I need suggestions for a radiant heat water heater

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Harold Pomeroy

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I have a 1000 square foot shop, with radiant heat tubing for water in the concrete floor. I have been heating the water with gas, but I want to switch to electricity. The reason for the change is that I had 33 solar panels installed on the roof this year, and make more electricity than I can use in the house and shop machinery.

My goal is to get the concrete floor up to 60 degrees. I have a gas hot air furnace in the shop, and I run that at 60 degrees. My thinking is to warm the floor up slowly, and heat the air with the gas heater. When the floor and cast iron machines are warmer, the gas furnace will run less. In other words, neither system will heat the whole shop, they will each do what they do efficiently.

I the past I have heated the floor water with a heat exchanger in the gas furnace. The sending water would be 70 degrees, the return could get up to 50 degrees.

I would like ideas of how to heat the water to warm up the floor. I was planning on using a 40 gallon electric hot water heater, but it looks like constant circualtion at low temperature doesn't work.

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Harold Pomeroy
 

Jadnashua

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Most electric WH have a 3500W element which equates to just under 12K BTU. That could be enough energy to heat that shop area. That's a pretty good load for a typical home solar array, though, depending on what else you have to power. Unless the WH is designed for that application, the constant circulation might make it wear out faster. It's hard to say what you really need without further analysis and specifics on whether the slab is insulated, the windows and type, the wall and ceiling insulation, and orientation of the building.
 

Harold Pomeroy

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Most electric WH have a 3500W element which equates to just under 12K BTU. That could be enough energy to heat that shop area. That's a pretty good load for a typical home solar array, though, depending on what else you have to power. Unless the WH is designed for that application, the constant circulation might make it wear out faster. It's hard to say what you really need without further analysis and specifics on whether the slab is insulated, the windows and type, the wall and ceiling insulation, and orientation of the building.

Thank you for your reply.
In Massachusetts,the grid acts as a battery, so called "Net Metering". All the excess power I produce counts against future electric bills, so I don't store any. The house has a 13 Kilowatt system on the roof, pointed in the right direction. The power I produce is twice as much as the house needs. The excess credit is transferred to reduce the electric bill in the shop. I still run a surplus, given that the shop uses 200 Kilowatt hours a month.

The electric heat in the shop will not be enough, because I vent dust collectors to the outside with no return air. I don't have the engineering ability to clean dusty air to a breathable standard, so I choose to use make up air. The walls have 10" of cellulose, the ceiling has 18" of cellulose, the slab has 2" of foam under it, and the windows are triple glazed.

I would assume that the water heater will get continuous use at a low temperature, for eight to ten hours a day. I shut the heat off when I'm not in the shop.

Using Google, I haven't been able to find anyone heating a building like this. I have a 4500 watt 40 gallon water heater that I got for free, so I'm willing to experiment. I would rather find out what other people have done, or know.
Thank you
HP
 
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Jadnashua

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A 4500W element equals just over 15K BTU. Your biggest unknown is how many air exchanges are caused when you have the dust collector on. What is the height of the shop and how many cfm is the dust collector? Why not run it through a filter, and not throw all of that conditioned air out?
 

Harold Pomeroy

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Conventional wisdom on standards for breathable air in wood shops has evolved over the past 30 years that I have been in the business. Generally, filters clean all the dust but the worst, finest, dust. I choose to shoot that dust out onto a meadow. I have three different dust collection systems, and efficient hoods and hoses, to reduce the excess air collected and dispersed. Working alone, I can turn the systems on and off as needed.

I don't need to calculate the heat loss for the building, because I can't. It depends on whether I'm doing hand work, with no blower, or running a rip saw, with several blowers.

Given that I want to run the radiant heat system continuously at low temperatures, and make up the rest with gas hot air, could a domestic electric hot water heater handle the load without suffering a catastrophic event?
 

Dana

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A small electric boiler isn't radically expensive, and would (unlike a hot water heater used only for space heating) can actually meet code in MA.

If the shop is insulated 2x4 framing and at least R19 in the attic/roof with double panes and a couple of insulated doors your heat load even a 0F outside, 60F inside will likely come in under 15,000 BTU/hr. A typical hot water heater running continuously without thermostat control would overheat the place, and would not be able to adjust it's aquastat temps down to what the slab would really be needing to deliver the temperature.

A 4.5kw electric boiler is still less than $1000, is designed for the job, could carry the whole load (if you wanted it to, or you could get a deliberately smaller one), and has the temperature adjustment range you're looking for.

A 2.5kw version is probably what you would be looking for if you expect the gas burner to share much of the load. (The 2500 watt boiler can be served by a dedicated 25A 120VAC circuit.)
 
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Harold Pomeroy

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Thank you, Dana. I would rather meet code than re-invent the wheel. The electric boiler sounds like a good option. I can also hire a plumber to hook it up, if it meets code. There is no running water in the building, and I run antifreeze and water in the system now. I will have to figure out how that works.

I have never heard of electric boilers, and didn't find them using Google. Unless one had a lot of solar power like I do, they probably wouldn't be cheaper than fossil fuel. Finally, wood heat is cheap, but having a wood fire in a wood shop violates lots of rules.
Harold
 

Jadnashua

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One thing to keep in mind is that the heat transfer with antifreeze in the fluid lowers the effective transmission rate...IOW, each pass won't deliver as much heat using antifreeze verses plain water. If you were expecting to fully heat the place, that might dictate the temperatures you run the system and the spacing of the tubing (but, it is what it is in your slab). The system needs to be pressurized, so you'll have to run maybe a hose or something similar to get the proper pressure on the system (a boiler needs at least 10 psi typically to trip the safety switch and allow it to run). Once it is pressurized, if it's tight, you should not need to add any water and it should hold its pressure, but it will shut down if the pressure drops too low.
 

Harold Pomeroy

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The anti-freeze is to worth using, given that the system will work fine at lower efficiency. The heat in the floor is supposed to save propane, and use up the excess credit on my electric bills. In Massachusetts, any excess solar power I produce beyond what I am using is stored as credit against future bills.
 

Dana

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Verify with the manufacturer whether anti-freeze (and what type) is compatible with their electric boilers.

At MA retail pricing an electric boiler isn't necessarily more expensive than propane burned at ~82% efficiency in a tank hot water heater. One kwh used in an electric boiler & pump delivers 3412 BTU of heat into the room. Normalizing to million BTU (MMBTU) it takes 293kwh/MMBTU, at a typical retail 20 cents/kwh that's $58.60/MMBTU. (Running that power into a pretty good 3/4 ton ductless mini-split heat pump would be $15-20/MMBTU ) .

A gallon of propane has 91,333 BTU/gallon, burned at 82% efficiency delivers about 75,000 BTU into the hot water (and later to the room), so it takes 13.33 gallons/MMBTU. At this week's statewide average of $2.87/gallon that's $38.27/MMBTU. That's quite a bit cheaper than an electric boiler, but a lot more expensive than a mini-split heat pump. Only a few years ago propane was bumping north of $4, and more expensive than the 16-18 cent electricity.
 

Harold Pomeroy

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Thank you everyone for your responses.

I am hiring my neighbor, a licensed plumber, to install something. We haven't figured out what yet. Given that the electricity cost nothing, and air exchange in the shop can be thrown off by a dust collector with an 11" inlet, calculating anything isn't conventional. My goal is to set up something that will use excess free electricity, and not burn up. I would be embarrassed to have my neighbors, the volunteer fire department, come undo a situation that I had caused.

Thank you for your help.

Harold Pomeroy
 

Dana

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The heat load of the dust collector can be calcultated. The specific heat of dry air is about 0.018 BTU per degree F per cubic foot. A 200 cfm blower draws (x60min=) 1200 cubic feet per hour. If you're keeping it at 65F while working, and it's 10F outside that's a 50F difference, so it's blowing away 1200 cfh x 50F x 0.018= 1080 BTU/hr. When it's only 30F outside it's a 30F delta, and a 648 BTU/hr load, etc. If it's 1000 cfm (like a large furnace air handler) you are looking at 5400 BTU/hr @ +10F outdoors.

I don't know whether your dust collection system is rated higher or lower than that, but clearly running the dust collector only when it's really needed makes a difference. Even at 60,000 BTU/hr it takes awhile to extract enough heat from the building, slab, and contents to lower the temperature dramatically, just as it takes awhile for a 60,000 BTU/hr furnace to raise the temperature of a house from a 5F overnight setback.

The "free" solar electricity still has a cost, and unless your array is ridiculously oversized for your normal usage, you would eat up the excess pretty quickly if heating primarily with an electric boiler. A ductless mini-split heat pump would likely eat it up too, but only about 1/3 as fast. In a high dust environment you'd be cleaning the filter-screen weekly, but that's not a difficult process, far easier than cleaning the coils, which may also be necessary annually in a really dirty space.
 
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