Replies

1. 
   user-1035959 | May 30, 2013 01:19am | #1

   Can your electrical system handle the amps required of an electric tankless? They have a big demand and often require some upsizing. Depending on how robust your enclosure is you could probably get the best bang for your buck doing a high efficiency electric boiler.

2. GBA Editor
   Martin Holladay | May 30, 2013 05:53am | #2

   Ed,
   When you refer to "solar panels," I assume you mean a photovoltaic array, not solar thermal collectors.

   If you have a grid-tied PV system, and if your local utility has a net-metering agreement (which is typical), you will be credited on your electric bill for all of your PV production -- unless you have an unusually large PV system that is already producing more electricity on an annual basis than your house can use, which is rare.

   Just because you have a PV array, doesn't mean that it makes sense to heat your house with electric resistance. Electricity is still a very expensive heating fuel, and there are usually better ways to heat your house.

   Concerning your question about whether a standard 50-gallon electric-resistance water heater has enough capacity to heat the average American house: the answer is, probably not. But the only way to know for sure is to perform a heat loss calculation to determine your design space heating load.

3. 
   Ed124 | May 30, 2013 09:21am | #3

   I just upgraded to a 200 amp panel, and had a photovoltaic system consisting of 17 sun power 327 panels, 2 inverters( SPR 4200 and SPR 3600 ) installed averaging 25 kw's a day. My house is approx 1500sq ft, located on Long Island.
   I currently use a Weil McLain 114,000 btu oil burner for a single zone baseboard system, and use the coil to heat domestic hot water. I have a central air system consisting of 3 1/2 tn RUDD condenser and air handler.

   I am not that fancy with calculations of heat loss etc.
   I was exploring actually 3 options:
   1. Electric hot water heater
   2. On demand water heater
   3. Electric heat strip in the air handler like the back up for a heat pump system..

   I tried to explain my system the best I possibly can. i may sound all over the place, but i am trying to achieve the most efficient but cost effective way to heat my home. I do appreciate any direction or advice, and am willing to try any solution that could eliminate the use of the burner. Thank you

4. 
   Ed124 | May 30, 2013 09:46am | #4

   Martin,
   I am not sure of this and the above info will help paint a better picture :
   I have produced approx 450 kw's in 3 weeks. Banking about 225 kw's . Lipa ( Long Island power authority only rebates 105% of your annual consumption. ( I used 6500 kw's last year) so I had my solar system designed with the 2 inverters to accommodate adding more panels next year to receive the rebates. My thoughts are if I am producing electric to handle my consumption and it balances annually then I would eliminate the cost of fuel oil. I know any heat/cooling that is lost due to insulation, doors, windows is wasted, so I am trying to research how to perform a heat loss calculation.
   Thank you for your time, any help is greatly appreciated..

5. Expert Member
   Dana Dorsett | May 30, 2013 11:58am | #5

   Every successful heating system starts with a heat load calculation. Without it you're just shooting in the dark.

   If you have mid to late winter oil bill with a "K-factor" stamped on it, and have been using only oil to heat your house, it's possible to get the sizing of the electric boiler (and NOT an electric tankless) sufficient to handle the peak loads, which will also determine the size of the electric service necessary. We'd also need to know your exact location to figure out the outside design temperature. (You'd need almost 3x the electric boiler in Edmonton Alberta that you'd need in San Antonio Texas.)

   The odds of a standard electric water heater having sufficient output in raw kilowatts to handle the heat load is pretty low unless this is a very well insulated house (say 2x code min), or a very mild climate. A 4.5kw water heater running flat-out is only about 15,000 BTU/hr, which is about half the heat load of a typical IRC 2012 code-min 2000' house in my neighborhood (central MA.)

   The temperature required is a combination of how much baseboard you have, and how big the heat load is. At 130F average water temp (140F out, 120F return) fin-tube baseboard puts out about 260-270BTU/hr per linear foot. If you have 75' of baseboard and a heat load of 15,000 BTU/hr you'd get there (barely) with a standard electric hot water heater at domestic hot water temps. If your heat load is higher than that you'd be much better off with an electric boiler. An electric tankless is not designed for the flow rates and duty cycles needed for hydronic heating, and are solely fixed-temp output, whereas electric boilers are far more rugged, and can be set up to run under outdoor reset control for higher comfort and lower distribution losses.

   Unless your local grid is mostly renewable, odds are that going to resistance electric heating would result in a net increase in your carbon footprint. If it's mostly coal-fired you'd even with heat pumps you'd be seeing a net increase. Depending on your, heating primarily with ductless air source heat pumps would give you between a 2:1 to 4:1 advantage over heating with electric hot water heaters or electric boilers (they use only 1/4 to 1/2 the power) and SHOULD be considered by anyone heating with oil in any region except those with a coal-dominated local grid, even in colder climate zones.

   More here:

   http://www.rmi.org/cms/Download.aspx?id=10410&file=2013-05_HeatPumps.pdf

   200A of 240V is probably enough power either way, but seriously, heating with resistance electricity isn't very green, and can only be rationalized when the heating loads are very low (like PassiveHouse low.)

6. 
   jj1 | May 30, 2013 02:47pm | #6

   Hi Ed: you should be able to make significant progress toward your goals in our Long Island climate. Step One: schedule a Home Energy Audit. LIPA has a program which subsidizes HEAs. Mine was conducted by Conservation Services Group, they have a Hauppauge office 631-590-5700, they are quite knowledgeable. They utilize specialized instrumentation including a blower door and infrared cameras, etc. CSG can estimate your home's energy usage, they will recommend the most cost effective improvements, they can recommend specialized local contractors and provide price bids for you, they will provide you with a written summary report. I suspect that air sealing and some additional insulation (such as in the attic) may be very cost effective; CSG will rank their various recommendations and estimate their BTU savings. Step Two is to implement the most cost effective strategies based on this data; you can do much of the air sealing work yourself for example.
   If your oil furnace is nearing the end of its useful life, then the Home Energy Auditor can estimate whether a single ductless mini split air source heat pump (such as the Mitsubishi Hyper Heat) could provide efficient heating in winter and cooling in summer. These units work well in our LI climate where winter temps rarely go below 10F and even then typically at night for short time periods. Using a coil with an oil fired large furnace can be quite inefficient re producing hot water during summer, as your oil bills most probably already attest. The Home Energy Auditor can advise you whether it might be timely to substitute a hybrid heat pump hot water heater instead. Long Island's 51F winter deep ground temps typically allow efficient operation of a heat pump water heater in a basement even during winter, for example. Lastly, the Home Energy Auditor can provide detailed information re the many (significant) rebates and subsidies which are available for NY state homeowners for all of these items and efforts.

7. Expert Member
   Dana Dorsett | May 30, 2013 05:18pm | #7

   Somehow I'd missed the reference to Long Island location. Your 99% outside design temp on L.I. is about +15F, and WELL within the efficient operating range of a ductless heat pump (you don't even need the HyperHeat series, and an Fujitsu AOUxxRLS2 has demonstrably higher efficiency than the MSZ-FExx NA hyper heating units:

   http://www.nrel.gov/docs/fy11osti/52175.pdf

   Most 1500' houses in a L.I. location with older 2x4 construction, with reasonable insulation & air sealing, with storm windows or double-panes will come in around or under 25,000 BTU/hr @ +5F, which is more like a 99.9% outside design condition, and could be heated with ~2 "tons" of ductless. A single head 2-ton would run about $5K installed, but depending on the floor plan you may need 2 or even three, but it's unlikely you'd have to break $8K total.

   Mark Rosenbaum heated his comparably sized (but better insulated) home on Martha's Vineyard with a single 1-ton Fujitsu (the AOU-12RLS2 ):

   http://blog.energysmiths.com/2011/03/out-with-the-old-in-with-the-new.html

   But he has noted elsewhere that he might have added a second head or an electric radiator for temperature balance for the other bedroom if they actually used it.

   AT L.I. electricity prices you absolutely DON'T want to heat the place with an electric hot water heater or electric boiler. But ductless heat pumps would be comparable in operating cost to natural gas heating, and you'd have about a 3:1 advantage or slightly better in your climate.

   A potentially cheaper (if less efficient) option would be to swap out the AC unit for a variable speed ducted heat pump. Even the smallest versions are likely to be oversized for you heating load, and neither the efficiency nor comfort would be as high as with ductless systems. With well-sealed and insulated ducts it might average better than a 2:1 advantage over an electric boiler, but with leaky or uninsulated ducts you're probably looking at about 1.5:1 (half the efficiency of heating with ductless.)

   If you have oil use numbers (or a K-factor) that could be used as a sanity check on the heat load, and a ZIP code to verify outside design temperatures it would be useful.

   Ground source heat pumps would edge out ductless on efficiency with a small margin, but only if they nail the system design perfectly. And cost-wise it's typically 3-5x the upfront cost, even after federal tax credit subsidies. Even with 25 cent electricity it would be tough to make a finacial case for ground source, particularly for smaller systems. Yours would surely be around 2 tons, 3 tons tops, and likely run north of $30KUSD before subsidy.