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Thread: Electric water heater for radiant heat?

  1. #1
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    Default Electric water heater for radiant heat?

    We are presently planning the construction of a new 1,500 square foot home in New Hampshire and we are thinking about using hydronic radiant floor as our main heating system. The size of the conditioned area will be ~3,400 square foot counting the basement and a 400 SF loft.

    We were initially thinking about using geothermal as our main heat source but the price of the system ($40,000 with the wells) even after a $12,000 tax credit made us change our mind.

    We are presently considering two alternatives:
    1- Propane with a Buderus GB142/30 water heater.
    2- Two 85 gallons Marathon electrical water heating tanks MR85238.

    We are hesitant to go with propane considering that the price of propane is going up a lot faster than electricity. In New Hampshire, propane went up by 117% since 1995 while electricity went up by only 35% during the same period. Public Service of NH also offer a ~20% discount on current electricity rates with their HeatSmart program.

    We are also likely to install a solar drainback system eventually with two or three series of evacuated tubes.

    What do you think about using the two 85 gallons water heating tank as the main source of heat?

    Thanks in advance,

    JF

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    NH has some of the highest electric rates in the USA...I would not consider electric for heat unless I was using solar panels and only needed it occasionally. Plus, I (and this is me personally) just don't like the thought of a WH as a boiler substitute. While you can buy WH with a large enough burner, the average ones aren't big enough for a typical house. Plus, unless you buy a commercial unit (and they have much shorter warrantees), they are designed more for batch operation, not continuous use as you could experience in NH. And, an electric one rarely uses more than 5kW, and that isn't enough to keep a house warm in NH in the winter. Take a week where it barely got above single digits last winter and see how stressed the things would be!

    I'd go with the Buderus and put in a propane tank, or make the investment to go solar with a good backup system if you can afford it. they haven't started to tax the use of sunshine (yet!).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    Default heat

    HOW FAST you heat the water is more important for heating purposes than how much water you have stored. That is why real boilers are small with big burners. If you are not reheating the water fast enough even the largest tanks will eventually be depleted and then the area will start to cool down.

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    Master Plumber master plumber mark's Avatar
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    Talking not electrical

    the electrical route will cost you a fortune
    probably more over the long haul than what it wouuld cost you for the geo-thermal

    the cost on the geo-thermal seems rather high... have you shopped that around ???


  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    An electric WH generally uses a 4500W element, so x2 is 9Kw. That's equivalent to about 31K BTU. That is likely too small for a house that size in NH! Now, if it was super insulated and really good windows, it might cut it, but it would be close, and if you ever set the house back, it would take many hours to days to recover since there'd be no margin to exceed that loss to the outside. Now any radiant can make you feel comfortable at lower temps, and that helps with the sizing, but if you're using PSNH, you better have a high paying job!
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    In the trades Dana's Avatar
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    Quote Originally Posted by jadnashua View Post
    An electric WH generally uses a 4500W element, so x2 is 9Kw. That's equivalent to about 31K BTU. That is likely too small for a house that size in NH! Now, if it was super insulated and really good windows, it might cut it, but it would be close, and if you ever set the house back, it would take many hours to days to recover since there'd be no margin to exceed that loss to the outside. Now any radiant can make you feel comfortable at lower temps, and that helps with the sizing, but if you're using PSNH, you better have a high paying job!

    For leaky not-so well insulated old construction maybe, but for new stuff? Au contraire!

    That's only a ~20btu/ft^2 design-day heat load for the primary space- it doesn't need to be super-insulated, just decent (insulated foundation, reasonably tight, R5 sheathing if it's frame construction, code-minimum double-pane windows) to come in at half that. On design day it won't take hours to recover- being in constant circulation it'll stay pretty much at the tank's thermostat setpoint. Heating applications aren't the same as "recovery" we talk about in DHW apps where you have 45-50F water in, 125F out. The return water on design day will be around 10-30F below the tank setpoint, depending on the hydronic design, never 70F below. The tank temp won't change much, the duty cycle of the elements will.

    My very similar sized & configured (~1800' first floor, ~350' loft/attic, ~1600' basement) ca. 1923 stick build house in central MA runs ~28kbtu (measured) at a outdoor design temp of +3F, and it's below current code for attic insulation for most of it, 16" oc.full-dimension 2x4 framing, no insulating sheathing, and known gaps in the wall insulation to be rectified. (It has an insultated foundation though.) Only slightly smaller, tighter, better insulated I'd expect it would run about 30K at -10F, which is a lower design temp than all but the very coldest locations in NH (not counting the summit of Mt. Washington. ;-) )

    9kW is undersized if it is intended to be combi space + hot water heating system though. I'm not sure about NH codes, but many/most places dissallow use of water heater solely for space heating. Multistage/modulating 10-12kw electric boilers are pretty cheap though, and the operating cost will be the same as an electric tank.

    If electricity is your heating source, it's worth spending more on the building envelope to get the whole-wall-R-values up at least into the R20+ range, if not a super-insulated R30+. See:

    http://www.buildingscienceseminars.c...se_Studies.pdf

    or in more detail:

    http://www.buildingscience.com/docum...alue_Walls.pdf

    At R30+ whole-wall levels your heating loads will be quite modest- under 2kw.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    Radiant works well. The response rate is often much slower than warming some air in a furnace. It doesn't lend itself really well to setting back the temperature for a period. If you were to go away for a weekend, and set the house back to say 60-degrees, when you got home, with only 9Kw of power, it will take a long time to warm the house up. Maintaining it at a temp is easier. You have to put in more energy to increase the temp than to maintain it, you won't have much of a reserve.
    An average portable electric heater uses about 1.5Kw. Six of them might keep the house warm, but maybe not. The radiant certainly should be much more even heat. When you figure all of the customer charges and costs, in NH, electricity costs around $0.17 per Kw...it's expensive. So, $0.17x9x24 = $36.72/day IF (and it might be a cold day) it had to run 100% of the time. That's over $1K in a month. Last winter we had over a week where it barely got to 10-degrees. that would be one nasty electric bill, and I think the system would need to be running nearly constantly which could reach that max. My small condo with only two outside walls and fairly decent insulation needs about 26KBTU on its design day. It's only 1100 sq ft. Run your numbers, and then figure the running costs. Electric is probably not the way to go, and using a WH for space heating may void the warranty. Check your specific unit carefully.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    It's true that the response time is slow (and PID algorithm thermostats are a good idea with staple-ups). But unless it's WICKED cold out the ramp up from a room temp from 60F to a room temp of 68F is only a couple of hours, assuming you don't have a huge amount of thermal mass, like 12" thick concrete walls for interior partitions, etc. Deep setbacks aren't the greatest way to run any sort of radiant floor, but it's not nearly as bad with staple-ups as it is with slabs. A small wood stove as supplemental heat for ramping it up quickly may make sense in some homes.

    But any electric-resistance heating is going to be expensive to run unless you get the heat load down. Geothermal heat pumps are all the rage, but on a house this size you may be better off with spending the money you would have spent on heat pumps & wells and plowing it into a higher-R envelope to cut the heat load in half or more. Alternatively, a propane fired boiler or combi-HW heater will be cheaper to run than electric-resistance in most (but not all) markets in New England.

    Still, you'll NEVER run it at the full 9kw for 24hours at a stretch. Even on design day if your peak load was 9kw at 6AM the load in the afternoon will be under 6kw, maybe even under 3kw. The daily cost is determined by the daily average load, which will always be under 9kw. If your true design heat load is the full 9kw, it's 9kw for only 3% of the hours (the coldest 3% of hours) of the heating season, which are typically short in duration, and occur primarily in the pre-dawn hours on the coldest days of the year. If you want to figure out what your average cost for the month will be, calculate based on the heat load at the average daily temperature for that month. In Concord, NH, the coldest monthly average occurs in January, and that average is ~19F....

    /http://www.climate-zone.com/climate/...shire/concord/

    ...and 19F is probably ~25 degrees warmer than the peak design load, and only about 2/3 of the peak load.

    Still that's pretty expensive at flat-rate residential in most of New England if the heat load is truly that high, which it probably isn't.

    Many/most utilities charge a different (lower) rate for those who heat with electricity, still others offer dynamic demand-rates, and when your heat load is highest (the overnight cold hours) your rate would be off-peak cheap stuff, and if you wanted to avoid the peak demand rates you could put it on a timer, let it coast during peak hours. (Some with dynamic rate structures can make out well using high mass radiation like slabs, and/or large buffer tanks to pre-heat during off-peak rates, etc.)

  9. #9
    DIY Senior Member Runs with bison's Avatar
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    Reliability is a factor.

    I grew up out in the sticks. When it got really cold or there was a blizzard, electric was iffy. Several times we went a week without electric following a winter storm. Fortunately, we had a wood cookstove and fireplace, plus a propane heating stove so it wasn't more than an inconvenience for us.

    Even when I lived in Northeast Texas in the city we were without power for 4-5 days during an ice storm and that wasn't very long ago.

    I'm not inclined to trust electric only heating in a northern or midwest climate when the mercury is hovering around 0 F. This reminds me that I need to get a panel installed for a generator so that I can run the furnace blower and draft fan periodically if we lose power for several days.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    As I said, last winter, and it wasn't a great anomaly, we had about 10-days straight where it never got to 10-degrees. At night it got as cold as nearly -20, which is much colder than most people run their heat load analysis. I would not rely on electrical heating appliances. If you want to go off-grid or maximize the economy, adding enough insulation and ensuring you've plugged all of the leaks will go a long way towards minimizing energy costs.

    If I was going to build a house, I'd build it with ICF walls and factory fabricated large panels for the floor structure and roof...a huge, insulated thermal mass. One school out in the cold norther mid-west built a new gymnasium with the stuff and their heating costs for the entire winter was $100. I really like the windows made by www.visionwall.com too.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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