furnaces are twinned now and using 1 transformer. On the terminal board of the furnaces the R/G/W/C (I think, I'll check when I get off work) are being used. 

The wire connected to R splits/wired nutted and goes to the thermostat and runs to the condensate pump->condensate float switch->secondary furnace R.

The wires connected to C/G/W split/wired nutted and go to the thermostat and to the furnace respective to the correct colors.

The twin terminal is not being used on the board. The furnaces have different breakers but from the same feed. It seems like the twin terminal should be used but it might not matter. What is the functionality of the twin terminal?

The furnaces both come on together and are working. I tried just turning on the fans using the thermostat but they didn't work. I put a jumper across R and G on the stat and they came on so I think that the stat could be defective. The current setup the secondary furnace can be shutdown and the primary will continue to run. This seems like a problem to be because the primary furnace will suck air back through the secondary and cause the motor to spin in reverse. The air circulation through the house will be minimal.

When I had the units installed the Trane Salesman said that the furnaces would be staged. Currently they are twinned but I want to have them stage but want to make sure they are wired correctly.

In this thread
http://www.diychatroom.com/showthrea...highlight=twin

beenthere states that the twin wire is to synchronize the blowers of the furnaces. So is in possible if the twin wire is connected to run a wire for heating to each furnace from the stat? Will that keep the blowers running at the same time but allow them to be staged? Will the twin terminal also shutdown both units if one goes down? I couldn't find anything in my manual to explain how the twin terminal is used.

If the furnaces are staged then the 1st stage will get worked more than the second. To avoid this would you recommend installing a timer relay and set the accumulation time to 500 hours to have the stages switch?

This seems like it would work:
http://web4.automationdirect.com/adc/Shopping/Catalog/Relays_-z-_Timers/Koyo_Timer_Relays_(KT_Series)/KT-V4S-D

I have a Honeywell stat and don't really understand when the second stage will be called for. I read that the stat keeps track of how often heat is being called for to determine how long it should wait to bring on the second stage. Does anyone know the program for the logic?

That is a lot, sorry :huh:


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## beenthere (Oct 11, 2008)

The thermostat alogorthym depends on which model sthermostat you have.

Yes, you can stage the furnaces. Must use the twin terminal.

Johnson has a twinning control that will do what you want as far as alternating lead lag furnace. It alternates the unit each heat call. 
It uses sensors that will shut down the furnaces, if one of the blowers fails to run.

Just curious, how many sq ft is your house, and what size furnaces, do you have.


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## MrControls (Dec 11, 2008)

Thanks

My house is 3600 sq ft (not including the basement), 3 stories (center row home), and old (storm windows). The furnaces are 74k BTU. My old furnaces were Rheem 80% efficient and 100k BTU.

What exactly does the twin terminal do? Does it just energize the blowers of both furnaces at the same time using contacts that open and close? If the 2nd blower happens to shutdown because of a safety trip the 1st blower will continue to run? If that is the case then the Johnson twinning control is necessary in my opinion and the HVAC company should install it.


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## beenthere (Oct 11, 2008)

Yes, it brings them both on at the same time.
And it runs them both at the same speed. 

Back draft dampers can also be used instead of the control. That way if one blower goes out, you still have some heat.

From the sizing you posted. It sounds like they just guessed at a size.
Instead of doing a load calc, and putting in a correctly sized single unit.


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## MrControls (Dec 11, 2008)

Yeah, the guy just guessed. Does it seem like the furnaces are oversized?

I think they are and staging would help when it's not that cold out. 

Using back draft dampers would be nice.

I guarantee the HVAC company is just going to use the twin terminal. That seems like a rig job but it will work. The one furnace will get used a lot more and there could be issues if the one shuts down. Live and learn.

Thanks for all your help :yes:


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## beenthere (Oct 11, 2008)

Your welcome.

A 3 story 3600 sq ft home takes less to heat it then a single story 3600 sq ft home.
When both are constructed of the same building materals.
So, while a single story may need an output of 144,000BTUs, a 3 story may only need 100,000BTUs.


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## hvaclover (Oct 2, 2008)

beenthere said:


> Your welcome.
> 
> A 3 story 3600 sq ft home takes less to heat it then a single story 3600 sq ft home.
> When both are constructed of the same building materals.
> So, while a single story may need an output of 144,000BTUs, a 3 story may only need 100,000BTUs.


Gotta explain that one to me been. i never heard that before.


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## MrControls (Dec 11, 2008)

I'm meeting with the HVAC company today in about an hour. I'm not sure what I should say about the way they are planning on staging the furnaces (just using the twin terminal). I sent Trane an email last night and this morning they sent me the installation guide that I already have so I wrote them back an asked them how to wire the furnaces so they can be staged. Still waiting for a response.

The twinning control/kit seems like it has 2 functions. If your furnaces don't have the twin terminal then it can be used to twin them and it can stage the furnaces. Is it really necessary if you have the twin terminal or just recommended?

Update:

I salesman came by and I asked him a few questions.

Twin terminal - He wasn't sure and started trying to explain the functionality but then ended up double talking.

Staging - Got stuck here too. Asked him about the twinning control and he said it wasn't necessary because of the twin terminal but somehow he couldn't explain the functionality of the twin terminal.

Current Wiring - Wasn't sure why it was the way it's currently wired.

Furnace Sizes - Used the sq ft of basement multiplied it by 3, ceiling height with no insulation, ave sq ft of windows, center type, and 0 degrees OA to 74 degrees inside so it seems rough but he did calculate the BTU.

He said that he'll send me an email today to explain everything once he looks over his documentation.


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## beenthere (Oct 11, 2008)

hvaclover said:


> Gotta explain that one to me been. i never heard that before.


You only have 1/3 of the heat loss through the ceiling as you would with a single story/rancher.
Only 1/3 of the loss through the floor.

Rancher/single story, 3600 sq ft ceiling.
3 story, 1200 sq ft ceiling.

3600 sq ft ceiling, R19 insulation, 0° outdoor, 74° indoor, heat loss = 14,021 BTUs.
1200 sq ft ceiling, R19 insulation, 0° outdoor, 74° indoor, heat loss = 4,673 BTUs.

Same thing with floor.


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## beenthere (Oct 11, 2008)

MrControls said:


> I'm meeting with the HVAC company today in about an hour. I'm not sure what I should say about the way they are planning on staging the furnaces (just using the twin terminal). I sent Trane an email last night and this morning they sent me the installation guide that I already have so I wrote them back an asked them how to wire the furnaces so they can be staged. Still waiting for a response.
> 
> The twinning control/kit seems like it has 2 functions. If your furnaces don't have the twin terminal then it can be used to twin them and it can stage the furnaces. Is it really necessary if you have the twin terminal or just recommended?
> 
> ...


He just used some basic rule of thumb.

Good chance a single furnace would be more then enough for your house.

But, make more money on selling 2 furnaces.


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## MrControls (Dec 11, 2008)

It's possible and this winter I'll find out. I guess I don't have to worry about getting cold but I'm definitely disappointed with the Trane rep and support. I'll try another brand next time.

I setup the staging and the 1st furnace is maintaining the temperature. The second furnace hasn't kicked on yet and it's 30 degrees outside. The air isn't mixing that great out of the units. The 1st furnace heat feeds more of the back of the house and the 2nd front/middle.

Right now I'm just using the twin terminal but will add a safety relay that will kill R (to the stat from 1st furnace)if the blower of the 2nd furnace isn't working. I'm going to wire R/C (2nd furnace) to the coil of the relay then have the contact of the relay in series with R (to the stat from 1st furnace). If the 2nd furnace shuts down because of an internal safety then it will shutdown 1st furnace. I'm surprised that this functionality isn't already setup. To vary the run time I'm planning on using a double pole latching relay. The furnaces will switch each cycle.


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## SKIP4661 (Dec 3, 2008)

Could use low temp (100 Degree) manual reset limits attatched to the blower housing. when one blower failed the other would blow heat back through the failed furnace. Could break the furnace low voltage and shut the furnaces down.


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## MrControls (Dec 11, 2008)

That would work and provide a good safety. Then I wouldn't be relying only on the internal safeties. Thanks

Update
To get better mixing I'm thinking of backdraft damper.
http://www.ruskin.com/catalog/servefile.aspx?id=94

Need to size the dampers before I do it but it will also allow me to run just one furnace without having the second blower going so it could give me even more control. I'll have to get a controller to do this but it might give better temp control when it's not that cold out. The static pressure could be too low to circulate the air through the house just having one blower on though. It will also allow one furnace to run as beenthere stated if something happens to the other one. Nice back up


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