What are you basing your calculations on? Are you not a TV repairman by trade?Originally Posted by 54regcab
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What are you basing your calculations on? Are you not a TV repairman by trade?
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Using the 35f temperature the OP stated in the original post and square footage. Of course a load calculation would yield more accurate results. I've seen way too many oversized furnaces, many of them cycling on high limit switch due to low airflow/poor ductwork/oversizing.
Got out of the TV business back when the prices of new TV's dropped to a point where there was no money to be made repairing them. Glad switched to HVAC, TV prices have just continued to plummet ever since I got out of the business. Saw an LG 60" flat screen for $800 recently, 42" TV's under $400.
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On the data plate it says "Bonnet Capacity=98000". So, based on what you said above, it would seem that this furnace is an 80% unit, with 98,000 BTU going into the house?
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Not really. Older furnaces, ones that do not use induced draft blowers, are rated differently. Bonnet capacity ratings are not actual efficiency ratings, but is the maximum amount of heat that furnace bonnet can handle. Most furnaces just prior to induced draft furnaces were in the 65-70 percent efficiency rating.
I just read over your original post for specifics about your area. The usual temperature range for the Tacoma, MD (District of Columbia area) is to provide 70 degrees indoor temperature when the outdoor ambient temps are at 17 degrees. Once again, your furnace may be fine for your area, but if replaced, should be of a lot less Btu input value.
In your area, if your home is comfortable with a 3.5 ton (42,000 Btu) cooling system, it will kept warm with less than double that amount of heat (84,000 Btu), which it is possible what your existing furnace is producing. If you currently have a 3.5 ton cooling system, I would recommend a 90k condensing furnace as a nice replacement. If your cooling system is a 3 ton, and that amount of cooling is keeping your house comfy, go with a 70k condensing furnace (it indicates your house is tightly built and decenlty insulated.)
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John01 understand that steady state efficiency and AFUE are 2 different things. Your furnace could very well be 80% while running, but cycling losses and pilot light (if your uses one) reduce AFUE to 65-70%. Newer furnaces have much lower cycling losses and no pilot lights therefore the 80% Input/Output ratio is closer to AFUE.
The 123k input rating was when the furnace was new, it may or may not be that now. Ideally it should be the same now as when it was installed, but gas burner orifices can partially clog, and the gas valve can loose it's calibration. You can "clock your gas meter" and see how much gas it's actually burning.
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Not Tacoma, Maryland, but Tacoma, Washington...
Yes, it does have a pilot light. And, I don't have any cooling attached. This is just a furnace.
Now, I just did a seat of the pants check of furnace cycling, because I don't have a data logger to capture the data. I did not clock the meter, as suggested by 54regcab. Didn't think to do it.
The current outside temperature is 39 f.
The thermostat set temp is 71 f.
When it drops to 70, the furnace turns on. It runs for fourteen minutes, and raises the temperature to 72 f.
It takes an hour and 25 minutes for the inside house temp to drop from 72 to 70, and then the furnace turns on again.
Is that helpful information?
Last edited by John01; 12-25-2012 at 10:09 PM. Reason: clarity
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For your area...Tacoma, WA (which I was not aware existed), it sounds like your current furnace is properly sized, assuming that the outdoor temps do get colder than it is currently. I know that Washington state is not as cold as it is in the Northeast, but I am assuming the temps do drop below freezing that far north. Are my assumptions correct?
I don't know what clocking the meter would tell you other than how much gas you are using per hour, which is strictly input data that has no relevance to heating the house.
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This information is innaccurate. Bonnet temperature has nothing to do with cycle rates and cycle rates have nothing to do with efficiencies.
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I ran into a furnace last week and took a snapshot of the label 100K input, 80k bonnet capacity. The bonnet can handle 80k but has a 100K firing rate? What technically is the "bonnet" of a furnace anyways? I've only seen this term used on the really old stuff.
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It's the way furnace manufacturer's used to rate furnaces, but is not an actual efficiency rating. Ratings were done with an 85% temperature rise and stack temperatures were allowed to be tested at 550 degrees. All furnace inputs were given a nominal output bonnet rating of 80%, based on the high temperature differentuals and the high flue gas temperatures. Bonnet capacities had nothing to do with efficiency ratings as they do in the methods of testing furnaces today.
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RoBoTeq, you live closer to the OP than I do, interesting theory on sizing furnaces based how much AC it takes to make a house comfortable. Here in Oklahoma we're closer to a 1:1 ratio. Normally the blower size dictates the minimum furnace required, which usually exceeds the 1:1 ratio. Some contractors take it one step further and use the high BTU for a given size blower because that's what the supply house has in stock. The result is a furnace that rides the high limit because the contractor doesn't upsize the ductwork when they install the oversized furnace. Unless the customer has a system powered thermostat that goes blank when the system hits high limit the customer have no idea the furnace is cycling on high limit unless they happen to be watching the burners. As long as the house is warm and warm air is coming out of the vents you will rarely get a call. Since the furnace is typically 2X the size needed the house will stay warm even when the burners are shut off 1/2 the time on high limit.
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Blower size has absolutely nothing to do with sizing HVAC systems for cooling or heating. Btu values in heat gain and heat loss are the only things that matter. In my area, it just so happens that if a certain amount of cooling will cool a house, just less than twice that amount of energy in heat will heat the same sized house. That is assuming there are no oddities such as extremely high ceilings throughout the house, massive amounts of windows or an indoor swimming pool or hot tub in the house.
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In our area the amount of cooling required dictates the minimum blower size needed. For Oklahoma any furnace that has enough blower capacity for cooling will only come in a size that will provide sufficient heating. To get a 3 ton blower means buying a minimum 50K furnace, 4 ton 75k, 5 tons 100K, etc.
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Equipment sizing has nothing to do with geographical areas and no where is the amount of heat needed for a home in any way dictated by blower size. Furnaces are designed with blowers capable of handling the capacity of the furnace, regardless of cooling needs.
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True, the heat needed isn't dictated by blower size. The installed furnace tends to be oversized in our area because a furnace with enough blower capacity for the air conditioner is only offered with BTU that exceeds the amount of heat loss in the winter. A structure may have a heat gain of 48,000BTU in the summer and a loss of 40,000 BTU in the winter. However finding a furnace with a blower capable of 1,600 CFM with less than 60,000 of heating capacity is difficult to find from most manufacturers, so a 60K or larger furnace gets installed. Happens a lot in southern climates.
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RoboTeq are you saying this explanation of AFUE/bonnet capacity is incorrect?
http://www.managemylife.com/mmh/ques...ed-air-furnace
If bonnet capacity is the maximum that the furnace can handle and not the actual output, how would bonnet capacity ever be exceeded without over firing the furnace?
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You are posting a home improvement, DIY site as where you get your information from????
There are many inaccuracies in that persons misunderstanding of furnaces.
Do you really think that the old natural draft furnaces with 6-8 inch flues and mammoth dilution air openings were operating at 80% efficiency just because they showed a bonett capacity that was 80% of their input rating? Why did the HVAC industry ever bother to develope induced draft furnaces if that were the case?
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Makes sense on the 6" flues and the gobs of additional dilution of air can no way be as efficient as a induced draft furnace that has controlled air volume, even steady state. The flue gases still have to be at a certain temperature to prevent condensation/rust out. I do find it odd that manufacturers listed a bonnet capacity without a means to change what the actual output of the furnace is. I don't see any way to increase the bonnet output without overfiring the furnace or doing other modifications. Not sure what the point of even listing bonnet capacity is since if it cannot ever be achieved.
For load calculations, rough guides like "Heat is 2X AC" you posted earlier tend to be the rule rather than the exception. For cooling Contractors typically use 500sqft per ton without regard to house construction, insulation, or even age. Only older homes (pre-1970) really need 500sqft per ton of cooling, new construction can get away with as little as a ton per 1,000sqft per ton if it's built right. A full load calculation is almost like a unicorn in our area. Correctly designed ductwork may possibly be even more rare, we see a LOT of ductwork "wall of shame".
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Interesting that you admonish me for my "rule of thumb" (incorrectly quoted while ignoring that it was posted strictly as a guideline for how to tell if your furnace is most likely too large), and then you completely ignore telling us how you make your determinations about furnace capacity in your area. Nice dodge....
Since your area does have average temperatures dropping below freezing with cooling averages during July and August in the low 90's, there is not that much difference between where you are and where I am.
Since you insist on continuing to admonish HVAC contractors on this site, at least have the courtesy to tell us how you come to your conclusions that so many of us don't know what we are doing. Hopefully, you can do that without directing us to another DIY home improvement site as your source of information.
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I do believe there is plenty of quality work being done by contractors who take pride in their work (see wall of pride forum). However I see a lot more work that qualifies for "wall of shame". Unfortunately the lowest bidders get too many of the jobs and the work is marginal at best. Apartments and rent houses tend to be the worst around here. I don't have anything against the HVAC profession or I wouldn't have chose it as a way to make a living. I just a have a pet peeve with work not being done right.
Furnaces around here are constantly oversized, but most customers are used to it and few complaints are generated from the uneven temperatures and higher utility bills. Customers only complain when the furnace starts kicking of on the high limit switch and it shuts off power to the thermostat (if it's hardwired, on battery stats they normally don't notice). Normally this is due to marginal ductwork, sometimes due to dirty or overly restrictive air filters.
I'm surprised to see that your high temperatures in PA are close to ours in summer, and your winter temps are also similar. I would have thought it would be much colder that far north. I don't know where you are in Eastern PA, but years ago I lived in Carlisle PA and remember having to wear a coat to my senior graduation in 1989. We had no AC installed our house and many neighbors didn't either. You don't find many houses in Oklahoma without at least a window AC. While in PA I also remember the lake freezing so solid you could walk on the ice, something that's never happened in Oklahoma in the 10 years I've lived here. Carlisle also plowed ALL the roads, not just the highways and snow routes. Perhaps global warming has made both climates hotter over the past 20 years, but part of me thinks global warming is government propaganda. We did however have 18 days in a row over 100 degrees last summer, and going into attics really sucked those days. Here's a link showing how hot it got the past 2 summers, how does eastern PA compare?
http://www.srh.noaa.gov/oun/?n=climate-okc-heatwave
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