Outside Make-Up Air

Recently, the building industry has become aware of the subject of assuring proper airflows in fireplaces (and all other heating devices) as it can affect the purity of the air in the house.

In 1980, JUCA engineers published several major articles in trade magazines on this subject. (See one at Editorial to the Industry.)

Various "experts" now express their understanding of the facts. They are mostly correct, but often incomplete.

We will discuss the situation of a fireplace, but the general concepts apply to any combustion-based heating system.

Since the chemical process of any type of combustion involves combining oxygen from the air with the carbon or hydrogen of the fuel, air is necessary for the fire.

In the old days, this minimal consumption of air was removed from the house (up the chimney) and it was replaced by "infiltration air" that snuck in around windows and doors. No problem.

As houses became super tight due to conservation and money- saving policies, a problem arose. When a traditional open fireplace operated, it often caused 400 to 800 cubic feet per minute of house air to go up the chimney.

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Newer, tighter houses wouldn't allow that much replacement infiltration air to leak into the house! This creates a low-pressure area (kind of a weak vacuum) in the house. This situation restricted airflow in the fireplace chimney, so it didn't work quite as it should. (Actually, it's overall efficiency was HIGHER, for several technical reasons) The limited chimney airflow caused the fireplace to be more susceptible to wind shifts outside. The fireplace might be a lot more likely to "back-puff" sending smoke (and pollution) into the house.

A collateral problem existed. In such a super-tight house, whenever a bathroom exhaust fan was turned on, where was the replacement air going to come from to replace THAT exhausted air? Often, the only source was ... down the fireplace chimney. This created a cold draft in front of the fireplace, more noticeable than when the air came in a little bit everywhere. Sometimes, this allowed the smells of the fireplace to be noticed in the house, even when the fireplace was NOT in use! And pollutants that should have gone up the chimney could now be sucked back down and out of the fireplace, to supply the air being removed by the bathroom ventilator. The backwards chimney airflow also made it difficult to start the fireplace. This is REALLY a bad deal for the fireplace! It gets blamed for a situation that's actually not even its fault!

A related subject can also be mentioned here. If the fireplace chimney happens to be on a west or north outside wall, another problem can occur. Prevailing winter winds from those directions bump into the house structure. This air has to find a way to get past the house somehow. Most will go around or over the house, but the high pressure area created just upwind of the house could force some of this air to go DOWN the chimney, through the house, and OUT through leaks on the east and south walls. (A taller chimney generally cures this)

The old "flow-thru" leaky house used to cleanse itself. In the modern super-tight house, once pollution gets in, it has nowhere to go and it stays in. Health concerns result. So, in some ways, the modern concern about indoor air purity is solidly based.

Two very different approaches have become popular to solve the problem. They are often confused together since they both are called "Outside Air".


This involves using the fireplace's or furnace's BLOWER to suck in a little additional "outdoor MAKE-UP AIR" into it's warm air system. This has NOTHING to do with the fire, and is not the same as using "outside COMBUSTION AIR" which enters inside the firebox (below). The "make-up" air concept is regularly used in commercial buildings, and is a residential application of that technology.

It can be a valid approach. By adding A LITTLE extra air to the house, the house interior is kept slightly pressurized. Air could only leak OUT of the living area, through cracks or INTO a fireplace and UP the chimney, eliminating any chance of smoke pollution coming into the house. The continual input of (pure?) outdoor make-up air will also cleanse existing indoor pollution, but it does this by PUSHING warmed house air OUT through leaks which defeats the concept of super-tightness and energy conservation. There are even house windows on the market today that have vent-holes in them to allow the purging of this warmed house air to the outdoors. That seems to be a weird result of making the house more energy efficient! JUCA owners are lucky in burning wood to heat their houses rather than having paid for gas or oil to heat this air that is intentionally pushed out.

A recent variation on all this has recently appeared. The state of Minnesota has even REQUIRED this new system (1998) of trying to clean house air, but they have CLEARLY not thought it through! That variation involves having a motorized blower continually SUCKING warmed house air out, and dumping it outside! First, it's wasteful, sucking warmed air out. But, far more importantly, it actively creates a LOW-PRESSURE circumstance within the house. (What were they THINKING!?) Since such a system guarantees a sibstamtial amount of air to have to leak INTO the house, it would CERTAINLY make ANY fireplace, woodstove or furnace dump some of its flue gases INTO the house. (Unless that product is an absolutely sealed unit, which eliminates all fireplaces and woodstoves and many conventional furnaces. On top of that, it creates cold drafts wherever the cold air could leak into the house, for example at the kind of windows mentioned above.

A funny part of this is, that since this idea creates such problems, they mention that nearly all houses would then NOT pass air purity tests, and that such failure would require ANOTHER motorized blower, this one to blow cold outside air INTO the house, and it must be rated with a greater flow rate than the TOTAL of the motorized exhauster mentioned above, PLUS the clothes drier, PLUS the bathroom ventilator, PLUS the kitchen ventilator, PLUS all other ways air could leave the house intentionally! THIS makes this replenishment air blower really HUGE, continually pumping a LOT of very cold outdoor air into the house, that all requires heating and humidifying! This could nearly DOUBLE the heat load of the house, wiping out whatever fossil fuel savings the homeowner might have gotten as a result of the very tight house. UNBELIEVABLE!!

By the way, this LAST blower mentioned above, is an example of a MAKEUP air source.

NORMAL Makeup air systems only add maybe 5% (or less) of new outside air to the re-circulated house air. This pressurizes the house, but doesn't adversely increase the heating load of the house or the necessity for additional humidification. So, in a super-tight house, a small amount of Makeup air might be advisable. If you house was built with one of those "Exhaust only ventilation systems", it would be tough for you to install ANY woodburning product, or even many other appliances!

It seems clear that after a few years, the folly of their ways will show up, and they will somehow modify that peculiar law in Minnesota. Until then, homebuyers in Minnesota will pay extra fuel bills, and additional higher costs for appliances (furnaces, water heaters, etc) without any benefit from it. We know this is not what you wanted to hear, but it is the situation.


The other approach is the Outside Combustion Air previously mentioned. By supplying air to the firebox for the fire from outdoors, it will not need to draw house air, so it will not create a low pressure situation in the house. The draft created in the chimney would draw air into the firebox through this intake, thereby greatly separating the airpath INSIDE the firebox from the airflows in the house itself. Thus, the smoke would not be drawn out into the room, so no indoor pollution would be created. This approach has the advantage of not bringing cold "make-up" air into the house which has to be heated. The cold outdoor air only comes into the firebox and then goes up the chimney.

JUCA's way of doing this is an Option that is Described Here

Our Suggestion

The "Make-up" air approach would make excellent sense if it was activated by a negative pressure sensor in the house. If it only brought in such air during the relatively rare periods of negative pressure, little extra heating load would be introduced, while still eliminating any problem involving indoor air purity.

The external combustion air approach works fine except in ultra-tight houses when a kitchen exhaust fan or other exhauster is on. This is the only circumstance where a little pollution could still get into the house. If this was a situation which could occur regularly, then sealed doors would be necessary on the fireplace. Since that is quite rare, we feel that non-air-tight doors are nearly always fine on JUCA products.

A contractor could use either or both approaches in a JUCA installation, depending on how tight the house is. In general, we do not think the "make-up" air approach is necessary except in ultra-tight houses or where asthmatic occupants were involved.

A few of the JUCA fireplace doors listed ARE air-tight for such installations. We feel it unnecessary to use sealed doors for any but the most extreme situations.

The "external combustion air" provision is often easy to do.

We prefer approaching the problem as follows. Install the JUCA. Use outside combustion air if appropriate. Use normal (non-air-tight) doors to allow proper operation of the JUCA system.

This should handle 99% of all installations. If further pollution concern is present, such as for an asthmatic child, include an electronic air cleaner in the furnace and/or JUCA house air path.

If even more preventative measures are deemed appropriate, a HEPA super filter could be used instead. We can help you get a residential version of Hospital-grade HEPA filtration for about $700. Such a unit ensures that your house air is as pure as the air in a Hospital!

If a make-up air provision is desired, hook it to a negative pressure sensor so it only brings in outside air when actually necessary.

For older or less air-tight houses, keep in mind that NONE of these considerations usually need be addressed at all.

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