Jump to content

Furnace Sizing


kurt
 Share

Recommended Posts

I screwed up a job. Furnace oversized to the point it blew out the secondary heat exchanger (Cat IV).

Nothing seemed out of sorts when I was there, I did the usual stuff. Now I have a customer that's being nice, but he's definitely in "Why didn't I catch this?" phase of our discussion.

I haven't decided what to do, although I know this is going to cost me. Well, not *what to do*, as much as *how much?".

Of course, I'm now flogging myself for not recommending he obtain a Manual J on the furnaces, but it just didn't seem necessary (then).

What do you folks do with furnace sizing?

Link to comment
Share on other sites

I screwed up a job. Furnace oversized to the point it blew out the secondary heat exchanger (Cat IV).

I'm not sure that I understand how an oversized furnace will self destruct, is this the report of a tech? Is the thought that it is short cycling and causing excessive expansion and contraction? How many sq ft is the home and how many BTUs is the furnace (if you have that info)? Any recalls on that brand of furnace? Something doesn't pass the smell test. I'd make a call to the manufacture to discuss further.

What do you folks do with furnace sizing?

When I check the BTUs of the furnace, for my report, if it is really skewed I make mention in the report. When I have seen a problem it is with oversizing but I've never told someone that a heat exchanger can fail. ???

Link to comment
Share on other sites

I, for better or for worse, don't concern myself in the slightest with equipment sizing. As Marc said, it's beyond our scope, and especially with cooling, HVAC guys have told me numerous times old rules don't apply with the new super tight and energy efficient homes - best left to the expert, which tickles me to death.

The problem is, that as inspectors, the more "expert" shoes you imply you have, the more you are expected to wear.

I always tell clients, "You simply can't expect us to be Licensed Plumbers, HVAC guys, Certified Electricians and Chimney Sweeps, etc. Clients always readily agree, that's unreasonable.

This is why I am such an ARDENT fan of the "Standards of Practice" They're there to protect us and to offer an official and reasonable expectation of our service for our clients.

Well meaning HIs, who are driven to know everything about everything, put themselves and us in hotter water than we're supposed to be in. I like knowing a lot too, but I tend to hold my cards pretty close to the vest regarding exactly what I know and understand to let the Standards of Practice be what they are to us - a standard and shield.

Kurt, don't accept the responsibility of an expert, if you aren't one! I've never understood, why an HVAC guy is so eager to say, "The home inspector should have caught this." Why should he have caught it - YOÚ'RE the expert, not the home inspector? It's a silly and illogical statement. I mean, the HVAC contractor CERTAINLY doesn't consider us his equal, yet suddenly he wants us to bear equal responsibility? I don't think so...

Why have "Standards of Practice" if we refuse to permit them to be the standard?

The installer is at fault.

Link to comment
Share on other sites

The installer is at fault.

Consider this scenario: The seller has a furnace on hand and to make his house more attractive, he installs it in a workmanlike manner prior to the sale.

An argument could be made that we've seen thousands of installations and by osmosis, if not formal education, we should know when a unit is dramatically over sized or under sized. I can't imagine Kurt making any real significant error though.

Several years ago I did an inspection on a nice ~2,500 sq foot home with a 55K btu, 93% fresh air breather that, at the time, was about 6 years old. I reported that I had serious reservations about the furnace's ability to heat the home on very cold days.

The realtors and the HVAC tech they hired, all insisted that since the furnace was six years old it must be capable of heating the structure. What the seller didn't say was he'd bought the furnace on Craig's list at the end of the previous season and installed it himself.

My clients bought the house in November and before the end of that year they had a new 85K btu unit installed. The seller fessed up and paid for 1/2 of the new furnace.

I'm glad I mentioned my reservations in the report.

Link to comment
Share on other sites

I screwed up a job. Furnace oversized to the point it blew out the secondary heat exchanger (Cat IV).

Nothing seemed out of sorts when I was there, I did the usual stuff. Now I have a customer that's being nice, but he's definitely in "Why didn't I catch this?" phase of our discussion.

I haven't decided what to do, although I know this is going to cost me. Well, not *what to do*, as much as *how much?".

Of course, I'm now flogging myself for not recommending he obtain a Manual J on the furnaces, but it just didn't seem necessary (then).

What do you folks do with furnace sizing?

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.

Link to comment
Share on other sites

And, Jim has stated it well - grossly - a no brainer.

No doubt we've all done inspections where something is crying out to us regarding comfort, under-dehumidification etc.

I used to do the ole' 600 to 800 SF / ton rule on cooling, but HVAC contractors convinced me it's just not a good idea any more.

Don't be too quick to roll over unless, as Jim says, there's more to this than you've shared.

Meanwhile, you certainly have my thoughts and more.

Link to comment
Share on other sites

I tried to search 'oversized furnace' and 'short cycling'. The first turned up lots of poorly written web pages for Mom & Pop HVAC companies, and the latter more information than I ever wanted to know about cycling shorts[:-yuck]

I did stumble across a few discussion forums and I can't tell what's scarier, the advice on the DIY sites or the stuff over at P.T. Barnum's [:-banghea

I gotta agree with the masses, sizing isn't your job. Besides, when did you look at it? It's entirely reasonable that it was functioning fine as recently as a few weeks ago and short cycled itself to death through the arctic blast we just endured or the thaw that followed it. Exactly how much can be gleaned from observing a single cycle?

Tom

Link to comment
Share on other sites

Terry's questions are good ones. Outrageous examples aside, you can't really gauge whether the furnace is oversized without counting the supply and return runs, and checking their diameters.

I don't hide behind SOPs, but including everything I know about HVAC systems would add an hour or two to every gig, which most folks don't expect and certainly don't want to pay for.

Link to comment
Share on other sites

Bain, "Hiding" behind SOPs is probably a bit strong and maybe even an inappropriate assumption.

I make my point, not based upon the standards as something to hide behind, but more as a standard that well-meaning inspectors de-value by grossly exceeding them. If one chooses to do that then charge more and be clear, "This isn't a home inspection". To do otherwise screws up the real beauty and intent of SOPs! Consinder the definition of the word "Standard".

Standards of Practice serve PRICELESS purposes:

1. They set a professional standard (I started to type "Industry Standard" and could feel Mike O brow-beating me...)

2. They offer the public a clear and reasonable expectation. And that's critical.

I've never hidden behind them, and frankly if you really screwed up, you can't hide behind them.

Competitors exceeding them has never actually harmed me. I just think it's a misrepresentation of the service.

Of course, we can always just do away with SOPs and let clients and attorneys have an eternal field day...

Link to comment
Share on other sites

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.

Thanx for the comments. I'd really appreciate some additional education on this stuff. I know Terrance and Bain know more than a bit about HVAC; any hints you wanna share?

On a brighter note, I'm now cleared by the Secret Service to look at the house next to Barack and Michelle's in the morning.........should be kinda cool......don't tell anyone.......

Link to comment
Share on other sites

I screwed up a job. Furnace oversized to the point it blew out the secondary heat exchanger (Cat IV).

Over sized in what way?

How does a direct vent furnace blow up?

I don't believe for one minute, that you screwed up, there is more to this story...care to elaborate.

You're not running some kind of covert psychological response subliminal test...are you?

I know it's only Tuesday and all. [:-paperba

Link to comment
Share on other sites

I screwed up a job. Furnace oversized to the point it blew out the secondary heat exchanger (Cat IV).

How big was the furnace and how big was the house?

Nothing seemed out of sorts when I was there, I did the usual stuff. Now I have a customer that's being nice, but he's definitely in "Why didn't I catch this?" phase of our discussion.

Why didn't you? Was the furnace grossly oversized?

I haven't decided what to do, although I know this is going to cost me. Well, not *what to do*, as much as *how much?".

$20 gift card to Starbucks?

Of course, I'm now flogging myself for not recommending he obtain a Manual J on the furnaces, but it just didn't seem necessary (then).

What do you folks do with furnace sizing?

I do a seat-of-the-pants look-see. If I think it's grossly over or undersized, I say so, otherwise I don't say anything.

- Jim Katen, Oregon

Link to comment
Share on other sites

You have a good attitude and ethic.

Hey, if I find myself asking, "How did I miss that?" I write the check too - no questions asked.

Thankfully, that's something that happens about once every two or three years.

"Beating the rap" will cost one far more than a meager check ever will. Word travels fast, if your don't do the right thing and a good reputation will take you a lot farther than a little extra cash.

May this cost far less than you fear and may your client have a conscience.

Link to comment
Share on other sites

I've read in HVAC trade publications that something like 50% of all furnaces are oversized. Having a 100K BTU in place rather than an 80K BTU really isn't that big of a deal. And . . . if you look at sizing calculators, you have to factor in glazing, insulation, and a whole bunch of other variables. There's no way we can do that during the course of an inspection. If the furnace were a 200K rather than an 80K, I'd agree that you screwed up. But you didn't.

Mgbinspect, this thread isn't about the SOPs. I responded the way I did because I know that Kurt, like myself, busts his ass for his clients so he can separate himself from other HIs who don't. We've had this discussion ad nauseum when it comes to walking roofs and similar issues, and don't need to have it again. I wasn't knocking anyone else. I was just responding to Kurt.

Link to comment
Share on other sites

[

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).

While it may be oversized it's still no reason for the furnace to crap out. I would toy with the idea of offering the inspection fee back, to keep the peace, but I certainly wouldn't offer to buy a new furnace (it's not that "grossly" oversized either).

How did the home owner come to find out it was oversized and how did he/she come to the conclusion that the furnace failed due to it being oversized?

I smell a skunk.

Link to comment
Share on other sites

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).

I can't believe that would be an issue. If you pay for a new furnace I'm coming to Chicago and taking the fin off your wind surfing rig with a hammer while you watch.

Link to comment
Share on other sites

Not to mention, there's always the possibility that the furnace size could be reduced

That is true, you can de-rate a furnace however you'd have to get the blessing from the manufacture (also the client would forever be suspect and blame you for the latest invasion of slab ants and Aunt Betty's lumbago because you de-rated their furnace).

Link to comment
Share on other sites

Not to mention, there's always the possibility that the furnace size could be reduced

That is true, you can de-rate a furnace however you'd have to get the blessing from the manufacture (also the client would forever be suspect and blame you for the latest invasion of slab ants and Aunt Betty's lumbago because you de-rated their furnace).

Maybe three times, I've found furnaces that had bolts screwed into one of the burners to lower the BTU output.

Link to comment
Share on other sites

Hi Kurt,

I thought Chi Town was the land of licensed contractors and Building Permits. My money says that the installation was, or should have been "approved".

The installer may have been concerned about the occupants perception that the new furnace delivers *colder* air. 25% over the calculated size is not uncommon. This would also not cause the secondary heat exchanger to fail.

I'm with Chad and the rest, there's either more to this, or there's nothing to this. Don't beat yourself up.

Tom Corrigan

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...