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  1. #1
    Join Date
    Nov 2009
    Location
    Vancouver, BC
    Posts
    1

    Advice: Lennox G61MPV or G71MPP?

    I'd appreciate some advice on which of the two Lennox furnaces named (G61MPV or G71MPP) might be best suited to our situation:

    1400 square foot bungalow with old (35 year+) inefficient furnace; cold air return is in master bedroom and a source of noise (in effect the furnace is directly below the opening, and with the current furnace fan, makes too much noise to permit easy sleep). We have programmed the thermostat so that the furnace does not run during the night -- which means that the morning house temperature is often below the 68-degree target. The furnace runs a couple of hours in the morning, but is set back (to 61 degrees, I think) during the day, and back to 68 for evening. So during fall/winter it never really reaches 68 until early evening.

    The G71MPP sounds appealing (since the claim is that it is "one of the quietest furnaces made") -- I expect that the same claim could be made of other modulating furnaces. But -- from those with experience of the G71MPP -- will it be quiet enough to leave running through the night so that the morning house temperature is closer to 68? (give the location of the cold air return)

    I'm trying to get an understanding of what a modulating furnace would do in this situation (morning temp significantly below 68) relative to a two-stage furnace -- would the G71 be any more likely to reach 68 faster than our current furnace? How does a modulating furnace determine what output level to start out at? Is the starting level different depending on the difference between current temp and target? And once it reaches 68, does it tend to stay on longer (at a lower output level) than a two-stage furnace like the G61MPV?

    Also: I'm starting to get the feeling that our current thermostat (a programmable Honeywell: not sure the model number) might not be suitable with either of these furnaces: will I definitely need to get a new thermostat with the G71MPP? (or the G61MPV for that matter) Or is it likely that our current one will be "just fine"?

    Thanks for your advice...

  2. #2
    Join Date
    Nov 2009
    Location
    Saskatchewan
    Posts
    492
    All good questions for the contractor. New furnace, new stat. Just my policy. It only makes sense to stay current on stats.

    I am not sure why you are so worried about noise. Your fridge will be louder than your furnace. And we don't unplug the fridge at night.

    I haven't done Lennox for 12 years so I cannot help with the product.

  3. #3
    Join Date
    Nov 2009
    Location
    Vancouver, BC Canada
    Posts
    19
    I had a G71 installed about one week ago (I'm just a regular homeowner). Here is my general advice:

    1. We all hear/read that the installer makes all the difference and having gone through this install I can vouch for this 200% (I did not do enough homework on this and regret it).
    a. Pick a good installer first...then pick your product.
    b. Get references, check out the installer's previous work. Does he use tons of aluminum tape (like mine did) or do all new sheet metal fittings where appropriate?
    c. Ask for a heat loss calc to properly size the unit, it may be that you'll need to go with a different furnace manufacturer because Lennox's offerings don't fit.
    d. Find out where he plans to run the intake and exhaust in advance of the
    install and make sure the PVC runs are well within the furnace venting specs.
    e. If you decide on the G71, ask your installer if he's comfortable configuring in variable capacity mode (many...as I've read are not...or too lazy to do so)
    f. Since you will likely need to get a new stat...ask the installer if he will run enough wires (if needed) to properly operate the stat (I had to be explicit with my installer)
    g. Ask for written guarantees that everything will be to spec (venting as mentioned about, static pressure etc.)

    2. G61 vs. G71? IMHO, the only difference is the G71 has a modulating gas valve. The logic on the G71 system board (when configured in variable capacity mode) allows the G71 to "learn" from previous heat calls/runs and subsequently adjusts the firing rate at very granular increments (e.g I've seen mine run at 46% or 67% burn rates). The G61 only has two fire rates 70% (I think) and 100%. I chose the G71 because the majority of the time while trying to maintain set point during the day (or night)...it runs at 40% (and is very quite BTW). The G71's staging logic is primitive compared to what I've read on other furnaces (such as Rheem's). It all spelled out in the service manual:

    http://tech.lennoxintl.com/PDFs/0804d.pdf

    3. Regarding the stat. You'll want a two stage stat at the minimum (for me the jury is still out on if a Lennox comfortsense 7000 is better suited vs. my current Honeywell TH8321).

    4. Noise.
    At low speed (525CFM) the furnace is practically silent. At high speed (1100+CFM...I think??), the sound of gushing air is unavoidable. I don't really mind. BUT...at around ~632CFM...(medium low...I think??) my motor makes an annoying whine. I cannot stand it. The Lennox tech told me the whine is due to the AC to DC conversion and is unavoidable but due to manufacturing variability can be louder or quieter on each motor. I have requested the Lennox tech to come and "listen"/measure this sound to see if it is within spec...since my useless installer told me it was completely normal. In summary: go listen to someone's else's G71 (if that's what you decide) and listen to it across the range of speeds if possible.

  4. #4
    Join Date
    Jul 2007
    Location
    Tulsa, OK
    Posts
    974
    At low speed (525CFM) the furnace is practically silent. At high speed (1100+CFM...I think??), the sound of gushing air is unavoidable. I don't really mind. BUT...at around ~632CFM...(medium low...I think??) my motor makes an annoying whine. I cannot stand it.

    Using a Baldor Ball Bearing motor available from "The Fan Handler" out of Eugene Oregon will not be noisy all the way from as low as 14 RPM to 2,200 RPM. Major Manufacturers do not want you to know this at all due to its embarassment to their design flows inherent in trying to accomodate federal efficiency laws. The truth is a Baldor fixed speed motor and "Fan Handler" set up out performs "any" branded system that is variable speed, if you base it's results on fan cure laws, not to include the gas train design. The only flaw I have ever found is if you lower the bottom RPM for steady state "fan on" to low or use a to slow fixed speed line winding you can get #1. To high of heat rise. #2. Trip a fire wall damper fuse-able link. 3. Fall below mixing stratified air table to stay steady state mixed and be an embarrassed idiot of sorts!

    There are several other brands that do what a "Fan Handler" does however my personal experience is it is the best overall for reliability etc.

    Baldor Ball Bearing Motor is your answer for rumble and or noise (whine) at RPM factors. The other Manufactures use lesser quality materials for windings, commutator and rotor and the bearings.

  5. #5
    Join Date
    Aug 2007
    Location
    louisville, ky
    Posts
    951
    a g71 properly installed and set up is the best. so quiet its hard to tell if its running on the lower end, onces it ramps up to 70%+ it will become noticable, but most of the time it will run lower than that becuase u typically only need high capacity one those very cold design days.

    As long as the stat has two outputs for gas heat and u dont have it coupled with a heat pump it will work. w1 and w2 terminals.

  6. #6
    Join Date
    Apr 2009
    Posts
    523
    Quote Originally Posted by collective View Post
    I'd appreciate some advice on which of the two Lennox furnaces named (G61MPV or G71MPP) might be best suited to our situation:

    1400 square foot bungalow with old (35 year+) inefficient furnace; cold air return is in master bedroom and a source of noise (in effect the furnace is directly below the opening, and with the current furnace fan, makes too much noise to permit easy sleep). We have programmed the thermostat so that the furnace does not run during the night -- which means that the morning house temperature is often below the 68-degree target. The furnace runs a couple of hours in the morning, but is set back (to 61 degrees, I think) during the day, and back to 68 for evening. So during fall/winter it never really reaches 68 until early evening.

    The G71MPP sounds appealing (since the claim is that it is "one of the quietest furnaces made") -- I expect that the same claim could be made of other modulating furnaces. But -- from those with experience of the G71MPP -- will it be quiet enough to leave running through the night so that the morning house temperature is closer to 68? (give the location of the cold air return)

    The longer it runs the louder it will be. Blower speed is determined by fire rate, fire rate is determined by an algorithm. If its THAT noisy you need duct work modifications. A manual J to verify proper unit sizing too. Dont buy this furnace trying to bandaid another problem.

    I'm trying to get an understanding of what a modulating furnace would do in this situation (morning temp significantly below 68) relative to a two-stage furnace -- would the G71 be any more likely to reach 68 faster than our current furnace? How does a modulating furnace determine what output level to start out at? Is the starting level different depending on the difference between current temp and target? And once it reaches 68, does it tend to stay on longer (at a lower output level) than a two-stage furnace like the G61MPV?

    I guess a none technical way of describing a modulating furnace at least like the G71 compared to a 2 stage gas furnace would be calling it a 60 stage gas furnace. It will not heat up faster then a standard furnace that defeats the point of modulation or stages. The G71's operation is a little technical but basically it uses the call from the thermostat, previous run time, and current run time to determine an output. The starting output will be different depending on thermostat call and previous run time. And technically yes it will stay on longer but its hard to notice a difference comparing it too a 2 stage furnace

    Also: I'm starting to get the feeling that our current thermostat (a programmable Honeywell: not sure the model number) might not be suitable with either of these furnaces: will I definitely need to get a new thermostat with the G71MPP? (or the G61MPV for that matter) Or is it likely that our current one will be "just fine"?

    It depends on the model of thermostat. To get the full potential out of the furnace you need a 2 stage heat thermostat. If your current thermostat does that then it will be fine. The only requirement of the 2 stage thermostat is it can not lock into 2 stage it needs the ability to drop back to 1 stage heat before ending the call. Most, if not all Honeywell stats do this.

    Thanks for your advice...
    I would definitely choose a G71MPP over a G61MPV any day. But I'm a geek about these things. The G71 has GE's 3.0 motor and I dont THINK the G61 does. You have the variable gas valve which is operated by the inducer speed which is a variable frequency drive control but a very intelligent (in comparison) control board. And we havent had a lick of trouble with them at all. I've seen install issues but thats about it, not break downs.

  7. #7
    Join Date
    Aug 2009
    Posts
    1,127
    Quote Originally Posted by deux View Post
    I am not sure why you are so worried about noise. Your fridge will be louder than your furnace. And we don't unplug the fridge at night.
    "We" may not, but some people actually do! I met one just last week. When you think about it, this may not be ideal for food preservation (although this guy said there was only a couple degree rise overnight), but it is very similar to a setback and may even save energy while reducing noise pollution.

    I shopped like crazy for my new fridge with the primary criteria of noise. My fridge may be better than average however it still bugs me at night, and even during the day at times, particularly the solenoid clunk. I've been tempted to turn it off myself at night, but I still have some solutions to trial, such as insulating my interior walls.

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