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Furnace capacity sizing based on observed run times
In another post I have mentioned we will be looking to replace a nat. gas furnace maybe next fall. It's been a cold winter in Northern IL. We had a couple of nights when it hit about 30 below. Our house it about 1,500 sq. feet and our furnace capacity is 60,000. I think our design temp is 4 degrees (zip 61071). I might try to figure out how to supply my run time chart from Ecobee to anyone interested if I can figure that our. Generally run times were about 35-40 minutes while well below zero degrees. Load calculations are a total foreign concept and not available through my preferred contractor. My wife and I are considering requesting a 40,000 btu replacement furnace. We keep our house at 68 degrees and don't do set backs. What say you?
Thanks! joe
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what efficiency is the 60k? what is "well below" zero degrees?
Nest is POO!!
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I would not put in anything that is below/less than design. Just because you want/like how that MAY run, the next owner might not.
If your preferred contractor does not do load calcs, find another contractor. If he does "seat of the pants" or "rule of thumb" on sizing equipment, what else is he doing it on?
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from an excerpt by Paul Jacob in Sun City, AZ
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Hi Mark,
Our current furnace is 97%. Would like new equipment near that efficiency also. Well, we had several days when it stayed below zero for the highs. An example the night it was -30 I think our daytime high was about -10 or so. Also, our attic insulation and is R50. And two bedrooms are above our attached garage we heat to 60 degrees with a " hot dawg". Two of the adjacent walls are then common to the living space also. Did I make sense?
Thank you,
Joe
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Originally Posted by
pacnw
I would not put in anything that is below/less than design. Just because you want/like how that MAY run, the next owner might not.
If your preferred contractor does not do load calcs, find another contractor. If he does "seat of the pants" or "rule of thumb" on sizing equipment, what else is he doing it on?
The next owner doesnt matter. They can change it how they see fit.
That said, your questions and mine should be answered.
Nest is POO!!
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Originally Posted by
joeselectric
Hi Mark,
Our current furnace is 97%. Would like new equipment near that efficiency also. Well, we had several days when it stayed below zero for the highs. An example the night it was -30 I think our daytime high was about -10 or so. Also, our attic insulation and is R50. And two bedrooms are above our attached garage we heat to 60 degrees with a " hot dawg". Two of the adjacent walls are then common to the living space also. Did I make sense?
Thank you,
Joe
Lennox and others make a 98% unit. If the run times are while at night you are only slightly oversized. Also, if the localized temperature averages over the past few years reflect these -10, -20, -30F temperatures on a regular basis then keeping the 60K may not be a bad idea. A 40k would very likely be too small based on this micro-climate data. Only a Manual J heat loss calc can tell you exactly what you need.
Micro-climate does NOT take into account the 4 degree design temperature. It would use one of the other negative temperatures.
Nest is POO!!
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Marc, thanks for taking the time. The temperatures we have the meteorologists termed "once in a generation" low temperatures. This will probably be our forever home. Thanks for sharing your thoughts.
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Originally Posted by
joeselectric
Marc, thanks for taking the time. The temperatures we have the meteorologists termed "once in a generation" low temperatures. This will probably be our forever home. Thanks for sharing your thoughts.
what were the past years' temperatures on average?
For example, my "area" usually uses 10 or 0 degrees. My area for the last few years has had long stretches of -10 to -20. I try to size the duct and system for -15. Oversized? Perhaps "by the book", but "by the comfort" Ive never had a complaint.
Nest is POO!!
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Marc,
I have no doubt you have had no complaints! I would have to dig into some climate data and do some research. If we would end up going the 40,000 route I would expect that we may lose a degree or two below the set point when those unusually low temps occur. Me thinks their would be some warm air circulating at all times without the stop and start.
Good thoughts, you're a thinker!
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Originally Posted by
joeselectric
Marc,
I have no doubt you have had no complaints! I would have to dig into some climate data and do some research. If we would end up going the 40,000 route I would expect that we may lose a degree or two below the set point when those unusually low temps occur. Me thinks their would be some warm air circulating at all times without the stop and start.
Good thoughts, you're a thinker!
If undersized, you'll lose a lot more than that. 20,000 btus is a lot of energy.
Nest is POO!!
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Thanks Marc. 40,000 not the way to go then.
Have a good night.
Joe
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Post Likes - 1 Likes, 0 Dislikes
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Like I said, only that calculation can tell you what you need at the given design temperature. You'll be able to dial it in easily with that.
Have a good night.
Nest is POO!!
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Duct leakage to unoccupied space will undermine a runtime engineering method of sizing.
If you're losing any significant amount of conditioned air which is ultimately replaced with outside air, that impacts the demand over a given period of time. Increase the runtime by lowering the capacity and you'll unknowingly increase the demand upon your equipment....which could put you in a bind.
People get hung up on equipment efficiency without thinking in terms of system efficiency. Duct leakage solutions can often lower your utility bills more than simply bumping your burner efficiency. A contractor who doesn't/can't/won't do a load calc probably doesn't know much about what impacts system efficiency and may not be the best outfit to guide you to the best decision. Good luck.
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Originally Posted by
btuhack
Duct leakage to unoccupied space will undermine a runtime engineering method of sizing.
If you're losing any significant amount of conditioned air which is ultimately replaced with outside air, that impacts the demand over a given period of time. Increase the runtime by lowering the capacity and you'll unknowingly increase the demand upon your equipment....which could put you in a bind.
People get hung up on equipment efficiency without thinking in terms of system efficiency. Duct leakage solutions can often lower your utility bills more than simply bumping your burner efficiency. A contractor who doesn't/can't/won't do a load calc probably doesn't know much about what impacts system efficiency and may not be the best outfit to guide you to the best decision. Good luck.
btuhack,
All of my ductwork is within the heated space. I absolutely positively believe in using the load calculation. Literally NONE of the contractors in my area that I have consulted perform load calculations. In absence of the load calculations I was attempting to use alternative data and seek professional consultation from the pros here that I respect. My "hangup" ( thanks) for the higher efficiency furnace was to have the most BTU output available for the below design temperature conditions.
Seems to be a fine line of tolerance for consumers that want to follow the "oversized equipment is not better" position in regards to hvac equipment.
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Some manufacturers make a modulating furnace, it is designed to match output to the needs of the house at the time. You would be able to keep the 60k and still not be over sized as the system will only run as needed. It may not be perfect but it lieu of a calc it may be your best option. Of course then the question becomes "is the duct sized correctly"?
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Originally Posted by
BNME8EZ
Some manufacturers make a modulating furnace, it is designed to match output to the needs of the house at the time. You would be able to keep the 60k and still not be over sized as the system will only run as needed. It may not be perfect but it lieu of a calc it may be your best option. Of course then the question becomes "is the duct sized correctly"?
Hello, indeed they do! I have one of the modulating furnaces right now. I did not want to "brand bash" that's why that was not mentioned. And it's has nearly the perfect btu output to match the load. You are absolutely correct. It's the most comfortable furnace I have experienced. However, my wife and I went back and looked at service calls for no heat over the past 9 years....( If we could get beenthere out her bet he could keep it going!) We are considering going to properly installed, properly sized and simple (probably a singe stage time proven) new furnace.
Thanks for your reply!
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I am a Trane dealer, I have close to a dozen in the field going back at least 10 years. 1 has been replaced due to a flood the replacement has been in 4 years no calls, 1 had a glitch in the thermostat that just needed reset, and a third had a dirty flame sensor.
How many problems have you had?
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Originally Posted by
BNME8EZ
I am a Trane dealer, I have close to a dozen in the field going back at least 10 years. 1 has been replaced due to a flood the replacement has been in 4 years no calls, 1 had a glitch in the thermostat that just needed reset, and a third had a dirty flame sensor.
How many problems have you had?
Well, we have had at least two no heat each winter. Sometimes three. I have tried all of the dealers that service my brand of modulating furnace. I have had updates done and both and after each of the updates the furnace had problems after the updates, ( condensate collector and pressure switches) no contractor has ever found an actual problem. When we have moved to other contractors I have had them do my fall maintenance and inspection... still same results. My wife is even proficient at reading the fault codes through the window and interpret them with the manual. I have no doubt that YOU and many others that opine here could diagnose my problem (s). What is not realized is that the contractors that give advice here are the exception to what most of us consumers have available to us. Including load calculations. I have no one near me on the professional locate map. So, we are leaning towards a simple furnace installed by a great local contractor and be done. I work 12 hour shifts and have plenty else to worry about.
Thanks Again!
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Sorry to hear that you have had so many problems, even more sorry to hear you don't have a person in your area that lives up to the standard. It saddens me that so many in my industry will go no further than the minimum in their profession. While I know installation is the key to success I also think some brands are a little better engineered/refined. The problem is that really doesn't matter if the installation is less than optimal.
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Originally Posted by
joeselectric
btuhack,
All of my ductwork is within the heated space. I absolutely positively believe in using the load calculation. Literally NONE of the contractors in my area that I have consulted perform load calculations. In absence of the load calculations I was attempting to use alternative data and seek professional consultation from the pros here that I respect. My "hangup" ( thanks) for the higher efficiency furnace was to have the most BTU output available for the below design temperature conditions.
Seems to be a fine line of tolerance for consumers that want to follow the "oversized equipment is not better" position in regards to hvac equipment.
A few values can get one to a Preliminary Load Calc:
1. Age of house: ___ / built 19_ _
2. Length, width & height ___ i.e. 50' x 30' x 8' = 1,500 sq feet - 12,000 cubic feet
3. # of floors
4. # of windows on Each side of the house: N, S, E, WEST
5. WINDOWS: Single or double pane, Tinted
6. ZIP _ 61071 Northern IL
____ 5'F / 90'F -- 6500 HDD / 800 CDD
Last edited by dan sw fl; 02-07-2019 at 05:11 PM.
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