Source
r/HVAC•
AT616

What is the expected operation of a 97% efficienct Trane two-stage furnace with multi-speed blower. Mine seems to run on high too much.

Last year, I purchased a Trane furnace through Lowes, and we've had some issues with it. First, the salesman undersized the furnace for our house, and for the first month, it seemed to run continuously without being able to maintain the temperature. I convinced the installer that it just wasn't right, and sure enough, they determined that it was undersized, so they replaced it with a larger unit. The new one works great at quickly achieving temperature after nighttime setbacks and maintaining the temperature in a much more comfortable manner than our old single-stage furnace.

My concern is that I think the programming may be off on the furnace because it seems to run on the high stage with a high fan speed way more than I had expected it would. During the day, to maintain temp with a 30-40 degree outdoor temp, it cycles on for about 5-7 minutes, then off for about 3. Over that 5 minutes, it sounds like it is on the low stage with a low fan for about 2 minutes, then full speed for 3 minutes, then back to low for about 2 minutes, then off. Does this sound typical? I was expecting it to run for a longer period of time at the low stage with low fan speed much more often, and really only go on the high stage with high fan speed when it was returning from a setback.

Also, our expected energy savings haven't really materialized. All of this makes me wonder? The furnace company didn't seem to particularly believe me when we had the undersized issue and it took some convincing him to change things (he said our house was leaky, which it is a 120-year-old Victorian, but our old furnace had no issue maintaining temp). I want to understand how these things typically operate so I can explain to him that I think our's isn't working properly.

What would you do? Thanks!

Posted Nov 13, 2019, 11:48 AM UTC

18
seamonkeys590•

What size did you come from and what size did u have before replacing it.

I have the same furnace 60k btu for a 3200 sqft home. 30 degree weather it normally runs in low stage for 30 to 40 mins. Set back takes a couple hours at this temp. Down around 0 it runs 100% of the time. Below 0 2nd stage cycles off and on. Below -32 to -38 depending on wind it can't keep up anymore an additional source of heat is used.

I undersized mine so it runs all the time during our normal winter.

4
seamonkeys590•

I will also add I have never come across an under sized furnace. Normally we have to up size to get the required blower for ac size.

1
jacecontrols•

So you had what was probably the proper size furnace (yes they run most of the time especially on low stage, yes they are slow to catch up when people have huge setbacks) but you decided to over size it now it short cycles like a *****, hmm interesting... Now because of all the cycle losses your not saving any money.. really interesting...

But seriously, they probably didnt upgrade you to a 2-stage thermostat and the up-stage timer on the furnace board is set to 5 minutes. So it stages up then goes right off and you are hearing the post cycle fan. I would recommend either increasing the timer or geting a 2 stage thermostat with the w2 wired to the furnace (prefered). They may have to run more wires but at least the furnace wont stage up when it is almost satisfied. Timers really cant predict the load.

3
4br4c4d4br4•

I wonder if your "low speed" is the exhaust fan.

2
shankartz•

Not familiar with the Trane 97% furnaces but on the lennox and napoleon ones I work on they have fan patterns. It will run the fan on a lower speed and slowly ramp up to a 100% set fan speed before tapering off after so many minutes. It is one of the ways it maintains efficiency and a comfortable air temp, this is adjustable through tabs or dipswitchs most of the time.

As for gas flow, typically if you do not have a 2 stage thermostat it will run on low fire for a length of time determined by the board settings then run on high fire for the remainder of the heating cycle. If you have 2 stage stat then the stat controls it based on demand. You will 100% not get same efficiency out of board control as you would a 2 stage stat. Best option is a communicating stat though they are more money. If you don't have one get yourself a honeywell t6 or t10 2 heat 2 cool stat and run a new 8 wire to hook it up properly.

2
terayonjfLocal 638•

Depending on the system and programming 2nd stage is usually on a timer. So after x amount of run time at stage 1 stage 2 operates. When temperature is reached it drops back into fan mode at a slower speed until remaining heat is removed and fan shuts down.

As for savings that's a touchy subject. You can't compare electric and gas bills from last year to this year unless you take into account cost of a therm, price of a kw and temperature. If the price of a therm went up and the temperature is colder your bill will be just as high if not higher

1
the_marshall•

The staging is controlled by either the thermostat, or the control board in the furnace. What thermostat do you have? Do you know if it is a 2-stage thermostat? If not is there a jumper between w1 and w2 on the control board? Or is the control board dip switch set to automatically upstage after 5mins? Unfortunately we need more info to give better answers.

1
bobbymcpresscot•

Keep in mind the efficiency of the unit is just how much btu output it has. A 97% 100k btu furnace is effectively 97k btu. An 80% is 80k btu. The stages are where you will see energy savings, but if you just have a dummy thermostat the system will run for 5 minutes, on average, on low, and if the thermostat hasn't satisfied it kicks it into high gear. Until it satisfies at which point it cuts the gas, and turns off the inducer, leaves the main blower fan on all by itself to cool down the heat exchanger.

In this regard no it would not be normal unless you have a very smart communicating thermostat and high end system to switch back to low stage after temp has been satisfied.

If it only runs on high we need to check some things. Is it even two stage? Do we have a jumper on the heat stages? Two stage thermostat acting up?

1