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Senior Member
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Lexington, KY
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 Furnace Sizing
[#27] Posted: Jan 19 2010 - 5:50:20 PM |  |  |
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Quote: Originally posted by Jim KatenQuote: Originally posted by kurt
Well, I'm still in discovery stage, but I did get someone to do a Manual J, and they indicated the furnace was approximately 25% oversized (100,000btu when an 80,000 would have worked). First of all, that's 20% oversized, not 25%. - Jim Katen, Oregon
It depends upon how you interpret the figgers. 25% of 80,000 is 20,000, so the 100,000 BTU unit would be 25% too large.
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Senior Member
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Lafayette, Louisiana
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Furnace Sizing
[#28] Posted: Jan 19 2010 - 7:03:25 PM | | |
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There are greater consequences to installing too much tonnage of cooling in a house. Cooling needs to be sized correctly for proper humidity removal (latent heat removal) but the heating system? Exceeding the J calc by a safe margin is the norm for me. It's insurance.
It's somewhat like sizing an electrical service entrance. If the load analysis says anything between 125 to 160, I sell a 200.
Marc
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"If Guam gets too overpopulated, it might tip over."
Congressman Hank Johnson (D) GA |
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Associate Editor - TIJ
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Charlestown, Massachusetts
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Furnace Sizing
[#29] Posted: Jan 19 2010 - 7:14:42 PM | | |
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Quote: Originally posted by kurtQuote: Originally posted by Jim Morrison
I don't do anything beyond commenting if a system strikes me as grossly over or undersized.
I can't believe you are going to write a check based on what you posted. There must be more to the story.
Well, I'm still in discovery stage, but I did get someone to do a Manual J, and they indicated the furnace was approximately 25% oversized (100,000btu when an 80,000 would have worked). It's all still got me rattled. Quote: Chad Fabry: I'm glad I mentioned my reservations in the report. Wish I had had some reservations. I usually (nearly always) do. I could have merely said "ya know, I'm not sure; get a Manual J", and everything would have been fine. I get gigs because I hold myself out as expert. Now I'm going to pay a price, as yet undetermined. Sure, I could beat the rap, but that ain't how I run.
Kurt,
You are just as honest and righteous as the day is long, and I'll have harsh words with anyone who says different. That said, the above statement is a bit misleading in light of the facts as you present them.
If you want to write these folks a check to protect your rep, that's one thing. But as near as I can figure, you don't owe them a thing. Not that you asked, but I suggest sleeping on it before making any decisions.
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Jim
www.AlMorrison.com
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Kenmore, WA
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Furnace Sizing
[#30] Posted: Jan 19 2010 - 7:30:52 PM | | |
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Hi Kurt,
Sorry you're going through this, Bud. If it's a Cat IV with a two-stage heat exchanger, should it not be able to anticipate this and dial back or increase fan speed to compensate for the higher Btu output? I should think that if it were properly set up that, even being over-sized, it's little brain should have dialed back.
That said, if the customer is looking at you as the good guy that made an honest mistake, and you want to preserve that good will, maybe the thing to do would be to spring for a new heat exchanger and burner assembly that will reduce Btu output of that furnace to 80,000. I see a lot of these suckers and I've seen plenty of them that are the same brand with identically sized cabinets where the Btu's are all over the map. I think the only difference has to be in the burners and heat exchanger configurations.
Changing the exchanger and burners will take a fraction of the time it will take to completely tear it out and put a new one in - especially if a complete new furnace will require sheetmetal work - and it's got to be cheaper.
ONE TEAM - ONE FIGHT!!!
Mike
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Gaston, Oregon
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Furnace Sizing
[#31] Posted: Jan 19 2010 - 9:59:57 PM | | |
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Quote: Originally posted by BainQuote: Originally posted by Jim KatenQuote: Originally posted by kurt
Well, I'm still in discovery stage, but I did get someone to do a Manual J, and they indicated the furnace was approximately 25% oversized (100,000btu when an 80,000 would have worked). First of all, that's 20% oversized, not 25%. - Jim Katen, Oregon It depends upon how you interpret the figgers. 25% of 80,000 is 20,000, so the 100,000 BTU unit would be 25% too large.
Hah! That'll teach me to get uppity.
- Jim Katen, Oregon
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Newberg, OR
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Furnace Sizing
[#32] Posted: Jan 20 2010 - 09:17:35 AM | | |
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Back in October, I looked at a 1-off new construction home around 2000sf or a little over when including the smallish 12x18 unfinished basement area which was advertised as 'Ready to Finish'. The furnace was 40k BTU's.
In my report I noted that although I am not qualified to properly size a furnace, I typically find larger units in home's this size, and recommended some further evaluation on the matter.
The war started soon after. The buyer was told by 2 HVAC techs they hired, that the furnace was in fact too small. However, the seller found his own HVAC tech who showed through their calculations, the size was in deed adequate. That's where it ended.
As fuel for the fire, I had previously inspected another new construction home built by this same builder. The previous home was about 1500sf and had a 60K BTU furnace installed. Unfortunately, even this information was unable to help my buyer over come the seller in this case.
In the end, I was very surprised to find how much of a variation there was, depending on who was doing the calculations. Relative to what I encountered here, Kurt, your variation on the other end, seems miniscule. Good luck.
Wayne G
Northwest Inspector
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Columbia, MO
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Furnace Sizing
[#33] Posted: Jan 20 2010 - 09:19:48 AM | | |
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Kurt,
I think Mike is right.
My two cents is if there isn't a air leakage to the outside and a duct air leakage to the outside testing on this home and add to your manual J, one would only guessing.
John
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Chicago, IL
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Furnace Sizing
[#34] Posted: Jan 20 2010 - 10:36:43 AM | | |
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I appreciate everyone's comments. This has been enlightening. I'm still trying to get to the bottom of all this, and how things went awry.
Right now, I'm taking Jimmy's advice, and sleeping on it.
This is a tricky ass job, and I just got a tuneup in how I approach looking over heating systems.
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Kurt in Chicago
Workin' the Inner City
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Frankfort, Indiana
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Furnace Sizing
[#35] Posted: Jan 28 2010 - 12:49:13 PM | | |
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I found some interesting information today regarding failed secondary heat exchangers in Carrier, Bryant, and Payne furnaces.
As parts of a monthly draw inspection I routinely visit a local Carrier warehouse; to look at and photograph off-site stored materials. Today, during the visit to the warehouse, I noticed a pallet of damaged heat exchangers and asked the salesman about them. He indicated that they had failed due to the furnace units being oversized.
He said the problem is from an excessive amount of condensation in the secondary heat exchanger due to the unit being oversized. The exchanger cannot drain the condensate properly and the result is excessive corrosion in the bottom of the unit and failure.
As I understand it, Carrier went through a class action law suit, brought on by 6-8 homeowners, in Wisconsin. In order to save face they implemented a program to repair, or offer a credit toward new units to the effected homeowners. The documentation I was able to secure also indicated the replacement coverage is for 20 years from the date of original install.
Bryan
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Lafayette, Louisiana
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Furnace Sizing
[#36] Posted: Jan 28 2010 - 3:54:04 PM | | |
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Does anyone understand by what mechanism does an oversized furnace of this manufacturer and model result in a failure of the secondary heat exchanger? Why is there a problem in which it cannot drain the condensate?
Marc
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Congressman Hank Johnson (D) GA |
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Gaston, Oregon
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Lafayette, Louisiana
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Furnace Sizing
[#38] Posted: Feb 09 2010 - 1:35:15 PM | | |
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If the duct system has adequate capacity for the amount of furnace btu/hr installed then the consequence of excessive btu/hr is short cycles, not high limit triggers. I'd check supply air temperatures after it's been running for a while and compare it the manual to confirm that it isn't getting too close to the upper limit. If it is, the air flow through the furnace may be inadequate. If it's not, then I'd check the high limit switch by substitution (substitute a new one).
If you hang around this forum for a while, there are other members with more experience than I who may post other suggestions.
Marc
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"If Guam gets too overpopulated, it might tip over."
Congressman Hank Johnson (D) GA |
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louisville, ne
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Furnace Sizing
[#39] Posted: Feb 11 2010 - 7:30:32 PM | | |
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is that the total sf of all floors? thats definately oversized if that is the case i would say.
Have you been keeping furnace filter changed,did the check the coil to make sure it wasnt clogged?Those are just a couple of the things that could cause an airflow/overheating issue.
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Calgary, AB
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