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  1. #14
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    Quote Originally Posted by hangfirew8 View Post
    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
    Yeah, and given our expected rate increase of 20% this summer, the electricity cost is only increasing. Are you as thrilled as I am about the possible $170 credit we each may get in return for the de-regulation debacle? Wow, what is that equal to... about the same amount as the average BGE customers electric bill has increased in a 1 or 2 month period. So in consideration of a one-time $170, BGE gets to increase our bills about $150 every month.

    Anyway, not at all surprised you're so happy with the dual-fuel. I would like to know what you have the lockout temp set to and which thermostat they installed. I had my lockout temp around 25 degrees this winter... unless we got wintery precipitation in which case I adjusted to more like 30 or 35 degrees due to the unit going in to defrost cycles a lot more because of frosting up quicker.

  2. #15
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Quote Originally Posted by platchford View Post
    Anyway, not at all surprised you're so happy with the dual-fuel.
    I'm also very happy with the low speed/ECM fan feature. Instead of cycling through a 4 degree temperature range, the thermostat keeps the temperature pretty much exactly the degree F it should be, without excessive on/off cycling. It really is true, it helps eliminate hotspots/coldspots and keeps a more even temperature throughout the house.

    As for the thermostat, I picked the Rheem model RHC-TST401MDMS, which is the subject of another thread: Rheem modulating furnace versus Honeywell HTH8320U1008. As it turns out, I received the RHC-TST411MDMS, which is just an updated version of the 401.

    Quote Originally Posted by platchford View Post
    I would like to know what you have the lockout temp set to and which thermostat they installed. I had my lockout temp around 25 degrees this winter... unless we got wintery precipitation in which case I adjusted to more like 30 or 35 degrees due to the unit going in to defrost cycles a lot more because of frosting up quicker.
    I didn't set it to anything, but the installer set it to 42. I'm thinking that's way too high, but it was useful for testing the dual fuel setup on some warm-ish Spring days right after installation.

    Is there a good web calculator I could use for finding the optimal setpoint?

    -HF

  3. #16
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    Quote Originally Posted by hangfirew8 View Post
    I didn't set it to anything, but the installer set it to 42. I'm thinking that's way too high, but it was useful for testing the dual fuel setup on some warm-ish Spring days right after installation.

    Is there a good web calculator I could use for finding the optimal setpoint?
    As far as I know, in order to calculate the optimal point, you have to know the "fuel economy" (BTUs per $1 or $$ per 1,000,000 BTUs) of the gas furnace compared to the "fuel economy" of the heat pump. The thing is, the the "fuel economy" of the heat pump is proportional to the OD temp. You should be able to get specification sheets that list how many BTUs the heatpump can provide at 47 degrees and 15 degrees. Calculate the "fuel economy" (for lack of a better term) at each temperature... then compare that to the "fuel economy" of your gas furnace. Your heatpump will be more efficient at 47 degrees but it is quite possible that it will be less efficient at 17 degrees. I estimated using a temperature curve the efficiency at 32 degrees in order to come up with my optimal setpoint.

  4. #17
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Quote Originally Posted by hangfirew8 View Post
    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.
    I think you missed his point. Most inefficient systems are reasonable during the mild months because they don't run much. Therefore, the electric bill won't be very high. You really should make sure the duct work is designed and installed correctly, including checking for leakage. That could be stealing a lot of energy. Also, the building envelope should be checked. How good is your insulation? You may also have excessive leakage past the envelope which will cause problems on those cold nights.

  5. #18
    Join Date
    Apr 2004
    Posts
    7,680
    The last balance point I figured for someone on BGE came to about 44 degrees. I'd leave it where it's at and forget about it.

  6. #19
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    Ok, I'm going to make some assumptions here in order to approximate your balance point. First, Dan previously estimated your propane cost at 27,000 BTUs per $1. He also calculated the electricity cost but I'm going to adjust that based on our expected rate increase of 20% this summer. A 20% increase will raise our rate to about $.16 per kWhr but I'll estimate $.15 since there should be no increase in the distribution charge (theoretically but not likely) this summer. At $.15 per kWhr with straight electric you'd get 22,750 BTUs per $1 and with the heatpump at 47 degrees you'd get about 68,000 BTUs. The specification sheets for my Rheem heatpump list roughly half the rated BTU output at 17 degrees... you'd get approximately 34,000 BTUs per $1.

    Gas - 90% - 27,000 BTUs per $1
    Electric - Straight - 22,750 BTUs per $1
    Electric - Heatpump 47 degrees - 68,000 BTUs per $1
    Electric - Heatpump 17 degrees - 34,000 BTUs per $1

    If this is true then even at 15 or 20 degrees your heatpump is more efficient that your furnace. The question becomes can your heatpump at half its rated capacity still heat your home at said temperature? In your case the balance point is probably going to have more to do with the heat gain as a function of a particular temperature rather than which is more efficient. The problem is, in order to size a heatpump to heat your home even in the coldest temperatures, you'd have to oversize it so much as to make it inefficient for humidity control in the summer.

  7. #20
    Join Date
    Apr 2004
    Posts
    7,680
    Unfortunately you still have to run the indoor blower...

  8. #21
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    Quote Originally Posted by docholiday View Post
    Unfortunately you still have to run the indoor blower...
    I'd consider that a negligible amp draw in most cases. Besides, it would only be fair to count the amp draw of said blower for the difference in period of time when using the furnace vs the heatpump. Example, furnace runs for 20 minutes each hour to maintain temperature and heatpump runs for 50 minutes... meaning only 30 minutes each hour should be counted towards additional energy use. Also, running 50 minutes per hour is preferable to 20 minutes per hour from a comfort standpoint anyway.

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