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Posted by geothermaljones on June 14, 2007, 11:35 pm
Wow...
Who on God's big greeny sizes cooling based on the Heat Index?
We've got -60 & colder wind chill factors here, but damned if I'm gonna add
another 40 degrees to my heat load calculations.
Size for design conditions... the 2.5% Ashrea column is what I use for
cooling... Heating 1%
(but we Minnesotans like our heat in the winter) Most run loads & add
10-15% for heat
Some loads software based on the latest ACCA standards had issues as they
started adding a % for cooling & the systems were being oversized...
(use the calculated cooling tonnage & not the "recommended" tonnage on Elite
Vers. #8)
ACCA data can be a bit large, especially when having to guess at any amount
of "worst case" construction materials, but it's the industry standard, &
even those who write there own load software base it on Man J.
Sizing for shade trees & awnings can be risky too, construction materials
are the only constant.
I lost the 300 yr old oak in Sept 2005 to a tornado, lost 75-80% of my 10am
to 7pm shade on the roof & back yard,
my 2-1/2 ton system is running longer & I'm rebalancing rooms as the heat is
much greater in a few areas.
I'm upping it to a 3 ton 2 stage heat pump when the dead presidents allow.
On the up side, I can now see my house on google earth, & we've planted a
garden for the first time since we bought the place.
As for "skin effect" uh...???
Grab a Psychometric chart & measure the actual heat content of a "pound of
dry air" at the different conditions.
It's called enthalpy (h).
Mpls design (89db/73wb) h= prox 36.3 room temp (72db/ 50% RH) prox 26.1
The wet bulb in Salt Lake & Vegas require adding moisture to reach 50
Relative Humidity.
I've seen systems in the high desert sized for 300-400 sq ft per ton
residentially because there's no humidity,
Boy you can cool a house down quick that way...
If you sized like that in our area, you'd have cool clammy & cantankerous
customers, & probably very little word of mouth or repeat business.
Most residential systems in my area (new or "updated" construction) tend to
fall in the 750 -850 sf/ton.
Undersizing for longer run times will wring out more moisture, & even if the
system starts to spiral up when the temps exceed design for extended
periods, it's dry air & isn't nearly as uncomfortable...
Of course it's good to explain this to your customer & get their okay.
Show them some Bin Data & the actual hours the temps exceed the design.
This will really explain the Ashrea 2.5% & 1% columns...
Finally, what did you pay for?
If you paid for a 2 stage, you ought to get it, though, if it's not in
writing, that may be tough.
2 Stage will allow for oversizing with the nuts to cover temps exceeding
design, & the lower capacity to cover the majority of the cooling hours.
If your sizing a 2 stage heat pump, by all means oversize, you'll get more
heat in the winter months, drive down the balance point of the system, &
still cover all the afore mentioned summer cooling needs.
But then again I could be wrong...
goodluck
geothermaljones
st.paul,mn
> Greetings,
>
> I am badly in need of opinions. Although I have seen this issue discussed
in
> several threads, I still am uncertain as to which direction to head.
>
> I recently replaced our AC. At the time of the quoting process, my house
was
> calced at 31/2 ton (1800sf, ss, 100+ summers). Upon selecting the
> contractor, I asked for the 16 seer. I was told it only came in 4 ton, but
> since it was 2 stage, it would not be oversized for my house. I went with
> it. 2 weeks after install, I was on the manufactures web site and saw that
> the unit was not 2 stage. I presented this to the rep and he got back to
me
> admitting he had made a mistake and that brand did not have a 4 ton 2
stage.
> Although, he claims they will make it right, he feels that the 4 ton
single
> stage will be OK for our house/area (of course). So far we have not had
> enough hot weather to heavily test the unit. We have had a few warm days
> where we ran it for a few hours. I did notice it being slightly humid, as
> far as what constitutes a short run cycle, I am uncertain. It didn't seem
to
> start/stop excessively.
>
> So, should I press for a change out now, wait for hotter weather before
> deciding, or is the difference between 3.5-4 ton so marginal that there is
> likely no problem? Thanks in advance for any advice.
>
>
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