domestic electric water heater for radiant floor heating

motownmike

Technically, what are pros and cons?

Jamie Hall

There are no pros that I can think of, other than the first cost being cheap.

If the electricity being used is generated in part by fossil fuels -- which most is -- their energy efficiency is horrible.

In most places (not all) they are a lot more expensive to run than any of the fuel burning boilers.

They are neither built for not intended for continuous duty heating.

Overall, they are not fit for the purpose.

I'm sure others will come up with more detailed cons...

Ironman

What is the btu load for the area being served?

motownmike

11702 at 0 degrees outside

Ironman

You'll get 15.3k btus from a 4.5kw element. Enough to cover your load, but very expensive for energy cost.

Is the floor in slab or staple up?

motownmike

Mine will be staple up.
I can't get natural gas.
Electricity to be a cheap to install back-up to a wood gasification hot water unit.
Everything I find on fuel costs is similar to this: http://www.csmonitor.com/Environment/2012/1207/Cheapest-way-to-heat-your-home-Four-fuels-compared/Heating-oil-2-526
What am I missing?

Gordy

I would take your kilowatt costs after fees, and see what your actual bottom line is per KWH.

Electric water heaters are much slower to recover than gas of same tank size. Look at it like this with 4.5 kW element you produce 15234 btus to heat the water on a 40gal tank where a gas WH typically uses 36,000 btus.

It may work. It's marginal at 0 degrees for the load. If it's for back up don't plan on breaking any records for catching up on setpoint if you fall below it by far.

In my mind for backup to wood boiler the Electric water heater sits cold until needed. It will take some time to heat up to a usable tank temp to give a worthy SWT for a stapleup radiant design. Just keep that in mind.

Edit: you are using a wood gasification boiler for a 12k load, or is there more, and you only want the WH to cover a portion of the total load?

Zman

I think that article is garbage.

Almost as bad as the one that I just read about Obama care reducing bankruptcies as the nation was coming out of the great recession. In that case the overall economy improving had more impact than Obamacare. You could use the same junk data to prove a connection to almost anything. Maybe the decrease in highway fatalities in women between 18 and 24 was the cause...
I don't have a strong opinion Obama care, I just hate misleading journalism.

In the case of this CSM article, they make zero reference to the cost of the fuel itself which varies greatly from State to State.

Use the attached spread sheet and compare your local rates and get the real math.

If electricity is the right choice, an electric boiler is a better idea than a water heater.

Oh, and sorry for the rant, the spread sheet is worth putting up with it.

hot_rod

Are you asking pros and cons versus gas or lp fired?

Or versus an electric boiler?

Jamie Hall

I do wish that people would read all the information about an article before committing to it. Did anyone happen to notice the date on that article? 2012? Anyone recall the spike in oil prices that year besides me?

The article is useless -- which isn't too surprising. Use @Zman 's spreadsheet to get somewhat more realistic numbers.

If you are using this as a backup to wood heat, well and good, but I would still get a proper heating boiler. Further, size it to the full load of the house. If you were to go on vacation during a cold week? If you had the 'flu (or worse)?

I note, too, that you do mention wood heat, which suggests to me the possibility that the environment is of some importance to you. Allow me to point out that in most parts of the country most of the electricity is generated by fossil fuels; electric heat in that circumstance has an overall efficiency of less than 40%, whereas any decent oil or gas fired hot water boiler has over twice that (85% to 95%).

motownmike

Thanks to all who responded. I understand what you are saying.
Just for grins -- Here is a more current look at heating fuel costs: http://www.mass.gov/eea/energy-utilities-clean-tech/misc/household-heating-costs.html

newagedawn

the newer electric water heaters have come down considerable on Kwh in the past 5 yrs making them alittle more atractive for a modest house to use radiant , i wouldnt use in a Mc mansion tho

Gordy

motownmike said:

Thanks to all who responded. I understand what you are saying.
Just for grins -- Here is a more current look at heating fuel costs: http://www.mass.gov/eea/energy-utilities-clean-tech/misc/household-heating-costs.html

funny electric is the most expensive in my area. I'd do the math using @Zmans material. Eye opener.....

Gordy

newagedawn said:

the newer electric water heaters have come down considerable on Kwh in the past 5 yrs making them alittle more atractive for a modest house to use radiant , i wouldnt use in a Mc mansion tho

Not sure what you are talking about. Cost of water heater, or how many KWs they use? If the latter a watt is a watt.

newagedawn

you are billed on killawatt hours used , check your bill next month, and i know for a fact that the Kwh's newer water heaters use is a considerable drop from say ones that are more than 5 yrs old and im taking in some cases 40% less

newagedawn

nobody pays more in Kwh than the people in CT, this state is out of control when it comes to electricity and considering how often a raidiant system will run, in my opinion it could be a consideration for a modest house

Zman

newagedawn said:

you are billed on killawatt hours used , check your bill next month, and i know for a fact that the Kwh's newer water heaters use is a considerable drop from say ones that are more than 5 yrs old and im taking in some cases 40% less

I think you may be confused. Electric water heaters have resistive heating elements. They are and always have been very close to 100% efficient. The only way to improve the energy consumption is to either put in a smaller element (less performance). Or insulate the tank better (same performance less heat loss).

Heat pump water heaters under light load are more efficient but not in a heating situation because they are just taking energy from the air in the room and putting it in the slab (see Peter and Paul analogy).

Jamie Hall

Zman said:

newagedawn said:

you are billed on killawatt hours used , check your bill next month, and i know for a fact that the Kwh's newer water heaters use is a considerable drop from say ones that are more than 5 yrs old and im taking in some cases 40% less

I think you may be confused. Electric water heaters have resistive heating elements. They are and always have been very close to 100% efficient. The only way to improve the energy consumption is to either put in a smaller element (less performance). Or insulate the tank better (same performance less heat loss).

Heat pump water heaters under light load are more efficient but not in a heating situation because they are just taking energy from the air in the room and putting it in the slab (see Peter and Paul analogy).

@newagedawn cheerfully claims that new water heaters may use up to 40% less electricity. Other than that being advertising hype, they do use less; perhaps not that much less, but less.

But...

The reason is much better insulation and thus much lower standby losses.

They aren't going to cost any less to heat a gallon of water from one temperature to another than the old geyser I once had which had no insulation at all

Which means that in a home heating situation, you are going to pay very nearly the same -- within the margin of error of the heat losses -- whether you use an electric boiler designed for the purpose (which might last a decade or two) or a domestic electric water heater (which might last two seasons) -- or a row of teakettles. As has been said, a watt is a watt.

The idea of a heat pump (hybrid) water heater for space heating is just funny, as the heat pump will industriously suck heat from the space you are heating to heat water to heat the space you are heating to provide heat to the heat pump which will suck heat from the... oops, I've said that.

jumper

On a closed system I don't see why a cheapo electric hot water heater won't last indefinitely in radiant heating duty. Pressure is lower. They're made for raw water so corrosion & scaling is minimal compared to what they're made for. Supply temperature is lower than domestic hot water. Return is warmer than normal input. Yes,the elements may run hotter but does that matter?

Decades ago night time electricity in Ontario was less expensive than gas when efficiency was factored in. I asked commercial water heater manufacturers about modifying their products for extra high temperatures.

Larry Weingarten

Hello, There is a newish heater by Rheem: http://cdn.globalimageserver.com/fetchdocument-rh.aspx?name=professional-prestige-hybrid-electric-spec-sheet--rheem-gen4-hybrid-electric-2017 It's a heat pump that can be ducted, which means you could use it in different ways, depending on the weather and how you arrange the ductwork and dampers. So, you could use it to do radiant without doing the Peter and Paul thing.

Yours, Larry

Gordy

All we know about the application is that it is a staple up radiant with just under 12k at design. What we don't know is required water temps, or if the staple up was plated to reduce water temps.

Will the electric WH support the required water temps for a staple up application which usually requires higher water temps than sandwich, or concrete embedment depending on the assembly design.

That's my concern, along with output, and energy consumption.

hot_rod

I've used 6 gallon, 12 and 20, 30 and 40 gallon gallon electric tanks on radiant. Virtually all residential tanks will run to 140- 145F.

The older style thermostats ran up to 160F, now a days they don't indicate temperature just Warm to Hotter.

If you want quick recovery from room temperature a small tank with a 4500W element would work.

All tanks these days have 1" screw in elements and you can find a lot of sizes and wattages. A common 240V 4500W give you just over 15,000 BTU.

Plates under a floor with 140F should get you 20 BTU/ ft or more, depending on the floor thickness and floor covering.

I do add a 30lb relief valve when I use a tank for radiant.