On Jun 15, 3:04=A0pm, rdad...@[EMAIL PROTECTED]
(Dick Adams) wrote:
> When I needed a new roof, it took me over a year
> to find a rep who knew roofing. =A0One rep told me
> the job would be a roof over and would need 24
> squares. =A0I knew the roof already had two layer
> and would be a tear-off - plus the roof was only
> 24 squares. =A0Needless to say I am leery of home
> improvement reps.
>
> But now every one with whom I have spoken about
> a new AC unit has a Master License (I checked).
> But not one of them has done a load calculation.
> And the lowest bid for a 2.5 ton, Seer-15 is
> $5,400.
>
> Where can I find the math for a load calculation?
>
> Dick
Here is the organization some application are modeled after
http://www.acca=
..org/
You can get the books from them on how to do a Manual J (load calc), D
(duct sizing), T (Air distribution basics), S (system sizing).
I have done most of the design through the manuals and my advice is to
hire a contractor that has the software to do the calcs. Be sure they
do a complete *****sment of the thermal envelope, a blower door test
pressure balancing and a duct test and possible an infrared camera.
Doing a heat loss/gain calc is worthless without knowing what you
really have. I do not know anyone that can guess at the natural Air
Changes per House of a house or the CFM duct leakage.
If your contractor only installs the AC unit without testing out it is
a second rate job.
1) Superheat or Subcooling depending on a TXV or not
2) Duct test for leakage
3) Static pressure in the ducts
4) Flows from each grill to balance the system
5) Pressure balance room to room
a. To be sure there is no back drafting of natural draft
chimneys if you have any
b. To reduce forced infiltration/ exfiltration by the fan
blower
6) Measure the system flow to be sure your getting what is rated (this
normally requires new ducts and a larger return)
7) Install dampers in each duct to do balancing
8) High efficiency filter in the return grill
a. Grill must be large enough to reduce static
b. Filter is at the return to keep the whole duct system clean
A properly sized system will be nearly silent, draft free comfortable
and low utility bills.
Does the contractor subscribe to Building Science, if not find another
that does. May things in a house are connected and fixing one can
upset another.
By the way it is unlikely the bid for $5,400 will get you any of this,
buyer beware...
Hope this helps
Andy
Whole House Performance is the right thing to do
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