Hello, All,
I am going to upgrade my old gas furnace to a new 90%+ efficiency unit and am wondering if I should opt for model with a variable speed blower. Any opinions out there if this feature is really worthwhile or just a fad? Any help would be greatly appreicated!
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Listeners write in about haunted pipes and building-science tomes, and they ask questions about roof venting and roof leaks.
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
what is the purpose of a two speed fan?
I don't know but,
It seems to me the faster you get the hot air from the furnace to the cold room the sooner the furnace shuts off. A slower the fan the longer the furnace will run and burn more gas.
Why don't you ask the supplier? The variable speed model may change speeds between heating and air conditioning. I have seen some models that do this.
We tested one in our lab and it is very quiet. It should also perform better since it will keep the air moving slowly at all times so stagnation and stratification will not occur. I know that I will seriously consider buying one when I buy my next furnace.
I have a variable speed furnace in my home and think it's great. It does a much better job of getting all the rooms to an equal temperature then my old furnace did. It is also much quieter then my old furnace.
Multispeed blowers are prepared for air conditioning.
On our furnace the fan is set for low speed for heating.
When the air conditioning is on the blower is set to
high speed. I am not sure, but I think this is because
the cooler air is more dense. These rules are not set
in stone, some people do not like the noise the furnace
can make with the high speed fan setting.
Rob,
I do not have an answer to the 2 speed fan question, but I would like to make some comments on the High Eff furnace. Depending on your house setup and your tolerance for noise, you might want to ask to hear one in operation.
About 3 years ago, the gas company ran gas lines down our street and gave us the opportunity to hook up for free if we converted to gas heat and hot water. I previously had electric hot water, and oil heat. Both units were about 22 years old and had never missed a lick. I wanted the gas heat mostly for quiet operation. Our furnace is in a closet in the lower level of our split level and the oil burner was noisy when running. Well, surprise , surprise...the combustion air fan of the High Efficiency gas furnace is just about as noisy ! So depending on your furnace location in the house, you might want to keep this in mind.
I have seen a very large savings in energy costs. I think that a lot of it was from getting rid of the electric water heater. I am not certain about the furnace...in three years I have had the igniter burn out twice to the tune of $2oo each time, so any energy savings were probably wiped out. If it helps any, my Jan gas bill was $83 for a family of three in the Washington DC area (Southern Maryland)
Just make certain to research it a bit to make sure you will be happy.
Hope this helps,
Robear,
I do not believe that it is a fad, but an evolution of the products available. Improvements come at a cost, though, and you should get some quotes to compare wether or not you're willing to pay for a superior product.
There are several options in this category to consider. First is the blower and the draft inducer. Then there is the burner/gas valve. There are two-stage multi-speed and there are two-stage variable speed furnaces available. A "standard" furnace has a single stage gas valve and a 4 speed blower. Two of the four speeds are available for operation, one speed for cooling (usually the highest) and one speed for heating (usually one of the lower two speeds). The reasons for this are more detailed than I want to get into now, but if your interested, we'll go there. A multi-speed furnace with 2-stage heat operates basically the same as the standard, but there are 3 blower operation speeds and two different "firing rates". The draft inducer fan in these furnaces operates at a single speed. In a variable speed furnace, the blower ramps up and down to various speed (that are pre-set, but changeable) and typically operates at 4 different set speeds: one for ventilation (if operated with the fan switch in the "on" vs "auto" position), one for low heat, one for high heat and one for cooling. The draft inducer fan also operates at variable speeds depending on the rate of fire.
The best you can get is a variable speed, 2-stage furnace. They are quieter and more efficient than any other type of furnace, and offer a more consistent comfort level, but are of course the most expensive. I have one of these in my home and would highly recommend it.
The next best option is the two stage, multi-speed furnace. Then there is the high efficiency single stage.
Tim
Tim,
As usual, great info. Are the variable speed, 2 speed draft inducer types all 90+, or do the 80+ come with that type of set up?
How might one distinguish between the "standard" two speed and the variable speed?
Bob_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
"How might one distinguish between the "standard" two speed and the variable speed?"
Other than the price tag? I went to Trane's web site to see how the furnaces were labeled, thinking the variable speed units would have it all over the case, (kind of a "look at me? look at me!") they don't distinguish them like I thought they would. Their page did spell out the features of the individual units however.
http://www.trane.com/Residential/Products/Furnaces/Furnaces.asp
I bet York, Carrier, Lennox would have similar web pages.
Bob,
I am not aware of any < 90% AFUE furnaces that have variable speed blowers or draft inducers.
Standard vs. multi-speed vs variable speed - all basic furnaces (of any consequence presently being manufactured in the USA, of which I am aware) have a four speed, permanent split capacitor (psc) blower motor. In heating or cooling modes, it will operate at a single speed, unless you physically change the tap in the blower compartment. This is what I refer to as "standard". A multi-speed furnace typically will operate at a single blower speed in cooling mode and two different blower speeds in heating mode. The speeds available are limited to a few preset selections. The true variable speed furnace has an AC to DC transformer and the blower motor is actually DC powerwed and is infinitely variable, within a range. The controllers limit how many operating speeds you can utilize. Mine has four sets of dip switches to select speed for vent, low-heat, high-heat and cooling.
Seems like the draft inducer fan is the critical component in determining the quietness of the furnace. Do manufactureres rate them at all on their products? Can you give a rough price differential percentage wise between a standard, a multi speed and a variable speed? Any particular make/makes you have good experience with?
Tom
Coupled with previous discussions of heat exchanger efficiencies (the boundary layer is where most of the temperature dorp is and that is roughly proportional to square of velocity, tho lots of other factor also). Coupled with that, the power needed to run the blower varies as the cube of the blower speed. Lots of trades to find the most efficient spot. Thus, fully variable speed (with the DC conversion step) can provide an overall efficiency increase of a few percent when coupled with a control system that takes into account the room temp, HE temp, and energy input.
Seems like the draft inducer fan is the critical component in determining the quietness of the furnace.
FWIW, the draft inducer fan is the little guy at the front of the furnace; they are typically pretty quiet (a noisy one is one in trouble.)
I believe you're thinking of the blower fan.
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
There are many factors that determine the noise level of a furnace (or any peice of equipment), the motor mounts (good ones are isolated via rubber or spring isolateors), cabinet insulation, air velocities, burner noise and duct connections and configuration.
Manufacturers seldom, if ever, rate the noise level of furnaces in any standard way.
List prices from my old 2000 price catalog for (100 MBH input) various types:
80% basic - $960
80% 2-stage- $1422
91% basic - $1847
92% 2-stage - $2328
94% 2-stage, variable speed - $2924
I own, recommend and usually specify Heil products. Local support (in Rockford, IL) is excellent.
List prices from my old 2000 price catalog
Thanks: now I've got an answer for the "how much will a new furnace cost me?" question.
Going from "a couple of thou, plus or minus" to "a thou to 3 thou, plus of minus, maybe." {G}_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Tim and Bob:
Thanks for all the info. I am thinking of getting a new furnace and A/C, this thread provided me with valuable info.
Tom
I think your question is "Why two speeds?". The answer has to do with the finer points of heat exchanger design and comfort. For any furnace, the designer has to balance efficiency and cost. There is no free lunch and so the only way to get better efficiency is to make a larger heat exchanger to better transfer the heat from the burnt gas to the air passing over it. Of course, a larger heat exchanger costs more and is larger, which is a packaging problem. Let me try to explain this so that it is easily understood.
The design of the heat exchanger must take into account three (at least) criteria; 1. It must be large enough to absorb the amount of heat being fed to it without burning up. 2. A major factor in 1 is how much air is being passed over the heat exchanger to cool it and prevent it from burning up. 3. The whole system must be designed to deliver the amount of heat necessary to heat the house.
I'm not an HVAC contractor but it seems to me that it would be impossible to make a furnace with a two speed blower without also giving it a two stage burner. When the blower goes to it's lower speed it would seem that the result would be that the heat exchanger would get very hot and have a shorter life.
The idea behind a two speed furnace is as follows. When you do not need the full output of the furnace (most of the time), it can run at reduced output to consume less energy. This has several benefits. The furnace will be operating in a mode where it effectively has a very big heat exchanger for a small output heater which is very efficient. By delivering less warm air to meet the lower demand the unit cycles less frequently which is also good for efficiency and makes the living space more comfortable by keeping it at a more even temperature.
I would guess that the 90% rating is based on the overall operation of the unit. It probably is about 80% efficient at high speed and 95% efficient at the lower speed. It is probably expected that the unit will operate in the low speed 2/3rds of the time and so the 90% overall rating.
Hope this helps.
Joe