# How off were my hvac guys?



## smeagol (May 13, 2008)

2 story 4900 square foot house. 9' sidewalls throughout. Regular fiberlass insulation. Central minnesota 92000 btu furnace 3 ton ac unit


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## fitter (Nov 25, 2010)

Way undersized. Call a well known contractor thats been around for years.


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## hvac122 (Dec 11, 2010)

Impossible to tell without a load calculation being done. If they did one have them show it to you. Sizing can not be dine from square footage.


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## thom (Nov 3, 2006)

It's a shame. We did rule-of-thumb sizing for 50+ years and it worked, then, some smart guys with computers told us it wouldn't work and we believed them. Now we've got to buy fancy software and enter all the information into a computer to find out what size to put in. For some reason it doesn't heat or cool as well anymore. Must be climate change.


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## NICKPAUS (May 11, 2008)

smeagol said:


> 2 story 4900 square foot house. 9' sidewalls throughout. Regular fiberlass insulation. Central minnesota 92000 btu furnace 3 ton ac unit


 
Must of took the cheapest bid by $5000 minimum. No red flag? Here in Southbay with the best weather in the world. 10 tons 2 5 ton units with 120-150,000Btu worth of heat depending. If its hot there 15 tons....


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## beenthere (Mar 5, 2009)

thom said:


> It's a shame. We did rule-of-thumb sizing for 50+ years and it worked, then, some smart guys with computers told us it wouldn't work and we believed them. Now we've got to buy fancy software and enter all the information into a computer to find out what size to put in. For some reason it doesn't heat or cool as well anymore. Must be climate change.


Load calcs have been around 60 plus years. They were done by paper and pencil. Unfortunately, some didn't learn that the load calc does not tell what size unit to use. Rather what capacity the unit must have.


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## beenthere (Mar 5, 2009)

smeagol said:


> 2 story 4900 square foot house. 9' sidewalls throughout. Regular fiberlass insulation. Central minnesota 92000 btu furnace 3 ton ac unit


Your area temps aren't real hot(maybe to you they are), so 6 tons of A/C maybe, give or take a half ton(don't know how tight your house is constructed, or anything about your windows or direction your house faces.

As for heat, might be a little low. 2-60,000 or 70,000 BTU input 95% efficient furnaces, (yep, should have 2 systems in that house).


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## BadgerBoilerMN (Sep 10, 2010)

Three years ago, I took out a brand new Buderus GB, which was three times too big for the job. The original 4-year-old Ultra went back in and has worked perfectly ever since. The Buderus was installed to satisfy a cold room (still cold, since a bare tube staple up will not heat and all-glass room with 2" angora rug covering 95% of the radiant floor). I think the original contractor and the second guy didn't do a heat load analysis.

Last week, I took out an "extra" boiler installed to serve a 360sq.ft. addition...you probably guessed already, the original boiler was 15% oversized WITH the addition. Working great now and the new boiler is on Craig's list.

If you don't do a Manual 'J' long hand or by computer, you don't know what you are doing. 

Every homeowner should ask any prospective HVAC contractor to produce a sample of a Manual 'J' load analysis. 

This is one of the few ways a hapless homeowner can best choose his HVAC or radiant heating contractor and a simple question that will eliminate 9 out of 10 HVAC contractors and sadly 999 out of 1000 plumbing & "heating" contractors.

Oversize equipment whether it be cooling or heating makes for higher installation, operating and maintenance cost while producing less comfort (modulating blowers and burners notwithstanding).

Those who know how will always work for those who know why.


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## thom (Nov 3, 2006)

BadgerBoilerMN said:


> If you don't do a Manual 'J' long hand or by computer, you don't know what you are doing.


I just assumed..... 

Is everyone not required to submit plans for permit:
... showing duct location and sizing
... showing duct inslulation
... including a manual J calculation
... showing sizes of equipment and minimum efficiency

Maybe it's just Albuquerque that requires all that.


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## BadgerBoilerMN (Sep 10, 2010)

They are required in the larger municipalities but wholly ignored or misunderstood by the administrative authorities knowing how, but not why, in many cases. Homeowners should not depend on an inspector to assure a sound design.

I was just called on the location and size of an expansion tank (both were correct), but the totally redundant furnace that was 50% oversized with 2.5 tons air on a 1.5 ton load, was somehow missed?


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## tccoggs (Dec 17, 2008)

I tore out my 168k downfired CI boiler and replaced it with a 59k triple pass unit 3 years ago. House is still warm, got plenty of hot water and my oil usage went down 50%.

I added central to my house 6 years ago. First contractor came in with 3 ton, second 5 ton, 3rd did the software and came up with 4 ton. I gave him the job and unit keeps the house cool and runs plenty to dehumidify the house during the muggy months.

A little insulation and proper sizing goes a long way. As for those complaining about software, HVAC Calc is not that expensive in comparison to some of the other tools that you currently use.


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## Cole82 (Nov 22, 2008)

thom said:


> I just assumed.....
> 
> Is everyone not required to submit plans for permit:
> ... showing duct location and sizing
> ...


 We just have to show size of equipment and efficiency numbers.:laughing:

Cole


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## flashheatingand (May 3, 2008)

We are required to have man. j & D, and duct leakage test. The thing is with retro, you might as well match the furnace to the existing ductwork. There are exceptions to this, so if things are grossly wrong, then yeah they should be fixed. But for the most part, if you want maximum efficiency, match the air handler to the ducts first.


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## SLSTech (Sep 13, 2008)

thom said:


> I just assumed.....
> 
> Is everyone not required to submit plans for permit:
> ... showing duct location and sizing
> ...


Ahh, one assumed :whistling 

In some locations it is required, but I would estimate that those places are in the minority or as Badger said, the inspectors don't have a clue. You should have heard the grumbling & the where are we going to find someone that does all that in the ENERGY STAR class being held in Atlanta - he11 most I have seen down here just change the address on a prior Manual J's they did & the duct work is left to the guy in the field


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## BadgerBoilerMN (Sep 10, 2010)

Be it ductwork or radiation, the heat and cooling loads dictate the appliance size and to a lesser degree flow (air or water). Cooling is more sensitive to flow naturally, but oversizing AC will lead to cold damp air and oversizing heat, short cycles and lower MRTs (comfort).

If the first guy made mistakes (using the rule of thumb engineering) the smart guy has a chance to improve nearly every system and his personal reputation...business success.


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## Dr Heat (Dec 25, 2008)

Manual J D S they all have there benefits but nothing replaces experience.

I can spec a house by looking at it. I have done hundreds of manual J's but there are only 4 or five sizes to choose from so....

My point: any one here who can do a manual J correctly can also rule of thumb correctly and conversely if you cant rule of thumb you don't have the skill to manual J correctly.

I address these comments to residential typical construction only.


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## Dr Heat (Dec 25, 2008)

. 10 tons 2 5 ton units with 120-150,000Btu worth of heat depending. If its hot there 15 tons....[/quote]


Really in Minnesota 

You may not be that far off. the heating seams quite low but ac is right on. If this is new construction and the house was built well. 

Manual J does not account for bad construction better do a blower door and duct blast.


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## beenthere (Mar 5, 2009)

Dr Heat said:


> Manual J D S they all have there benefits but nothing replaces experience.
> 
> I can spec a house by looking at it. I have done hundreds of manual J's but there are only 4 or five sizes to choose from so....
> 
> ...


There are 6 standard sizes of A/C for residential, and 6 gas furnace sizes by most manufacturers in residential. So its common to have guesses of 60,000, 80,000, and 100,000 BTUs on a house that only needs 50,000 BTUs of output. And guesses for A/C anywhere from 2.5 ton to 3.5 tons on that same house.

A 2 story seldom needs as much A/C as the same size single story would.

There is a guy in Texas with a 4200 sq ft house, that is cooling it with a 2 ton A/C. Rule of thumb would have it grossly over sized.


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## BadgerBoilerMN (Sep 10, 2010)

A heat load analysis simply qualifies the qualified. This string clearly points out the obvious and makes choosing a heating or cooling contractor easier for the confused homeowner with 5 _different _furnace or boiler sizes bid for the same house. 

Ignorance is bliss.


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## Dr Heat (Dec 25, 2008)

I will concede that "rule of thumb" was a poor choice of terms. I work consistently with Engineers who double check my design build drawings, so manual J is always a part of every system I speck. What I was getting at is even the manual J is not a science without objective components. 

A house in Texas with a two ton load on a 4200 square foot home has something special going on. I doubt a typical heat gain heat loss annalist would come to Two tons simply using a manual "J". Someone took a risk based on previous experience.

Manual J is not the only heat loss heat gain tool we need to use.


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## beenthere (Mar 5, 2009)

Dr Heat said:


> A house in Texas with a two ton load on a 4200 square foot home has something special going on. I doubt a typical heat gain heat loss annalist would come to Two tons simply using a manual "J". Someone took a risk based on previous experience.
> 
> Manual J is not the only heat loss heat gain tool we need to use.


The load calc was done with Manual J version 8.


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## MechAcc (Feb 27, 2005)

We are only guessing. That unit could have been sized correctly and the required insulation was not installed. Canned ceiling lights not insulated. Air infiltration etc. At one of our RSES educational meetings one of members performed a blower test. The local general contractor had a long talk with his crew and the HVAC contractor about all the defects that were found on this new house. Air gusts around window frames. Air being pulled from attic into the wall cavity returns etc.


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## BadgerBoilerMN (Sep 10, 2010)

That is a point well taken, but no argument for rule-of-thumb load calculation. The designer is not responsible for as-built performance but can only go by the spec.

A SIP home in new construction and a foamed home in old are pretty sure bets, but nothing beats the combination of a good program and field experience.

Naturally a blower test reveals one of the critical elements of heat load analysis.


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## acefurnacefixer (Dec 7, 2006)

thom said:


> It's a shame. We did rule-of-thumb sizing for 50+ years and it worked, then, some smart guys with computers told us it wouldn't work and we believed them. Now we've got to buy fancy software and enter all the information into a computer to find out what size to put in. For some reason it doesn't heat or cool as well anymore. Must be climate change.


LOL.............and after going through all that, they don't make furnaces that are exactly the btu's that that fancy software calls for............so what do you end up doing?.......putting in what you would have put in anyway. 

On a side note..........if you have to put in a "over sized" furnace, put in a two stage so that the system will be "in the range".


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## Texahoma (Feb 10, 2009)

*Load Calcs*



thom said:


> I just assumed.....
> 
> Is everyone not required to submit plans for permit:
> ... showing duct location and sizing
> ...


It's is coming to Oklahoma and about time. Plan on spending at least 1K for the program. I use Wrightsoft.


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