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Combining electric cove heaters with PV

Lutro | Posted in Mechanicals on

In a recent blog, Designing an HVAC System for a Cold Climate, Malcolm Taylor is quoted as recommending electric cove heaters, and a link to the Comfort Cove line from Radiants Systems, Inc. is provided in the blog text. Searching Green Building Advisor turns up half a dozen other references to cove heaters, all very short, and all positive. I wonder if there is more useful advice on choosing and using this technology for supplemental heat.

In my case, we have grid-tied PV at the regulatory maximum of 10 kW, and we have another 2 kW of PV panels currently sitting idle. We would like to displace some of the propane burned to heat our inefficient early 70s adobe and frame house (originally built for Bob Dylan). Our solar contractor has proposed connecting the idle PV panels to baseboard convectors. I’m leaning toward cove heaters, both because I prefer to have a radiant component to the supplemental heat, and because furniture blocks the majority of the wall perimeter.

The solar guy is talking about running the direct current output of the PV panels to the resistance heaters, avoiding an inverter, and increasing efficiency (he says). I’m not sure if the heater manufacturers will approve the use of DC current, and I’m interested in GBA opinions. Later, we hope to connect these PV panels to an electric water heater, as we continue to try and improve the efficiency of this problematic house (don’t buy a house designed by a poet’s manager). Perhaps a mini-split is in our future, after air sealing and insulation are complete, but does any of this make sense as an interim approach?

The house is near Santa Fe, NM, a bit over 6,000′ in elevation, climate zone 5. The Heating Degree Days (base 65) listed on various sites range from 4,500 to a bit over 6,000, with roughly 5,600 being the mode.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Derek,
    Yes, you can operate an electric resistance heater designed for AC with DC -- as long as the DC voltage is close to the voltage rating of the appliance (usually 120 volts or 240 volts). Here is more information:
    https://www.oemheaters.com/t-dc-powered.aspx

    Using DC power directly (without an inverter) is more efficient than running the power through an inverter, because no inverter is 100% efficient.

    Needless to say, your plan may not meet electrical code requirements -- so check with your local building department, or proceed at your own risk.

  2. wjrobinson | | #2

    How many sqfft is the home? What is the energy required? (manual J loads per room etc)
    How open is the home? Can you post floor plans and pics?

    As to miss use of cove heaters, if they are not designed for DC then if your home burns to the ground, someone better have good liability insurance to cover such. A quick internet search looks like DC units exist. Myself, I would keep it simple and use an inverter. The juice from this next bank of solar is only going to be available when the sun shines, not sure how the heck you figure out how to use it, and not have to baby sit it. Might be best to heat your domestic water with the 2K panels.

    In a Bob Dylan house.... hope you play the rightiously right tunes...

    As an aside, I'm sure

  3. charlie_sullivan | | #3

    Conceptually, that;s a great thing to do with that surplus power. However, it's a little complicated in practice.

    As Martin says, the heater itself doesn't care whether it's ac or dc. You'd want to make sure the highest voltage you get on a cold sunny day is not more than 10% higher than the rated voltage of the heater--otherwise it could overheat and create a shock or fire hazard. You could use a solar controller or a controller and battery bank to be sure of that, or just use specs or measurements to make sure. (It's a good time of year to find worst-case cold/sunny combinations!)

    That only works for heaters that don't have a fan. Applying dc to an ac motor will not spin it and will burn it up.

    However, if the heater is controlled by a thermostat, and the contacts in that thermostat are designed to interrupt ac, they may arc when used with dc and may fail much more quickly, possibly even immediately. So you might need to shop around for a temperature controller that is rated to switch the dc voltage you are using, or find a heater with a built-in thermostat rated for dc as well as ac. You can probably find 12 V or 24 V heaters rated for dc more easily than 120 V or 240 V heaters rated for dc.

    There are also code issues that require the wiring work be done to a different standard with 240 V dc vs. 240 V ac. You might need to run the wires in conduit, for example, and you need dc rated fuses instead of regular circuit breakers.

    If you use an inverter, and the inverter is inside your thermal envelope, the efficiency loss there is not a problem as it still goes to heating your house. But there are a few disadvantages--one is the cost. And the other is that inverters come in two flavors--grid connected or off-grid designed to be used with a battery bank. You'd have to use the battery bank, since you done want to connect to the grid with this. But then the inverter would run the battery down keeping the heaters on even when the sun wasn't shining, and it would run them at the same power level independent of how much sunlight was coming in. You'd need to add a layer of control that would cycle the heaters on and off to maintain the battery level. In principle, someone could make a simple inverter that would allow the output voltage to vary according to the power coming in from the array, but I don't know of any such product on the market.

    So even though the inverter losses are not an issue, that doesn't seem very practical, and I think you are better off running dc heaters. The code issues may be easier at 12 V or 24 V than at 120 V or 240 V, and you may have an easier time finding appropriate heaters.

    With a dc setup, you can get more power out over a range of conditions if you have a solar controller that has a "maximum power point tracker" built in. That would help you both get the best power output and it could also serve to prevent the voltage from going above the rated voltage of the heater.

  4. wjrobinson | | #4

    Charlie and all,12 volt heaters... will take exactly 20 times the amp capacity of 250 volt heaters to put out similar BTUs. That's rediculous.

    I think the whole idea is interestting... but stupid.

    Sell the home for a profit to a fan of Dylan and build a net zero home from scratch via an experienced crew.

    I love to experiment, but it is important to know it is not a path to spending less in most cases. It's like be a gambler and saying you make money... you do.. and... you also lose way more than you make which is usually not talked about. Why that's no fun is it.

    The practical solution is to sell off the extra panels and go skiing.

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