Should you set a thermostat differently for a two stage furnace?

I guess I should have asked the installers, but they just left a minute ago. I’m the proud owner of a 14.5 SEER two stage gas pack unit now, with a new programmable thermostat (not sure what was wrong with my old one, but whatever.) So. The way I’d been setting the thermostat, in the winter I let it get way down during the day and at night when we’re asleep, and then it kicks in for the evenings and to wake me up warm in the morning.

Is this still economical, now that the furnace won’t be going at full bore if it only has to maintain a temperature? If the furnace is using less fuel in its maintenance setting and has to use more to change the temperature significantly, am I wasting gas when I run it on the other setting?

Of course, other changes have happened - they enlarged my return so I should be getting about 25% more air volume coming out of the registers, plus I went from either a 9 or 10 SEER to 14.5. Would they affect the calculation?

If it’s strictly a furnace, that is, a heat generating unit, the setback will save energy. If it’s a heat pump, that complicates matters. A heat pump can enjoy efficiencies higher than 100% when operated as such, but if the thermostat setting is more than 2 degrees higher than the temperature of the room will kick on electric heaters that have efficiencies lower than that.

It’s a gas furnace (and electric air conditioning.) Not a heat pump.

Your thinking is entirely logical, but thats not really how things work.

Your furnace is an innocent bystander, so to speak. Its responding-----via your thermostat----to BTU losses from your home to the environment. So…your needs are your needs and the furnace has little to do with it. It simply responds to things it can’t control.

So whether the furnace needs to go “full bore” or not has more to do with the outside temperature (actually…both the inside temperature you set the stat at, and the outdoor temperature—and specifically the “delta” or difference between the two…But I digress…) and the quality of your walls, attic, windows and doors than the fact the unit is 2 stage or not.

In other words, when the temperature outside is mild (and you have fewer BTU losses…) it is possible that “first stage” is able to heat the home. As it gets colder, the BTU losses are greater than the “first stage” output. Ergo, the home [would otherwise] gets colder. But the thermostat seamlessly calls for “backup”----second stage heat. Now the “full bore” output of the furnace is as great-----and most often greater-----than the BTUS lost to the environment.

However-------that 2 stage furnace has only one heat exchanger. And, obviously, it has to be sized for the “full bore”; in other words, the full capacity of the furnace.

As a result, our testing has [consistently] found that these appliances are at their most efficient during full fire----second stage.

If economics are your primary concern I would jumper the 2 stages together and turn this unit into a single stage (full bore) unit.

Because the fact is, 2 stage units are a benefit to your comfort, not your wallet.

To answer more specifically…

Theres no such thing as “Now that the furnace…” unless you did the windows, attic insulation etc also. In other words, assuming you haven’t changed the thermostat settings you normally used previously, your heating needs haven’t changed one iota. See below for an expansion of this answer…

You will likely be using less fuel because it is a higher efficiency unit, not because it has 2 stages. (because in either stage it is likely much more efficient than your previous unit)

Heres an example: tonight, based on the setting on your thermostat, the quality of your “envelope”—walls windows etc…---- and the outside temperature, your house will lose 50,000 BTUS per hour to the environment.

If you had a 2 stage unit that had 50,000 BTUS on stage one, and a full 100,000 BTUS at full bore, or stage 2, it wouldn’t matter from a dollars and cents perspective, because at full bore (100K) the unit would run only half as long as first stage (50K).

Because in the end you’re simply “replacing” the 50,000 you’re losing each hour through the envelope.
**Now that’s not 100% true, because there are some efficiency losses from “start up”—which suggests that the first stage is better on mild nights because it will run longer and cycle less.

OTOH, the second stage is probably more efficient at full fire than first stage. In the end, I believe its a wash.

IME/IMO, “two stages” are good for your comfort principally. The efficiency of the unit (including SEER for HP and AC units) is good for your wallet.

In A/C yes, and if its a heat pump.

If its not a HP, the savings will only be in the summer. But those are big savings.

So…you should see big savings in gas for winter, and big electric savings in the summer.

Very good move on the return air.

I was shocked when he told me how much more I could have been getting just out of my older unit with the return air enlargement. Who knew? (Well, you. I didn’t.)

So, what you’re saying is, BTUs is BTUs, and it’s going to take a certain amount regardless?

Well sort of…

There are expensive BTUs and inexpensive BTUs.

Your envelope, for the most part, determines *how many you need.
The efficiency of the furnace determines how much you pay for each one of them.

Does that make sense?

In other words, the way you buy a cheaper BTU is by sending more of them into your home as opposed to up your chimney.

An 80% efficient furnace means that 80% of the BTUs go into your home, and 20% up the chimney.

A 95% efficient furnace means 95% of those BTUs end up in your home and only 5% “escape” up your chimney.

So technically you are using less BTUs because you are wasting less of them. Your BTU needs----a function of the weather and the quality of your envelope----haven’t changed. You’re sending the same amount of BTUs into the home as you did before.

But you’re sending less up the chimney.

If you want to close the loop and send less into your home, improve the windows, doors, insulation etc.

Another thought on set back.

The amount of heat (BTU’s) your home will loose is a function of the difference between the indoor temp and the out door temp.

Is the efficiency of a two-stage furnace the same whether it is running using one stage or two?

That makes sense. I’m working on the insulation - the house was built in 1928 and has original windows, which I’m replacing slowly. They’re incredibly expensive, though. I’m adding insulation to the attic next, also building a foam box thing to insulate the attic access. (Was going to work on that yesterday while I was stuck at home all day, but I didn’t want to be in the way of the installers in that hallway.) I was surprised that Lowe’s didn’t have those little light switch and power outlet insulating things - that was going to be my other “trapped in the house with the workmen” project, but I had to order them online. Might as well do the easy cheap stuff when you can, even if the heat’s running out the windows like a Kenyan marathoner, right?

As a practical, published matter, yes.

However, we have tested 2 stage units and have found that a 2 stage unit at full fire (2nd stage) is nominally more efficient than at 1st stage.

Remember that you can set the degree difference trigger on the thermostat for the different stages too.

Our first stage can be set for half a degree to 2 degrees trigger from the set temperature.
Our second stage can be set in a similar manner, but always a greater difference from the set temperature.

Set the 1st stage to .5 degree trigger and the 2nd stage to 2 degree trigger.
Set the 1st stage to 1 degree trigger and the 2nd stage to 2 degree trigger.
Set the 1st stage to 1 degree trigger and the 2nd stage to 3 degree trigger.

It a personal comfort choice.

Your DC continuous blower fan can be set for different speeds at different stages too, but that is on the furnace board not the thermostat.

What thermostats have these adjustments?

The one they installed with the furnace is a Braeburn model 5100.


I didn’t know any stats that do that.

I don’t… think my new one does, but I only skimmed the manual.

I don’t see 5100 on their site any longer, but 5300 has that feature. I didn’t know it wasn’t basically standard on two stage systems.

I install Honeywell, although White Rogers and others make a good stat. (and I have a lot of experience with all of them)

I am a big fan of Honeywell, and the stats I install don’t have this feature.

raindog is obviously more knowledgable about this stuff than I - but here’s a quick layman’s guide to temperature setbacks and how to achieve high efficiency. Check out, specifically, question #6 in the FAQ’s.

YMMV due to all the aspects previously mentioned - window quality, insulation levels, thermal mass inside the house, outside temperature, etc, etc. The link is a starting place, but each situation will require tweaking.