Fixes for oversized furnace


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Old 12-22-05, 09:37 AM
disco
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Fixes for oversized furnace

OK, OK, I made a mistake. Despite reading this forum on and off for over a year, in preparation for a furnace replacement, I didn't ask the critical question "How did you decide to size this furnace?" I made an assumption. Now we're trying to remedy it.

Assumption- that the furnace would be 50,000-75,000 BTU, as recommended in the 3 other bids and 4-5 other conversations with contractors. (I was fooled by the model number- a G50 Lennox).

A 90,000 BTU 80% efficient furnace with a 4-ton fan was installed. To his credit, the owner (not the installer/bidder) is "working with me" to remedy the issues (seems hot, blower comes on like hurricane Wilma, short-cycling, a bit noisy). He admitted today that it "probably should have been a 70,000, with maybe a 2 or 3 ton blower." And "40 BTU per square foot should be plenty." 850 square foot house, no basement, 1 floor. Equates to 34,000 BTU, account for unheated crawl and altitude downrating (5,000ft in Denver) and maybe you get to 40,0000?, 45,000?. Output on current furnace is 71,000 before downrating.

I'm sure he wants to do anything but replace this furnace. Verrrry tight crawl space installation.

His solutions; put flex duct in to slow the air flow, maybe cut another duct into a currently unheated, but insulated and sealed porch/laundry room, ensure the "heat anticipator" is properly set on the thermostat, possibly cut another duct to heat the crawlspace (currently unheated, no return air vents, only combustion air vents).

What I really want is a right-sized furnace. How hard can I push, where are the weak points? What solutions should I accept? What ones will really work?

Advice?
 
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Old 12-22-05, 09:42 AM
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Originally Posted by disco
What ones will really work?
A new furnace.
 
  #3  
Old 12-22-05, 10:17 AM
disco
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Thanks, shank. That is my preferred option, but I'll need some more ammo, or more Wheaties, to move this guy off of the "fixes." I'm not particularly good at head-butting.

Lennox tech support suggested a heat-load calc by someone else to prove that it's oversized. I can get one, but would rather not pay if I can get this guy to cop to the size issue, or admit that it's a real problem.

He did say, essentially, "being oversized isn't going to hurt you that much."

What could be the long term problems to equipment, comfort, safety of oversized furnace?
 
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Old 12-22-05, 10:20 AM
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bottom line... btu's are dollars used. even it you choked it down to get rid of the gust and redirected the flow, you're still burning too much fuel. you have to either replace the burner with a smaller one and the blower as well, or get the correct size heater. i'd rather have the correct size new unit than a modified incorrect unit.
 
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Old 12-22-05, 12:53 PM
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Get a new unit.

Closeing down the air flow is hard on the equipement.. Short cycling, over heating, waste of gas is NOT good. With the all that, the furnace will NOT last.

You can down load a program and pay $50.. be best thing to do if no one wants to do the calc.

http://hvaccomputer.com/talkref.asp
 
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Old 12-22-05, 01:37 PM
disco
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Thanks, Jay.

I actually did an on-line calc at Heatload.com, which is Weil-McClains' on-line sales tool. Don't know if you've seen it, but it seems moderately complex (not having seen a full blown calc program). It came up with just under 60,000 btu, not downrated for altitude. There were a few other minor issues, but it seemed reasonable to maybe a bit high based on insulation factors. They recommended a 75,000 btu 80% furnace.

Have you seen this calculator? Does it cover the ground or is it too simplistic?

Also, would his admitted "rule of thumb" of 40 btu per square foot seem reasonable? I have been told 2- 3 times in seeking other bids from local contractors.

I guess my bottom line is that I know this guy is going to resist, and I'm looking for the best leverage to get him to suck it up and put in a different unit. BTW- I still have half of his $2,500, pending my satisfaction. Also, they do work for my brother's contracting company.
 
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Old 12-22-05, 05:30 PM
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new unit.
do not pay him.. it's the only leverage you have.
get the right sized unit. period.



that unit will work if you leave all of your windows open in the winter.
 
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Old 12-22-05, 08:08 PM
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Wow, there is no fix. The furnace must come out. I will guarantee you the manufacturer will not warranty this thing when it is junk in 3 years. Also, with it running that hot its just a matter of time until the exchanger cracks and it starts spewing carbon monoxide into your home. Initially, I would say you need a 45000 btu. Im not sure of your climate and heat loss though.
 
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Old 12-23-05, 02:55 PM
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I have to agree with all of the above replies. When you purchase a replacement system, you are making a long-term decision and hope to have 20 years of trouble free operation. You are entering into situation where you will have an instant problem and a long-lasting one at that. You may feel some compassion for the person who will not get paid for their work but that is why the payments are split, so nobody gets cheated. You deserve the system you thought you were getting. A furnace with an oversize output will get too hot and cycle on the high limit control. That is inefficient because the heat exchanger is always at its max. allowable temperature. When the blower is oversized you get the 'Wilma syndrome' and that throws comfort out of the question. You should probably have a net output of about 55,000 btus and a blower that ranges from 400 to 800 cfms. Variable speed is worth the extra money in my opinion. Good luck snd return here any time for unbased information and opinions.

Ken
 
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Old 12-23-05, 04:37 PM
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850 sq ft........ a candle should heat that
 
  #11  
Old 12-23-05, 06:14 PM
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Candle--LOL

After all, we are talking about Denver. I think it would take a mighty big candle.
A very accurate (from what I have read) heat loss program is the one available from http://www.hvaccomputer.com.
The cost is about $50. But at least you have something to stand on. A problem I have found with some of the equipment manufacturers heat loss calculations is they tend to oversize equipment. I guess it's a CYA thing.
 
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Old 12-23-05, 07:01 PM
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Heat loss calcs were made to help us. The only problem is they are so complicated that they only confuse us. Lets face it, with todays energy efficient materials, there is not much difference Between a house with 10 windows and one with 15. Bottom line......new construction, attention to detail, use 30 BTU/sq. ft. Then do the heat loss calc and negotiate from there. I replace many furnaces each year because of oversizing yet the new smaller units work just fine and more efficiently I might add.
 
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Old 12-26-05, 09:18 AM
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The HVAC Computer program is easy to use- just follow the directions. They have a telephone help line if you need it.

Because your house is on a crawl, the heat loss is probably around 30k btu or less if the walls have reasonable levels of insulation.

The correct solution is a properly sized unit.
 
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Old 12-27-05, 10:05 AM
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Exactly how short are the cycles?
 
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Old 01-03-06, 02:46 PM
disco
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Update to "Fixes for Oversized Furnace"

Wow-

Thanks for all the input to everyone. Sorry to be slow in replying, but I have been out over the holidays and thankfully not thinking about my furnace.

The cycles vary with weather, but the "on" cycle is pretty consistently at 8 minutes. The longest I have clocked is about 11 minutes.

"Off" periods (always measured at 68 degree stat setting) vary between 14-18 minutes at 30 degrees outside, down to 6-8 minutes during a very cold snap (5 below). Note that minimum design temperatures for Denver are usually 0 degrees.

I plan on doing the full blown heat load calc that is found on hvac-calc.com as my next step.

I do not plan on paying. By the owner's admission "40 btu per square foot should be enough". I think the owner just delegated this job to the wrong guy...
 
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Old 01-03-06, 08:46 PM
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Do NOT go by the 40btu per sq ft.

do the calc and see what we come back at.

The cyeles you said is way too short.
 
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Old 01-04-06, 08:29 AM
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How long should the cycles be?

Reason why I am asking is because I suspect that I also have an oversized furnace (slightly). I leave the heat off most of the time and only use it for 45 minutes in the morning so it's warm when I get out of the shower. It somehow manages to cycle on and off for those 45 minutes though and I suspect that it should stay on the whole time. My furnace might be oversized becuase house was uninsulated when I bought it, and I insulated the attic and fixed the air leaks at the doors, etc. I also had an issue with the blower fan limit switch and haven't had it fixed long enough to say if it's still cycling on and off like it was before though.
 
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Old 01-04-06, 06:32 PM
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Cycle length

At design temperature (outdoors) the furnace should run almost constantly.
 
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Old 01-06-06, 04:36 PM
disco
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Heat Calc on Oversized Furnace

OK- I bought the HVAC computer heat loss calc and have done calc's with a few different variables (infiltration, especially). Here's where experience comes in. Experience I don't have.

Note that room by room measurement equals 782 square feet, not 850 I was working with. Also, the laundry is an enclosed porch, well insulated and sealed, never heated. I've assumed I want to heat it, and included in calc's, though it's always been fine with natural circulation from the kitchen.

Right now, it looks like the best number is about 45,000 BTUH heat loss. Range is 41,000 to 49,000 depending on infiltration variable and best guess on wall insulation (No cavity insulation, but 3/4 inch styrofoam under new vinyl siding).

If I have a 88,000 BTUH furnace at 80%= 71,000 BTUH output.

But I have to downrate for altitude? Contractor sez that is .8 per 1,000 feet over 3,000 feet altitude. (At 5,000 feet that's 2 x .8=.16. From 100 leaves .84)

.84 x 71,000= 59,640 output with no "safety factor"

If I go with "high side" heat loss calc (49,000) and a safety factor of 10% (=53,900 loss) we get only 6,000 BTUH oversized.

If I go with "low side" heat loss calc (41,000) and safety factor of 10% (=45,100 loss), we get 14,500 BTUH oversized.

If everything is correct (math, calc, etc) the "high side" heat loss doesn't seem to be significantly out of whack.

Are my calc's right? Anything I overlooked? Are these figures enough to account for what others on this site have called short cycling, and other symptoms of an oversized furnace?

Some final stats:
Heat rise= 62 degrees (specs call for 40-70 degrees)
Supply air temp at exchanger= 122 to 124 degrees
Blower capacity 4 tons (1600 cfm according to Lennox, but I dont' know on what fan setting).

I'm out of brain power. Supply air still seems hot to me, blower seems too much, house seems uncomfortable and unevenly heated, and furnace cycles seem short.

Final thoughts before I report these findings to contractor?

Have good weekend. See you Monday....
 
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Old 01-06-06, 04:59 PM
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Final thoughts

Looking at the temperature rise & the temperature of the supply air, they look good; BUT, I would almost be willing to bet dollars to donuts the fan speed is cranked up to bring the temp rise into spec. If this is right, it's no wonder you have a hurricane going on in the house.
 
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Old 01-06-06, 06:14 PM
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over sized

850 sq is small i have 1500 and a 3 ton unit
It sounds to me like your way over sized on the fan.
1. what is the temperture setting on the fan switch. 120 degrees?
2. what type gas are you using. could be under fired.
don't block off air flow that will cause your fan to run at a higher rpm and draw more amps.
try a rheostat ( fan speed controll er)
 
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Old 01-06-06, 07:47 PM
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You should have a blower with half the capacity of your present one. 800 cfm would be plenty for the size of your house. That will be a tough one to correct. It seems like a mis-match between output and airflow but I am not familiar with Lennox equipment.

Ken
 
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Old 01-07-06, 07:28 PM
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The Numbers are clear

Everyone seems to agree that based on numbers, you have the wrong furnace.

I would have to wonder if the Manufacturer has a regional service manager or design engineer near you who you could invite out. I suspect that if they came to your home, it would be clear to them and they would help you work with the dealer who may have just sold you an extra that he already had in stock.
Good luck
 
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Old 01-08-06, 07:01 AM
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Assuming you were truly duped into this. Don't pay a dime and get a Lennox rep out to see it. They should not be handling Lennox equipment if that's the work they are doing. I wouldn't even imagine a hack putting a furnace that big into a home that small.

Don't except any modifications to the furnace as that will void the UL rating and any warranty.

What size was the original furnace ??
 
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Old 01-09-06, 01:37 PM
disco
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A bit more on spec's

Thanks again for helping me understand the complexities here, and for good ideas. I know it is time to get some equally competent advice, on-site. I like the thought of a regional rep or design engineer.

Answers to a few questions:

I agree that temp rise and supply air temp seem to be in spec, but fan speed is set at the LOWEST setting (although owner wanted to verify). Still, it clearly blows much harder than old furnace.

I don't know temperature setting of fan switch, but my understanding is that fan operation is set by timing, not temperature. Fan shut off is at factory default, which is 90 sec. after burner-off, I think.

The furnace runs on natural gas.

Old furnace was a 90,000 btu Janitrol, with 75,000 btu output. It was hot, cycled on and off a lot, and I never dared turn it up. And that was with a pretty good sized hole in the supply plenum, and probably running nearer to 50% efficient. You still could have roasted a pig with that thing.

Seems like I have the same situation now, only this time I have a much stronger fan....Not what I wanted for my $2,500, which I'm still very willing to pay for the right unit.

As for being duped, well, I'd like to write it off as an honest mistake. Installer just put in the same size furnace without doing any kind of heat loss calc, and never asking me how the old furnace worked or if I wanted any different quality of heating. This is where we return to my mistake. Since all the others spec'd 50 to 75,000 btu units, I thought that was what this guy was going to do
 
 

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