Home & Garden Columns

About the House: The Problems with Forced Air Heating

Matt Cantor
Friday June 16, 2006

In the 19th century and the very early parts of the 20th, coal burning was a common way of heating our homes. It seems amazing to us now that such a wasteful, dirty and downright dangerous method of heating would be, not only the choice of a generation, but literally built into the homes of the era as permanent systems. 

I’m quite sure that carbon monoxide poisoning was rife in society in much the same way that lead poisoning was commonplace and unidentified amongst the Romans. Many, I’m sure were killed by the noxious gas but far more lived depressed lives of inexplicable lethargy (as is common with carbon monoxide poisoning). 

Future scientists may judge our forced-air heating systems in a similar way. Although these were considered state-of-the-art in the 1950’s, we’ve moved on in many ways and discovered many things about these systems which cause me to question the logic in retaining this technology. 

The main reason for my dislike of this extraordinarily common heating method is its lack of efficiency. This starts with the whole idea of heating the air, as opposed to, say, the floor or the walls. When we heat air and blow it around, there is a great deal of heat loss.  

These systems, for the most part, have relatively low efficiencies with loads of our hard earned Therms going up the chimney. While there are higher efficiency forced-air heating systems available (90 plus or condensing furnaces), these have a range of efficiency problems as well. 

These systems are heavily dependent upon a complete lack of leakage in ducting in order to maintain their efficiency and leakage is extremely common in these systems. They also tend to lose loads of heat straight through the ducting surface where the insulation is loose or missing. 

Many older systems (even those which include a new replacement furnace) have little or no insulation on the ducting and these lose loads of heat to crawlspaces, basements and other unused areas thus reducing the efficiency and using heat, Natural gas and money unnecessarily. 

Forced air heating systems also blow air around in the house and with the air comes noise, dirt, dander and other pollutants. Although it is possible to make very quiet forced-air systems, most are not so well-designed and many actually whistle or make other noise. The fans also make noise, although I confess this is a small part of my dislike of this technology. The detritus stored and blown about by these systems is, on the other hand, a major complaint for me.  

I’ve been inside these systems on many occasions and, as a rule, they’re filthy and their contents are constantly being blown back into the living space. Sometimes system which come to contain moisture through leaks or condensation end up harboring airborne microbes such as Legionnaire’s Disease, although this is uncommon on the West Coast. 

While such systems can effectively filter the air as they run, most are not properly outfitted in this regard. Many have ancient filters that should have been changed long ago and rely too fully on owner maintenance (this also causes the furnace to struggle and overheat). The typical filtration methods are poor and filters are often located in places that are hard to reach and often ignored (although these issues can be addressed by dogged or thoughtful technicians or owners). 

A heating system which doesn’t suck and blow air needs no filtration and can keep allergens (and house cleaning) to a minimum. Of course, a forced-air system that has great filtration can actually lower indoor air pollutants but these are few and far between. Overall, I would tend to prefer a system that doesn’t blow anything around my house in the interest of heating. The more direct the form of heat transport, the better. 

Another, and perhaps central, failing of the forced-air system is the lack of engineering in the flow of air. It is not enough to merely cut some holes, here and there, in the floors of your house and then to connect them up to the two ends of a furnace. The flow of the system must be considered. Otherwise, as is all too often true, one room is well heated, while another is quite cold. Also, without good return flow from every space to the intake (the big grill usually located in the living or dining room), the entire system will run inefficiently and the furnace can also overheat. 

Many houses that have heating registers (the ends of the heating ducts inside the rooms) have little or no space below doors for the heat to circulate back the intake and thus heat slowly and poorly. A forced-air system is a circulatory system and a blockage in a circulatory system is precisely what you don’t want. It has been suggested by some that forced-air systems should have a small cold air intake in every room. 

But so far, this has remained theory in virtually all applications. While some houses do have more than one cold air intake, the most I’ve ever seen has been three and they were all outside of bedrooms. 

Forced-air heating systems also have some degree of inherent hazard as they can, under certain conditions, take exhaust gases (including carbon monoxide (CO)) and put them into the ducting system along with the warm air. This is the main reason that these units should be examined regularly (once a year is a good practice). 

This is not true for all heating systems, although exhaust leaks into living space are certainly an issue with gas wall and floor furnaces. 

My final area of complaint with forced-air heating is the problem of where to put it. While the furnaces are not so very large and condensing units can employ plastic flues which exit near the unit, the ducting still can take up hundreds of cubic feet of interior space and make construction and remodeling very complicated. 

They can also keep you from doing the architecture you really want to do. For professional designers, this remains a source of real frustration. The huge hoses have to be run through every room of the house and greatly complicate and inhibit the shaping of spaces. Often, entire second floors end up without registers due to this complexity and few systems end up being truly balanced. 

Given these issues (and there are certainly more we could trench up if there were time) I suggest that it’s time for all of us to revise our notions about heating. Sadly, solar heating methods have not been given much attention in the last two decades despite a fair amount of research in the late 1970s under Jimmy Carter. We should all continue to push for advances in passive and active solar heating since, ultimately, it is the logical and most parsimonious realm of home heating. In the absence of this, I think that hydronic, or hot-water heating, is the next best thing. 

Modern hydronic heaters, like the little Munchkin are very stingy with BTU’s (British Thermal Units) and use plastic tubing to communicate heat through the dwelling. These tubes are so small that they can be threaded through the floors or walls of the home without any other deliberate modification. 

In other words, you can put the heater exactly where you want it and you don’t have to worry about where to put the soffits, walls or hallways. You can also heat the underside of a floor providing one of life’s great pleasures … a warm floor. If you keep the floor at 70º, the ambient room temperature can be 65º and you’re going to feel nice and warm, especially if you like to walk around in your bare feet. It’s nicer on your lungs too. The warmest part of the room should be the floor since ultimately, it’s going to convect upward to the top of the room. 

Hydronic systems can also provide heat using radiators and the remodel of a house need not involve the removal of the living room ceiling when radiators can be placed upstairs to serve the bedrooms. 

Since there is no way for CO to enter a room through the water in the tubing, the greatest danger inside the house is a leak. Hydronic heat is also more efficient and less costly in the long run because it’s heating something that doesn’t dissipate so rapidly every time the door gets opened. 

When we heat a radiator or the floor, those things don’t rapidly cool when someone opens a window or a door. When you heat the air, the ability of that heat to escape the house through door, window or other means is far greater. 

The reason that forced-air has won out for so many years is that it’s fairly cheap to install and hydronic is not. A hydronic heating system is generally going to be over 12 grand even for a very scaled down system and can be 20 grand for a larger house. Forced-air, on the other hand runs from 5-8K for most small to mid-sized systems. Also, with so many forced-air systems in place, people will tend to continue to upgrade parts of this system rather than junk the whole think in favor of a better system. But two kinds of people tend to choose the hydronic alternative, the rich and the smart. 

People buy these systems for their great comfort and their long-range cost profile (they’ll actually save you money if you own them long enough). Some also choose these systems when members of their families are allergy-prone. High tech folks like engineers tend to choose these systems because they make sense despite (and perhaps because) they’re not being the common fare. 

It’s funny how this thing has come around from the distant past. Hot water, radiator-based systems were quite popular a hundred years ago, although they were far less efficient and much more expensive to install. We had our flirtation with forced-air and I’d say that the romance has soured; so we trudge, somewhat chagrined back to our old flame.