electric water heater for shop addition
Last Post 15 Oct 2014 01:51 PM by Dana1. 4 Replies.
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Phil BUser is Offline
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13 Oct 2014 08:27 AM
I have put an addition onto my shop and I am looking for a little advise for a heating unit to use for my in slab system. The slab is insulated with 1.5” foam under and about 3” on the perimeter. I have flash & bat insulation on the walls, a 10’ ceiling with blown in probably r38, the pex is 0.5” id. The area is 576 sqft. The location is central IL. I’m thinking that a domestic ELECTRIC water heater could work but I know that’s not typical. The reason I’m thinking electric is that it gives me more options for placement and I don’t need to vent it which would be nice for this installation. So the question is: will either a 20 or 40 gallon electric water heater work okay for this space, I would want to keep the temp about 66 degrees. A little additional background fwiw. The main shop is 1792 sqft with a large attic. That space is heated with a domestic 40 gallon LP water heater (in slab) and it does keep the space about 66 degrees even in the 2013 winter.
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13 Oct 2014 04:37 PM
Short answer. NO.

Check the output of the two. The stored water has nothing to do with the output.

First, an ACCA Manual 'J' to find the load, then an electric boiler purpose-made for space heating, if you think you can afford the fuel bill.
MA<br>www.badgerboilerservice.com
Dana1User is Offline
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14 Oct 2014 03:30 PM
A typical 40 gallon LP HW heater has about 26-30,000 BTU/hr of output.

A small electric tank delivers about half that.

A dual-element 40 gallon tank can run about 30,000 BTU/hr with both elements firing 100% of the time, but they're not designed for that sort of duty.

An electric boiler is a much more rugged and appropriate piece of equipment with all of the necessary certifications & ratings, and it won't break the bank. It's more expensive than the first electric hot water heater, but it will outlast 3-5 hot water heaters that are cranking as hard as your apparent loads.

Run a heat load calc, and buy the right sized electric boiler for running the whole thing (shop included) if that's the route you're taking. LP is pretty expensive, basically a wash against an electric boiler in many markets. (How much does propane cost, and how much does electricity cost in your neighborhood.)

If you want to save money on operating expenses, you can get about 3.5x the efficiency (1/3 the power use) out of ductless mini-split heat pumps, but you won't have the comfort of a warm slab. A pair of 1-ton units or a single 2-ton (possibly with 2 heads) would probably do you, but run the heat load numbers. It's a lot more money up front, but it's a much lower marginal operating cost. But you also get high efficiency air conditioning out of the thing too. (Not that you would EVER need air conditioning in central IL... :-) )

If you're putting ductless for just the addition you can probably get by with a 3/4 ton brand-X single-head unit, but if you want to cut propane costs (and still keep the warm slab in the shop area) you can get a dual-head unit and put a 6000-7000BTU head in the addition, and a much bigger head in the shop. In that scenario you would use a floor thermostat to control the slab temp, and adjust the room temp with the ductless.
Phil BUser is Offline
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15 Oct 2014 12:07 PM
I've thought of replacing the existing LP heater with something that would run both areas, the floor pipes are reasonably close enough to do it, about 12’. Maybe when the existing heater goes down – could be soon. I’m wondering why a boiler over an electric tank though on the original question ? Seems like 15kBTU or so would be fine for 576sqft and 15k from a boiler is (forgive my ignorance if this is stupid) is the same as 15k from a boiler. A domestic electric water heater has almost nothing to fail whereas boilers have electronics and presumably other more complicated guts to fail. The only benefit seems to be less liquid in a tank and a smaller – cleaner install. I’m looking at something like this as a boiler: EMB-S-5 Electro Industries Electric Radiant Heating Boiler-15,000BTU, 240V,4.5KW http://www.amazon.com/dp/B0057GJYKE/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=2CG9TQHY6H2E8&coliid=I3PY3VQEX982K0 I would not use anything but slab heat now, the addition has a spray booth so that’s another reason for slab heat there.
Dana1User is Offline
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15 Oct 2014 01:51 PM
You really need to start with a heat load calculation to know where it really lives, using your 66F indoor design temp and whatever the 99% outside design temp is in your area, and the U-factors & areas of the different exterior wall/ceiling/window/floor/door assemblies.  A 15,000 BTU/hr heat load on a 576 square foot addition would be an unusually high number- it could easily be half that (or less.)

Electric boilers are far more robust than electric hot water heaters- count on it.  They're designed to handle higher flow, and they're designed for operation at a high duty cycle. Your presumtion that they are more complex under the hood is mistaken- they are in fact simpler, more rugged beasts than a electric hot water heater, and can be operated over a wider temperature range. The controls are different reflecting the different type of use, but they're still box o' rocks stupid, nothing like a modulating condensing boiler or a tankless hot water heater, but pretty much everthing about them is designed for high-reliability. 

The average 3 zone hydronic zone controller has more (and more sensitive) electronics in it than an electric boiler.

http://www.pexheat.com/site/images/argo/at_boiler.jpg

The failure modes on electric boilers are a bit friendlier too, with very low probability of a leak or flood.

What does propane cost, and what does electricity cost in your area?
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