@Tom, low static return paths ensure a central return in the basement or in any other locations won’t cause negative pressure. This is very easy to test & verify. There shouldn’t be any open combustion appliances in today’s tight homes (my comment was in response to someone building a new home), and you definitely don’t want open combustion appliances (or even a seal combustion appliance) in the same room with an open return plenum.

The other systems you mention can create negative pressure, but that has nothing to do with having a central return. In any case, basement homes in areas known to have radon should have sub-slab venting and/or exercise vigilance when it comes to testing. Again, my comment was in reference to new construction. If someone wants to convert to a ductless return in an existing home, they need to pay attention to the caveats I mentioned.

This takes me to my next question, in (good) residential mechanical design, are houses ever designed positive pressure? I always see them negative or switching between negative and close to zero, depending on what is happening with combustion air, clothes dryers, kitchen and bath exhaust. I work in commercial construction. In hospitals, labs, even down to a small office building we always have minimum outside air and it is designed and balanced to exceed all extract air, making the building as a whole slightly positive. I have never seen an outside air duct running to a residential system except for combustion or HRV/ERV. These devices are still designed just as an even air exchange, right? Shouldn’t there be a fresh air duct to the return to bring in (x) cfm of outside air? I mean it’s not going to fix the people’s houses who demanded their kitchen be outfitted with 3000cfm commercial fume hood without make-up or DOAS. But why are our houses all negative by design (or lack thereof)?

Since Andrew asked about the need for vents vs semi-conditioned, I assumed he was referring to heating & cooling supply vents. If atmospheric combustion appliances are present, those obviously require a pair of outside air vents as per code.

As an aside, I can’t believe code still allows atmospheric combustion furnaces and water heaters. Code (in most locations) has required builders to meet verified air tightness thresholds for many cycles now, but still allows open combustion appliances with big holes in the air barrier to keep from killing the occupants with poisonous gases? Go figure.

Allison wrote: “If the gas water heater is of the direct vent, power-vented, or sealed combustion type, it doesn’t need its own isolated room.”

I was referring specifically to using the mechanical room as the return plenum (i.e., ductless return). In that case, even a sealed combustion appliance cannot be in the room. A sealed combustion furnace would obviously need to have a ducted return, but you can still get nearly all of the benefits of a ductless return by installing a large central return in a wall facing an open area in the basement.

To clarify part of what David said, not all gas water heaters and gas furnaces need to be treated separately the way David describes above. If the gas water heater is of the direct vent, power-vented, or sealed combustion type, it doesn’t need its own isolated room. For furnaces, the atmospheric combustion type also should be isolated just as a natural draft gas water heater would. Sealed combustion don’t need to be isolated but as David said, it’s not a good idea to have an open plenum return with gas appliances in them in case of a gas leak.

Here’s an article I wrote a long time ago about creating an isolated room for atmospheric combustion appliances. Keep in mind that it’s nearly impossible to do this in an existing home.

Making Your Home Safer with a Sealed Combustion Closet

Tim

Same as my answer to Andrew

What kind of venting?

If the Mech room has a natural draft chimney which it should not, it would need combustion air. Assuming the room is tight as it should be the volume of the room cannot be used, it would need a dedicated opening to the outside. Again, in todays world there should never be a natural draft chimney. If there is a powered exhaust vent such as a dryer or exhaust fan or the range hood or bath fan could make the room negative in pressure and backdraft the chimney. This would be a “BAD” idea.

If the venting is to remove unwanted heat this would require an assumed delta T from inside to outside. This would only work if the outside temp is less than the Mech room temp. When times of low delta T it would require a large air flow to keep the room cool or at least close to outside temp. If there is an exhaust fan to do this where will the makeup air come from? Likely unfiltered, not ideal. If supply it needs to be filtered and maybe dehumidified.

If the venting is to remove moisture it can have the same issues as removing heat plus when the air outside has more water in the inside it will be adding water and not removing it.

This would need more info to decide on what to do.

@Andrew, a mechanical room should not get a supply vent; indirect conditioning will be more than adequate. But I recommend a central return located in the basement. In the case of a heat pump, the mechanical room can be a return plenum (i.e., ductless return). I typically mount the air handler on a riser with filters on two sides, no grilles.

If you have a gas water heater, it must be isolated – for example, by framing a closet within the mechanical room (use exterior type door & threshold). If you have a gas furnace, the mechanical room cannot be a plenum. In that case, install a large central return filter grille in the wall facing an open area.

A central or ductless return has several advantages: it saves the cost of installing return ducts, simplifies supply duct routing through the floor system (i.e., no return-supply crosses), and improves blower efficiency (i.e., by reducing external static pressure). Most importantly, in homes with basement mechanical rooms, routing the return air for the whole house through the basement greatly improves air mixing. This allows the A/C to keep basement humidity in check when there’s little or no sensible cooling load.

Providing for low-static ductless return paths costs very little if planned for during the design phase. See my comment in Allison’s article The Central vs Dedicated Return Vent Debate for additional details and external links on how to design low static return paths.

Good idea!