Question for the group:
I moved into a house with a 1990s Lennox whisper heat furnace. The
installed filter is the 4" thick kind, not the usual 1" kind. So I've
been paying 30 bucks every six months for new filters.
However, I dug up the technical furnace manual on the net. Turns out,
it is specced for the 1" washable filters.
Would I be safe rigging up the furnace to run 1" filters?
I was contemplating welding in a little bracket to do so. The smaller
filters, even the nice ones, are so much cheaper even with the more
Let me get this straight, you want to redesign the filter system to go to a
1 inch pleated filter that has a higher static pressure (resistance to air
flow) to save $10.00 a year?? It will cost you at least twice that much in
higher utility bills to make that change. BTW... just because it says that
the 1 inch filter is good "up to" 3 months, doesn't mean it so..... like the
insurance commercials where they say you could save "up to" $500 on your car
insurance. Don't believe it. The 1 inch filters need to be replaced *EVERY
MONTH* (about 10 hours of actual run time)
You must either be a landlord, or an EE... neither of which has a clue about
how HVAC systems are designed to work, or why they work that way.
Wow, what a HELPFUL post. This tells me EVERYTHING I need to know and
isn't flamebait at all. You really have me pegged. How are you so
I can't WAIT for your next post.
yup, another EE trying to re-invent the wheel.
The 4 inch pleated filter was installed because of lower airflow
restriction(static pressure), it exceeds factory spec, and you only have to
replace it twice a year. If it isn't broke, don't *fix* it.
As far as the dirt and dust in the 55 year old ducts, get them cleaned or
replaced. They don't last forever.
Some people change the oil in their vehicle unnecessarily. Fleet
operators actually test their oil to see if it really needs changing.
Get yourself a Filter Minder, there are several different devices on
the market to let you know when it's time to change your filter. Here
are a few links to examples:
That is a nifty idea. Cheap too. In my situation it would definitely
pay for itself within a year.
Do you have any thoughts on why my system was installed with the big
4" pleated filter? is that a factor of overall CFM, pressure, fan
size, etc? I am also concerned that a pleated filter had been
installed when the tech manual didn't call for it.
If it helps, I had to open a return air duct this week, and it was
filthy inside. Granted it probably hadn't been opened since it was
built in 1955, but there was a think cohesive layer of dust on the
bottom of the horizontal cold air return duct.
That is probably the whole reason for the 4" media filter right there. The
system had too high of a TESP and they used a better filter to help
compensate for it rather than fix the ductwork issues that caused it in the
first place. TESP is total external static pressure, AKA how restrictive
the return and supply duct systems are to airflow. The TESP has to be
within a certain range specified by the equipment manufacturer. Too low and
the blower motor overloads and burns out as the torque required to turn a
centrifugal blower varies directly with the airflow through it. It can also
cause the primary heat exchanger to run too cool and cause condensation in
it thus rusting it out. In cooling mode too much airflow can make the
condensate fly off the evaporator instead of draining to the pan thus
wetting the inside of the supply duct where it re-evaporates and raises the
humidity in the house and also drains out and causes water damage. Too high
pressure and the airflow drops causing the blower motor to overheat from
lack of airflow, and causing the heat exchanger to overheat and crack. In
cooling mode the coil runs too cold and can freeze and the reduced
temperature lowers the suction pressure and causes the compressor to pump
less thus reducing the cooling capacity of the system.
As you can see if the 4" filter was the easiest way to make the system work
within the specified limits I would keep using it. It is either that or
have extra returns and supplies added as necessary and have the existing
ductwork cleaned to lower the TESP enough that the 1" filter will work.
Common sense...doesn't exist. It is what you believe others should
know, based on your years of experience. Its a myth. Nobody learns
anything without asking questions and doing stuff, and that includes
What is an EE?
I cant help but feel sorry for people who are territorial about their
profession. There is nothing an HVAC tech can do that I couldn't do
without the proper knowledge and equipment. You've admitted it
yourself: it's a series of calculations and some book knowledge. It's
basic mechanical engineering with maybe a touch of fluid dynamics. And
there are those who insult people who haven't learned it yet and are
stupid enough to wonder about it.
The only difference between home electrical and home HVAC, is that
there aren't a bunch of DIY books on the subject. And someday that
will change. Just like with all the other trades, people will hire a
pro only when the job is too much to bite off. And they'll have the
knowledge to properly tackle the jobs safely. No longer will we have
to suffer $75 service calls just to get browbeaten.
For those who helped me, thanks! You've confirmed that my project is
best left untackled. And I will get a pro to help balance the system
before I finish the basement.
For those with disdain for the DIY'er... You might be better served by
specializing in fixing amateur jobs. Because all your secret knowledge
is being gradually pried open. There will be a day when people won't
have to take abuse from guys because they happen to bend sheet metal
and run calculations for a living.
Common sense tells you that if you hit your hand with a hammer, its going to
hurt, or if you stick a fork in an electrical outlet, its going to knock the
crap out of them. We have a lot of Darwin Award nominees that come in here
that haven't figured that out yet.
The ones who get insulted are the ones who won't listen to good advice and
council, and get nasty because they get told what they don't want to hear.
We get a lot of that in here because the folks asking the questions are
trying to do things that are inherently dangerous, without the proper tools,
and will put themselves, their familys, as well as their home at risk.
They can't print the books fast enough to keep up with new technologies. $75
only gets a tech there, and basic diagnosis. Repairs are extra.
Unfortunately, you can bring your broken air conditioner to our shop, so we
have to bring our shop to you in the form of a service truck.
Its more than the calculations, the technologies are changing very rapidly
indeed. To a point that there are only a handfull of techs are trained on
them. 2 of the systems I worked on today have serial controlled, variable
frequency, inverter drive heat pumps and modulating gas furnaces..... and
these are residential systems. You can now add "electronics tech", and
"computer tech" to the long list of vocations that are encompassed by the
Do a little homework on warranties, pricing, and problems. My wholesale cost
for a 3 ton, 4 cassette Mitsubishi mini-split heat pump is considerably
higher than a 15 SEER conventional system including ductwork. Just because
it looks good on paper, doesn't mean that its going to be better, or last
longer, or cost less less to install. Then there is a small problem of
technologies... the higher tech the system is, the fewer choices your going
to have for finding a tech that is actually qualified on it, and knows what
he is doing with it. OTOH, if you got more dollars than sense...
I'm not saying don't go with the mini-split, I'm just saying that you better
have a tech that has had the training, and understands the digitaly
controlled, variable frequency inverter drives that are used on them.
But you didn't take issue with is statement that the installed cost would be
about the same.
I'm only trying to clarify the issues. I have a choice between adding a
(single) mini-split unit and extending one of my air handlers. The air
handler is actually in the area I want to add (an attic), but the mini-splits
look quite attractive, for many reasons.
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