Comparing furnaces


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Old 03-30-09, 11:35 AM
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Comparing furnaces

I am considering replacing my current furnace (an 80% efficiency 70K BTU tempstar that is roughly 18 years old) with a new high efficiency furnace.

I'm open to other options, but at this point I am considering the Lennox G71MPP and the Trane XV95, and I am interested in getting opinions or references to reviews of these two systems. I realize that the Trane is a two stage and the Lennox a graduated stage system, but I do not know if this a gimmick or actually helpful.

I live in the Seattle area, winter temps typically are around 20-40 degrees. We do get down to the single digits every now and then. I use natural gas. The house is approximately 1500 square feet, a rambler, has a small crawlspace and is currently fairly poorly insulated. There are registers in each room, but my impression is that they are undersized for some. The furnace is in the converted garage, the furthest register is roughly 75 feet from the furnace. With the current furnace I get very uneven heating.

In addition to the furnace, I am planning on adding air filtration (e.g. cleaneffects/accuclean) and steam-based humidification. I will be running thermostat wiring and replacing the thermostat, presumably with a Honeywell IAQ. With the air filtration, I would like to be able to circulate air throughout the year, so the systems fan will be in very regular use.

I am primarily interested in the overall quality/reliability as well as even heating and efficiency of these units. Given the lower number of registers I would like to be able to run the fan at a higher than normal rate for a home of this size.

Any information or pointers is much appreciated!
 
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Old 03-30-09, 11:41 AM
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"has a small sub-floor and is currently fairly poorly insulated"
If you plan on improving the insulation/efficiency of your home, do it before you size the furnace. With substantial improvement, your heat load can drop in half and you end up with an oversize furnace that has to short cycle on demand.

Bud
 
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Old 03-30-09, 03:01 PM
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I agree with Bud, definitely improve the insulation and air sealing of the house before sizing the new furnace. I do question the necessity of a humidifier in the Seattle area, we're not exactly known for having dry air. In fact, my own house (Bothell) is running about 40% RH lately and I have never seen it below 30%. The mere fact of having a few people breathing along with normal cooking and showers will probably keep the humidity within normal limits, last winter I had more trouble with high humidity than low.

I have a home similar in size and and arrangement and I had a Lennox G60DFV-70 installed three years ago. My biggest complaint is that the furnace is too large, even on the first stage, most of the time. If you go with the G71MPP you will likely have a bit better control on the low end where you will need it during the not-so-cold periods. I don't know how Lennox controls this (probably time based like York) but you may be able to have it set up to limit the high-end output. One thing that you might consider is that the 70,000 BTU furnace is the smallest size that Lennox makes with the variable speed blower so it would be okay for your house as it is and still would be able to run at a lower heat output after you employ more insulation and air sealing.
 
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Old 03-31-09, 10:27 AM
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Thanks for the advice on the insulation. I'll look into my options on that. My concern is I may need to replace the furnace before I get that project complete.

Average humidity in the house is around 35%. I live at the top of a hill in Kirkland, WA if that is relevant. I prefer quite a bit higher humidity, especially in the winter, due in part to nasal/sinus issues that humidifiers help alleviate. I notice a significant improvement in some symptoms when I stay in 50%+ humidity. I've decided I want to increase the whole house humidity somewhat, not just my bedroom and office.

I'm fairly confident that the two systems proposed are sized about right for the current house, and the fact that insulation may help so much makes me lean towards the Lennox G71MPP, as it can function as low as 42K BTU (60% of 70K) and as high as 67K BTU. I just verified, and the tempstar is actually an 80K, so its putting out roughly 64K at 80%.

Does anyone have a comment regarding the variable output of the Lennox vs the two-stage of the Trane? The engineer in me likes the idea of the efficiency (and that its variability can be controlled by the IAQ or a later upgraded system) of the Lennox, but I also know that the more complicated a system, the more likely it is to fail in a difficult to repair mode.

Thanks for the information and advice so far!
 
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Old 03-31-09, 11:20 AM
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The term for humidity if "relative humidity", which means while you are enjoying 50% humidity in the center of your home, in the winter the colder wall/window areas will be much higher, causing sweating and moisture damage. Inside walls, closets, and behind drapes where there is poor air circulation condensation will form providing a place for mold to form. The threshold for mold is 50% humidity, above that it can grow.

Do some reading on air sealing, and how it can improve the humidity level in your home as well as reduce the heat loss. If you do like the higher moisture levels, it might be best to do as you are now with selected areas only.

Good Luck,
Bud
 
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Old 03-31-09, 01:29 PM
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I suggest that you do a Google search using the terms Lennox G71MPP and see what others are saying about this particular furnace.

Also, the minimum output of the G71MPP is 40%, not 60% of maximum. This will give you a BTU input range of 28,000 to 70,000 or an output of about 26,600 to 66,500 BTUs per hour. With a range like that you would still be okay to install this unit before upgrading the building envelope.

I did notice that the modulating feature is time-based rather than by a modulating thermostat. I would prefer a modulating thermostat but such would require a significant increase in the cost of both the thermostat itself and the furnace control board.
 
 

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