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  1. #1
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246

    Questions taking Rheem HP to dual fuel

    I'm really glad to have found this site and look forward to hearing your views.

    After abusive local electric rate increases, I'm going dual fuel to cut next Winter's heating bill. I got 5 quotes on adding LPG to my 10 year old Rheem 13 Seer A/C/Heat pump. Of Trane, Lennox and 3 Rheem quotes, all the Rheem guys came in lowest. The winning quote replaces the thermostat with a Honeywell (no model noted) dual fuel thermostat. I did upgrade from the Classic 90 to the Prestige 90 updraft modulating furnace, plus opted for the Honeywell HE265 Humidifier. I live in Central Maryland (got down to 6 degrees many nights in Jan/Feb this Winter).

    I know that the 10 y.o. A/C+Heat Pump will probably go up in a few years, but I'll fix it at that time. I'm too busy recovering from $600 monthly electric bills to replace everything at once.

    After signing on the dotted line, I realized one reason the winning bidder (a listed Rheem dealer) came in lowest- he is not replacing the "coil" like the other guys. If this is not 100% optimal, I can live with that, but I wonder if it matters that much. One guy said replacing it would turn my 13 SEER into a 14 SEER. I'm thinking I'll need the HP/A/C replaced in a few few years and I'll do it then, but perusing the Rheem parts web sites, I see they have a coil optimized for dual fuel setups. Is this coil replacement considered optional or necessary?

    One other caution I have is that my new contractor said Rheem doesn't make a dual fuel thermostat. After googling (for 5 seconds) I found the RHC-TST401MDMS.

    I'm going to find out what thermostat he's using before the install next week, but I'm wondering how much that matters, too, since it is "dual fuel".

    Thanks
    HF

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,125
    Replace teh coil, when you replace the OD unit.
    If you replace it now, you may be limiting yourself to the choices of which OD unit you can use later.

    Any of the Honeywell thermostats, th6220,th8320mth8321, or yth9421 are good duel fuel stats.

    But, I'll let one of the Rheem guys answer if the Rheem mod stat can do duel fuel.
    If it can't, Then your modulating furnace will be a 3 stage furnace instead of the 13 step modulating furnace it was designed to be.

    If the mod stat can't handle duel fuel. Ask your contractor to upgrade you to the YTH9412(commonly known as the IAQ stat).
    It can stage the gas furnace, most other stats can't, and the firnace will use its timer to stage up weather you need it or not.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,194
    Yes, the Modstat can do dual fuel with its outdoor sensor. With that you get the full modulating capability. With the IAQ, it becomes a 3 stage.

    The coil should be matched to the heat pump. There is no reason to change it now. Wait til the HP dies then get a coil that matches whatever heat pump you choose.

    I probably have the same HP you have. It is on dual fuel but I use the pump until it just can't keep up since our electric rates are cheap and natural gas rates are steep.

  4. #4
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Thanks, beenthere & BaldLoonie! Thats a lot of piece of mind for me about the coil. I feel pretty good about the furnace and the contractor, now I feel a lot better about his coil-less quote, but I like to trust AND verify.

    Sounds like I have to know exactly what thermostat is going in before we can pass judgement on that one. I'll update the thread as soon as I find out. If y'all don't like it, I'm sure he'll take me up on an offer for an upgrade (for a few $$ more, of course.)

    I forgot to mention it, but an outdoor thermometer IS part of the quote, so he's got that covered. All the quotes got that right (for different amounts, of course).

    -HF

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295

    Question BG&E rate at ~ $0.14 ?

    Quote Originally Posted by hangfirew8 View Post
    I'm really glad to have found this site and look forward to hearing your views.

    After abusive local electric rate increases, I'm going dual fuel to cut next Winter's heating bill. I got 5 quotes on adding LPG to my 10 year old Rheem 13

    I live in Central Maryland

    I know that the 10 y.o. A/C+Heat Pump will probably go up in a few years, but I'll fix it at that time. I'm too busy recovering from $600 monthly electric bills to replace everything at once.

    After signing on the dotted line, I realized one reason the winning bidder (a listed Rheem dealer) came in lowest- he is not replacing the "coil" like the other guys.

    I'm going to find out what thermostat he's using before the install next week, but I'm wondering how much that matters, too, since it is "dual fuel".

    Thanks HF
    Did you ever investigate specifically
    WHY your electric bills are so enormous? $500 at $0.14 / kw = ~ 3,500 kwh

    Will adding LPG equipment actually save money
    in comparison to a properly operating, efficient heat pump?

    Will use of LPG at a high (> 30'F) balance point
    actually increase your overall energy costs?

    http://bge.apogee.net/homesuite/calcs/rescalc/

    Energy Prices Used by BGE:
    Electric $ 0.1316 ($/kWh)
    Natural Gas $1.159 ($/therm)
    Propane $3.05 ($/gal)
    Oil $3.26 ($/gal)

    LP at 90% efficient provides ~ 80,000 BTU/Gallon ... 27,00 BTU / $1

    3412 Btu/kW
    $0.1316 $ / kW
    3.0 COP at ~ 35'F
    $0.0439 effective $ / kW

    77,781 BTU / $1 at 35'F COP 3.0
    26,000 BTU / $ 1 straight electric

    ... Sounds like a back-up electric strip might be providing
    a significant portion of the heating capacity.

    MD climate
    4,300 Heating Degree Days
    1,400 Cooling Degree Days

    http://www.wunderground.com/history/...q_statename=NA
    Last edited by dan sw fl; 03-08-2008 at 06:37 AM. Reason: CDD and Energy Price updates
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Dec 2007
    Location
    Ohio
    Posts
    2,876
    Don't let money be a reason to not change that heat pump. There are pleanty of programs available to you that would allow you to change the entire system at once. If you are paying 500 bucks a month then the savings alone of a new system will pay for any financing you may choose. IMHO I would strongly recommend doing the entire system....a completely matched and properly set up system is going to save you alot more money than just replacing half of it...but then again...whatever
    I need a new signature.....

  7. #7
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Quote Originally Posted by dan sw fl View Post
    Did you ever investigate specifically
    WHY your electric bills are so enormous? $500 at $0.14 / kw = ~ 3,500 kwh
    Good question. My professionally maintained, serviced once-a-year HP system is a model of economy in the mild heating months of October and May. The electric backup does indeed come on, every night during the Winter, as soon as the temperature goes below freezing. I used to heat to 70 occupied/69 when away, since the rate increases it's 68/67.

    My total kilowatts during a cold month are about the same as they were 5 years ago, excepting one month 4 years ago when I needed to get the HP serviced, which I did.

    Quote Originally Posted by dan sw fl View Post
    Will adding LPG equipment actually save money
    in comparison to a properly operating, efficient heat pump?
    That remains to be seen. I do have a good indicator. My in-law apartment at over 1/2 the square feet has LPG-only heat, and the total energy bill (electric+gas) averages less than 1/3 of my bills. Like my house, same weather, it has a lot of glass, is modern, well insulated construction, etc.

    I will confess I have two other reasons to go with gas. One is the unreliable power we have in the area. (The local pastimes here seem to be drinking and driving over utility poles). Gas is much more amenable to running off of backup power then a HP. The second reason is perceived comfort. Even though my HP has no problem keeping up with the thermostat, I keep finding my family over at the in-law apartment enjoying the gas heat.

    Quote Originally Posted by dan sw fl View Post
    Will use of LPG at a high (> 30'F) balance point
    actually increase your overall energy costs?
    Probably. Why would I do that? I plan to go with my installer's initial recommendation.

    Quote Originally Posted by dan sw fl View Post
    Thanks, I'll take some time to check this out. I saw the calculators linked on this site, but they want to compare oil to gas or somesuch.

    I would also point out that BGE has a vested interest in selling Heat Pumps. They've been doing it since the gov't subsidized their rates for saving energy by doing so, and they still offer financing to sell them at BGE HOME. They are very, very big fans of heat pumps.

    Quote Originally Posted by dan sw fl View Post
    Energy Prices Used by BGE:
    Electric $ 0.1316 ($/kWh)
    Natural Gas $1.159 ($/therm)
    Propane $3.05 ($/gal)
    Oil $3.26 ($/gal)
    My initial fill is $2.79 a gallon. Who knows what it will be next fillup.

    Quote Originally Posted by dan sw fl View Post
    LP at 90% efficient provides ~ 80,000 BTU/Gallon ... 27,00 BTU / $1

    3412 Btu/kW
    $0.1316 $ / kW
    3.0 COP at ~ 35'F
    $0.0439 effective $ / kW

    77,781 BTU / $1 at 35'F COP 3.0
    26,000 BTU / $ 1 straight electric

    ... Sounds like a back-up electric strip might be providing
    a significant portion of the heating capacity.
    Below 30 degrees, yes it does, which is every night, and this Winter, most days during the morning and evening.

    [QUOTE=dan sw fl;1786686]



    MD climate
    4,300 Heating Degree Days
    1,400 Cooling Degree Days

    Haha... the common Maryland misconception... BWI has nothing to do with MY weather. Think 8 to 10 degrees colder, 200+ meters higher and put the house on a windswept hill. Also, no Chesapeake Bay heat sink nearby here. I have snow when they have rain.

    -HF

  8. #8
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Quote Originally Posted by I_bend_metal View Post
    .a completely matched and properly set up system is going to save you alot more money than just replacing half of it...but then again...whatever
    Thanks. I am getting a professionaly installed, matched and, hopefully, properly set up system. Whatever I do will take a few years to amortize.

    I guess your assumption seems to be my heat pump is broken- both my pro's, and their equipment, disagree with you. Comparing notes with my neighbors, it seems my house is rather on the efficient side, as would be expected of modern construction with a 13 SEER until compared to older stuff.

    -HF

  9. #9
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Quote Originally Posted by dan sw fl View Post
    Did you ever investigate specifically
    WHY your electric bills are so enormous? $500 at $0.14 / kw = ~ 3,500 kwh


    http://bge.apogee.net/homesuite/calcs/rescalc/
    I couldn't resist, so I stayed up late to use the calculator. According to it, if I lived in warmer Baltimore, I should be paying $300 to $400 more per month on my January bill then I do now, depending on whether I count my basement as heated area or not.

    I'm feeling pretty warm and fuzzy about my heat pump efficiency right now!

    By switching from electric heat to propane, BGE thinks I will save about $50 during January. I doubt it, but time will tell. I suspect it will be better than that. If that is the only savings I get, I still have the comfort factor and the generator backup option- good enough for me.

    -HF

  10. #10
    Join Date
    Apr 2004
    Posts
    7,680
    If you're on BGE, the electric rates are pretty high, I've seen economic balance points as high as 45 degrees. I just dont know that there is much value in a dual fuel there considering the payback and all. Now of course thats with Natural gas as the fuel, I am not sure where the oil economic balance point would end up.

    The Mod is great, use the stat you selected to get full benifit. If you still go dual fuel, all they need to do is add an outdoor sensor. I've got the mod with a 16 seer HP, but where I am at, electic is less than half per KW so it definately pays.

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295

    Wink Weather and whether it affects you ?

    Frederick Degree Days:
    Heating Degree Days 4679
    Cooling Degree Days 1162

    PREVIOUS Year
    Heating Degree Days 4948
    Cooling Degree Days 1034

    Baltimore Degree Days:
    Heating Degree Days 4333
    Cooling Degree Days 1466

    Previous year Degree Days:
    Heating Degree Days 4369
    Cooling Degree Days 1314

    Hagerstown
    Heating Degree Days 4726
    Cooling Degree Days 1297

    Previous Year
    Heating Degree Days 5001
    Cooling Degree Days 1063

    I don't see a a large HDD difference,
    but there is a little ... ~ 10 -17 %

    There is enough temperature difference to
    proportionally affect overall heat pump C.O.P.

    So BGE /Baltimore calculator As-Is will not be real accurate ( < 20%) for all of MD.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    First off... that $0.13 per kW is probably just the electricity "supply" cost and probably therefore does not include the "delivery" cost as well. Figure more like $0.15 per kW or just take your total electric bill and divide by your total kW used to get a more accurate amount. Eh, maybe it is right... took my bill and did just that and came close to $0.13 or $0.14 per kW.

    As far as the value of using a gas or oil-fired furnace for the secondary heat, you will probably save money vs using the electric resistance heat of a typical air handler given the direction our electric rates are still headed and how quickly. However, have you considered oil? An oil furnace (even the Rheem 80% efficient units) will provide about 110k BTUs which is almost 34000 BTUs per dollar. Of course, if you don't have a chimney then that adds other costs as well. Ultimately, given the current state of affairs in the energy field everything is a bit of a gamble.

  13. #13
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Thanks for the food for thought, platchord.

    I now have my 90+ mod furnance, and aside from a couple of minor installation annoyances, I love it. Where I live, the electric backup came on practically every night from the end of October to the end of March.

    I know that LPG is not the cheapest stuff, but it is a dual fuel system, the heat pump works better then ever now, and because I got the modulating furnace, when I'm using expensive propane, I'm using as little as possible.

    Why is my Heat Pump working better then ever? I had it checked before, the refrigerant pressures were perfect and I had it serviced regularly and replaced filters often. However after almost 11 years, it had a layer of lint on the A coil that was like felt! No, it couldn't be cleaned in regular maintenence, as it was completely concealed by the electric heating coils and buried in a metal cabinet with no doors.

    I wonder how many poorly performing heat pumps get replaced due to a layer of felt-like lint blocking airflow? I'm sure there's some kind of vacuum test that would detect the condition, but since fixing it requires draining the refrigerant and pulling the whole system apart, I wonder how many techs even offer to do the test, and even if they did, would want to mess with a near-11 year old coil and resist the urge to just recommend replacing it?

    Do I regret getting Rheems' best gas furnace after finding out why my old heat pump over electric was so inefficient? Not a bit! I now have:

    1. The ability to catch-up and heat my house in minutes when it's near Zero degrees outside. This is VERY important because we have frequent power outages here, ESPECIALLY when it is cold.
    2. My old 13 SEER A/C and Heat Pump will now run more efficiently than ever for the last few years of its life,
    3. I can run my entire system off of a fairly small electric generator (see 1.)
    4. Every time Constellation/BGE cranks up the electric rate, I can fiddle with the set point
    5. My money goes to the local HVAC guy and the gas delivery company instead of paying Constellation to build Coal plants in WV and ship the power via very high tension lines to New Jersey through Maryland thus avoiding all those local environmental laws, and getting the consumer (me) to pay their carbon offsets.

    Am I letting my personal feelings influence my energy choices? You betcha.

    -HF

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