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Need Furnace advice
Old 10-18-2008, 10:21 AM   #1
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Need Furnace advice

My old furnace needs some expensive repairs to get through the winter. It was "high effeciency" in it's day but is now about 30 years old, so a newer model should be more efficient. There also seem to be a number of new options I am not familiar with. Apparently I can add-on an automatic humidifier, or an electrostatic filter that's more efficient that the regular filters, or a UV microbe killer, or a fancy variable speed fan to make heating (and cooling) more gradual, or motorized louvers to make multiple heating zones, or a variety of schemes to replace the pilot light, some of which sound dangerous to me. It's a gas furnace that heats by forced hot air.

I'm a bit overwhelmed with choices. Any suggestions for what kind of replacement furnace (efficiency) is worthwhile? Anyone have any experience with any of these options and whether they (or other ones I don't know about yet) are something I should consider.
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Old 10-18-2008, 10:31 AM   #2
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I am a duct cleaner and work with heating guys all day. Go ahead and get a 90% or so efficient furnace. More than that and the repairs get costly as they are still very new and being tested long-term.

NO electrostatic filters, please. Just a plain old 1" disposable ACCORDION style filter, NO fiberglass, will do the trick. The dirtiest hot air ducts we see are in systems with the permanent filters.

PLEASE.....PLEEEEEASE dont get the humidifier. I'd guess that one in 3 mold jobs we work on are caused directly by a whole system humidifier. If you need humidity, use an in-the-room unit. Just think what dumping gallons of water will do to your METAL ductwork over time. Think what mold needs to thrive. Dont do it!

DO get your ducts cleaned at the same time the furnace is going in...it's worth it if its been more than 10 yrs or so

UV killers do work, but only if you are ultra-sensitive do I reccommend them. Keep in mind that the unit's cost to the heating guy are usually $100-$125....and they want to charge WHAT

Variable speed fan is more to break, more expensive to fix


Heating zones can be great if done properly....but only if you need it. The furnace will be running no matter what....so it's not an efficiency thing...more of a luxury

Good luck....let us know what comes of this
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Old 10-18-2008, 11:39 AM   #3
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Having done HVAC for 20 years my recommendation is KISS. The Fed's recommendations are right on. This piece of equipment heats your dwelling if it fails, it will be when it is COLD outside. The more bells and whistles on the furnace the less likely the repair guy will have the repair parts "on the truck" so you will be colder longer and pay more for repairs.
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Old 10-18-2008, 11:45 AM   #4
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The fed is correct, the simpler the better, and yes those fancy filters, what a waste.
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Old 10-18-2008, 12:31 PM   #5
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All of the above ++ Insist on a heat load calculation. Especially if insulation has been added over the years.

Most furnaces are grossly oversized. Be sure the burner capacity is matched to the heating needs. Oversized furnace dumps a lot of heat up the chimney. No need to heat the neighborhood. Heat rise should be no more than 70 degrees F from return duct to supply duct.

Ideally the furnace should run about 90 % of the time on the coldest days.

In heating as in air conditioning bigger is not better.
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Old 10-18-2008, 02:23 PM   #6
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1) Well, there are different types of "fancy filters." I agree that the fancy electrostatic (i.e. ppwered by AC current with grids to attract the dust) are not worth the money and are ineffective against certain types of dust. But, you probably want a good quality high-efficiency filter. There are two ways to go:
-- Have the guy install a standard furnace with the standard space in the cabinet for a 1" thick filter, then buy a high quality one and replace it every 3 months (at the longest). Price these at Home Depot--they are typically $8 each.
-- Have the guy install a cabinet or filter holder for a "media" filter. These have very deep folds and are overall about 4" deep. They cost about $25 and you replace them once per year.

I prefer the thicker, "change once-per-year" filters because it s slightly less expensive, less hassle, and the larger filter surface area leads to better furnace airflow overall--which saves money and improves furnace efficiency.

2) Depending on the cost of natural gas (and what you think it will eventually climb to, and how long you'll be in the house) a high-efficiency furnace might be worth the money. If your furnace is really 30 years old, then you'd need new vents for a high-efficiency furnace (they use plastic PVC pipe for the exhaust vent, and also have a PVC pipe to bring in the combustion air from outside your home, which improves their efficiency). Installing these new vents might add quite a bit to the cost, or it might be very cheap depending on your situation (they can often go right out the wall instead of going up through your roof--works great).

Here's a simple calculator to let you figure the payback time for a more efficient furnace. I bought a high-efficiency model (93%, which was the best at the time) and I've been happy with it.

3) Don't worry about the safety of a furnace without a pilot light. The gas won't flow if the ignitor doesn't operate.

4) Have you got central AC? Is it working okay? If you are happy with it, you'll probably want to get a furnace with the same blower size as the one you have now.

5)UV light--I've gone back and forth on this. It does keep your AC coils from growing muck (if this is a problem, and if the UV light is installed in the right place) and does help kill nasty stuff in your air--but is any of this really a benefit to the average family? Also, the bulb has to be replaced every year--price the replacement bulbs before you buy a unit.

6) Definitely get a heat load calculation (or do your own). You can also figure out some things based on the unit you've got now--on the very coldest days, is it unable to keep up with your heating needs (too small)? Does it have so much extra capacity that it shuts down for 50% of the time even on the coldest days (too big)?

Do a search for posts by CFB on his furnace purchase--he gave a lot of good tips here.


Good luck
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Old 10-18-2008, 02:32 PM   #7
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Great advice above in all respects.

Best recommendation is the heat loss calculation, you can do one yourself too to get an idea of what size furnace you need. 30 yo furnace is invariably oversized. The most efficient furnace will run all the time on the coldest day the heat loss was figured for...there's a lot of confusion on that point but lots of cycling on a furnace (what you get from oversize) is inefficient. Invariably the 90 pct run time on coldest days is a great thing to shoot for.

If you've got AC, then your furnace size may be constrained by the blower size necessary to suit your cooling requirements.

For more info all on one site: HVAC-TALK.com
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Old 10-18-2008, 03:16 PM   #8
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No AC, so no idea about blower rating from that. On coldest days the current furnace runs often, but I'd guess less than 90%. Unfortunately I never checked for that and I will likely want to replace before another coldest day occurs, so I may have to guess on that.

Quote:
3) Don't worry about the safety of a furnace without a pilot light. The gas won't flow if the ignitor doesn't operate.
Maybe. Ever device I'm aware of (cooktop, grill, lantern) that I or my family have had that used an electronic ignitor instead of a pilot light has been a miserable failure and required manual intervention with matches in order to light after a year or two at most. While I can see it could be safe still, if the interlock still works, I'd also be concerned about the repair rate and costs if they fail on furnaces anywhere near as often as they fail on other appliances.
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Old 10-18-2008, 03:34 PM   #9
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Quote:
Originally Posted by growing_older View Post
Maybe. Ever device I'm aware of (cooktop, grill, lantern) that I or my family have had that used an electronic ignitor instead of a pilot light has been a miserable failure and required manual intervention with matches in order to light after a year or two at most. While I can see it could be safe still, if the interlock still works, I'd also be concerned about the repair rate and costs if they fail on furnaces anywhere near as often as they fail on other appliances.
If it makes a difference to you, the technologies used by the ignitors are different. Your stove, grill, lantern etc use a piezoeletric ignitor, while the furnaces generally use a "hot surface" ignitor. I imagine it isn't as reliable as the trusty old pilot light, but they have a good reputation.
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Old 10-18-2008, 03:42 PM   #10
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Here's another quirky thing about furnaces: The BTU rating is for the amount of fuel that goes IN, not te amount of heat that comes OUT. So, a 80% efficient "75,000 BTU furnace" actually puts out less heat than a "65000 BTU furnace" that is 95% efficient.

So, if you get a more efficient furnace, you could go with a smaller size than the BTU rating of the less efficient model you have now, and still get enough heat.
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