Make up air Issues

These questions/comments were gleaned from various forums:

Does it matter how far away the make up air is (as long as it is 10 feet away from the exhaust vent and furnace hot water heater etc.?) Or does it work better if I am on the same side as the open window and therefore closer, but not too close!

Make up air from the 2 open windows way on the other side of the basement. Keeping both doors open, one for the vent hood and one for the furnace and hot water heater…oh question…my appliances have large fresh air returns so do I still need both windows open?

If I put in a more power[ful] fan, would I have to have more make up air that what the windows would supply?

Well, the furnace and hot water heater should already have their own fresh air sources in place regardless of anything else.

A glass studio in the basement has some very special issues that artists should be aware of.

The first involves the presence of other gas-burning appliances such as a hot water heater and/or the furnace. These appliances will always use room air (especially hot water heaters). It has been code on houses built since 1980 or so that a combustion air (make up air) duct be run into the room where the hot water heater/furnace are located. You should never consider this to be part of your fresh air source when making your calculations for your glassworking exhaust system. Additionally, you should also check to see if a fresh air duct actually exists! I have been seeing a lot of reports where this code-required ducting has not been installed, and it makes me wonder where the building inspectors are.

Houses built before 1980 and have not had their furnace upgraded to a more current model usually do not have a fresh air supply. They relied (and still do) on general household air for make up air.

Certain furnaces, mostly those called “90+ efficiency”, supply their own fresh air from the outside. There is usually a double PVC pipe system that brings in outside air and exhausts the used combustion air. As far as I am aware, hot water heaters are not this advanced yet (bringing in their own outside fresh make up air).

There is some confusion among some people, especially as evidenced above, calling the cold air returns “fresh air returns”. This is incorrect terminology. The “cold air returns” are the other half of the household ventilation system, which returns air (cold in the winter, hot in the summer) to the furnace to be run through the plenum (heating in the winter, cooling in the summer). They (the “cold air returns”) are run through the entire house, and serve to act as the return side of the forced air system that heats and cools the house.

There is also a new(er) additional device, called by many names, but usually an air-to-air ventilator. this system removes excess moisture in the house during the winter heating period when windows and doors are closed. It brings in the dry cold outside air, usually in low volumes, and exhausts an equal amount of warm moist air from inside the house. Sometimes these systems are part of the house ventilation system, providing higher volume exhaust for bathrooms and kitchens. These systems should also never be part of the glassworking ventilation system.

Before the glassworking ventilation system is installed in the house, you need to know that the balance of air flow inside the house has been designed to provide fresh air for the existing gas-burning appliances and nothing more.

As has been noted elsewhere in this blog, for every CFM of air that you exhaust from your glassworking ventilation system, you must provide the same CFM of fresh outside air. The fresh system should at all times (especially in an in-the-house studio) be passive, that is, let the exhaust fan pull what it needs.

The main requirement for the fresh air supply is that it needs to be a minimum of 10 feet from any appliance exhaust duct.

In non-temperate areas of the world, I advocate ducting the fresh air supply to the underside of the workbench, then bringing it into the actual workspace with floor register adapters, then capping the opening with an actual floor register.

If a furnace relied on and functioned safely with household air before a lampworking vent system with its own fresh air supply was put in, why would it be any different after that vent system is installed? If the vent system has its own fresh air supply, it is not taking anything away from the gas burning appliances already in place, is it?

Not necessarily. There were a LOT of houses built that didn’t properly account for the need for fresh combustion air, and typically run negative pressure when the furnace/hot water heater are running, especially in deep winter. Houses that have been “tightened up”, with additional insulation added, new windows, etc. usually haven’t added a fresh air source for the furnace and hot water heater (who thinks about what the furnace and hot water heater need???). You can’t assume that the furnace and hot water heater were getting along just fine.

It is very conceivable that a fresh air duct installed for a glassworking ventilation system is going to be also supplying fresh air for the rest of the gas burning appliances in the house. A good way to test for this is to turn off your ventilation fan, but leave any dampers open on the newly installed fresh air duct. The turn on all (and I mean ALL) of the gas burning appliances in the house. Hold a tissue over the fresh air duct. If it doesn’t move, then the house has enough fresh air for all the appliances. If the tissue moves (and I believe that *most* will) then there is not enough fresh air being supplied into the house and an HVAC professional will need to be consulted.

How critical is it to have a good/abundant fresh air source for your appliances?

Well, as an example: backdrafting a furnace exhaust flue will condense the sulfur dioxides into a liquid: mild sulfuric acid. This acidic condensate will eat at any metallic exhaust ducting (which can be identified as rust stains running down the outside of the exhaust stack from the furnace), AND can eat away at the heat exchanger inside the furnace. Once the cast iron (and sometimes stainless steel) heat exchanger is perforated, you will get carbon monoxide circulating inside your house with the heated air.

Fresh air is always going to follow the path of least resistance. This means it will backdraft an exhaust flue, the plumbing vents (ewwwww!), suck from cracks around windows and doors — anywhere it can to return the inside of the structure to a neutral pressure.


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