Lennox Furnace--Question on Dual Heat Mode

[yelbike]

I'm not sure you can accurately say that low fire is more efficient. But a properly controlled and operating multi stage furnace will provide a more comfortable environment.

 

[Sigarms]

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?

Two types of "two stage gas valves"

1 - one that is timed, that brings second stage heat automatically on.
2 - one that brings on second stage heat when needed.

If you're using LP, 1st stage will use less gas, saving you money (more so than NG which is usually less expensive).

In NC, most guys size a house for heating at 20 or 17 degrees design temp. Fact is though, although it may get colder than 17 degree in NC, the run time below 17 isn't as great as a run say at 45 degrees (in NC).

Second stage (high fire) on the gas valve produces more heat (uses more fuel). However, when heating set point is met, valve can drop down to first stage, using less fuel due to the net output of the furnace.

With LP in general, a non timed 2 stage gas valve can save you money for heating because a single stage valve burns a max capacity when heating is needed whereas a two stage valve can drop into the low fire mode.

Generally, you pay more for a "true" two stage gas furnace than a single stage gas furnace.

What I don't understand is why you paid someone to install a two stage gas furnace, yet it was wired up incorrectly not to bring second stage on? Sorry, makes little sense.

 

[npalen]

The installer let me know up front of the need for another stat wire (eight total) to utilize both stages of heating and cooling. Being retired with time on my hands, I told him I could run a new stat cable and probably will.
I was trying to decide if there was a need for high fire given my furnace size, home size etc. I will probably go ahead and pull the new cable since it will be fairly easy.
The new Lennox stat allows setting degrees differential and stage delay (up to two hours) That will let me play with the settings. The downside is that there are not separate settings for heat and cool but no big deal to change for the season.
Probably should get one of the new Lennox S30 stats. It uses GPS in addition to WiFi.
iComfort S30ｮ | Smart Thermostat | Lennox Residential

 

[kenmac]

I'm not sure you can accurately say that low fire is more efficient. But a properly controlled and operating multi stage furnace will provide a more comfortable environment.

I can.. If you are burning less gas than is needed to heat (43 CFH vs 64 CFH). You are saving $$$.. Saving $$$ is efficiency in my book..

Now, if it's real cold and you keep it on low fire trying to save $$.. that's a different story

I'm with Sigrims.. I would have let him run the wire ,and check everything out to make sure the unit went from low fire to high fire, and didn't have any glitches in the system..You wire it up wrong ,or something.. He may not want to warranty it

 

[npalen]

I would guess that any efficiency on low fire comes from the fact that the furnace and blower is not cycling on and off nearly as much as on high fire. A spike in electrical energy consumption occurs each time a blower is started especially if it's not "soft start". But whether heat output per BTU input is better on low or high, I don't know.