My house was built in 1995. I have been trying to correct a poorly design/poorly installed HVAC (gas heat and central A/C) system
for my home over the past couple of years. My house has two systems. One
for the main floor and basement and one for the upstairs. Thanks to your previous advice I was able to
correct errors in the ducting in the room over the garage. I did the heatpro calculations and purchased
a CFM meter to check how the actual delivery aligned to calculated.
I won't repeat all the history but the addition of another return (8" round) and closing off a couple supply vents (in rooms closest to furnace that
had two supplies one was shut down) made a large difference
and now the comfort in the over the garage bonus room is much better. I also had the bonus room floor spray foam insulated as well from the garage ceiling.
I still had a strange problem that whenever the second story blower went on the basement door would slam shut. I've also had problems with gas water heaters
backdrafting in the summer and the furnaces sometimes backdrafting in the winter. A couple years ago I installed a Tjernlund Draft Inducer and control system to
fix the backdrafting problem when either of the furnaces or hot water tanks fired. I haven't had backdrafting since this was installed. I still have
slamming basement door though.
The symptoms only seemed related to the second floor system (blower) not the main floor system. I assumed shoddy return air trunk sealing was the problem.
Today I decided to take a closer look. As expected the 6" round take offs from the basement trunk were just elbows dropped on top of trunk
with a hole slightly larger than the diameter of the pipe. As much as 1/2" of gap most of the way around the elbow/trunk.
So that is certainly contributing to the return leakage and negative pressure in the basement. What I found next really threw me however.
Hidden on the top of the return plenum I found a 6x14" grill mounted providing air intake from the basement directly into the
return plenum. Recall this is the HVAC system for the second floor not the main floor or basement. Worse still the grill was directly on top of the
horizontal plenum right above the vertical trunk running from the air handler (the point of maximum static pressure).
The return grill was deliberate by the installer but I can't think why he would want air supply from the basement for the second story unit when
he didn't do the same on the main floor system. Also the vent is large and so close to the blower it must really compromise the return effectiveness
from the furthest returns which are 60-70 feet away.
So I removed the grill and covered it over with sheet metal and duct tape (the proper metal type tape). I then sealed the take offs on top side of
plenum except for two that are buried in a finished ceiling. The door no longer slams shut when the blower goes on.
So this has been a long preamble.
1) Is there a legitimate reason the installer would put a cold air return direct to the basement?
2) Could he have meant to put this return on the main floor unit to be a return for the entire basement area?
3) Isn't it a well known problem when you put returns in the same area as gas burners due to negative pressures and
the increased probability of backdrafting?
4) Was this extra return vent just to allow blower to meet CFM target instead of properly sizing the return plenum
and duct system? A band aid if you will to a half done duct design/install?
5) The return grill was on the top side of the plenum just under the joist so not visible. Its almost as if it was
being hidden. If it was deliberate why not mount it on the side of the plenum so it could be in view and cleaned. Its location is very suspicious.
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