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How Off Were My Hvac Guys?

 
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Old 03-07-2011, 06:32 PM   #1
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How Off Were My Hvac Guys?


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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Old 03-07-2011, 08:24 PM   #2
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Re: How Off Were My Hvac Guys?


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

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Old 03-07-2011, 10:49 PM   #3
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Re: How Off Were My Hvac Guys?


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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Old 03-08-2011, 02:00 AM   #4
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Re: How Off Were My Hvac Guys?


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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Old 03-08-2011, 02:17 AM   #5
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Re: How Off Were My Hvac Guys?


Quote:
Originally Posted by smeagol View Post
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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Old 03-08-2011, 05:03 AM   #6
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Re: How Off Were My Hvac Guys?


Quote:
Originally Posted by thom View Post
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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Old 03-08-2011, 05:15 AM   #7
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Re: How Off Were My Hvac Guys?


Quote:
Originally Posted by smeagol View Post
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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Old 03-08-2011, 02:45 PM   #8
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Re: How Off Were My Hvac Guys?


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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Old 03-08-2011, 03:49 PM   #9
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Re: How Off Were My Hvac Guys?


Quote:
Originally Posted by BadgerBoilerMN View Post

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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Old 03-08-2011, 04:39 PM   #10
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Re: How Off Were My Hvac Guys?


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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Old 03-08-2011, 06:24 PM   #11
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Re: How Off Were My Hvac Guys?


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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Old 03-08-2011, 06:56 PM   #12
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Re: How Off Were My Hvac Guys?


Quote:
Originally Posted by thom View Post
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.
We just have to show size of equipment and efficiency numbers.

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Old 03-10-2011, 07:53 PM   #13
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Re: How Off Were My Hvac Guys?


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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Old 03-10-2011, 11:50 PM   #14
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Re: How Off Were My Hvac Guys?


Quote:
Originally Posted by thom View Post
I just assumed.....

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

Maybe it's just Albuquerque that requires all that.
Ahh, one assumed

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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Old 03-11-2011, 07:02 AM   #15
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Re: How Off Were My Hvac Guys?


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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Old 03-12-2011, 11:22 PM   #16
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Re: How Off Were My Hvac Guys?


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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Old 03-12-2011, 11:30 PM   #17
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Re: How Off Were My Hvac Guys?


. 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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Old 03-13-2011, 06:48 AM   #18
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Re: How Off Were My Hvac Guys?


Quote:
Originally Posted by Dr Heat View Post
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.
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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Old 03-19-2011, 11:58 AM   #19
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Re: How Off Were My Hvac Guys?


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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Old 03-27-2011, 01:43 PM   #20
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Re: How Off Were My Hvac Guys?


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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