I have some weird and what I believe to be abnormal duct under my home. I recently installed a brand new HVAC system and it's not running or performing as desired. I believe I am losing a ton of efficiency due to this duct work being concrete and cinderblock. It looks to be sealed from the few pictures i could get after removing the furnace. HVAC installers said they hadn't seen this before. Now that I am using the system my in home temp rarely reaches the desired temp, and in the 100 degree heat it's only getting about 20 degrees cooenter image description hereler inside, 23 degrees cooler at most. I expected this system to get my house to 68 degrees no matter how hot it was outside! Does anyone know if there is any spray/easily installable coating that can be applied over the concrete/cinder block to help seal/insulate this duct. My coil temp has air at 44 degrees, yet by the time it flows through the ground back out of my vent's its up to mid 60 degrees. Losing 20 degrees in the short span underneath my house seems like a ton to me. Is it normal, or is this due to bad duct work? and do I have an option? Please Help! I am at a complete loss!

  • Is your furnace pumping air directly into this "ductwork"? Is this considered your "trunk"? How big is this space? It appears to be 24"x24"?
    – MonkeyZeus
    Commented Jun 18, 2021 at 17:25
  • What part of the country are you in?
    – MonkeyZeus
    Commented Jun 18, 2021 at 17:30
  • @MonkeyZeus I am in Albuquerque New Mexico. Yes the air is pumped directly into this ductwork, which I imagine is exactly what you measure at about 24x24. Unfortunately the new unit is now on this opening, and I have no access to it other than floor vents! I think I'm just pretty screwed. Velocity of air can't be good as this opening is so huge, and losing 20 degress in an extremely short distance seems a lot but maybe I'm wrong.
    – TannerR
    Commented Jun 18, 2021 at 18:15
  • How big are the floor vents
    – JACK
    Commented Jun 18, 2021 at 20:06
  • 24x24 would suit a 4,700 square foot house. However, assuming all of your registers are sized properly then that space will simply fill with conditioned air until it pressurizes and it will be pushed through the vents at the correct velocity. You likely have to wait longer than normal to get conditioned air in your vents but it should be the correct temperature.
    – MonkeyZeus
    Commented Jun 18, 2021 at 21:08

1 Answer 1


If the air coming out of your registers is about 20°F colder than the temperature of your home then things are operating properly.

See https://iwae.com/resources/videos/cold-air-conditioner-ask-expert-episode-69.html

However, if the volume of air coming out of the registers supplied by this cinder block ductwork is considerably less than comparable registers then you have a turbulence issue affecting air velocity most likely triggered by that corrugated "ceiling".

You should get a static pressure test done which compares the pressure of your main supply versus your main return. Any HVAC technician worth their salt will be able to tell you exactly what the problem is. However, the installer was probably just a laborer so their inexperience can be ignored.

I think fixing the corrugated ceiling will be more beneficial than sealing the cinder block.

Although, if you're already working inside the tunnel then might as well get the wall surfaces as smooth as possible too. I'm not sure which products will provide a long term solution but also not be hazardous to your breathing health even after fully cured.

  • Using the ground for environmental temp stabilization is actually a very old method and trying to further cool may not be as efficient. 20 degree that sounds good to me also but if the op wants cooler faster mini split may be the better way to go or insulate the duct and loose the natural stabilization that a duct in earth provides.
    – Ed Beal
    Commented Jun 18, 2021 at 17:33
  • 1
    @EdBeal Looking at all that "natural" dust I'm starting to think the inventors of that very old method really hated their client's lungs: dust, radon, literal dirt, etc...
    – MonkeyZeus
    Commented Jun 18, 2021 at 17:35
  • I guess you haven’t looked at many modern ducts. Believe it or not there are quite a few home owners that do not realize they need to change there furnace filter and do so only after things quit working.
    – Ed Beal
    Commented Jun 18, 2021 at 18:38
  • This looks like it was the space where fibreboard ductwork, flexduct or metal pipe ducts should've been, instead of airflow going into an unsealed space.
    – user113627
    Commented Jun 19, 2021 at 2:13
  • @EdBeal Oh don't worry, I believe it. However, just because the homeowner is likely to neglect their system does not warrant the original installer to be so irresponsible of OP's duct system; it's a surefire way to get radon directly to where it's needed least. However, I wouldn't doubt that OP's situation was contrived by a homeowner ¯\_(ツ)_/¯
    – MonkeyZeus
    Commented Jun 21, 2021 at 14:38

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