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  1. #1

    Question Heat Pump in San Diego?

    Hi, first time poster, would appreciate your advice...

    We are in the process of having a good sized grid-tied solar PV system installed, and have planned to intentionally over-size it to make room for further increases in electric usage (an electric car, HVAC).

    I wanted to explain my thinking on the HVAC side and see if I'm doing anything stupid.

    Background:

    The house is was built in early 1980s and is wired & plumbed for Air Conditioning but doesn't have it. The gas furnace is original. Natural gas is used for heating, hot water, clothes dryer, and the fireplace (which is rarely used). The house is fairly well insulated.

    Climate is near-coastal San Diego, so AC usage is typically only a week or two a year. However this summer was really hot -- since my wife and I work at home a lot, it's nice to have AC for those rare times when we need it. (We put in a crummy window AC and it did ok at keeping one room cool, but it was noisy and added about 100KWH to our bill.)

    Our gas furnace is probably 65% efficient (based on its age). Our comfort heating load looks to be 30 therms per month, or 880KWHt/month. So if we just ran electric space heaters, we'd go broke. However, modern heat pumps can do about 3.0 COP (ratio of heat out to electric in) in a mild climate. So we'd need 880*.65= 650KWHt of heat, which would require only 215KWHe of electricity for several months of the year (if my calcs are correct)

    Price-wise this works out to about $30/month for gas vs. $30-60 for electricity at today's prices. Thus if we were paying SDGE for electricity there'd be no financial benefit to buying a heat pump and using it for heat. But with solar, the balance changes. (for reference SDGE charges about $1/therm for natural gas and $0.14 to $0.33/KWh. Our solar system should be big enough to just zero out our bill on average year round, we believe)

    For cooling, I figure we'd probably use 100-200KWH for 1-2 months of the year (that's a WAG number).

    Questions:
    * are my calculations roughly correct? any big errors?
    * I assume that we'd get rid of the gas fired furnace entirely (our HVAC closet is pretty cramped) - is that dumb? Typical winter low temperatures here are about 40F, perhaps 35F on a couple days.
    * any other issues with heating with a heat pump? As I understand it, they tend to deliver air that isn't very hot, so they run more often.
    * I'm a big fan of humidity control (I can stand 80F when it's dry, but if it's 70F and humid I'm not happy), and we are concerned about noise - does that mean we should look into a 2-stage system? Any recommendations?

    Thanks in advance

  2. #2
    Join Date
    Jul 2006
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    a heat pump would be perfect with your climate at 40 degrees outside temp the air will feel warm from the vents . . make sure that the auxillary heat is locked out to a low temp as you wont need it
    We really need change now

  3. #3
    Join Date
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    Probably don't matter what we tell ya. You got it all figured out.

  4. #4
    Quote Originally Posted by Tommy knocker View Post
    Probably don't matter what we tell ya. You got it all figured out.
    I can't tell if that's sarcastic or serious...can you explain?

  5. #5
    Join Date
    May 2000
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    Indianapolis, IN, USA
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    Quote Originally Posted by Tommy knocker View Post
    Probably don't matter what we tell ya. You got it all figured out.
    Let's keep it professional. This wasn't called for.

  6. #6
    Join Date
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    You also have to take into account the added value to the home as far as resale. Equity is hard to come by these days.
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Not sure what you mean by over size the PVs. Does that mean 10 KW instead of 4, or 15 KW instead of 10.

    Best to find out what size heat pump you really need first, before making any calculations on PV size. Don't for get to include water heater and dryer if your going to electric with them also.
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  8. #8
    Not sure what you mean by over size the PVs. Does that mean 10 KW instead of 4, or 15 KW instead of 10.
    Good question - in this case, neither - we are limited by roof space, so "oversize" in this case means going to go for the higher efficiency panels (SunPower E20s) which should get us a little under 5KW system and generate roughly 600KWH/month-ish. That's about 50% more KWH than we are averaging now but we think that would roughly cover the heat pump usage if my #s are right.

  9. #9
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,319
    Something to think about...if you put in a heat pump, you will probably use the a/c side of it more than one or two weeks a year. Why? Because it's there. Whereas before you might hold out on the window shaker until you just could not stand it, now you can flip a switch and everything's cool again.

    When I lived in San Diego I serviced heat pumps. Few of them had emergency or auxiliary heat strips. And this was back in the 80's. I know heat pump performance and efficiency has improved since then. But if I had gas heat I would carefully weigh the costs of natural gas vs. heat pump. For a PV array I would lean toward keeping gas on the water heater, dryer, and maybe even the furnace. During my time in SD I lived in Linda Vista, El Cajon, and Lakeside. Only in El Cajon and Lakeside did I find a need to run a/c more than just a few hours a day and a week or two per year. Closer to the coast, most of the time having the windows open kept everything fine. Something you can only do where I live now maybe a week or two out of the entire year.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #10
    Join Date
    Jan 2004
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    Quote Originally Posted by Shophound View Post
    Something to think about...if you put in a heat pump, you will probably use the a/c side of it more than one or two weeks a year. Why? Because it's there. Whereas before you might hold out on the window shaker until you just could not stand it, now you can flip a switch and everything's cool again.

    When I lived in San Diego I serviced heat pumps. Few of them had emergency or auxiliary heat strips. And this was back in the 80's. I know heat pump performance and efficiency has improved since then. But if I had gas heat I would carefully weigh the costs of natural gas vs. heat pump. For a PV array I would lean toward keeping gas on the water heater, dryer, and maybe even the furnace. During my time in SD I lived in Linda Vista, El Cajon, and Lakeside. Only in El Cajon and Lakeside did I find a need to run a/c more than just a few hours a day and a week or two per year. Closer to the coast, most of the time having the windows open kept everything fine. Something you can only do where I live now maybe a week or two out of the entire year.
    Depends on what size array he puts in.
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  11. #11
    Join Date
    Aug 2003
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    Fort Worth, TX
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    Quote Originally Posted by beenthere View Post
    Depends on what size array he puts in.
    True.

    If I still lived in coastal San Diego I would still focus on the building envelope first before considering PV. No need to go super insulation crazy in SD, as the climate is temperate. However, paying attention to the envelope can reduce both the intensity and the need for energy consuming climate control. During my SD days I serviced many coastal homes with no a/c and only wall furnaces for heat. These old homes were leaky and marginally insulated; I can only imagine how even modest improvements in that climate would reduce costs and improve comfort overall to homes like these, and even the newer tract homes like the OP may have.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  12. #12
    Quote Originally Posted by Shophound View Post
    True.

    If I still lived in coastal San Diego I would still focus on the building envelope first before considering PV. No need to go super insulation crazy in SD, as the climate is temperate. However, paying attention to the envelope can reduce both the intensity and the need for energy consuming climate control. During my SD days I serviced many coastal homes with no a/c and only wall furnaces for heat. These old homes were leaky and marginally insulated; I can only imagine how even modest improvements in that climate would reduce costs and improve comfort overall to homes like these, and even the newer tract homes like the OP may have.
    The house is stucco with concrete roof and quite well insulated. If we lived closer to the ocean we wouldn't need A/C, and if we lived further away we would need it without question. We are kind of in the transition zone climate-wise.

    A mini-split located in the office has been suggested, but I'm not sure if this would do the trick, especially in the winter when we'd want downstairs heating.

  13. #13
    Join Date
    Nov 2004
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    SW FL
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    6,272
    Quote Originally Posted by soylentgreen View Post
    Hi, first time poster, would appreciate your advice...

    We are in the process of having a good sized grid-tied solar PV system installed, and have planned to intentionally over-size it to make room for further increases in electric usage (an electric car, HVAC).

    I wanted to explain my thinking on the HVAC side and see if I'm doing anything stupid.

    Climate is near-coastal San Diego, so AC usage is typically only a week or two a year. However this summer was really hot -- since my wife and I work at home a lot, it's nice to have AC for those rare times when we need it. (We put in a crummy window AC and it did ok at keeping one room cool, but it was noisy and added about 100KWH to our bill.)

    However, modern heat pumps can do about 3.0 COP (ratio of heat out to electric in) in a mild climate. So we'd need 880*.65= 650KWHt of heat, which would require only 215KWHe of electricity for several months of the year (if my calcs are correct)

    Price-wise this works out to about $30/month for gas vs. $30-60 for electricity at today's prices.
    Thus if we were paying SDGE for electricity there'd be no financial benefit to buying a heat pump and using it for heat. But with solar, the balance changes.
    (for reference SDGE charges about $1/therm for natural gas and $0.14 to $0.33/KWh.
    Our solar system should be big enough to just zero out our bill on average year round, we believe)

    For cooling, I figure we'd probably use 100-200KWH for 1-2 months of the year (that's a WAG number).

    Thanks in advance
    So, paraphrased,
    Is the question,
    you wish to invest in a ~ 4,000 watt solar system and a small heat pump [ total ~ $27,000 ]
    and try to justify a savings of $200 a year on Heating (150 therms N.Gas) & Cooling (100 kwH) in SanDiego?

    That SIMPLY does NOT work out even using CALIFORNIA _GREEN_ MATH.
    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

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