Lennox Furnace--Question on Dual Heat Mode

[zzvyb6]

Will any of the wireless 'stats work in your application ?

 

[npalen]

I had checked into the wireless but didn't find any that had the needed capability.

 

[dadster4]

Letting the furnace control low/high fire rather than the thermostat is usually preferable. Forcing the furnace to run on low fire when your heat loss is too great (colder weather) will result in a drafty feeling due to the warm air temp being too low. On many systems I installed the main thermostat recorded the time that the furnace ran in low and high fire as well as low and high cooling among other statistics. I found that the system ran in low fire about 70% of the time. It's also worth mentioning that the variable-speed blower will adjust it's speed to match the heating mode (and provide the proper air temp) saving a significant amount of $ for electricity. Please do NOT use the G (green wire) terminal to test the use of W2 (2nd stage heat) as mentioned above...it can cause numerous problems.

 

[yelbike]

Letting the furnace control low/high fire rather than the thermostat is usually preferable. Forcing the furnace to run on low fire when your heat loss is too great (colder weather) will result in a drafty feeling due to the warm air temp being too low. On many systems I installed the main thermostat recorded the time that the furnace ran in low and high fire as well as low and high cooling among other statistics. I found that the system ran in low fire about 70% of the time. It's also worth mentioning that the variable-speed blower will adjust it's speed to match the heating mode (and provide the proper air temp) saving a significant amount of $ for electricity. Please do NOT use the G (green wire) terminal to test the use of W2 (2nd stage heat) as mentioned above...it can cause numerous problems.

What numerous problems?

I install furnaces in cold climate all the time, Manitoba, Canada. I haven't seen problems with a 2 stage heating thermostat controlling a two stage furnace, ever. The only reason why the control board on a furnace has a time delay for the different heat stages is if its not practical to install a two stage heat thermostat.

Typically the low firing rate has a lower blower speed so the home doesn't feel drafty, the reason you don't want to permanently lock the second stage of heat, is your furnace may not be able to keep up with the heat loss load in the coldest design weather for your region.


Btw, yes the low fire stage is more efficient, not in combustion rate but for the fact that your using a smaller output. Think of using two vehicles, a grocery getter for daily driving than a pickup for weekend towing.

 

[npalen]

It would seem that the low fire on the new Lennox at 44K BTU OUTPUT should perform as well as the old single stage Lennox at 45K BTU OUTPUT. (See post #10 above) The fan speed on the new furnace would probably be much lower than the old while both are producing about the same output. It would therefore seem that the temperature at the registers would be higher with the new furnace. Or am I missing something?

 

[kenmac]

well, If you weren't going to use the furnace as intended , you shouldn't have spend the xtra $$ for the more efficient furnace that has low fire / high fire ..This feature will save you some $$

 

[npalen]

What I'm trying to understand here is that if the furnace is more efficient running on low fire and can handle the heating load year around on low fire, what is gained by going to high fire?

 

[s219]

What I'm trying to understand here is that if the furnace is more efficient running on low fire and can handle the heating load year around on low fire, what is gained by going to high fire?

It's a function of the difference between the set temperature and the current temperature (temperature differential) and/or the amount of time the furnace has to run to reach the set temperature. Basically, when more aggressive heating is needed -or- low fire isn't getting up to the set temperature fast enough, it kicks into high. All depends on how the controller is setup.

When I have seen these systems (or similar) they can often run in low most of the time, but would kick into high in situations like maybe in the morning when you turn the thermostat back up after setting it low for the night. There would be a larger temperature differential to overcome, so the furnace will kick into high -- maybe immediately, or after some time.

 

[dadster4]

s219 hit it right on the head with the explanation of why you want two stages of heat. The efficiency will remain the same on both low and high fire, but the BTU output and wattage will be greater on high. All I was trying to say is that there is no reason to run an extra wire to have the thermostat control low/high fire, the furnace controlling when to kick up to high fire is fine.
The problems I was referring to concerned using the green wire (fan) to use as W2 (2nd stage heat)...not that a two-stage stat is a bad thing.

 

[kenmac]

when ever it goes to high fire it cost you more $$ to operate.. Some can stay on low fire and heat just fine.. just depends on how yours was set up...
Like a heat pump going from heat pump to 2nd stage resistance heat.. If you can stay on heat pump you'll save $$ over the 2nd stage elect