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Posted by on January 20, 2007, 2:38 pm
> Dam it! I was afraid to hear that! Thanks for your time and
> suggestion.....
>
> When it was suggested "someone who knows distribution systems" is this
> referring to another HVAC technician or an Engineer? What do I look for
> in the Yellow pages?
That's your first problem... call and ask around for a competent company!
Who you say? Friends, family, neighbors, co-workers, HVAC Distributors,
etc... who they use and are they happy with the results?
> Was that one 20 x 16 duct for supply and another 20 x 16 for return?
>
> I'd prefer to have the duct system designed and do the work myself as I
> have full access to duct work...... and no money.
It's not likely that you will find a company that will properly design a
system and than hand over the plans to let you "do-it-yourself".
If you do, you should expect to pay for their time and design layout.
Remember, you want it done correctly. Not cheap as possible.
> I'm still puzzled why the furnace has only a 14"x14" supply duct;
> however it suggested I'll need a 20" x 16"?
>
> Please bare with me, I'm really trying to understand how all this works
> so I won't make that same mistake twice.
>
> Thanks again for your suggestions.
>
>
>
>
>
> gofish@gonefishin.net wrote:
> > "Double A-Ron" wrote:
> >
> > >Recently replaced my forced air oil furnace with a 5 ton dual speed
> > >WaterFurnace Envision. My HVAC guy told me the not to worry about my
> > >ducting as the ECM motor will overcome the lack of duct size. I mainly
> > >concerned about my return air vents.
> >
> >
> > your hvac guy is a clueless brain dead moron for giving you such
> > false advice.
> >
> >
> > >
> > >Here are some details;
> > >
> > >- 2 story house - 2800 sq/ft
> > >- Near Toronto Ontario
> > >- Two supply ducts 12"x8" and 12"x7"
> > >- Two return duct 14"x6" and 12"x8"
> > >- 18 heat registers feed by 5" dia ducts
> > >- 4 return air vents 15"x7" (2 up stairs, 2 downstairs)
> > >- Design heat loss of 63,312 btu/h
> > >- Heat gain 30,000 btu/h
> > >- Furnace blower settings - 1st stage CFM=1500, 2nd stage CFM=1950
> > >
> > >Quite honesty everything seems to be working fine...... slight noise
> > >from downstairs return vents, however we're warm. Defiantly much long
> > >run times over oil.
> > >
> > >I've read many times the CFM should be 400 per ton (cooling?)..... I'm
> > >nowhere close. So I having trouble sleeping at night not know if I my
> > >ducting is incorrect or I shouldn't worry as the ECM motor overcomes it
> > >by pushing more air. Also adding to my confusing is the supply duct on
> > >the furnace measures only 14"x14".
> > >
> > >I'd very much appreciate your thoughts and advice,
> > >Thanks
> >
> >
> > airflow is dependent upon the restrictions imposed by friction losses
> > of the duct system and obstructions such as filters and air
> > conditioning coils. also, the grills themselves impose restrictions
> > to the air flow.
> >
> > to achieve the nominal 400 cfm per ton, its obvious you cant change
> > the coil, although you can change air filtration and ducting. but why
> > sacrifice air filtration because your hvac tech is brain dead?
> >
> > the solution is to design the duct system (both supply and return) so
> > that it imposes a very nominal total static pressure on the fan motor.
> >
> > now a variable speed blower will help overcome some of these
> > restrictions, but it will not correct a faulty design. and it will
> > cost you more money in operational costs than if the duct system was
> > designed correctly from the git-go.
> >
> > my ductulator says that if i want to move 1950 cfm at .08" wc, my
> > round duct size must be 19" OR a 20 x 16 duct. OR two 15" round
> > ducts.
> >
> > why .08" wc? well, the maximum your fan will deliver is in the .5-.7
> > range. Now add in a wet coil factor of .35 to .45, throw in a dirty
> > filter or whatever, then the grills, and whats usually left is a
> > number under .1 for the duct system. there are different ductulators
> > for metal or flex ducts. A smart ccokie will design around the .08
> > figure and be very very close 99.9% of the time.
> >
> > my suggestion is to find somebody who knows how to properly size air
> > distribution systems.
>
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