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  1. #1
    Greetings... First post here!

    I am researching a new furnace for my 30 yo house in Calgary. I want the most efficient unit available, and am debating between the Carrier/Bryant 58MVP/+90i vs. the Rheem/Rhud Modulating unit. Carrier seems to have the slight edge on the AFUE rating, but the Modulating feature seems compelling for our long and cold heating season, but certainly the Carrier will do the job too.

    But, one factor I am having trouble finding facts about is the electrical consumption of these two units. Furnacecompare.com rates the Carrier/Bryant at Total Annual Auxiliary Electric (kW/hr) of 105kWh, which is low, but that site just gives me a proxy error for the Rheem. The Rheem web side tells me that the motors are 373W at 6.8A, but that doesn't tell me annual energy consumption.

    One of my goals is to participate in Canada's 'One Tonne Challenge' goals, and electrical usage in Alberta is all coal fired, at a whopping 925 grams greenhouse gas (GHG) per KWh. So, the less electricity, the less GHG, not to mention, the smaller my investment in solar panels when I eventually make that investment. Electrical consumption becomes a significant deciding factor for me that the industry seems to completely ignore rating.

    Anyone know the facts of electrical consumption on these units?

    Peter Straub
    Calgary, AB

  2. #2
    Join Date
    Mar 2004
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    1,042
    I don't have a firm answer for you, but here's what I do know.

    With variable speed furnaces, the biggest factor affecting electrical consumption is how much restriction there is against the airflow. To minimize the consumption, you will need free-flowing ductwork and a low-restriction filter setup.

    They use the same motor technology. The one thing that comes to mind, though, is that the modulating furnace runs the blower at the slowest speed possible to maintain the temperature rise within the upper limit. The Carrier variable speed units seem to demand unusually high airflow rates when they are on high heat, especially in a few particular sizes.

    What input size furnaces are we talking about here?

  3. #3
    Originally posted by wyounger
    To minimize the consumption, you will need free-flowing ductwork and a low-restriction filter setup.
    What input size furnaces are we talking about here?
    That makes sense. I was contemplating one of those Carrier electronic air filters. At .01 micron, they seem to offer the best filtration, but like the furnaces, I am having trouble finding facts about these units... electrical consumption. Is there any rating for air filters in terms of the energy needed to move a given amount of air through them? There should be!

    The furnaces in question are the smallest they make... 45,000BTU Rheem - 40,000BTU Carrier. According to my Energuide report, my before design heat loss is 48,700 BTU, the 'after upgrades' is rated at 34,700BTU. My current furnace is 118,00BTU and been given a nominal rating of 63% seasonal efficiency in the my report.

    Peter Straub

  4. #4
    Join Date
    Jan 2004
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    I would think the Rheem mod would use less electric.

    The blower will be at lower speeds most of the time.

    You might want to check into media filters, they use no electric, and are availible in several ratings. The higher the merv rating, the more it filters.
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  5. #5
    Join Date
    Mar 2004
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    Media filters use no electricity directly, but they are more restrictive than electronic air cleaners, and the restriction will affect the blower's power consumption. But if you use an oversized media filter (or a pair of them), the restriction drops drastically.

    It's enough to make your head spin! I for one would still vote against the electronic air cleaner just because they require so much maintenance.


  6. #6
    Join Date
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    wyounger's right, they are more restrictive, so they must be sized/paired to keep the static low.

    For the first 6 to 8 years in this trade, I liked electronic air cleaners, then I wised up.

    Hate those things now.
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  7. #7
    Join Date
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    OK, the 58MVP040 is set for 585 cfm on low fire and 800 cfm on high fire. It's certified for 25-55 degree temperature rise on low fire and 30-60 on high.

    The smallest Rheem modulating furnace is variable over the 40-100% capacity range, of course, with blower speed varying to maintain a 65 degree temperature rise. Rheem states an allowable range of 320-795 cfm. But its input capacity is 60,000 BTU, not 45,000. Rheem does have some 45k furnaces, but not in the modulating series.

    I do have a lookup table for the power consumption of the Carrier based on static pressure, but not for the Rheem.
    Carrier 58MVP-040 (total wattage)
    Low heat, 0.1" ESP-0.6" ESP (0.1" steps): 55 71 91 108 129 147
    High Heat, 0.1" ESP-1.0" ESP: 97 119 141 165 187 209 230 252 274 292

    That won't tell you what an average consumption rating might be, but if can come up with a comparable table for the Rheem we can still make a better educated guess about overall power consumption.

    It's hard to compare the two because of the difference in modulating versus nonmodulating. Low fire in the little Carrier isn't much more than the 40% point in the mod, though- 25k versus 22.6k- so even the mod will do a fair amount of cycling in this application. The Rheem will always have a higher temperature rise than the Carrier, BTU for BTU, regardless of conditions, so it will be moving less air around. In this application it will probably never see 100%, because it's actually rather oversized for the load, so it will probably never exceed even 700 cfm. That again suggests lower power consumption.

    The Carrier is just barely enough based on your load calc- it's rated 38k output for upflow (37k in other positions)- and you get a 5% derating for altitude in Calgary (3550 ft.). That gets you an actual output of 36.1k. It will be just efficient as Carrier claims (96%) but will run a great deal in the winter, and in record cold, may not quite keep up running nonstop. That also means that it will take a long time to recover from a programmed setback (sleep, away, etc.), and if the house should get cold as a result of a power or equipment failure, it could take well nigh forever to catch up. To achieve the 96.6% efficiency rating, Carrier made this particular unit huge for its capacity; it is 7" wider (24.5" wide for a 40k input!) and 6" taller than the Rheem, despite being of less input capacity.

    The Rheem has 50% more capacity; its output is 55.8k before derating. After the 5% derate, you get 53k maximum output. You will have plenty of capacity to warm up promptly after setbacks or failures. It could even cope with your load before the "improvements" listed in your load calc are made, though just barely. Because of the excess capacity it may not quite achieve its rated 94% efficiency, though. Some may argue that the excess is small, but on a percentage basis it's 50% more capacity than is required, which is not trivial.

    None of this really answers your question, I know, but maybe it'll help you make a decision based on some other criteria.

  8. #8
    Join Date
    Apr 2002
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    The AFUE ratings are good in the lab but a little different when installed out in the field. As well your furnace will be derated for altitude.

    The efficiency ratings are also when a furnace is warmed up and reaches steady state. The efficiency is lowest when they first fire up each time the thermostat calls for heat.

    The rheem will have a lot less starts and stops and will be getting 'highway mileage' during most of the winter.

    Other furnaces will start and stop more often similar to 'city mileage'.

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  9. #9
    Join Date
    May 2000
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    Indianapolis, IN, USA
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    Only diff I could see is Carrier uses a variable speed inducer which could have a electrical usage advantage. But when it goes out, there goes any savings

  10. #10
    Join Date
    Oct 2004
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    Both of those 90+ furnaces have the DC variable speed (ECM) blower motor. There should be little (if any) difference in comsumption...

  11. #11
    Join Date
    Oct 2005
    Location
    Buffalo, MN
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    I agree on the electronic air cleaner, they don't trap as much as claimed. Not to mention if you're trying to conserve electricity why would you consume power to filter air when there are better methods. I personally like the Aprilaire Spacegard 2200 filter system, it's a very wide media filter, low restriction, high efficiency.

  12. #12
    Wow! Great information.

    I realize that the sizing and specs I was looking at for the Rheems were for the RGRA models... not the modulating RGFD models. Indeed, the smallest is a 60K BTU input. Same blower motor power, though.

    Seems that the Carrier has the slight edge on pure power rating... max 292W vs. Rheem's 373W. But that really means nothing in terms of total real world energy consumption.

    So, if I got the whole picture here... The Rheem will run longer, but at lower blower speeds and lower fuel utilization rates, and therefore longer at it's rated AFUE. The Carrier will cycle more, try to blow more air and use more energy there, and because it cycles, run at max efficiency less often.

    I guess all in all, I am leaning towards the Rheem. The modulating technology does seem like a significant move forward in the HVAC world.

    Thanks for all the excellent info. Very good stuff.

    Peter Straub

  13. #13
    Join Date
    Nov 2005
    Location
    Kansas
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    4,421
    more influence on the efficiency of any equipment has the installer, right there is 50% of the efficiency. More important than the different brands is your contractor.

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