Furnace Sizing

[Jim Katen]

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%.

Next, that describes the vast majority of furnaces out there. Most are oversized.

By what mechanism, exactly, would oversizing a furnace damage it? It might lead to shorter cycles, but that's about it. Lots of other things lead to short cycles, poor thermostat placement being a common one.

Also keep this in mind: even a perfectly sized furnace is oversized *most* days of the year. An oversized furnace is just oversized *more* days of the year. Oversizing a furnace by 20% will affect comfort and efficiency, but it will have little to do with longevity of the furnace.

Now if the furnace is too big for the ductwork, that's a different story. Air flow will be inadequate and the heat exchanger will run hot. But even then, the hi-temp limit switch would kick in an the cycles would be governed by hi-limit instead of room temp. It wouldn't self destruct until the hi-limit failed (and they often fail in safe mode anyway).

One other thing. A Manual J calc involves several guesstimate factors. Ask your guy to run the calculation twice. Once with reasonable low guesses and once with reasonable high guesses for each factor. That will give you a range of acceptable furnace sizes that might clarify the fact that furnace sizing is largely about guessing.

- Jim Katen, Oregon

[Bain]

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.

[Marc]

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

[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.

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.

[hausdok]

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

[Jim Katen]

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

[Wayne Gardner]

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

[J Howland]

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

[kurt]

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.

[Bryan]

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

Download Attachment: Carrier Secondary heat exchangers.pdf

179.49 KB

[Marc]

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

[Jim Katen]

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

No. The explanation makes no sense.

- Jim Katen, Oregon

[Marc]

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

[plummen]

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.

[RobC]

Help Appreciated: new member

I just had a routine service call on a 5 year old Trane Furnance installed in our 2800 sq ft home. Tech says that our heating and cooling is grossly oversized for our home. Has a 120 BTU furnance and 5 T AC unit. He suggested replacing both units to the correct size and is working up an estimate on costs? Do not have specifics today, but can get them. This just occurred yesterday 2/8/10. System is working but had issues in Dec with High Limit Switch trips when weather got cold.

Mike posted that downsizing burner/etc would be a cheaper fix than a complete system changeout.???

How about if you delete this post and start another thread.