I am about to replace my 30 yr old forced warm air heating system. I'm
considering adding central ac & had the gas company come out yesterday to
give me an estimate. The main plenum (or hot air duct..whatever it's called)
is 8"x8" square sheet metal with 7 branch lines (which look like maybe 8"
round sheet metal) running in my 44'x24' split ranch. The gas company HVAC
tech told me that, while the setup was fine for heating, the ductwork was
"maxed out" & he wouldn't add central ac to it.
(The first HVAC company that came out didn't mention any such problem but
that doesn't mean that they just weren't out for $$$ & would install ac in a
My question is this:
1.Is the gas company correct in saying I don't have enough duct size to
connect a good ac system too it?
2. If so, short of ripping out the old ducts & starting from scratch, is
there a way to overcome the main plenum size problem?
(I am fairly handy & was even considering running another main duct from
just above the furnace into my attic (short run as I own a split ranch) &
then running branches off of this new line into ceiling vents to each room
just for ac) Then maybe the HVAC pros could connect a baffle so that some of
the air could be diverted into this new trunk line when I run the ac in the
summer??? (I live in Mass so our cooling season is only a few months a year)
That is very possible, since AC uses higher air volume and velocity.
or they dont know..and that will create problems.
First, not being a smartass, but no one here can tell you at all.
Neither I, nor anyone here has near enough information to run a manual J,
and then a D to calculate your duct runs, and tell you what would be proper.
I can tell you that 8X8 wont carry much air, and while it might be fine for
heat, depending on the actual size of the unit, it probably wont be near
enough for AC.
90% of the time, if the original duct is sized correctly and the home only
was set up for heat, the addition of AC requires new duct to be properly
Remember, the duct system is part of the system total. It must be correct in
order for all that money you just spent to work right.
Well...I dont know the laws in your state, but, here, any installation of a
heating and air system, requires permits, and the permit inspector will
check things like the duct ssytem. Should it be undersized, the permit
inspection process stalls. Homeowners here are not allowed by law to modify,
install, or screw around with ducts, or the systems installed with the
Your best bet, is to get estimates, and the ONLY company you need to go with
is the one that does a proper duct, and system sizing set of calculations.
Forget the name on the equipment he sells, since that is secondary.
If you dont have the right size ducts, particularly on a heater, since
todays heat systems are not like the ones from just 15 years ago, you can
end up with a bigger mess than you can imagine. It is possible that the
ducts you have now are not large enough for just a heater. Too restricted,
and the heat rise across the unit will be huge, and this will lead to
cracked heat exchangers fairly early in the units life if it is a gas, or
oil fired unit.
My home ~1925 single level has a central trunk that is 18" by 10" there
abouts. The air handler feeds into this plenum and all of the trunk lines
come off that. I have a 5 ton ac with a variable speed fan. Bit noiser than
the old unit I took out, more volume. 8X8 seems pretty small. Get prices on
doing it right and then evulate. There is nothing worse than spending money
and then finding out just a little bit more would have solved the problem
for ever. As others have said before the equipment is equal, it is the
installer that makes the system
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