Can I use the cold air return in the house as my fresh air source?

It sounds like a good idea…

The cold air return in your house should be more than enough to change the air with a good ventilation system.

Never EVER rely on the cold air returns of your household HVAC system to supply fresh air for your lampworking ventilation system. You **MUST** have an open window/door or ducted fresh air supply from outside. This is code and required in all parts of the United States.

Using household air for make up air will lower the pressure in the house as air is being exhausted and not replaced. https://mikeaurelius.wordpress.com/2007/12/27/makeup-air-or-your-fresh-air-supply/

It has also been commented that the duct for the furnace/hot water heater is good enough: No, it’s not. That ductwork is sized specifically for the furnace/hot water heater and any other appliance that burns gas.

Keep in mind that all fresh air being supplied to the house is passive. When the air flow in a duct exceeds its capacity, back pressure starts to build up — only so much volume can flow through at any given time.

Think about it this way: take two pieces of pipe, one 1/4″ inside diameter, the other 1″ inside diameter. Hook them up to identical water lines with identical pressure. Which pipe is going to pass more water (volume)? The larger pipe of course! Yes, the pressure of the water will be lower, the stream flowing out will be shorter, but it will put out more volume.

Air behaves the same way water does. You can only push so much air through a give duct without pressure consequences.

So what happens when you don’t supply additional fresh air for your LEV is that the appliances that rely on that designed-in fresh air source become air starved. The LEV will continue to exhaust, the single fresh air duct can supply only so much fresh air, and the balance has to come from somewhere else.

LEV fresh air needs to be wide open freely available fresh air in sufficient volume to supply the needs of the exhaust fan. If your exhaust fan is running at 700 CFM, the fresh air source needs to be able to supply 700 CFM. And because of this, it needs to have a separate source.

Household fresh air sources simply do not have sufficient available extra flow to allow for high volume exhaust systems like glassworker LEV systems. Do they have sufficient extra flow for bathroom fans and kitchen hoods? Yes certainly, but again, remember, the bathroom fan is usually run on the average of 5-10 minutes, the kitchen hood for maybe 20-30 minutes, and that both of those examples are very low flow rates — usually no more than 100-300 CFM each.

Now, you may be thinking to yourself “but the furnace only runs for 10-15 minutes at a time. So what’s the big deal?”

Furnaces use a tremendous amount of fresh air — just look at the burner on those things! Unless you have a 90+ furnace with its own fresh air supply, you are talking a substantial amount of fresh air being used by the furnace alone.

And just so you, my fair reader knows, I will always take a worst-case scenario when I am discussing safety issues such as ventilation. I absolutely try to avoid assumptions unless I’m assuming worst-case.

Remember what happens when you assume…

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