AFUE, real world and Sweet Point
I am going have my FWA NG furnaces replaced this spring and I am looking for some background information.
I know what efficiency is, amount of usefull energy out (in this case heated air) to the energy in the NG. Simple and straight forward.
But they use AFUE is which is based on some kind usage over a year. What I want to know how reliable this is real world. Is it like car MPG stickers? Or can can I depend on the numbers; ie if I it cost $100 for gas to run an 80% unit would I expect to spend $89 for 90% unit or $83 for a 96% unit.
And how can I compare this with my existing (1979) standing pilot furnaces?
A quick look at some of the manfactures show 2 classes of efficiency; 80%, 90-92% and at least one very high efficient unit at 96%.
What do you think is the “sweet spot”. That is where the fuel savings will pay for the difference is cost in a reasonable time period, say 5 years.
BTW, I am in the Kansas City, MO area (I think about 5000 HDD) and looking at my bills for the last 3 years it appears that we have 3-4 months of “real winter” and 2 months where the bills are about 1/2 ot 1/3 of the colder months. Totals have run about $450 to 625.
Also what is the realatively increase in problems and maintance issues when going to the higher eff units?
I found that you can get both variable speed blowers and 2 stage blowers in both 80% and 90% units.
But only 90+ units can be setup for external combustion air. Is this true or are there 80% units that can do this.
Here are the problems that I am trying to solve.
When I built in 79 there was talks of a NG shortage and in some areas new connections where banned. There where pilotless furnaces available, but they seems to be overly complicated for the technology in use at that time and I passed on them. But an alternate idea was to put the furance in an “enclosed” space with outside combustion air so that conditioned air would not be sent up the flue. However the details of that did not work out and area is really not sealed from the rest of the house and outside air vent spills cold air into the house. I would like to get rid of that and bring air directly in to the furnace.
Also it is a 2 story on slab with the lower level partially in the ground (about 30%) with lots of windows. But I had to fight long and hard to find an HVAC constractor that would do more than “stick a couple of vents in the ceiling for the ‘basement'”. I ended up with a separate down draft furance supplying ducts under the slab for the 1st flr.
That works, but the unit was way, way oversized. On a zero day it only runs about 25% of the time. However, because of the thermomass of the slab it does not short cycle. But the fan, even on the lowest speed, blow enough air to fly a kite.
So one of my options is to go with one variable speed, 2 stage furnace and zoning system.
Replies
Bill, what I tell customers is that I guruntee that on an IDENTICAL day, they will save 10% going from an 80 to a 90. The beauty is there is no such thing as an identical day.
Now for the fly in the soup.
We just got a bulletin from the NRC that says a variable speed furnace "may" burn more gas than a belted fan furnace. The vs may draw 1/2 amp and the belted may draw 8 amps. Those extra amps actually add heat to the airstream, and more gas needs to be burnt.
However, the lower power consumption offsets any possible increase in gas consumption.
The other kicker is that I've yet to see a furnace that wasn't oversized.
If you are going to stay in the house for any length of time, I would go for the vs,2-stage,90+.Zoning is a bonus if the contractor knows their stuff.
Get the best warranty you can, some of the parts are real expensive
Edited 12/30/2004 3:55 pm ET by rich1
First, in terms of the AFUE rating, spark ignition will affect an increase of abou 2.5% over a standing pilot in equal devices.
At a present level of usage, it will take you about 7 years to pay for the difference in price between a 2-stage variable speed 80 (AFUE = 80%) vs a 90+ (AFUE = 93%), all other things being equal. The price of gas will NEVER go up, right?
All but the cheapest equipment will outlast the payback period.
The main problems with maintaining the higher efficiency equipment is that when some moron plumber shows up to do a service call and doesn't have a clue how to troubleshoot anything more complex than a pipe, you get billed hundreds of $ for circuit boards. In reality, the higher efficiency equipment is very reliable and sedlom fails. Up unitl recent history, the ignitors tended to fail most of all, but they cost only a few $.
Size the equipment to meet the load. The oversized equipment that you now have is less efficient than properly sized equipment. I sized the equipment for my house with no margin to spare. Design conditions were -10 to 70. Last week when it was -5, it ran 80% of the time.
Timbo, have you ever reached -10? Just curious on the performance.
I did see -10, but I was under construction and can't count that time as "real" performance. The furnace is undersized and I supplement with electric and wood heat.
Something interesting to note, though. I found that the furnace ran less in a 24 hour period when set to a constant temp., as opposed to the programmed setback.
I think that it is a little warmer here in KC and than where you are, but not by much.I think I have seen that the design temp is zero. but I want it at least -10.I remember Dec 89 where it got down to -23 and I think about 10 days where it never got above -10.
"The main problems with maintaining the higher efficiency equipment is that when some moron plumber shows up to do a service call and doesn't have a clue how to troubleshoot anything more complex than a pipe, you get billed hundreds of $ for circuit boards. In reality, the higher efficiency equipment is very reliable and sedlom fails. Up unitl recent history, the ignitors tended to fail most of all, but they cost only a few $."Well I am an electrical engineer by training and problem solver by nature.The first level first level of an trouble shooting will be YOURS TRULY. And I will get a HSI spare to keep on hand.And any board that gets replaced I will keep and reverse engineer unless there is a core value (or under warantee)."Size the equipment to meet the load. "But if I do that I will have to give up my hoby of indoor kit flying.Thanks for the info.
Using AFUE should be reliable.
The purpose of AFUE is to give a realistic measure of the quantity of natural gas used. Here's how it works. Simple efficiency compares output to input. If the furnace takes in 100,000 BTUs of natural gas and delivers 80,000 BTUs of heat to the house (the rest going up the chimney as waste), then the furnace is 80% efficient. But these figures formerly were measured and expressed as steady state operation. In other words, if you operated the unit uninterrupted for an hour and measured during that time, that's what you'd get. But a real furnace doesn't operate that way. It cycles on and off. During the warm up period it runs less than peak efficiency. And when it doesn't run at all you still lose heat up the chimney. The AFUE ratings adjust for all of that by specifying a standard cycle of starting and stopping under a standard set of temperature and humidity conditions. Therefore, it is a fairly good measure of what you might see in actual use.
When this system was first adopted during the Energy Crisis, the fastest way most manufacturers could get a higher efficiency furnace to market was to install motorized flue dampers. When the furnace was off a damper shut, reducing heat loss up the chimney and increasing the AFUE. This saved actual money, too, of course, which was the intent of adopting this more realistic measure.
As a fairly good estimate of savings, you could do it this way. Take your current annual gas bill. Deduct the portion used for your water heater, if you have one, by deducting 12 times your lowest (usually July) bill. Say it's $800. Then, comparing a new furnace of, say 80% efficiency with one of 92% efficiency, you would use roughly 12% less gas with the higher efficiency furnace, which amounts to $160 per year. If you are good at math you could work out the figures more precisely with algebra, using the proper reciprocal relations and so forth.
Your standing pilot furnace probably has a fairly low AFUE because the pilot uses gas year round whether you need heat added to the house or not, and the flue is open all the time. I don't have a figure, but you will certainly lower your gas bill a considerable amount with a modern furnace.
>>During the warm up period it runs less than peak efficiency. I'm not sure that's true.In my testing (and I've only had my combustion analyzer a couple of months) with 80+ and 90+ the flue gas temps rise fairly quickly following light-off, as does the temp of the made air, but the temp of the flue gases seem to continue to rise higher and longer than the rise of the made air temps - suggesting that efficiency is dropping after several minutes.I don't have enough info and tests to draw firm conclusions, but I've got several other HI's who are going to be generating some hard numbers with me to try to come up with some real world experience.BTW, my testing also suggests that the quality of the service technician has a lot more to do with efficiency than the "rating" of the unit.And I am very suspicious of 2 stage burners - I suspect the low fire rate has poor heat transfer to the made air. I can't wait to get my hands on one to test, but there aren't many being sold around here.In the couple of months I've had my Fyrite combustion analyzer, I've come to realize how much information they can give as to combustion efficiency - without the CO, O2 and temp readings, it seems to me that tuning gas appliances is like throwing dice with a blindfold.I don't know about yours, but my church isn't a hotel for the holy, it's a hospital for sinners
Sojourners: Christians for Justice and Peace
Combustion efficency is only part of the equation.On most furnaces, all you can do is set manifold pressure. Heat transfer to the airstream is a bigger factor.If you ever have a chance, do a temp rise before and after cleaning a fan. 10 degree drop is not uncommon.I think, and I have no proof, that lack of proper airflow far out wieghs any increase you could get in combustion efficency.
Good points.Do you know of any easy way to measure air flow with a draft guage?One of the things we'll be measuring is temp rise of the made air, but I bet getting air flow readigs would give some valuable info.I don't know about yours, but my church isn't a hotel for the holy, it's a hospital for sinners
Sojourners: Christians for Justice and Peace
There was just a long discussion over at h-talk, I gave up early:)
Realistically,if the fan and filter are clean, the returns and supplies are not restricted, most techs will check the manifold pressure and then the temp rise. If you are in the middle of the range, you are good to go. Checking for actual cfm would cost to much,and for what benifit?If your rise is out of the range, then you need to do more checking.It's amazing how many return air vents are blocked by furniture.
All induced draft appliances will produce co, I'm more concerned about any ambient levels above 0ppm.
Rich
Zero CO isn't the goal; in fact, it indicates a significantly underfired appliance.I follow the CO analysis protocol adopted by the Building Performance Institute and National Comfort Institute (see, e.g., http://home.att.net/~cobusters1/coprotocol.htm)The goal is under 100 ppm and steady (testing over at least 5 minutes), and a draft between .01 and .02 wcin.I think very highly of the CO Analysis and Combustion Analysis courses taught through the National Comfort Institute: http://www.nationalcomfortinstitute.com/I did a very old furnace the other day which looked pretty good for the first 2 minutes - CO peaked about 70 at light off and dropped to 35. Draft was . tad high: .025 wcin. Oxygen was low, though, 4.5% (should be at least 6%) CO started to fluctuate at about 2 min, 35-50, and went up and down for a minute or so, and steadily rose over the next 5 minutes to over 2200 ppm and 0.1 O2%!Every standard I know of doesn''t allow more than 400ppm in the flue gases.There wasn't any CO in the ambient air, but above 400ppm is grounds to disable the appliance. A unit might be drafting OK at the time of the test, but conditions may easily change and change the draft performance.And I have found you can't tell anything about combustion efficiency or CO production from flame color.I don't know about yours, but my church isn't a hotel for the holy, it's a hospital for sinners
Sojourners: Christians for Justice and Peace
Hi Bob.I agree, 0ppm ambient, under 400 in the flue. Did you find out what caused the spike in the co? Sounds like a crack opened and started to disturb the flames.
Be aware of the outside temps when taking draft readings. When it is hot outside,(80+) the draft can be.01 or less.
No, I don't know what caused that rising CO - a "heating contractor" (I use the term loosely for that fellow) said he took it apart and it didn't have any cracks, but he also seemed to be of the school of thought the the only cause of high CO is a cracked heat exchanger (which my testing doesn't support - I've only had one test where a crack was clearly the cause of high CO and many wehere the cause is overfiring, flame impingement, insufficient air supply or a blocked vent) and he said "I don't care how much CO ius in the flue gases - it just gopes up the chimney."If I find pout what caused it I'll revert.I don't know about yours, but my church isn't a hotel for the holy, it's a hospital for sinners
Sojourners: Christians for Justice and Peace
Bill:
I'm guessing you already know this but I'm gonna say it for other's reading this thread, and BTW - I'm no HVAC expert, but I did learn a few things while building Energy Star certified homes for a builder I worked for...
Correct sizing of equipment was mentioned above. I'd like to add that if you want your equipment properly sized don't hire any HVAC contractor that will not do a "Manual J" calculation on your home and system. Require the Manual J calc report be physically provided to you. Manual J is a somewhat complicated set of formulas based on heat loss/gain and a whole bunch of other stuff that makes equipment selection a science. The good news is that there are computer programs out there that greatly simplify the Manual J calc task. The bad news is that computer illiterate people are basically excluded from doing Manual J calcs.
Another minor point is that the installed system has to be balanced. In short that means that if you have a 10x10 room with 2 windows and 1 supply register and a 5x7 bathroom with no windows and 1 register, guess which one needs more supply air? In my area, all new systems are installed with a damper in each supply duct and these need to be "cut back" for the above mentioned bathroom. I believe that the manual J report lists how many CFM of air each register requires - if I remember correctly.
I am going to get the homeowner license for HVAC-CALC and do my own verification.http://www.hvaccomputer.com/main.aspBut after living here 25 years I have a good idea of what I need. The AC is spot on, but I do want to ask about humdity removal with the higher SEER units.But the manual J and my experince will be a good tool weed out HVAC contractors.As far as balancing they each floor seems to be OK. Which is good, becaue the only way to adjust the air flow is the register dampners. The duct work for the lower level is in concrete and the upper level trunks are all in a soffit and the branch lines all covered by the 1st floor ceiling.I am also in real good shape with returns. 3 bedrooms up and each has there own return.The lower level is all open except for a small bathroom/utility area and has one bank of 4 returns. Reading too many messages I have learned that returns are an area that is often under designed.
I hear you about the return thing. In my home I have a 3 zoned system with 4 returns - no where near a return in every room but I'm very happy with the uniform temperatures in all parts of the house.
Most houses I work on have 1 return per floor. Probably underdesigned, but another thing I earned in the Energy Star program is that in this minimal return scenario, you must provide for circulation between rooms and the hallway (typically where the return is) either through significant spaces below the doors or via thru wall registers. Matt
Hi wildbill :)
I was wondering if Jim was going to comment.Don't let his comments about 80% and low fire worry you too much. It's another ongoing discussion. There are only a handful who know co like he does though.