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Community and Q&A

2-speed vs. variable-speed furnace blower

thrifttrust | Posted in Mechanicals on

I’m about to order a replacement 40,000 BTU gas furnace for my house in Traverse City, MI, CZ6. It will have a 2 stage burner with an AFUE of 96%. It is available with a 2 speed ECM blower and one with that is continuously variable.

The house is 900 sq. ft. It has R10 basement walls, R20 walls and R50 ceiling.

What difference does a variable speed fan make, and is it worth $200?

Douglas Higden


  1. GBA Editor
    Martin Holladay | | #1

    That's an excellent question. In theory, if you have a two-stage burner, it would make sense to optimize the delivery of the heat at the two firing rates by providing two possible blower speeds, each optimized for one particular burner output.

    That said, there may be a reason why there is an advantage to investing in a variable-speed blower -- but I just can't think of the reason.

    GBA readers?

  2. charlie_sullivan | | #2

    It's interesting that it's an ECM either way--that makes it sound like the hardware capability is there in either model and you just pay $200 extra for them to enable that feature in the variable speed one.

    I would think that the advantage of the variable speed would be primarily in commissioning the system: if the duct resistance is a little more than the default settings are designed for, you could boost the fan a bit, or if the ducts are short and generously sized, you could actually turn the fan down a bit, and get quieter operation with less electric consumption, while still achieving the design air flow.

    Some, but not all, ECM systems automatically vary the speed to maintain the airflow at the specified value. If the 2-speed model does that, then the advantage I suggest for the variable speed model does not make sense--it's already automatically doing what I suggest.

  3. thrifttrust | | #3

    Thank you for your insight. In studying the manufacturer's data I find only vague references to variable speed providing more "comfort". I did discover why the two speed uses an ECM. It has a feature that increase or decrease each standard speed 5 or 10 percent to tune it to the duct system. (Thanks Charlie, I hadn't considered the commissioning thing.) This should help me because the ducts are no doubt oversize in that they were installed with the furnace I'm replacing in 1960 when the house had R3 insulation and single glazed windows. I suspect that the two furnaces are identical save the control board. My experience with electronics tells me that it is cheaper to make a controller with discrete steps over one that is variable.

    I think I'll save the $200.

  4. thrifttrust | | #4

    When I posed this question the only choice was between 2 stage and modulating. Last week I was about to place an order for the lowest input (40,000 BTU) 2 stage, only to find they now offer it in 800 and 1200 CFM versions. I don't know which fan speed to get. What are the considerations? Obviously the slower unit is quieter. This is a replacement for a 1960 instillation. I've improved insulation and air sealing enough to justify the new smaller furnace. The house is 900 sq. ft. ranch. The all metal duct system is larger than I see in new installations. Does this affect the speed choice? In this CZ6 spec remodel I'm not installing AC but if future owners wanted it would the slow fan be a problem?

    Douglas Higden

  5. Expert Member
    Dana Dorsett | | #5

    Even at low fire a 40KBTU 2 stager is probably ~2x oversized for the heating load of the house described. It's a small house with decent R-values, and a much lower load than the 25-28KBTU/hr that a typical 40K-2-stage furnace puts out at low-fire. Without knowing too much about the place other than what has been described above I'm going to hazard that the true load is under 15,000 BTU/hr @ 0F, and that 18-20,000 BTU/hr would be a more appropriate maximum output number for the heating equipment.

    Get a handle on the load numbers using this methodology- there may be more appropriate options:

    Use +2F as the outside design temperature- it's the 99th percentile temperature bin for Traverse City. Of course it gets lot colder than that sometimes, but that's the appropriate place to put a stake in the ground, and use an appropriately modest oversizing factor from there to cover the Polar Vortex lows.

    The cooling load is also probably less than 3/4 ton. A hydronic air handler with the appropriately sized cooling coil and a heating coil that can deliver your design day heat at domestic hot water temperatures running off a condensing water heater would probably be a better overall solution.

    Let's see where the numbers come out before going too far down that path, but a 40K furnace is kinda silly- it's not as if the outdoor temperatures will get down to -100F any time soon.

  6. bennettg | | #6

    I own a Trane vs air handler and have studied the Goodman and Tempstar (ICP) variable speed air handler docs. At least for those, the VS AH offers 1) Maintaining the CFM vs pressure more tightly than the multispeed ECM AH. 2) A "DS" or similar terminal that drops the flow by 15-20%. Handy for humid AC environments when set up with a humidistat, probably not so much for Z6. 3) Some offer a +/- 5, 10% CFM trim to fine tune. My Trane does not. 4) Low speed, efficient nearly silent fan only, like 70-100w. Some can be set to 25/50/75% of nominal CFM. I have mine set up to circulate the air a part of each hour if the heat or cool doesn't.

    The VS AHs also offer fan profiles that ramp up and down the fan before-during-after the heat/cool. I prefer the more detailed control my thermostat offers to ensure a bit of start-up delay, reasonable run-time, slow-off for heat to extract most of the available heat and quick-off for cool to minimize re-evaporation in my humid environment.

    I learned this stuff from the install manuals for the air handlers.

    So is it worth it? Probably not, particularly if you get a nice quiet start-stop ramp up / down and lower fan-only with the two speed.

  7. lance_p | | #7

    Dana, I was thinking the same. Based on Martin's recent blog post I've been looking into Dettson furnaces. They make several models of 15k and 30k BTU gas/propane furnaces with AFUE of 95%+, they modulate down to 40% output, and they have variable speed ECM motors. The heat load numbers should definitely be scrutinized for a 900 sqft house before settling on a relatively huge furnace.

    I live in a 1800 sqft end unit townhouse (one long wall exposed), and our ~40k BTU furnace hardly runs at all. Even in the coldest weather (-22F/-30C sometimes colder here in Ottawa) the furnace only seems to run about 50% of the time. The front bedrooms on the second floor farthest away from the furnace are uncomfortably cold by morning, and the only way to fix this is by leaving the doors open or using auxiliary electric heaters. This problem persists even though I blew R60 insulation into the attic this fall (up from ~R20-R30), not that I expected it would fix the issue, just a data point.

  8. Expert Member
    Dana Dorsett | | #8

    Lance: Smaller burners= longer run times, and a greater likelihood that the heat will actually make it to the rooms at the end of the run. Sounds like your system might respond to some balancing vane tweaks.

    The Dettson approach is to micro-duct a home-run supply duct to each room from a short fat plenum, then tweaking the flows on each. That's pretty different from typical pre-existing duct systems.

    Goodman makes some ~30K 2-stager furnaces with a min-fire in the low 20s, which might work better for THRIFTTRUST, but a right-sized modulating cold climate heat pump will probably work better (but with a bigger upfront cost.)

  9. Reid Baldwin | | #9

    I suspect the 800 cfm and 1200 cfm version are so they can support 2 ton and 3 ton AC units.

    Our two stage 40K furnace has an ECM blower with five speeds available. You associate particular functions (fan only, low heat, high heat, and AC) with various speeds by connecting particular colored wires to various pins. It is something they expect will be done by installers, not homeowners.

  10. thrifttrust | | #10

    Thank you guys. I've been successfully cooling with a 10K window unit so I guess the 800 CFM should work if central air is added.

    Dana, you've confirmed my feeling that the 30K unit will suffice, but you've thrown a monkey wrench into my plan. The house has a condensing water heater. I experimented with a small scale open direct heating system using a handful of fin tube radiators. within weeks the check valve seized from lime scale. For a year I had to run the pump or the taps ran cold. That and the hassle of installing a full set of radiators or under floor pex put me off the plan. I worried that if I added a heat exchanger it too would suffer from lime. Besides, I have a perfectly good duct system.

    I just looked into hydronic air handlers and found a 30K 600 CFM unit for about 1000 USD. The water heater has side taps for heating. Can an air handler coil be safely connected directly to the heater side taps without a separate heat exchanger? I still have the pump and domestic hot water tempering valve. What else would I need? Would water from the heater be hot enough?

  11. lance_p | | #11

    Dana, without hijacking the OP's thread, I will say that I've been adjusting registers in an attempt to get more heat to the front upstairs rooms without much success. I think the basic layout is poorly conceived, none of the ducts have thermal insulation, and the ducts I can't get to are probably as leaky as the ones I could to seal... all contributing to poor comfort in those rooms.

  12. Yupster | | #12

    To go back to the question of the 2 speed ECM vs. the variable speed ECM, it may be helpful to understand the motor technologies. The 2 speed ECM is most likely an X13 ECM motor, meaning it has been pre-programmed to operate at specific torques. If additional static pressure is induced on a system, it will work harder to maintain it's torque. This is not the same thing as maintaining airflow. The airflow will still drop off, although not as drastically as older PSC motors. It also uses significantly less energy to operate than a PSC motor. The variable speed ECM is a bit different. It can adjust over a very wide range of speeds. It usually is programmed to ramp up softly, eliminating that bang when the furnace kicks on. It also will adjust it's speed to maintain airflow if additional static pressure is induced on a system, such as a filter getting dirty. It is the most efficient motor technology available. The downside is that if a variable speed ECM motor fails, you have to buy a replacement from the manufacturer that is programmed with the same settings as the failed one. This usually means many $$$$. The X13 is an excellent middle ground. If you want a more detailed explanation, just google X13 vs. ECM and there are plenty of longer winded explanations.

    In regards to attaching a hydronic air handler to a condensing water heater, if your existing system can't keep up with the load, (have to run the numbers and find out your heat load for heating and DHW to know) consider adding a flate plate heat exchanger and a buffer tank. This would increase the capacity of your system and allow you to have a closed system for your heating.

  13. thrifttrust | | #13

    Our DHW load is quit low and it's been suggested here that our heating load is perhaps 15K BTU. Our 50 gal condensing water heater input is 67K BTU.

    I checked fin tube radiator specs and found that heat transfer at 190°F is twice that of 140° F. Wouldn't that mean that I would need a significantly oversize hydronic air handler with greater air flow than a boiler fed handler or gas furnace? I actually considered a DIY system. The 1960 vintage furnace that came with the house was a lovely big old thing with rounded corners and hammertone finish. The welded steel heat exchanger was huge. I'd planned to fill it's space with a coil made out of fin tube radiators. I abandoned the plan due to the aforementioned lime scale issue.

  14. Expert Member
    Dana Dorsett | | #14

    FirstCo makes all sorts of air handlers that have output specifications at domestic hot water type entering water temperatures, and designed specifically for using water heaters at the heat source. All have multiple blower speed settings, some have ECM drive and a wide range of speed. Some have air conditioning coils too.

    eg: The 18HBQB has a 1.5 ton cooling (or heat pump) coil, and it can deliver about 24,000 BTU/hr at an entering water temp of 120F at the highest blower speed setting. They don't specify the output at lower speeds in the short-sheet, but the blower has about a 3:1 turn down ratio, which would allow it to set to the right range for heating loads under 20K:

    The 18HBQBR puts out about 20K @ 120F EWT at the high blower speed, also with about a 3:1 ratio between high and low speed:

    There may be more suitable versions with even smaller coils if you search Wall / Closet / Attic Air Handlers tab the catalog:

    If you keep the water heater at 140F (recommended by code), set the blower speed to a lower speed, and use a thermostatic mixing valve to deliver set the water temp to the air handler it's possible to set the output over a very wide range while tweaking it in for best comfort. At entering water temps much below 120F it may still heat the house, but the exit air temp at some of the registers might feel a bit tepid when it drops below 95-100F.

  15. lance_p | | #15

    Be careful with high water temps. The higher the tank setpoint the less efficient it is at heating the water, and if you don't get your return water temps below about 130F (the lower the better) your tank will not condense the combustion exhaust resulting in a huge efficiency penalty.

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