Power Vented Central Heating Systems: Pro's and Cons
Jan 12, 2002
The Bottom Line High technology furnaces and boilers are more expensive and less reliable than conventional systems. Until that changes, I'm proud to remain conventional. Don't be a Guinea Pig.
The following information was going to be a short email to answer a question I received and it turned into, what I thought of as, a suitable topic to review on a general subject matter. It also covers another alternative to buying unvented gas heaters. Any reason to discuss alternatives to unvented gas heaters is good enough for me.
This is for you Pete!
Thanks for your email. A couple of good questions and questions that I really need more information to answer properly. There are quite a number of power vented systems on the market today. Some power vent the appliance up an existing chimney and others are designed for sidewall venting. I assume, since you are concerned about staining on the outside wall, you are going to be sidewall venting it. Unfortunately, due to the higher Btu input of furnaces there will be a much higher volume of flue gases being expelled than from something like a small fireplace or a space heater.
On extremely cold days there will be a steady stream of condensing flue gases and under the right wind conditions or lack of wind conditions, some of it will condense on the side of the home. If you have white siding, especially aluminum, there is a good chance of the exhaust gases leaving yellowish stains on the side of the home, usually, in a larger, arc shaped pattern, above the vent outlet. Yellow for natural gas and sometimes a nice sooty grey for propane models.
The extent of the staining will depend on a couple of things. The static pressure of the exhaust fan, the location of the vent termination and the design of the appliance and the vent termination. If the design is such that the flue gases are blown away from the home, there is obviously less chance of it condensing on the wall. If you are located in a colder region of the country, there is a greater likelihood of it freezing on the side of the wall.
To answer part two of your question, power vented systems are somewhat more efficient than conventionally vented systems, since the off cycle heat losses are lower.
There are two basic kinds of furnaces and boilers that use power venting. Mid-efficiency appliances, which are generally only about 5 to 8% more efficient than present day conventional appliances. Mid-efficiency appliances have higher exhaust temperatures and will tend to condense less readily on the wall and will melt ice and snow built up on the termination. The bad news is that the high exhaust temperatures (350 to 450 F) are very hard on the power ventor fan and housing, as well as the motors, which have been known to seize up as the oil in the sealed bearings burns off. Small amounts of condensation can also cause the squirrel cage to go out of balance and I've even seen some that begin to corrode within a few years of operation.
I'm not trying to be too confusing for you, but there are also two methods of power venting. Induced draft and forced draft. The drawbacks to induced draft I've noted above, they are located on the hot exhaust side of the flue gas passageways. Then there are forced draft systems which are better in my opinion, since they only drawing in fresh air and delivering it under positive pressure to the combustion system, they are not subjected to the hostile hot and moist conditions that an induced power vent system is . They tend to be made of durable plastics, are lower cost and have a much better chance of living a longer life.
High efficiency, condensing furnaces also use both kinds of power venting systems, induced and forced draft. Since a high percentage of the heat is extracted in the tertiary heat exchanger, (condensing coil) these products tend to be in the mid to high 90% steady state efficiency ranges. The good news is that a condensing furnace will generally have an induced power venting fan that is subjected to less punishment from the lower temperatures in the exhaust gases. (I.E. 100 to 140 F) They are generally constructed from high temperature plastics and this technology has come a long way towards making them more durable. Unfortunately, they also tend to be the most wet flue gases, as the exhaust has already been cooled below dew point. They also have a little drain line linked to the floor drain for condensation that builds up in the fan housing.
In colder regions, it isn't uncommon to see stalagtites and stalagmites of ice either on the ground beneath the vent termination or hanging from the termination itself. You have a better chance of getting stains on the side of your home from these products.
I should point out that some manufacturers offer wall shields to help reduce the chances of staining. Some of them appear to be pretty effective, some are not. It would be best if you could get your contractor to show you a couple of installations with similar siding materials that have been installed for a few years. The wall shields are sometimes included with the vent kits and sometimes they're an optional accessory. If you decide to go with sidewall ventiing, get the shield. It may be a bit ugly, but, no uglier than a stain is.
Make sure you pick a sidewall location that isn't close to any favourite shrubs or small trees, because any shrubs within a few fee of the termination are usually dead within a year. Also don't let them vent it towards your driveway, unless you really don't care about the paint job on any cars that are going to be parked there. Of course, the vent termination has to be located away from windows, doors, gas/electric meters, air intake vents and soffits and in most places it "must" be at least 12" (24" in some area's of North America) abvove grade and kept free of leaves and snow/ice build up.
If you do get out to see an actual installation of one or two, maybe even ask the homeowners, if they're happy with the product. Of course, your contractor isn't going to refer you to the ones who have been ringing his phone off the hook with complaints. This may sound like a lot of extra work to put into simply buy a new furnace, but, this is a big investment and you don't want something that's going to heat your home nicely but leave stains on the wall.
Incidentally, the condensation is mildly acidic, so if you have any bricks and mortar near the vent termination, expect deterioration as well as staining.
With a condensing high efficiency appliance, you have a better than average chance of having to have an emergency service call within the first five years of installation, in addition to extra routine maintenance. Drain lines for the condensed flue gases plug up, neutralizer cartridges clog up and overflow, pressure switches, relays, electronic ignition modules, hot surface ignitors, post purge and pre-purge fan timers and all the extra "stuff" that go into a high efficiency product are effective, but, not yet quite reliable enough for the long term. (In my humble epinion.)
So the short answer to your question about efficiency is yes, you can expect a little more efficiency from power vented systems. They "may" use less air from the home for dilution in the vent and less air is drawn out of the house while the furnace is off. The heat exchangers are usually designed a little tighter, since the flue gases are either being blown through or drawn out.
MY SUGGESTION: (For what it's worth)
However, if you have an existing chimney, I wouldn't recommend going with a sidewall, power vented, mid or high efficiency furnace. I would suggest a modern conventionally vented furnace, using the existing chimney. You can expect steady state efficiencies around 80% from most of the current conventional products and fewer service problems down the road.
Again, if you can use an existing chimney, you will also eliminate any chances of staining the sidewall of the home with a conventional furnace. "I" guarantee you will have fewer service problems over the life of the product. Routine maintenance is simple and the appliance has fewer controls to go wrong on a cold winter's night.
Last and not least, is that the capital costs of a conventional appliance is lower than the mid and high efficiency.
I'll have dozens of salespersons who disagree with me on these last points and frankly, I don't care. I've been around long enough to see which of my friends and family who went for my recommendations are the happiest and which are not. Those who bought conventional appliances haven't had to call me, in the middle of winter, to get their furnaces running. (They do call me for other things though.) They paid less for the initial furnace and they've had fewer problems.
Sure, they may pay a little bit more each year for fuel costs. However, what does a 10% lower annual fuel utilization efficiency cost? In my case, it is about $60.00 per year in higher fuel costs, based on a furnace that carried an initial installed retail price that was $400.00 lower than a high efficiency furnace. That alone represents about a seven year payback on the equipment costs. However, after ten years, I have never had a mechanical break down and the furnace is still running at it's peak efficiency.
Those who have opted for high efficiency furnaces will rarely be able to make the same claims, in my experience. At average service rates, ranging from $60 to $90 per hour, it doesn't take too many emergency calls and expensive replacement parts to make up the difference in the fuel savings.
Another thing to remember about high and mid efficiency furnaces, is that they are always changing. High efficiency furnaces designed 10-15 years ago, are no longer in production in most companies, parts are no longer available for many of them. So, ten years down the road, you may just find yourself replacing the whole thing again due to a critical part being out of production.
Thanks again for reading "don't be a guinea pig or a canary" (see, I can spell guinea pig) if it gave you some food for thought about the heating appliance industry in general, it served it's purpose. If you have any more specific questions, about your application, that I failed to cover here, please feel free to write again.