how is a 2 stage furnace suppose to work?

[nospam_coloradotrout]

I've read the "benefits", and I've seen how mine is installed. But is this really practical or at least optimal?

I have a 100 BTU furnace to heat 1700 sqr feet mainfloor and about

1300 basement (partially finished). The 2 stage functionality works like this. I have a normal programmable thermostat. On the furnace they installed a "timer". After the thermostate turns the furnace on, it will run at low heat (3 burners) for 1 to 10 minutes (depending on the manual setting of the timer). After that time it will kick into high heat (2 more burners). It runs at high heat until the thermostat kicks it off.

Theoretically, on milder days, the low heat will be enough and it will never go into high heat. Then for the fewer cold days it will crank up the heat. However, we set the temp to 65 for the night and 69 for the day. So I can be sure that in the AM, it will crank up to high heat to get to 69 as that will take 30 minutes or longer.

Does this really matter? Would a 2 stage thermostat make a big difference? This all seems pretty questionable to me. Sounds like marketing hype.

[Martik]

This has already been discussed here. Do a google groups search. You need a

2-stage t-stat to obtain the benefit of lower output for a temp controlled period of time rather than a fixed period.

[Bubba]

??? What? Who installed a timer on your furnace? Is your furnace new or some pieced together self help project? 3 burners for 1-10 mins? What controls that? Then 2 more burners? At this point I have no idea what you have and Ive seen a lot of "chit". Yeah, its all marketing hype. Keep your "project" just the way it is. Bubba

[Hello Friend]

'I have a normal programmable thermostat. On the furnace they installed a "timer".

I think by 'timer' , you mean an automatic Time Delay Relay (perhaps ?) whereby the signal to the 2nd stage is delayed for 10 minutes.

'After the thermostate turns the furnace on, it will run at low heat (3 burners) for 1 to 10 minutes (depending on the manual setting of the timer). After that time it will kick into high heat (2 more burners). '

From this description, it would only be possible if you had a split manifold in your furnace with one gas valve controlling 3 burners, and another gas valve controlling the remaining 2 burners. I doubt this is what you have very seriously. Typically on a 2 stage gas furnace, you would have ONE gas valve that works in 2 incremental stages : the first stage produces a smaller flame thru ALL burners...then, when second stage is demanded, the same gas valve would let more gas into the ALL the burners thus increasing the flame (and hence , heat output) until the thermostat is satisfied then all gas is shut off to the burners. A

2 stage setup is advantageous , especially in areas where there is a significant milder winter period . Personally, a 10 minute time delay before 2nd stage is engergized..is too short of a duration ; 15-20 minutes would be better to give low stage a chance to satisfy the thermostat.

[Bubba]

Not at 6am on a butt-ass cold morning after you've programmed the thermostat to go back to 62 or 64 degrees at night. In the morning, you want that stat to kick in high immediately and stay there till its warm. THUS, the need for a 2 STAGE STAT! If you dont have a 2 stage stat you dont need a 2 stage furnace. Bubba

[nospam_coloradotrout]

It's a Maytag 2 stage 80% eff, variable speed, 100btu, new furnace. I posted previously with some related Qs, but this 2-stage stuff is bugging me.

The "timer" (relay, delay, whatever) does this: The igniter starts, gas is delivered to 3 of the 5 burners. After the time set on the "timer" (1 to 10 minutes), if the stat is still wanting heat, the other 2 burners will start.

Near as I can tell my "fan-only" blower speed remains the same during "fan only" and low heat. The speed increases with high heat.

Without sitting around all day and watching and listening to this think I cannot measure how ofter the low heat get the job done before the high heat kicks in.

I guess waiting up to 10 minutes before the high heat kicks in is not all that bad, although this seems pretty rudimentary, but I'm not sure how a 2-stage stat could really be all that more intelligent. It would come down to the stat having some kind of knowledge systems that would tell it when to kick into high asap vs wait 5 or wait 10, etc. It seems like the characteristics of the furnace, the current inside temp, the outside temp, the desired temp, and the efficiency of the furnace/ducting would all have to be constantly analyzed to sort out the best approach. Seems like too much to ask of a $100-$200 piece of electronics.

[Duane Bozarth]

...

...

Some (well at least one :) ) simply look at the temperature differential (setpoint-actual) and the rate of change of the actual and use that...if delta_T is so big, then both stages or if dT isn't quite so big but T isn't catching up, etc...is an application of "fuzzy logic" algorithms....whether it is any more efficient in the end, I have no data to help...but it doesn't use any additional inputs, just a simple microprocessor and some logic.

Don't recall which thermostat used it, saw it in a new product section in a mechanical engineering trade mag some years ago...

[MLD]

If you look at the manual that came with the furnace you will note that there are some very small switches called "dip switches" the "On-OFF" settings of these switches dictate (at least on my Trane furnace) how the furnace will operate---Motor speed time delay: that is,:how long at low speed before ramping up to high etc. The manual shows a number of switch combinations and what function(s) it will perform. The default settings are also shown. MLD

[Bubba]

I can honestly say Ive never seen this in a new furnace yet and I think Ive seen every residential furnace. All the burners fire on low or high. 3 of 5 burners dont burn unless 2 are stopped up. The only relays/timers/delay that control this are on the main circuit board of the furnace.

Usually true

and if you do know this answer then I will know you have NO LIFE.

A 2 stage manual mercury stat will sense the temp in the home and turn on one or both stages as it feels is needed by sensing actual room temperature. A digital thermostat will basically do the same with a little more algorithm knowledge of its own.

By knowing the proper temperature rise of your furnace, the actual heat stage you are in and the proper gas pressure and mixture. Simple, eh? Bubba

[Sexytom976]

Look around a bit, lots of people have asked the same questions.

The 2 stage thermostat basically decides if high heat or low is necessary. (as opposed to the furnace timing out to high from low)

Depending on the heat loss of a house, you might short cycle the furnace too often (not good) plus the temp variance would be noticable (not good either)

Lets say it takes 11 minutes to heat the home from 64 - 66 after 10 the unit kicks into high. Its basically putting the unit into overdrive and then boom, its off. Over and over isnt a good thing, plus it wastes energy to boot.

Another advantage is this.... When you have a call for heat from say 60 - 68 it should go right to high and then within a 3 degree range (65) it go to low and slow the heat process down to the desired range. Makes it much more comfortable.

I have a 2 stage furnace and love it. (Once I had the thermostat dialed into my preferences)

I personally use Honeywell thermostats (I have the new touch screen series, its freekin cool lookin) and they served me well so far. Check out ebay for 2 stage units

Tom