[imbored66]

Hello,

I had a Carrier Performance 96 (2 stage, 60,000 btu) installed in August, and now that it's got a couple months of winter under it's belt, i'm wondering if it maybe wasn't the right choice for my home.

I've noticed that the only thing it uses 2nd stage for, is when the thermostat is programmed to increase the temperature (in the morning for example). It's never called on 2nd stage for maintaining the set temperature...even last week when it hit sub -35C temps outside.

Does this kind of behavior indicate an oversized furnace? The installer didn't do a heat calculation of any kind... none of the 6 companies I got quotes from did.. however they all independently came to 60k as the size.

 

[JonesHVAC-R]

Hello,

I had a Carrier Performance 96 (2 stage, 60,000 btu) installed in August, and now that it's got a couple months of winter under it's belt, i'm wondering if it maybe wasn't the right choice for my home.

I've noticed that the only thing it uses 2nd stage for, is when the thermostat is programmed to increase the temperature (in the morning for example). It's never called on 2nd stage for maintaining the set temperature...even last week when it hit sub -35C temps outside.

Does this kind of behavior indicate an oversized furnace? The installer didn't do a heat calculation of any kind... none of the 6 companies I got quotes from did.. however they all independently came to 60k as the size.

You only need 2nd stage when in high demand. This is why you only notice it kicking in when coming back up to temp after the night program setting. The whole reason for two stage is to save you money... why use resources you don't need? Kind of like throwing money in the trash.

Also, a slower heat is more efficient than a faster heat.

 

[dunkman]

Sounds normal to me.

I actually replaced my furnace last fall too. My house was built in 1997, 1240 sq ft, + another 1240 basement below grade rambler style. Minnesota, design temp about -20 F. Original unit was a 80% 100,000 btu single stage. I replaced with a Trane XV95, 2 stage variable 60,000 btu nat gas.

Bottom line is the other morning it was -15F and I never noticed it going to second stage yet, just runs a steady 1st stage when it's cold like that. Just like yours it only hits 2nd stage when coming out of setback, or if I bump up the temp a couple degrees.

In my professional opinion, that's exactly what you want it to do.

 

[motoguy128]

FIrst, was a load calculation done? Can you give use some infor on the home (age of construction, # of levels and size?)

Funny... I did a search for " HVAC design tmeprature saskatoon " and it came up with a powerpoint presentation talking abotu furnace sizing in that region. You desing temp is 35C. So if sized perfectly, high stage would be running almost constantly at that temprature on an overcast day or at night.

Evne if you heat loss was lets say 45k BTU based on a laod cal, a 45k BTU furnace would have been a little small. But even then, the low stage of your furnace is around 37k BTU's. That tells me that a smalelr 45k BTU furnace would have been adequate. But I suspect the contractor sized for a lower design temp, added hi own safety factor, ot just replaced what was already there. What was already there might have been a lower effciency furnace too. So the new furnace is putting out more heat.

THe good news is that you're gas bills probably aren't that high sinec your heat loss si aparently only aroudn 35k BTU's, but you're not getting the full benefit of a 2 stage system, which is longer run times and comfort and you paid a little extra far a larger furnace and a little more in electricity to operate a larger blower on the same size ductwork.

Let see how I'm doing on my reverse gas bill estiamtion. If you average montly temp in Dec. was 20F, then based on hte run times you told me at design temperature, if gas costs you $0.80/therm (100kBTU's) then you gas bill should be around $110-120 for heating if you use setbacks and the average indoor daily temp is 65F. If you actually needed a 60k BTU furnace, your bills would be closer to $200 at that average monthy temp.

Yo can take average daily indoor and outdoor temp, estiamted heat loss at design temp and you gas rate, and calculate you monthly bill. You cna also take you montly bill and work backwards into a furnace size.

 

[skippedover]

I do not consider the operation of your new furnace to be normal at the temperature you indicated. Your -35°C converts to -31°F. I don't know how cold you expect it to get in your region but if that's a normal low, then your furnace is definitely oversized. Below is how it should work.

The thing to understand is that there are actually 2 design temperatures for a 2-stage furnace. The higher design temperature is that at which 1st stage is running constantly and just holding the indoor set-point. When it gets any colder outdoors, then second stage begins to bump on/off/on/off with the on cycles becoming longer and longer. When second stage is also on constantly, you've reached the actual maximum output of the furnace. Glancing at the OAT thermometer at that point would tell you what the actual furnace capability is and it should be somewhere in the general vicinity of the actual design temperature.

So let's for example, assume your home needs 2,235 Btu's when it's 15°C outdoors. Let's further assume that your home needs an additional 3,450 Btu's per 2°C decrease in outdoor temperature. If the furnace size (we'll forget efficiency deductions for this example) is 40,000 Btu's and is 2-stage @ 60% for 1st stage, then your furnace can deliver 24,000 Btus using first stage. Based on the needs of your home, then doing the math reveals that your first stage of heat should be on constantly at roughly +2.4°C, when your home requires about 24,000 Btu's. Any OAT below +2.4C requires additional Btu's, which are provided by 2nd stage heating. As the OAT drops to 8, 7, 6 and below, the 2nd stage heat will continue. And in my example, at -35°C, your indoor temperature would be mighty cold due to the under sized furnace selected. If the Btu design needs of your home were actually what I used as an example, then you'd need in excess of 88,000 Btu's at -35°C.

As you can see, the mere fact that the only time your furnace kicks to 2nd stage means that the OAT has yet to sink to a temperature low enough to prevent 1st stage from satisfying the t-stat. So...unless your design temperature is on the order of -50°C, I think your furnace is over sized.

 

[JonesHVAC-R]

Lets back up a minute here....

1.) Your furnace is maintaining temp @ 1st Stage
2.) Your furnace uses 2nd Stage when it needs to bring temp up more than +4 degrees
3.) The whole design purpose of a 2-Stage furnace is to save money by maintaining temp with less than 100% resources.
4.) Your furnace is doing exactly what it is designed to do

It is quite obvious to me that you are sized just fine and here is why in layman terms...

1.) If undersized 2nd Stage would run predominately
2.) If over sized 2nd Stage would almost never run as the 1st stage would take care of your demand
3.) By using Motoguys $.80/therm (100k BTU)... would you prefer to maintain temp @ 2nd Stage (60,000 BTU) being $.48/therm or would you prefer to pay $.29/therm for using only 1st stage (38,000 BTU) to maintain?


There is a lot more to sizing a furnace than just outside air temp and square footage of your home. When completing a heat loss/gain some of the variables taken into consideration are insulation, windows, doors, wall thickness, amount of exterior wall, etc. Then you need to ensure your ductwork is adequate and calculate CFM to each supply and return.... These are just some variables. I could list a lot of technical equations but to answer your question easily and effectively what I have above will suffice.

 

[beenthere]

Lets back up a minute here....

1.) Your furnace is maintaining temp @ 1st Stage
2.) Your furnace uses 2nd Stage when it needs to bring temp up more than +4 degrees
3.) The whole design purpose of a 2-Stage furnace is to save money by maintaining temp with less than 100% resources.

2 stage is for comfort. Not for savings because its running in first stage most of the time. Any savings generally comes from the customer setting his thermostat lower because of the longer run times in first stage. First stage is often less efficient then second stage.

4.) Your furnace is doing exactly what it is designed to do

It is quite obvious to me that you are sized just fine and here is why in layman terms...

1.) If undersized 2nd Stage would run predominately
2.) If over sized 2nd Stage would almost never run as the 1st stage would take care of your demand
3.) By using Motoguys $.80/therm (100k BTU)... would you prefer to maintain temp @ 2nd Stage (60,000 BTU) being $.48/therm or would you prefer to pay $.29/therm for using only 1st stage (38,000 BTU) to maintain?


There is a lot more to sizing a furnace than just outside air temp and square footage of your home. When completing a heat loss/gain some of the variables taken into consideration are insulation, windows, doors, wall thickness, amount of exterior wall, etc. Then you need to ensure your ductwork is adequate and calculate CFM to each supply and return.... These are just some variables. I could list a lot of technical equations but to answer your question easily and effectively what I have above will suffice.

OP. A single stage 40,000 BTU 96% efficient furnace would have a high BTU output then your 60,000 BTU 2 stage does in first stage. If you want 2 stage, your contractor may have gone with the 60, cause the 40 doesn't come in 2 stage.

 

[JonesHVAC-R]

2 stage is for comfort. Not for savings because its running in first stage most of the time. Any savings generally comes from the customer setting his thermostat lower because of the longer run times in first stage. First stage is often less efficient then second stage.

I agree 1st stage deals with comfort as well but it was not the #1 reason for design, it is just a bonus. Also, I disagree with your last comment... "First stage is often less efficiant then second stage." Here is why I, and many others, believe a slower heat is more efficient than a faster heat. Example only, If you bring a room temp up 5 degrees in 5 min (stage2) vs. 7 min (stage1) = furnace will fire a second time after shut down (stage2) because you heated the air too fast. Now you have just ran full resources twice and for a longer amount of time vs. running once for partial resources. That is another reason for 1st stage being used to maintain.

 

[skippedover]

I agree 1st stage deals with comfort as well but it was not the #1 reason for design, it is just a bonus. Also, I disagree with your last comment... "First stage is often less efficiant then second stage." Here is why I, and many others, believe a slower heat is more efficient than a faster heat. Example only, If you bring a room temp up 5 degrees in 5 min (stage2) vs. 7 min (stage1) = furnace will fire a second time after shut down (stage2) because you heated the air too fast. Now you have just ran full resources twice and for a longer amount of time vs. running once for partial resources. That is another reason for 1st stage being used to maintain.

A furnace or boiler running on less than maximum output often has an energy efficiency substantially lower than at high fire. Do some efficiency tests at both settings and you'll begin to understand. That said and put aside for a minute, the house has a Btu loss. The furnace replaces those losses with new Btu's. The needs of the home do not change if the furnace delivers the heat in 10 minutes of run time every hour or 60 minutes of run time every hour. The Btu's escaping from the home are the same, thus the length of run time is to replace only those Btu's lost. A single stage furnace running at rated input/output will cycle because it's replacing the lost Btu's with a much larger capacity and so the Btu's needed are limited by the T-stat. Sure, there's a slight loss of efficiency for each start-up and shut-down but you'll be hard pressed to see any difference in the gas bill based on cycling times.

The purpose of the multi-staged and now modulating furnaces is to deliver a more constant heat than is possible with a single stage furnace that's always oversized until the OAT gets to design temperature. That said, the output of the furnace at design temperature should be based on highest input, not lowest. Therefore, a 2-stage furnace that has an input of 80,000 Btu per hour should be selected for a home that needs 74,000 Btu's/hr. at DESIGN temperature. That 80,000 INPUT is the maximum and what should be going into the burner/furnace at design temperature. When the OAT is substantially above design, then the INPUT to the furnace will be the same, 80,000 Btu's until the controller (t-stat or user interface) determines that 1st stage will do the job absent any second stage input. In other words, the furnace is derated by staging down to some lower input in Btu's per hour.

In the OP's case, the furnace runs constantly on 1st stage. That tells us that the 1st stage input is sufficient to heat the house at the coldest (design) OAT and that the furnace has a higher maximum Btu/hr input that is not being used. So it's acting like a single stage furnace. Cycling on 1st stage only and never needing 2nd stage. I don't know how you do load calcs or furnace selection but if you're sizing furnaces based on design temp and lowest input, you're over sizing. And beenthere is correct. Staging and modulation of furnaces is for comfort. AC and HP is a different story. The compressor actually uses less electricity to operate on 1st stage over 2nd stage and the new modulating units are even more energy efficient. So there's a double gain for AC and HP. Both the comfort AND the efficiency improve. Not so with furnaces.

 

[beenthere]

I agree 1st stage deals with comfort as well but it was not the #1 reason for design, it is just a bonus. Also, I disagree with your last comment... "First stage is often less efficiant then second stage." Here is why I, and many others, believe a slower heat is more efficient than a faster heat. Example only, If you bring a room temp up 5 degrees in 5 min (stage2) vs. 7 min (stage1) = furnace will fire a second time after shut down (stage2) because you heated the air too fast. Now you have just ran full resources twice and for a longer amount of time vs. running once for partial resources. That is another reason for 1st stage being used to maintain.

Do some combustion testing on 2 stage units. And you'll see what I'm talking about.

 

[JonesHVAC-R]

LOL, I see what you two are getting at... maybe I should clarify what I meant as far as "efficient," or maybe I should just put the word COST in front of it.

Yes, a 95% efficiency furnace whether 2-stage or 1-stage is still 95%. But it is definitely more "COST EFFICIENT" (gas) to heat the degree or two (maintain) with less resources than it is full resources as you are using 40% less gas.

Did that help?

skippedover, This particular case the HO stated they only see 2nd stage in the morning when bringing back up to daytime temp. Also, I size my furnaces on output BTU and not input. I also do heat gain/loss calcs as well as ensure proper duct design. I instruct my guys to use a velocitor on all installs when adjusting dampers as well to each supply to ensure proper CFM is to each.

 

[beenthere]

LOL, I see what you two are getting at... maybe I should clarify what I meant as far as "efficient," or maybe I should just put the word COST in front of it.

Yes, a 95% efficiency furnace whether 2-stage or 1-stage is still 95%. But it is definitely more "COST EFFICIENT" (gas) to heat the degree or two (maintain) with less resources than it is full resources as you are using 40% less gas.

Did that help?

.

Doesn't use 40% less gas to heat to the same temp. Simply runs that much longer. And uses a bit more gas.

If the house is losing 20,000 BTUs an hour. A single stage 60,000 BTU input 95% furnace and a 60,000 BTU input 2 stage 95% furnace, both have to put the same amount of BTUs into the home. The 2 stage running in first stage simply runs that much longer to do it.

If the single stage had to run 21 minutes. Then the 2 stage in first stage would have to run a min of 32.5 minutes(based on first stage being 65% of second stage). And with a lower combustion efficiency, it would actually run longer yet, using more gas then the single stage.

The savings comes from most people setting their thermostat lower because the longer run time gives them a more constant feeling of warmth, and tends to help them not feel a cool draft.

 

[JonesHVAC-R]

Doesn't use 40% less gas to heat to the same temp. Simply runs that much longer. And uses a bit more gas.

If the house is losing 20,000 BTUs an hour. A single stage 60,000 BTU input 95% furnace and a 60,000 BTU input 2 stage 95% furnace, both have to put the same amount of BTUs into the home. The 2 stage running in first stage simply runs that much longer to do it.

If the single stage had to run 21 minutes. Then the 2 stage in first stage would have to run a min of 32.5 minutes(based on first stage being 65% of second stage). And with a lower combustion efficiency, it would actually run longer yet, using more gas then the single stage.

The savings comes from most people setting their thermostat lower because the longer run time gives them a more constant feeling of warmth, and tends to help them not feel a cool draft.

I don't agree. Heating the room faster using 100% resources doesn't mean you have changed the ambient temp. You have to take into consideration the difference of room temp (air temp) vs. ambient temp (surroundings temp). A slower heat will bring the ambient temp up as well. That single stage will fire a second time because you will drop a degree due to the ambient temp vs. the slower heat of the 2stage in stage 1 bringing up the ambient temp as well. Some techs believe ambient temp isn't that big a deal though and that it makes no difference... it is my opinion that ambient temp is important though.

 

[beenthere]

Since were talking about maintaining the temp. The surroundings temp is not effected like you think(guessing you mean furniture and other room objects).

If we were talking about recovering from a set back, then yes, a single stage would come on a second time. Of course, recovering from a set back, the 2 stage would go into second stage to recover until close to set temp, then drop back to first stage, and still just have a long run time at a lower efficiency.

 

[JonesHVAC-R]

So based on your comments I take it you believe that a single stage operates more efficiently than a 2 stage?

 

[beenthere]

So based on your comments I take it you believe that a single stage operates more efficiently than a 2 stage?

Yes, it does.

Do some combustion testing on single and 2 stage equipment. See how the 2 stage compare when they are in first stage.

Don't confuse comfort, with efficiency.

 

[mayguy]

When we get below zero, my 2nd stage cycles on and off, and 1st stage runs steady almost non stop.

I think you furnace may be over sized if it only cycling on and off on 1st stage with that temp you are posting.