Carrier Infinity furnace low heat option?

[DK]

We just had a new Carrier Infinity 80% furnace installed. It has a two stage heater and continuous fan. Our installer recommended against the special Carrier thermostat because he says it's not worth it, and we were glad after seeing the $250 price tag.

Anyway, the brochure states that the furnace is supposed to provide a "near continuous flow of warmth in low capacity operation for a steadier, more consistent indoor temp. Longer gentler heating cycles combined with lower fan speeds help keep your utility costs low."

So we were expecting fewer temp swings, with lower heat coming on for longer periods. And we do run the fan constantly.

So far, this new furnace has operated exactly like our 20 yr old furnace: heat coming on full blast, then cycling off for half hour or so, before it kicks in again and does the same thing. And our installer said that after so many minutes of the furnace kicking out heat, it'll go from low heat to high heat. But I don't feel any noticable change in the temp of the heated air after the furnace has been running for a long time.

It has been cold outside since we had the furnace put in, with the outside temp being -10 to +20 outside, so is that why we are feeling high heat cycling on and off all the time rather than the more prolonged low heat? Even so, the heat will cycle off for 30 minutes between cycles. Then I just feel the fan pushing air out of the vents that seems to be room temp. Will the "nearly continuous" low heat feature be more noticable when it warms up outside?

Anyway, how is the continuous fan and two stage heat settings supposed to work if you don't have the special Carrier Infinity Thermostat? Will it work OK without the big-buck thermostat, or is the only way to get that near continuous flow of low heat to get the Carrier thermostat? Or does it sound like it should work fine with a regular thermostat but that our installer doesn't have the furnace's settings correct?

Any advice would be appreciated!!

[Todd H]

Yeah, it's pretty steep for what it does. There are much cheaper aftermarket units that evidently do the same stuff.

He should've left the installation manual with you. Dig in. Without the Carrier Infinity stat (identical to my overpriced Bryant Evolution control I believe), at least on my 94% 2-stage unit, the CPU in the furnace controls this stuff.

It's entirely possible the installer screwed up a dip switch setting and it's firing on one speed only. But, it's also possible though that your heating needs right now are such that high heat is being called for, based on the CPU's stored run history. Is it cold there now? How long are your heating cycles?

On my Bryant high efficiency furnace installed a few months ago, there's a switch SW1-2 for "adaptive heat mode." described as "allows

2 stage operation with a single stage thermostate. Turn on when using 2 stage thermostat to allow low heat operation when..."

The description the installation manual for my furnace says that in the absence of a 2-stage stat, and in adaptive heating mode,

V. Sequence of Operation A. Single stage thermostat and two stage heating (adaptive mode) NOTE: This furnace can operate as a two stage furnace with a single-stage thermostat because furnace control cpu includes a programmed adaptive sequence of controlled operation, which selects low-head or high-heat operation. This selection is based upon the stored history of the length of previous gas heating periods of the single stage thermostat.

Your mileage may vary, but I wouldn't be surprised if yours works very similarly.

So, if you're really hearing only the 2nd stage and high fan speed when yours kicks on, it could be because adaptive heating mode is disabled by this switch being on (or its analog on your furnace) or because run time was such in prior cycles of operation that high output got selected, and because of the CPU's algorithm and hysteresis, it's selecting the high setting because it hasn't encountered a short enough run cycle to tell the CPU to do anything differently.

On mine at least, if you power down the system, it resets/erases the history, and wll start on low heat mode and stay there for up to 16 minutes before going to high if needed.

If I were in your position, I'd look for the relevant portion of your own install manual, eyeball the switch settings, and if you can confirm the adaptive heat mode isn't set correctly for a 1-stage t-state definitely get the installer back out there to fix his installation. If you still can't figure out why you're always seeing high heat, hell call him anyway, other things could be wrong (including "they may have undersized the entire unit").

Hope that helps some.

Best Regards,

-- Todd H.

[DK]

Thanks for the advice, Todd. I dug out my furnace install manual, most of which I don't understand, and saw the same explanation as you quoted. I also powered down the furnace and restarted, and for the first time felt the low heat coming out of the vents. So if that worked, then I'm guessing the switch is correct and the only reason it's always on high heat is because of the cold outside temps.

Glad to hear I don't have to buy a new expensive thermostat!!

[Todd H.]

Cool.

Yeah, your conclusions make sense to me--"normal operation."

After enough cycles of low-heat timing out to engage high-heat (to keep up with your desired temperature), I suspect that the furnace will begin starting immediately on high heat again, and you won't see a low heat cycle until the weather warms up enough such that your high-heat-only cycle times get too short for its liking.

The only thing I'd be concerned with potentially is that if it's not _that_ cold outside for your area, and/or your furnace seems to be running really long cycles or almost continuously, I'd be concerned that the installer put in an undersized furnace.

Best Regards,

-- Todd H.