HVAC Insane part 2
Sorry, I could not figure out how to add to my original post.
[Editor’s note: To post a comment on a Q&A thread, just type your comment in the box at the bottom of the page labeled “Post new answer.” You have to be registered and signed in to do this. Note that registering on GBA is free. The first half of this thread — the one that Mid Michigan started — is here: HVAC guys. Is this insane?]
Ok so the furnace is a 96 percent and gets its combustible air and exhaust from 2 1/2 pvc pipes. The two 4″ vents are for the occupants. The one plumed into the cold air return and I kinda like the idea, but think 4″ is too much vent air. The other hole is basically a passive 4″ hole in the basement. I guess if the house gets depressurized then it will let some outside air in. I trying to go around and seal tiny little leaks yet ignore this one?
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How large is your house, how many occupants, and do you have the results of a blower door test?
Mid,
OK, so we know that the duct has nothing to do with combustion air, since you have a sealed-combustion furnace.
The two ducts you describe are an extremely sloppy attempt at providing ventilation.
The duct that brings outdoor air to your return-air plenum sounds like it's part of a central-fan-integrated supply ventilation system. (For more information on this type of ventilation system, see this article: Designing a Good Ventilation System.)
If the system is properly installed, it needs (a) a motorized damper that is controlled by (b) a Fan Cycler control (also called an Air Cycler control). If the duct lacks a motorized damper and a Fan Cycler control, your house is likely overventilated when the furnace is running, and underventilated when the furnace isn't running. When the system is overventilating, it will be wasting a lot of energy. So get the missing parts installed.
As for the duct that is wide open: remove it and seal it up.
A 4" hole near grade will contribute to stack effect convection infiltration, even if it's passing through some duct work. An operable damper (powered or not, part of an automated ventilation system or not) can fix that.
A couple of 4" holes doesn't guarantee a ventilation rate either. Ceramic core HRVs can guarantee the rate, as can exhaust only systems (with caveats about uncontrolled ventilation air paths, which may be contaminated with soil gases or worse.)
Going back to the original thread and pictures, is the foundation (or band joist) insulated AT ALL? It doesn't appear to be, and could be the next real energy conservation project to take on, since it's bound to be a double-digit percentage of your heating energy loss.
https://www.greenbuildingadvisor.com/sites/default/files/20170930_144556_HDR.jpg
Martin , I click the link you gave me and you were right about the cheap mans supply side ventilation system being installed in my house. So now I'm kinda in a pinch, since I don't have a lot of free funds right now.
I have a temporary solution of installing a spring loaded back draft damper and then put a lever operated damper in front of that. At least I will be able to shut off some of the vent when it starts to get really cold. Or, do you think I should just seal this one off also?
If you want to use the air handler for ventilation, a 4" round lever-tweakable balancing vane + 4" butterfly backdraft preventer would be just fine.
If you want to do more targeted and better controlled ventilation, you might just go ahead and seal it all up, then budget for doing a more targeted ventilation scheme next year (and just crack open some windows if/when it gets too stuffy in the mean time...)
It's really your call, but I'd opt for the balancing vane + backdraft option, tweaking the balancing vane open if the mid-winter indoor air is approaching 40% RH @70F, throttle it back down if the winter indoor air too dry. Then just leave the balancing damper at that setting and see how fresh the indoor air feels in the shoulder seasons & summer. (That's what I'd do- YMMV.)
If it needs more ventilation than that using additional ventilation in a more directed manner is probably going to be more satisfactory than seasonally tweaking the balancing vane.