New furnaces - what should I know?

Factual Questions
1

Well, it may be time to replace the furnace in our house. It’s 23 years old, and in the 6 years we’ve owned it, it’s had various problems. The burner fan went out, and fried all the electronics, the humidifier was jury-rigged when we moved in, and now, something is partially blocking the supply fan, making it iffy if it will turn.

What questions should we be asking when we talk to salespeople about a new furnace? What features are available, and what do they mean? Are there any features that are pretty useless and just designed to jack up the cost?

Variable speed, 4-speed, 5-speed blowers?
Single stage, 2 stage heating?

2

One thing to check is whether your town offers rebates for “energy efficient” furnaces. You can get several hundred dollars back in some cases, which offsets the higher cost of such a model.

3

Ask what the EnergyGuide rating is on the furnace you are looking to buy. It’s required by law, and tells you how efficiently the furnace will run. The higher the rating, the more efficient. Also, look for consumer reports that tell you how quietly the furnace will operate. Also, compare the service contracts of various heating companies, because a furnace should get maintenance annually.

4

Since you’re replacing a 23 year old furnace, it’s a safe assumption that a conventional chimney is in use, be it masonry or metallic. Depending on the age and condition of the chimney, you may wish to consider a 90% efficiency unit and power vent it through a sidewall with PVC. If you go that route, pay attention to water heating, as you shouldn’t leave a chimney vent water heater as the sole appliance feeding a chimney-it won’t draft properly.

While going through this process, it would be wise to spend time closing up leaks in the supply and return ductwork. Feeding conditioned air into void spaces and sucking unconditioned air into return plenums makes the unit work that much harder for the same level of comfort, while sending $ up the flue.

You don’t mention if this unit is coupled to an A coil or packaged evaporator. New and higher SEER ratings for central air just went into effect on 23 January 2006 (13 minimum SEER) and if your central air is also old, or if you’re considering the addition of central air, it would be very wise to install/replace both together, as blower speed(s) and volume moved have a bearing on the efficiency and comfort derived from central air operation, and maximum efficiency would result from an engineered furnace/evaporator pair, as opposed to separate units cobbled together.

5

Don’t buy more then you need, if your current unit heats the house fine then don’t upsize it. Ideally you would like it to run 100% of the time on the coldest days to maintain the temp you want on the inside, If you want to be even more efficient get one that runs 100% of the time at a temp higher then the lowest temp in your area, but this will mean that when that day hits you will not be able to maintain normal ‘room’ temps.

Also remeber that a higher efficiency furnace will put more heat per btu then your old one, so you can get away with less BTU’s.

6

I just had to replace an old furnace (not sure how old, but in the house since at least 1977).

Some things I learned:

  • Your old furnace was at best, probably about 60% efficient. A new one will be better than 80% efficient. So your new furnace will probably be smaller in BTU output than the old one. (Physically much smaller too, but that’s not very important.) Have them measure the square footage of your house and calculate the size of furnace you need. My new one is exactly half the BTU’s of the old one, but warms the house quite well. Resist the temptation to go a bit bigger (closer to your old furnace) – you don’t need it.
  • many of the new more efficient furnaces will not connect to your chimney, but need new PVC vent pipes out of the side of your house (like danceswithcats said). That’s because the new furnaces try to recover all the heat in the ‘smoke’, so the ‘smoke’ ends up much cooler. Cool enough that when it hit the cold upper part of the chimney, it would condense and that water would run back down your chimney, and eventually eat out the bottom of the chimney. Pretty quickly too, since that water also contains acidic byproducts. I think that recovery is the stage 2 they talk about. I do notice a sort of whistling noise from that pipe now, compared to the chimney noises of the old furnace. You get used to it.
  • Current furnaces come in roughly 3 ranges of efficiency: regular (80-89% efficency) (note that this is still 1.5 times better than your old furnace), high efficiency (90-95% efficient), and ultra-high (96-99% efficient). I would not get anything in the regular range, because it isn’t really that much cheaper than the high efficiency ones, and with increasing energy prices, the savings from the more efficient ones will just get better. I’d also hesitate to go with the ultra-efficient ones. That last 4-5% of efficiency seems to come with a big jump in price, and it would take many years to recover the cost in fuel savings. In my house, putting that money into replacing old, leaky windows or better insulation would have been much more effective. But the ultra-efficient furnaces also often have the fanciest features, like multiple zones, continuaously variable fan speed vs. 2-3 speeds, variable burner flame, etc. If these are important to you, you may want to look at the ultra-efficient types. Given the design of my house, they weren’t real important to me.
  • Expect some differences in the way the furnace works. I notice that my new furnace seems to come on more often, but run for shorter periods of time. Indicates that the furnace size is more closely matched to the house, I was told. And the fan seems to run longer, even after the burner has gone out. This does seem to keep the house a bit more comfortable.
  • do check out energy-efficiency rebate programs. Often you can get rebates from the energy company, the city or state, and the furnace manufacturer. Takes some paperwork, but it’s worth it. Your furnace salesman will probably be able to point you to these.
  • ask about warranties, but I didn’t pay much attention to them. In my experience, every furnace lasts for years longer than the warranty. The warranty is just important if you happen to get a lemon that starts having problems in the first year or two.

Be glad you have some time to replace your furnace! I didn’t – it went out abruptly one night. Note: it gets dang cold in a house in Minnesota in January when the furnace is out for 2-1/2 to 3 days!

7

A key factor: How long do you plan on living in your house?

8

I replaced a furnace a few months ago, my second in this duplex.

I learned that, with a few bells and whistles aside, the medium-priced furnaces are the same as the higher-priced furnaces. They are made with the same parts, often made by the same company. The difference is in the length of the warranty. A Carrier is a Payne with a better warranty, but a Payne is a lot cheaper.

t-bonham is right - size your furnace to the size of your house. A too-big furnace is more expensive to buy and to run, and also may not last as long - something about the stress on the heat exchanger, don’t remember exactly. A variable speed blower is good if you are also running central air and the house is more than one story. 90+ efficiency is the way to go.

And maybe most importantly, the knowledge and ability of the installer matters more than the brand or type of furnace you buy. The best furnace will be a bust if the ductwork isn’t done correctly. Ask everyone you know for a referral. I’d gladly recommend the guys who did this last job, but not the ones who did my first furnace.

You may want to check out the HVAC forum here

9

Here’s the questions the salesperson should be asking you.

  1. How much longer do you plan to live there? (not all features will give you dollar for dollar appreciation, so your choice is important if you plan to move earlier rather than later)
  2. Do you have issues with dry skin, sinus infections etc in the winter? (Humidifier and/or variable speed blowers would be helpful)
  3. Are there hot and cold spots in the house? (Perhaps duct issues, “air balance”, or variable speed would be helpful)
  4. Are your utility bills objectionally high? Age of house? New windows? Quality/type of insulation in walls/roof? (Not all homes need high efficiency units, given the extra cost involved–although they are usually a good idea)
  5. What is the indoor air quality? Pets? Smokers? Kids? (speaks to filter selection)
  6. Do you have a budget you’re working with? (helps you prioritize features)
  7. Ever lived with a heat pump? (more on that later)
  8. Does the family have a predicatable schedule—especially one where mom/dad/kids are out of the house for 5-10 hours a day? (programmable/setback thermstat)
    Features:
    **Blowers: ** When using a Multi-Speed Blower, you’re only using 2 speeds. Typically High Speed is used for A/C and Low Speed for heating. The other ‘speeds’ are “parked” on dead terminals and not used. The benefit is that you (or a service tech) can move a couple wires and say, change the heating speed from Low to Medium Low, or Medium High. It’s helpful if you have an older 2 story, or have rooms far from the furnace where delivering the air is a challenge. OTOH, Variable Speed Blowers have no speeds. (Think dimmer switch) They ramp and down to meet the comfort need; in that respect they have hundreds of speeds. They do a much better job of managing dehumidifaction in the summer, and if you have a humidifier, humidity in the winter. If you’re interested in managing indoor air quality and humidity, it will be imcumbent upon you to leave your blower on 24 hrs/day. A Variable Speed Blower is outstanding for this; when there is no “call” for heating or cooling the blower will default to a very low speed and provide continuous circulation for humidity and air filtration. The downside? High first cost and higher maintenence. VSB will cost you $700-1000 more and if there is a failue the controls are very expensive. If you buy a VSB, make sure you have extended warranties on those controls; do not confuse the heat exchanger warranty with the controls/components warranty.
    Gas Valves
    A 2 stage gas valve uses a lower “firing rate” on a call for heat and steps up to high heat if the thermostat can’t be satisfied. You save gas. My concern is that the heat excanagers are designed for the higher firing rates so I’m a little dubious that the unit is at peak efficiency firing in low heat. The 2 stage feature is nice, but I wouldn’t pay more than $100-200 for it. If the furnace is sized apporpriately for the house, a single stage gas valve works very well.
    Thermostats
    If you have a predictable schedule that would allow for energy savings by setting back your thermostat , it’s a no brainer. If not, you can still have the same benefit by turning back the stat yourself when you leave each day. The Honeywell Vision Pro is an outstanding programmable stat.
    Filters
    This post in this thread will give you some information on filters. Make sure access is easy. Often the filter is ion the furnace and changing it is a pain—if they need to modify the duct to make a “filter track” that is preferable than fighting with the furnace.
    Brands
    Most brands are pretty good. The first tier is usually Trane, Carrier and Lennox. Other brands that are excellent are Bryant, York, Ruud, Westinghouse, Maytag, Ducaine, Payne, American Standard, Coleman, Armstrong, Arcoaire and others. The only brand I would stay away from is Goodman/Janitrol. It is true that some models (not the entire brand!) are the same—that a some Carriers are the same as their Bryant counterpart. It is not true however that an 80% efficient furnace is the same as a 90%–they are different in almost every way.
    Humidifier
    Aprilaire is the best known maker of them for furnaces. If you add humidity you will be more comfortable, at the same temperature. Your skin/sinuses won’t be as dry, and you’ll even be able to lower your stat a degee or 2.
    Air Conditioning/ Heat Pump
    Believe it or not, a Heat Pump, (HP)coupled with a standard 80% furnace is much cheaper to operate than purchasing a 90+% furnace. This is a paradigm shift for HVAC contractors and most haven’t gotten the message. But the cost of gas has made Heat Pumps much more efficient and attractive as a means of heating a house, even when natural gas (NG) is present. A HP/NG combination is a very cost effective way to heat—whether the NG furnace is an 80% or 90+%. I would strongly suggest you replace the A/C system if there is one—but you might ask the HVAC their opinion about a heat pump coupled with your furnace. If they replace the A/C, make sure they replace the Lineset.
10

Most likelythis is due to a new thermostat, one with a narrower turn on- turn off set points:

Old - set to 68F, Turn on at 68 off at 70
New - set to 68F, Turn on at 68 off at 69

11

One really common misconception I’d like to clear up. It’s common to believe that new furnaces are smaller [in Btus] because they are are more efficient.

That’s only partially true.

It is the Btu output that makes you comfortable—the Btus that go into your home. (the rest going out your chimney for the benefit of the birds warming themselves there)

If your home needs 60K Btus to keep you warm (based on where you live and the quality of your home’s construction etc) and the furnace is 60% efficient, than you need a 100K Btu furnace----60KBtu into the home, 40KBtu up the chimney, ergo 60% efficient.

So, if a contractor deems the [existing furnace] 60K to be the right size, and he is putting in a 92% efficient furnace, he will select a 65.2K input furnace, which will produce 60K Btu output. In this example the input was reduced, however the output (vs the old furnace he is taking out)remained the same: 60K Btu.

The common misconception is that a more efficient furnace means you can purchase a furnace with a lower output, or that a new higher efficiency furnace will put out more heat per btu then an old one.

A Btu is still a Btu. If a contractor downsizes the input------and maintains the same output-----it simply means you now have a more efficient furnace; that it takes fewer input Btus to provide the same output Btus.

Otoh, if he reduces [both the input and] output it’s not because those Btus are somehow more powerful than the old ones (a Btu is essentially the energy present in a match) but that he has determined that not only is your furnace inefficient, but oversized—too big.

It was quite common fior many, many, years to install furnaces that were much bigger than they needed to be. Who cared? You were comfortable and gas was cheap. But years later you’ve put in new windows, a new roof, blown in insulation etc—and the fact remains that even without those changes the original furnace was almost certainly oversized. (assuming it’s 20+ years old)

So, if your contractor reduces your input it’s because he has a more efficient furnace. If he reduces the output while he’s at it, it’s not a function of the furnace’s efficency—a Btu is still a Btu. He has simply determined that the existing furnace is/was larger than it needed to be so he is going to size it corrrectly.

12

Not in my case, the same thermostat was used unchanged.

But it does seem to me that the new furnace seems more responsive to the thermostat. At first, I was worried that it kept running even after the room had warmed up to the set temperature. Eventually, I realized that it was the fan that was still running, circulating heated air, while the burner had shut off. Seems to do that for a couple of minutes more after the furnace burner is off. I presume this is to circulate the heat that is still in the heat exchanger after the burner has shut off.

13

Is your existing thermostat a mercury bulb stat?

It is fairly common for a mercury bulb stat to act erratically with a new furnace. If it is a MB stat, I would replace it. The digital stats will do a much better job.

Your furnace should cycle 4-6 times per hour ideally. If it is cycling rapidly the common causes are:

1)The heating anticipator.While this is adjustable on MB stats, (by a service tech) it is usually a hassle and doesn’t work very well. Replace the stat w/ a digital stat.

2)An oversized furnace. Not likely from what you’ve described.

  1. Too much air. Reduce the speed for heating to Low Speed. (a service tech can do this)

  2. Stat location. If the stat is near a supply register, in sunlight or on an exterior wall, move it to a hallway near the return air register.

My guess? It’s the stat. If it’s cycling substantially more than 8 times an hour or so, you might replace it. The better ones (like the Honeywell linked above) actually allow a tech to configure them to limit short cycling. (short cycling will reduce your furnace’s efficiency)

14

[QUOTE=danceswithcats]
Since you’re replacing a 23 year old furnace, it’s a safe assumption that a conventional chimney is in use, be it masonry or metallic. Depending on the age and condition of the chimney, you may wish to consider a 90% efficiency unit and power vent it through a sidewall with PVC. If you go that route, pay attention to water heating, as you shouldn’t leave a chimney vent water heater as the sole appliance feeding a chimney-it won’t draft properly.
…
QUOTE]

Thanks, I was wondering this exact thing. If one is in this position (gas water heater, conventional chimney), what do you do? Get a HE water heater that also vents to the side?

Brian

15

You obviously know what you’re talking about, so this question isn’t a challenge to your credibility but sincere: If what you say is the case, then how come there aren’t any drafting issues during the seasons where the furnace isn’t used at all? I’ve got gas hot water which is running all year, and gas heat which is only use seasonally. I don’t know if I have multiple flues or not, just because it hasn’t occurred to me to look. In my previous house, there were multiple flues, so I’ll venture that I have multiples now.

Elsewhere in the thread it was mentioned that a newer, high efficiency unit would require horizontal exhaust – is this something that can be run in the plenum above a drop ceiling? The furnace is in the basement, so the exhaust would have to be drilled through the foundation, and now this hot gas is coming out at ankle level?

Any opinions on geothermal? I’d always thought of it as a cool, geeky thing to have, even if the initial investment is higher. I don’t know if it helps resale, because not everyone wants geeky things for geeks’ sake. But how about the other super high efficiencies? One of the questions to be asked is how long one expects to be in one’s house prior to deciding on an efficiency level. So is there any anecdotal evidence that a higher-efficiency furnace aids in resale value? Admittedly all I cared about when I got the current house was that the furnace worked and wasn’t spitting out CO or CO2 or other nasties. But on the other hand knowing it were super-higher or whatnot would have scored some points.

16

I’m not an HVAC specialist, but I’ll take the stab that in the winter your chimney is substantially colder than in the summer, and is thus more likley to cause the condensation issues. In the summer, you don’t need as much draft because it takes longer for condensate to form.

17

That’s exactly what they did with my new furnace. Depending on the length of the run, they might have to go up into your ceiling to provide braces to hold those pipes in place.

There are 2 exhaust pipes. One isn’t quite horizontal – it slants downward a bit, I guess for water to drain outward. But the other one turns up when it gets outside, and runs stright up for about a foot or two, like a mini-chimney. That’s the one that has smoke/steam coming out of it.

And my installers left the old chimney with only the gas water heater venting into it (along with an old fireplace that hasn’t been used in at least 30 years). They did rearrange the venting for the water heater somewhat, but it’s still the only thing venting into the original chimney. I sure hope it doesn’t have the problems danceswithcats mentioned!

18

Now wait a minute. We have a dual energy (electricity when the termperature is above -12 C and oil otherwise so that for about 10 months a year only the oil-fired water heater is using the chimney. Are you saying that is dangerous?

You may wonder (as I do myself) why we heat water with oil, but the house mostly with electricity. It is because when we moved in, we had too little hot water and it took to long to heat and we switched to oil and now she won’t consider switching back. Since we pay about 4c/kw for electricity (it triples when the temperature is below -12) we are paying a lot for that privilege.