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Whose Right? Hvac Quotes

 
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Old 02-07-2015, 07:33 PM   #1
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Whose Right? Hvac Quotes


Hi guys, I'm getting quotes on adding a heat vent in my house and I've received two quotes. One of the guys says add a 5" pipe on the end of the square box (trunk?), while the next guy said from the top of the box (trunk?), this way will require se 90's. He said not to add to the end because it will cause static electricity and change the flow.

Sorry hvac isn't my lingo, tried to make this sound as sensible as possible.

Also, my pipes are 6", one said replace them with 5", the other said leave the 6's ... It will add comfort.
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Old 02-07-2015, 08:29 PM   #2
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Re: Whose Right? Hvac Quotes


We just finished an addition where the new was run straight off the end.

How far of a run are we talking?

Also, I don't see a reason for needing to switch your existing.

It would be interesting to hear from someone with more knowledge what benefits a 90 and downsizing might have.

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Old 02-07-2015, 08:48 PM   #3
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Re: Whose Right? Hvac Quotes


I usually get enough bids for them to cluster around some price. I throw out the high & low bids and think over the others.
All bidders may come up with different explanations for what you should have, and all explanations sound credible. And maybe they are, depending on the unspoken assumptions each one makes.
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Old 02-07-2015, 09:20 PM   #4
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Re: Whose Right? Hvac Quotes


Quote:
Originally Posted by Antropovsky View Post
Hi guys, I'm getting quotes on adding a heat vent in my house and I've received two quotes. One of the guys says add a 5" pipe on the end of the square box (trunk?), while the next guy said from the top of the box (trunk?), this way will require se 90's. He said not to add to the end because it will cause static electricity and change the flow.

Sorry hvac isn't my lingo, tried to make this sound as sensible as possible.

Also, my pipes are 6", one said replace them with 5", the other said leave the 6's ... It will add comfort.
Not my thing, but maybe this will help. BTW, are you sure they mentioned static electricity, or was it static pressure?

http://www.energyvanguard.com/blog-b...ay-Be-to-Blame

Last edited by MarkJames; 02-07-2015 at 09:27 PM.
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Old 02-07-2015, 10:53 PM   #5
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Re: Whose Right? Hvac Quotes


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Originally Posted by MarkJames View Post
Not my thing, but maybe this will help. BTW, are you sure they mentioned static electricity, or was it static pressure?

http://www.energyvanguard.com/blog-b...ay-Be-to-Blame
Very possible it was static pressure, thanks for the links.
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Old 02-07-2015, 10:55 PM   #6
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Re: Whose Right? Hvac Quotes


Quote:
Originally Posted by klintala View Post
We just finished an addition where the new was run straight off the end.

How far of a run are we talking?

Also, I don't see a reason for needing to switch your existing.

It would be interesting to hear from someone with more knowledge what benefits a 90 and downsizing might have.
I'll measure as soon as possible.
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Old 02-07-2015, 11:12 PM   #7
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Re: Whose Right? Hvac Quotes


I would go off the top of the trunk as opposed at the end. Static pressure thing. Bitchin air off that run is likely, but, it might effect the flow to the rest of the system.
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Old 02-07-2015, 11:14 PM   #8
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Re: Whose Right? Hvac Quotes


the reason the trunk duct is capped is so it builds static pressure (bursting pressure). Taking a heat run off the end of the trunk reduces the static pressure in the duct and therefore affects the other heat runs on the line. It is poor trade practice to install a takeoff on the end of a duct.

As far as changing the existing runs, they should be sized to deliver the correct cfm to the rooms they serve, regardless of their size. This might mean a 6" per room, 2 - 5" per room, etc. In residential, this commonly ends up being 5" runs, and lots of installers only put in 5", and some obviously think it has to be 5".

a round duct 90 elbow produces as much resistance as 5' of straight pipe.
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Old 02-08-2015, 06:58 AM   #9
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Re: Whose Right? Hvac Quotes


A start collar of take of on the end cap will cause lots of turbulence, can actually cause less air flow in the supply. Better to take off of the top of the duct. The fewer ells the better, as it is less restrictive to air flow.

A 5" round doesn't flow much air, so a 6" will be better. Its easier to damper down a 6" to decrease air flow, then to get a 5" to flow more air. Smaller supply runs don't increase the comfort in a house.

He was worried about static pressure.
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Old 02-08-2015, 01:08 PM   #10
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Re: Whose Right? Hvac Quotes


Quote:
Originally Posted by klintala View Post
We just finished an addition where the new was run straight off the end.

How far of a run are we talking?

Also, I don't see a reason for needing to switch your existing.

It would be interesting to hear from someone with more knowledge what benefits a 90 and downsizing might have.
The takeoff from the plethum is approx 10' 6", the distance from the end cap to the room I want to heat is approx 15', and will require 2 90's.
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Old 02-08-2015, 01:32 PM   #11
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Re: Whose Right? Hvac Quotes


Can a side takeoff be used to eliminate 1 of the 90s.
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Old 02-08-2015, 05:42 PM   #12
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Re: Whose Right? Hvac Quotes


Quote:
Originally Posted by Antropovsky View Post
it will cause static electricity and change the flow.
So he should ground the pipes or use those things that are at the trailing edge of airplane wings?
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Old 02-08-2015, 09:09 PM   #13
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Re: Whose Right? Hvac Quotes


Quote:
Originally Posted by GettingBy View Post
I usually get enough bids for them to cluster around some price. I throw out the high & low bids and think over the others.
Lol. That's an interesting strategy. Just get a bunch of bids and the guys in the middle probably have it right.
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Old 02-08-2015, 09:29 PM   #14
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Re: Whose Right? Hvac Quotes


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Originally Posted by slowsol View Post
Lol. That's an interesting strategy. Just get a bunch of bids and the guys in the middle probably have it right.
And if two or more middle prices are the same it may be price fixing. And if a guy says he'll match a lower bid it is a close cousin to price fixing.

All bidders in your area are using the same labor pool and available materials. Way low or way high outliers are suspect.
http://en.wikipedia.org/wiki/Outlier

I had a dent in my car. The mean (average) price was ~$200 and one guy quoted me $600 while intently watching my face, but I deadpanned it.
He didn't get the job.

Last edited by GettingBy; 02-08-2015 at 09:32 PM.
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Old 02-08-2015, 09:44 PM   #15
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Re: Whose Right? Hvac Quotes


Quote:
Originally Posted by Antropovsky View Post
The takeoff from the plethum is approx 10' 6", the distance from the end cap to the room I want to heat is approx 15', and will require 2 90's.
That's about what I was picturing.

My (inexperienced in hvac) opinion is that you're not going to want the additional resistance of a 90 coming off the end cap if you're run is going to be 15ft + 90s. ...Unless all of your other runs are of similar distances.
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Old 02-09-2015, 09:35 AM   #16
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Re: Whose Right? Hvac Quotes


Quote:
Originally Posted by klintala View Post
That's about what I was picturing.

My (inexperienced in hvac) opinion is that you're not going to want the additional resistance of a 90 coming off the end cap if you're run is going to be 15ft + 90s. ...Unless all of your other runs are of similar distances.
So if it comes off the side or top a few feet from the end cap, and still includes 90's.... that is going to be more effective (Just curiosity, since It's the alternative method)? The quote that came off the side needed a couple 90's, while the one of the end, eliminated at least one.

Maybe i'll try to diagram it.

Last edited by Antropovsky; 02-09-2015 at 09:42 AM.
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Old 03-01-2015, 09:50 AM   #17
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Re: Whose Right? Hvac Quotes


Coming off the end cap is a code violation.

The reason it is a violation is because according to "SMACNA" and "ASHRAE" it changes the design protocol from an "extended plenum" or velocity pressure system to an octopus or straight velocity.

In an extended plenum all branches can be balanced; because the pressure builds in the duct and is equal (if installed correctly) at all branches. In a straight velocity system each branch is depending on its location of origin for it balance.

The reality is when you put a run out of the endcap in an extended plenum you change the characteristics of the whole system, making it very difficult to balance.

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