Crazy idea

Smith19

Hello All,

I've gotten a crazy idea from the way commercial HVAC has been done for many years. Perhaps some of you could tell me if It'd possible, insane, dangerous, or why it doesn't exist. Why is it that residential hot air furnaces haven't got make up air supply ducts going to the return plenum? Just a damper that's controlled by the temp of the air exiting the supply plenum, and is supplied by a small vent in an exterior basement wall. I think this sounds like a great idea. Any reason why it hasn't been done?

Rich_49

It is

As houses are getting tighter mechanical ventilation has become necessary . They are called HRV and ERV . Unfortunately the hvacks are working for greedy builders and will continue to skip them due to cost .  These also should not be combined with the traditional ducted system but should be independent , see DOAS .

Bob Harper

Passive MUA

ASHRAE conducted a study and prove passive MakeUp Air (MUA) is very unreliable. However, there are a number of problems with simply ducting outdoor air directly into a return plenum:

-if the air is too cool, you can actually damage a heat exchanger. The MUA would have to be brought in far enough back so it gets tempered with stale indoor air. This requires a metering system.

-Outdoor air can be laden with moisture, odors, noxious fumes and chemicals, bugs, etc.

-If you bring in too much MUA close to the unit then that forces a reduction in return air from the far reaches of the building. The result is air stagnation and a drop in IAQ.

-if you bring in too much MUA, you can over-pressurize a room causing air to be forced into interstitial wall cavities. This can lead to mold in walls and "soot" stains on carpets.

-if the MUA is too cold, it can throw off combustion analysis which can lead to CO production, sooting, and a drop in efficiency

-If you bring in too much MUA, your TESP will fool you into thinking your lousy ducts are great and you'll adjust your fan curve improperly

-The duct you use to introduce MUA would need to be insulated to discourage condensation, pitched for drainage of said condensation, preferrably have an in-line duct heater to temper the air, have a fan to force the MUA in so that slight pressure gradient shifts won't adversely affect flow, slave the fan to the AHU's fan speed (multi-speed fan), need a high limit switch to provide a safegard on the inline heater, provide an thermocouple/ snap disc/ RTD to monitor incoming air and turn on the inline heater only when needed, provide an activated removable charcoal filter, provide a MERV 8-12 particulate filter again with access, have all joints sealed with UL 181 closure systems and provide dehumidification for summer months.

-the intake must be well away from grade, exhaust penetrations, lot lines, noxious fumes, stinky things, etc. yet be accessible for inspection and service

-the intake would need a backdraft damper to close it off when not in use

-the intake would need to be very strong with a fine mesh to keep out critters and bugs yet still have sufficient net free area to provide adequate air flow

-be large enough to supply the requisite air flow.



Other than that, there's nothing to them!

Smith19

Plan

Here's what I'm thinking. The furnace is in a crawlspace, and the crawlspace has ventilation soffit like vents in the walls. I would run insulated flex duct from one of the far vents, allowing the intake from the outside to become tempered by inside air. The intake is far from the furnace exhaust, and far from anything harmful. It is also accessible. There would be a CO2 sensor (pneumatically operated or using DDC) in the supply plenum to control a damper at the return plenum. It would only modulate when the furnace is in use, otherwise it would remain shut. There would also be a secondary filter there.