Lennox Furnace--Question on Dual Heat Mode

[npalen]

Had a new Lennox with variable speed blower installed about a year ago and it has the low and high cooling as well as low and high heating capability. We've been enjoying the dual cooling modes but am short one wire in the thermostat cable to allow use of the dual heat modes. My question is whether it's worth the trouble of running a new thermostat cable to get the dual heat functionality. Is the furnace going to be appreciably more efficient when running in low heat mode when high heat mode is unnecessary?
It's the Lennox EL296UHV and 70K BTU heating a 1600 sqft ranch style in Kansas where it gets below 0 F a few times during the winter.

 

[Dave M7040]

I looked at the furnace spec's.
I dont see the dual heat feature offering you a lot and it may result in cold rooms far from the furnace.
When the fan is running at low speed, as it does on low heat, you are not going to get the same balanced air circulation you achieve with the high speed fan setting.
You really need a zoned system which has its own shortcomings. A zoned system will have motorized dampers in the ducts which blocks flow to those areas not needing heat.
If you want to experiment, just run a wire in the open without fishing it into the walls and see how it works before you do a lot of work.
Dave M7040

 

[yelbike]

If wired correctly, it will fired on low fire than automatically switch to high fire after a predetermined set time, approximately 13 minutes.
If you prefer the thermostat to control the second stage of heat you could switch "G" (fan control) to the w2 at the furnace and the thermostat. You will lose the fan control capability but it works as a test until you decide if pulling a new cable is worth it to you. There will also be a jumper or micro switch to flip as well.
The thermostat does a better job of driving the furnace than the time delay function, you can also lock out the high fire rate if you desire.

 

[npalen]

Thanks for the feedback guys.
I assume that loosing fan control means doing without the automatic circulation of air a programmable percentage of the time when neither heat or cool is running? Otherwise, I don't see a manual fan control on my thermostat which is a ComfortSense 7000 Series. I'm not sure the automatic circulation is of much benefit other than perhaps keeping temperatures even throughout the house and also more "filter time". We have never had a problem with uneven room temps even with the old (40 years) Lennox.
I would be interested in locking out the high fire rate if that would produce more efficiency. I suppose the question would be if low fire could keep up in sub zero temps.

 

[Dave M7040]

one issue with only using low fire is that you may feel a cool draft if the discharge air temp is low.
Do you have a heat recovery ventilator in your home?
If so it would be helpful to know how it is ducted.
Dave M7040

 

[npalen]

Negative on the heat recovery.
Can anyone answer the question of whether the furnace is more efficient when running on low fire? What, exactly, is the purpose of the low fire?
Just asking as I'm new to the world of high efficiency furnaces.

 

[murphy1244]

Negative on the heat recovery.
Can anyone answer the question of whether the furnace is more efficient when running on low fire? What, exactly, is the purpose of the low fire?
Just asking as I'm new to the world of high efficiency furnaces.

Some furnaces will runn on low fire for 10 minutes before going to high or normal fire settings. The goal is to bring it up to temp using less gas

 

[ray66v]

Mine hardly ever just ran on low. So, I locked the low fire out.

Probably is a nice feature, if you live where it doesn't get real cold.

 

[s219]

I am guessing it's already using low and hi heat modes based on how the controller is wired on the furnace itself. In many cases, the dual mode systems can have the modes controlled by the thermostat -or- based on number of zones calling for service -or- based on a timer. If the dual heat isn't wired to the thermostat, then the installer may have set it to be kicked on via the controller based on time. It may be running on low for a few minutes and then kicking onto high if needed. Hard to say. Need to know a lot more. You may be able to flip through the options on your controller to see how it is running the furnace.

 

[npalen]

Correction by OP: I mentioned in the first post that the new furnace was 70K BTU. It is actually 44K BTU on low fire and 66K BTU on high fire. That translates into 43K and 64K output based on 96% efficiency.
The old furnace was (IIRC) 60K BTU input at 75% efficiency which translates to 45K BTU output. So it would seem that low fire on the new furnace should output nearly the same as the old.

Edit: The negative might be less balanced air circulation as mentioned above by Dave M7040.