Furnace Runtime

[rws2011]

Hi all,

I had a new 100k XR95 Trane furnace installed in November in our 2600 sf house and all in all I am quite satisfied with it. I also had an Ecobee thermostat installed and I was playing with the numbers it has given me. On Thursday morning from 2am to 10am our average outside temperature was -3.6f and my furnace ran for 3.8 hours during that 8 hour period. Set temperature was 67f from 2-6am and 69f from 6-10am. I was just looking for some ballpark feedback on this since this is probably close to the coldest temps we will experience this winter.

Thanks, Ron

[selfemployed]

Since this is a 2 stage furnace http://trane.com/Residential/Products/Furnaces/XV95-Furnaces
there is no way of knowing whether it ran on high or low heat during the 3.8 hrs. or how much gas it used, CFH. I would look at your gas bill for last season in the same month. Ours have the previous year usage and average temp. for that month/billing peroid. Sorry I can't be of more help.

[rws2011]

Actually the XR95 is a single stage furnace, the XV95 is multi-stage. I was just wondering if someone could tell me if this level of performance is typical or if there are other factors, either good or bad that are in play.

Thanks,
Ron

[air2spare]

longer run times (or even continuous) at the typical lowest temperature for your area would indicate a properly sized furnace. Short run cycles indicate an over-sized furnace and compromises efficiency

[garya505]

For a ballpark evaluation, it sounds like you're considerably oversized. However, if your circulation is OK and comfort is good, then it doesn't matter much.

[beenthere]

Sounds like its almost twice the size you need.

[rws2011]

Thanks for the replies,

Although this is probably about the coldest we will get this winter, it is about 20 degrees warmer than the coldest temps I remember in this area. So I suppose that should be factored into the equation. I don't really know how the remaining capacity would keep up if the temperature was -25f, but that has happened only once in my lifetime and I'm in my mid 50's so maybe it isn't that important.

-Ron

[BaldLoonie]

3.8 hours out of 8 is well oversized.

At a glance, 100K 95% for 2600 sq ft in a cold climate sounds fine but your house must be extra tight for the furnace to run that little around 0 out.

[rws2011]

Thanks Baldloonie,

That was what I was looking for, a back of the envelope estimate based on the square footage, the climate, the size of the furnace and the runtime. The house was built with a 90% 100k Tempstar 16 years ago but the combustion air came from within the house. The new furnace gets its combustion air from an outside intake duct. That is probably the difference. The company that replaced it asked if we were happy with the output of the old furnace and since we had no complaints about it, they replaced it with the same size. Comfortwise the new furnace is fine, but it took a little time experimenting with the t-stat controls to get it that way.

-Ron

[RandomNick]

...Although this is probably about the coldest we will get this winter, it is about 20 degrees warmer than the coldest temps I remember in this area. So I suppose that should be factored into the equation. I don't really know how the remaining capacity would keep up if the temperature was -25f, but that has happened only once in my lifetime and I'm in my mid 50's so maybe it isn't that important...


If you were curious, you could gather additional datapoints and extrapolate down to the colder temps.

One question that recently came to my mind is: if you knew exactly what you needed/wanted in terms of heating capacity to offset heat loss and maintain a constant indoor temp, how much additional capacity would you add for a) setback option/flexibility, and b) initial spare capacity to make up for lost efficiency during the lifetime of the unit. Some amount of over-sizing would seem appropriate.

[lynn comstock]

If you were curious, you could gather additional datapoints and extrapolate down to the colder temps.

One question that recently came to my mind is: if you knew exactly what you needed/wanted in terms of heating capacity to offset heat loss and maintain a constant indoor temp, how much additional capacity would you add for a) setback option/flexibility, and b) initial spare capacity to make up for lost efficiency during the lifetime of the unit. Some amount of over-sizing would seem appropriate.NONE
a. In the worst weather it is foolish to insist on setback.
b. Maintenance will preserve the efficiency of the unit. Dirt is the second greatest enemy of efficiency and greatest cause of equipment failures (after poor installation)

[beenthere]

If you were curious, you could gather additional datapoints and extrapolate down to the colder temps.

One question that recently came to my mind is: if you knew exactly what you needed/wanted in terms of heating capacity to offset heat loss and maintain a constant indoor temp, how much additional capacity would you add for a) setback option/flexibility,

ZERO, None, Zilch, Nada bit.


and b) initial spare capacity to make up for lost efficiency during the lifetime of the unit. Some amount of over-sizing would seem appropriate.

ZERO, None, Zilch, Nada bit.

How much capacity do you think a furnace will lose in 20 years? Why aren't you having it service regularly so it doesn't lose its efficiency? It cost less then the gas bill oversizing can give you.

Why would you want a furnace that recovers from set back in 10 minutes when its 40 outside?

[RandomNick]

NONE
a. In the worst weather it is foolish to insist on setback.


Customer insists on setback... says they want it 63F overnight and warmed up to 70F when they get up at 8am (I know someone like this, but FWIW I don't set back). Now what?



b. Maintenance will preserve the efficiency of the unit. Dirt is the second greatest enemy of efficiency and greatest cause of equipment failures (after poor installation)

Maintenance as in routine cleaning of whatever nature is typically done in the industry or are you including more involved procedures? Absolutely ZERO loss of efficiency over 15 or whatever years? Not even 1%?

Is there something special about newer higher efficiency furnaces which make them trivially easy to keep running at their factory efficiency rating? I ask because one guy, who I thought seemed pretty knowledgeable overall, inspected my old 80% furnace and said it was in very good shape for its age (I haven't owned it since day one, but I had summer & winter maintenance done every year). However, when I asked that guy what efficiency it might be running at he sort of mumbled out loud "how old is this again", looked at the tag, paused, and said "about 70%". Someone else offered 60%, although they weren't out to service it and thus didn't take a good look at the burners or anything. So I have those datapoints to suggest that even with proper cleaning a furnace can for whatever reason lose efficiency over the years. Just bad datapoints or a function of furnace or ?

I just noticed beenthere's reply... same answer. OK, so this is to both of you :)

[beenthere]

Were you asking that guy how efficient it is as in steady state efficiency, or as in AFUE.

He could be right, and its only 70% efficient as AFUE. Of course, that may be all it was when it was brand new also.

A dirty A/c coil will do more to lower its efficiency then the age of the furnace will.

[RandomNick]

Were you asking that guy how efficient it is as in steady state efficiency, or as in AFUE.

He could be right, and its only 70% efficient as AFUE. Of course, that may be all it was when it was brand new also.


Ah, I see what you are saying. The question about efficiency was part of a larger question along the lines of: this is a 40000 BTU/HR input, 80% efficient furnace with an output capacity of 32000 BTU/HR. But it is 20+ years old. Does output BTU/HR drop off over time? How efficient do you think it is now?". In other words, I asked what may have been a poorly worded question about (not qualified) efficiency hoping to arrive at a guesstimate of the furnace's steady state output now. I think with at least one of those guys I went on to speak the math as in "ok so 70% of 40000 BTU/HR input... its putting out about 28000 BTU/HR?" and got a confirmation nod. In any case, that is how the "steady state output drops off over time" idea got solidified in my mind. Thank you for drawing my attention to the potential for miscommunication regarding efficiency.



A dirty A/c coil will do more to lower its efficiency...

As in optimal heat transfer requires optimal airflow and the later can be greatly impeded by a dirty coil? By extension, also a dirty filter?


then the age of the furnace will.

What contributes to age based lower efficiency in the furnace cabinet? Dirty fan blades? Dirt on the outside of the heat exchanger? Soot or rust on the inside? Residue buildup somewhere within the burner assembly? All of the above or some or none?

[rws2011]

If you were curious, you could gather additional datapoints and extrapolate down to the colder temps.

One question that recently came to my mind is: if you knew exactly what you needed/wanted in terms of heating capacity to offset heat loss and maintain a constant indoor temp, how much additional capacity would you add for a) setback option/flexibility, and b) initial spare capacity to make up for lost efficiency during the lifetime of the unit. Some amount of over-sizing would seem appropriate.

I was going to do that, the Ecobee makes it pretty easy to pick out data and see at what outside temperature the furnace should be running at 100%. I don't have time today but when I do I will post the result. I suppose I would rather have the furnace a little oversized than undersized. Actually from an efficiency standpoint my gas bill last month was significantly lower than my electric bill so I'm not sure longer run times at a lower rate of gas usage would save me much.

-Ron

[Rewind]

3.8 hours out of 8 is well oversized.

At a glance, 100K 95% for 2600 sq ft in a cold climate sounds fine but your house must be extra tight for the furnace to run that little around 0 out.

Got to be cautious when looking at runtimes at different OATs. The wind speed and direction along with the structure condition makes a lot of variables.

[RandomNick]

I was going to do that, the Ecobee makes it pretty easy to pick out data and see at what outside temperature the furnace should be running at 100%. I don't have time today but when I do I will post the result.

You HVAC geek! Just kidding :) FWIW, while I was logging usage this past winter I encountered one obvious thing. Once the sun comes up and starts warming your house it will quickly throw off your attempts to correlate usage with outdoor temperature. I also saw two curious things. Even on nights when the 12am to 6am outdoor conditions were stable (temp, etc), my usage would slowly ramp up and level off around 3 am or so. As if it took the structure that long to reach an equilibrium with the night time conditions. However, on several occasions after it had stabilized I saw what appeared to be a cycle starting early (which would lead to increased usage). As soon as a cycled started I pulled outdoor conditions from the wunderground weather station up the street. Each time I saw that early cycle, the temperature had just dropped by two degrees. So on one hand it seemed like it took the structure a while to stabilize to outdoor conditions, but on the other hand it seemed like the structure rapidly responded to small temperature changes. This is probably more than anyone wants to know, but I thought I'd be a geek for a second :)

[beenthere]

What contributes to age based lower efficiency in the furnace cabinet? Dirty fan blades? Dirt on the outside of the heat exchanger? Soot or rust on the inside? Residue buildup somewhere within the burner assembly? All of the above or some or none?

Part wear. Dirt on blades, in burners, etc.

Low air flow is the bane of any forced air heating system. Proper maintainence keeps parts clean and more efficient.

I've seen more then one tech agree with a customer that an old unit now has less efficiency then when it was new. Just to make a sale. Or to agree with the customer so as not to have to take time to explain why its still as efficient as it was 25 years ago.

[lynn comstock]

What contributes to age based lower efficiency in the furnace cabinet? Dirty fan blades? Dirt on the outside of the heat exchanger? Yes and yes unless maintenance is done, then no.

Soot or rust on the inside? Residue buildup somewhere within the burner assembly?Not a normal problem. Proper installation and maintenance should prevent this as well.

[rws2011]

You HVAC geek! Just kidding :) FWIW, while I was logging usage this past winter I encountered one obvious thing. Once the sun comes up and starts warming your house it will quickly throw off your attempts to correlate usage with outdoor temperature. I also saw two curious things. Even on nights when the 12am to 6am outdoor conditions were stable (temp, etc), my usage would slowly ramp up and level off around 3 am or so. As if it took the structure that long to reach an equilibrium with the night time conditions. However, on several occasions after it had stabilized I saw what appeared to be a cycle starting early (which would lead to increased usage). As soon as a cycled started I pulled outdoor conditions from the wunderground wee, the temperature had just dropped by two degrees. So on one hand it seemed like it took the structure a while to stabilize to outdoor conditions, but on the other hand it seemed like the structure rapidly responded to small temperature changes. This is probably more than anyone wants to know, but I thought I'd be a geek for a second ather station up the street. Each time I saw that early cycl:)

I plotted 6 points of varying temperatures and came up with a definite curve to fit the data. It is not linear. I took the same 8 hour period between 2am and 10am and summed the runtimes and averaged the outside temperatures. The outside temperatures varied from 40.5f to -3.61f. Of course this didn't account for wind, I suppose if I worked out enough points then the wind should average out but I'm not motivated enough to do that. The interesting part of the curve which is the part below 0f is really lacking enough data points so I just tried to fit the curve by eye. I know, not really scientific. Anyway the area around -20f +- 10 degrees seems to fit the curve pretty well for a maximum runtime of 100% and that fits the lowest temperatures in the area as well. So I would say that the furnace looks to be correctly sized for my house.

-Ron

[RandomNick]

I'm trying to understand how you approached this. Did you calculate this for multiple days:

Sample Period 2am to 10am = 480 mins
Ave outdoor air temp during that period = AOAT
Total furnace runtime minutes during period = TFRM
TFRM/480 mins = RuntimePct

Then plot RuntimePct at different AOATs, visually projecting that down to lower temperatures?

[rws2011]

I'm trying to understand how you approached this. Did you calculate this for multiple days:

Sample Period 2am to 10am = 480 mins
Ave outdoor air temp during that period = AOAT
Total furnace runtime minutes during period = TFRM
TFRM/480 mins = RuntimePct

Then plot RuntimePct at different AOATs, visually projecting that down to lower temperatures?

Yes, that is basically what I did. The Ecobee records temperature and runtime every 5 minutes so that is what I used. I used the same time period from 6 different days and my graph plots temperature over RuntimePct. I suppose if I were and engineer I could try to arrive at a mathematical function to predict the bottom of the graph. If my memory serves it appears to be a hyperbolic function of some sort but I'm really getting in over my head here and there probably is no good substitute for some additional data points. Attached is my graph.

-Ron

[KCA]

I am an engineer (nuclear). :)

Anyway, the function should be linear as the heat loss is directly proportional to the temperature difference between the inside and outside. If you were to gather more data, I think that you would find that some of these points are outliers.

The other part of the problem is that you really would need the actual time the burner as firing or energy released. If the furnace is only firing for short durations, there will be a larger % of time that the furnace is "running", but might not be putting out the full BTUs (think about the heat up and cool down of the heat exchanger). This might be throwing off your data points during mild weather where the furnace doesn't have to work very hard to maintain the temperature.

Yet another factor is wind. As the wind blows, the convective heat transfer helps to pull the heat away from your house and will cause it to cool off faster. In addition, the wind can cause a strong draft in fireplaces, WH vent, furnace vent, bath/kitchen vents, etc. and pull heated air out of the house. The bath/kitchen vents usually have dampers, but sometimes they stick open.

It does look like you are about 2x the required size. At around 0F, you are averaging something a bit under 50k BTU. How much under depends on how long it is running per cycle (heat up/ cooldown). Let's say that your numbers are accurate. Taking some rough averages...

At around 37F, you have a duty cycle of ~31%

At around 0 F, you have a duty cycle of ~48%

Based on these data points, we can do a linear fit:

y = -0.459459x + 48

To get 100% usage, you would need an outside temperature of ~ -113 F. Brrrr... :)

[rws2011]

I am an engineer (nuclear). :)

Anyway, the function should be linear as the heat loss is directly proportional to the temperature difference between the inside and outside. If you were to gather more data, I think that you would find that some of these points are outliers.

The other part of the problem is that you really would need the actual time the burner as firing or energy released. If the furnace is only firing for short durations, there will be a larger % of time that the furnace is "running", but might not be putting out the full BTUs (think about the heat up and cool down of the heat exchanger). This might be throwing off your data points during mild weather where the furnace doesn't have to work very hard to maintain the temperature.

Yet another factor is wind. As the wind blows, the convective heat transfer helps to pull the heat away from your house and will cause it to cool off faster. In addition, the wind can cause a strong draft in fireplaces, WH vent, furnace vent, bath/kitchen vents, etc. and pull heated air out of the house. The bath/kitchen vents usually have dampers, but sometimes they stick open.

It does look like you are about 2x the required size. At around 0F, you are averaging something a bit under 50k BTU. How much under depends on how long it is running per cycle (heat up/ cooldown). Let's say that your numbers are accurate. Taking some rough averages...

At around 37F, you have a duty cycle of ~31%

At around 0 F, you have a duty cycle of ~48%

Based on these data points, we can do a linear fit:

y = -0.459459x + 48

To get 100% usage, you would need an outside temperature of ~ -113 F. Brrrr... :)

Thanks KCA for the reply,

I thought about your post and logically it seemed that the relationship should be linear however the curve fit so well it seemed like there was something non-linear going on. I chose the time period because that is generally the calmest time of the day from a wind standpoint and over an 8 hour time period it should somewhat average out anyway and I believe the runtime that is reported by the Ecobee is the actual time the burner is firing. However your point about air infiltration made me think. I have a 6" duct that brings fresh outside air into the cold air return on the furnace. We had it installed as part of a passive radon mitigation system shortly after we built the house. That would certainly be a non-linear factor since the more the furnace runs, the more cold air it will bring in, etc. I would think that that would be a non-linear factor in any house that does not have an outside intake duct for combustion air as well.

Hopefully I will never have to test the furnace capacity at -113f :)

-Ron

[nchvac]

"The company that replaced it asked if we were happy with the output of the old furnace and since we had no complaints about it, they replaced it with the same size. Comfortwise the new furnace is fine, but it took a little time experimenting with the t-stat controls to get it that way".

-Ron

Classic! Same size, different efficency, no load calculation?
Sounds about right. Shame on him for installing the unit without doing a calc and shame on us for discussing the units sizing without doing the same. You could live in a glass house built on posts for all we know at this point.

I bet if you post your house measurments, insulation values, construction info like what the walls are made of, carpet, type of windows and all of these good things that someone on this post will run a calc for you. Any idea what the design temp is where you are? Picking out a number really doesn't cut it with load calcs. If not, someone here will find it in manual J, just tell them where you are in the state.

Have you hung around the furnace while it is running to make sure it isn't short cycling. Just because EcoBee said the furnace was running doesn't mean it was, it was just sending a signal to it, unless these units are way more advanced than the commercial control units most outfits sell and install. You may be going out on a limit if you don't have enough airflow on an oversized unit. You may want to monitor it in person for a cycle or two.

You must have this unit serviced regularly by a competent tech or you will have trouble way sooner than you would have on the older unit. And keep the filter changed regularly, and don't use a 1" pleat filter. The unit isn't an air purifier, so don't try to make it one. That's should be enough free info.