I have a Trane furnace.
One of the heat registers in my livingroom is underneath my sofa.
There are two registers in that room.
Is it safe to completely close off the register, or would it be better
to leave it open a tad?
I am not sure if this could damage my unit, and thought I would get some
expert advice here.
Properly designed, modern units can handle a 10-15% blockage of discharge
air. So if that furnace has 10 total vents closing one is no big deal.
It might whistle a little if it is on the high flow side of the plenum. If
it does you may want to reconsider.
If it is an older system or you experience any noticeable change you can
open it back up. It isn't going to kill the system on a trial basis no
matter what. The worst thing that can happen is the furnace will shut down
via the high limit switch.
Ive been in the hvac business almost 30 yrs now colbyt.
Could you please point me to where you read that: "modern units can
handle a 10-15% blockage of discharge air."
Im not arguing that you cant close one register. I want to know where
you pulled that number from.
........and if its a newer system, you can split the heat exchanger
with repeated limit switch shutdowns.
That would be a quote from a certified engineer who was working on
redesigning some systems a few year back so "modern" is a relative term.
I do agree that excessive high limit shutdowns can cause problems with the
modern tin heat exchangers.
but for other elderly family members I put sofas and chairs, and a
couple beds on platforms so the elderly could still stand up....
my high school buddy did this for his wife after she ..had knee
Kate, you might consider getting a long deflector for the register under
your sofa. For example:
"Use the heat register deflector to channel the heat into your room and
not into the bottom of the couch. Our PVC air register deflectors sit
over the register and re-direct the cooled or heated air from under the
furniture. Simply place air register deflectors over the register. The
air register deflectors click securely onto the floor register and
extend from 22" to 37" bringing the air out from under your chair,
couch, bed, cabinet, curtains or end table.
"The heat register deflector is a 2-piece unit 11-1/4" wide and 1-1/2"
high to deflect air discretely. The air register deflectors come with
one long clip to lock onto the register and two short clips to lock the
I have some of these leftover. I had placed one under my sofa, but the
skirt on the sofa looked funny then. But, someone else on this
newsgroup suggested putting the sofa on blocks a bit. I will try this
and then install the deflector. Many thanks.
The real question here is what's the point to closing it off? The
heat you get out of it eventually makes it's way into the room. If
the room was too hot relative to other rooms, or the sofa is getting
hot, then it would make sense to try to balance it. But if it's ok
in that respect, then I don't see any issue with leaving it open.
On Fri, 06 Feb 2009 22:51:46 -0500, firstname.lastname@example.org wrote:
There are two registers in my bedroom BECAUSE the dresser is over one.
You guys saying it being blocked is going to damage my furnace?
All closing one or two registers is going to do is make the rooms with
the closed registers cold and make the other registers in the house
provide a little more air. By blocking one 4" pipe out of ten you are
NOT reducing the flow by ten percent. Not likely even by two percent.
The pressure in the plenum will possibly go up .1 of an inch of water
The blowers are generally rated at 1200-1700cfm at .3 inches water
Doubling the static pressure reduced the efficiency of the blower by
half. - not the delivery, but the efficiency in CFM/watt. The average
furnace fan is 10-15% efficient, and produces something like 2
cfm/watt. Actual airflow dropped by one third when plenum pressure
was doubled from .6" to 1.3"
In a study of furnace installations, plenum pressures were in the
range of .25 to 1.9".
That same test shows that a brushless PM blower motor is SIX times as
efficient as an AC split cap motor at low speeds (used in constant
flow systems extensively here in Canada). This amounts to a 74%
savings in electricity used by the furnace - and as much as 25% of
whole house use. The loss of heat output from the inefficient fan
motor increases the gas consumption by 14%.
(I guess that's why I didn't see ANY gas savings when I put in the new
Medium efficiency (80+%) furnace but saw a huge difference in
If you doubt what I'm reporting you can read the report yourself at
The test was done under the auspices of the Ernest Orlando Lawrence
Berkeley National Laboratory at the University of California by Ian
Walker and Jim Lutz. for the California Energy Commission and the US
Department of energy.
VERY interesting reading if you are into that type of thing.
Apparently YOU did NOT READ this research draft.
I suggest you DO READ page 13 wherein the TEST SETUP is described.
Here's an excerpt pertinent to the issue of whether a furnace can be
damaged by blocking heat registers:
"The burners did not operate during the test and no gas was hooked up.
No cooling coils or filters were installed. In real field installations
the cooling coils and filters will act as additional flow resistance
leading to increased pressures across the blowers."
In other words, ONLY the air-handling characteristics of the furnace
BLOWERS were being examined in the test reported in this study, NOT the
heat characteristics of the furnace and their effects.
Blocking supply registers can result in damage to a furnace by
decreasing the heat dissipation from the heat exchanger and its
I did read it, and I did understand it. And what's more I can test
the static pressure in my system if I want to. I know how and caould
make the required test equipment very quick;y and cheaply.
And the airflow change (which can be tetermined by the pressure
change) is all we need to know to know if there will be a problem.
Accorsing to the information shown in the graphs included, it is
obvious it is not going to cause a problem unless there are fewer than
something like 5 outlets and you block one.
In my house there are mo less than 12. For most of the time I have
owned the house (26 years - 20 with the original (at that time 8 yr
old) furnace) at least 2 of those registers were completely closed,
one has been under/behind a couch, and 2 are shut off about half the
time when 2 bedrooms are not needed (daughters not home).
Add to that the FACT that many homes have balancing dampers installed
in the branch runners - my house has at least 3 (exposed in the
finished basement) Two of them are set to about 2/3 flow (on the six
inch pipes heading to the upstairs of the 2 story) to restrict the
flow to the upstairs so enough heat gets into the main floor.
This was all checked when the new furnace was installed about 7 years
ago, and the plenum pressure/airflow was all well within the specified
parameters. Both the pressure difference across the exchanger, the
heat rize across the exchanger, as well as the pressure in the plenum
was checked, and all were well within limits.
I don't have the pressure numbers but IIRC both pressure readings were
well under an inch of water.with the fan speed adjusted for the
desired heat rize. The installer did say the ducting was "generous".
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