DC water heater.

[Giantkiller]

I recently found out that you can get up to 3000watts DC water heater inserts that I can replace the AC inserts in my water heater. With panels so cheap. I want to put up enough panels to equal what the inserts would require. If I can power them directly from the panels would obviously be cheaper/more efficient. What would be the minimum required/safety equipment between the panels and the heating element?
Could I use the temperature control already on the water heater ?
Thanks for any help!
Dan McCoy
I installed all of the wiring to code, inspected. On my just finished self built shop/house. But I still don't understand most things electrical.

 

[BobandSharon]

Hi. If you look at Missouri Wind and Solar website you'll find heating elements and thermostats like what you're looking for.
I think the best approach is to use a solar charge controller that has dump-load capability to send power to your heating element(s), the charge controller will only divert power to the element(s) once the battery bank has been fully charged. Rather than allowing the charge controller to simply "turn off" the input from the panels when the batteries are fully charged, you could use more of the potential of your solar panels by having that extra energy diverted to your hot water heater. I suspect there's a lot of threads on this forum regarding this (I'm a newbie here).
My experience is with Victron equipment. You can program the MPPT charge controller to either power a load directly (once the batteries reach a programmed charge point), or to switch on a relay, it depends on the size of the load. I use relays as they handle all of the current (and associated heat) rather than the stressing the charge controller. You don't have to buy Victron, it's just what I'm familiar with.
Appropriately sized wiring and fuses will need to be installed. And since you're powering the heating element with DC, you'll want a fairly short cable run. You didn't say what voltage your system is, but 3000 watts is over 60 amps at 48vdc, and 125 amps at 24vdc.
I once saw an installation where the owner had modified an LP hot water heater by brazing a threaded bung onto the tank, allowing him to thread an electric heating element into an LP hot water heater. His propane usage went down drastically in the summer months, while allowing for normal LP operation during winter. Pretty cool.

 

[OzSolar]

I recently found out that you can get up to 3000watts DC water heater inserts that I can replace the AC inserts in my water heater. With panels so cheap. I want to put up enough panels to equal what the inserts would require. If I can power them directly from the panels would obviously be cheaper/more efficient.

Welcome to the forum Dan!

I would offer there's another side to that which is with panels so cheap there's really no reason to do DC direct appliances, lights, water pumping etc.

Now perhaps you're trying to limit the load on your Inverter(s) which is understandable but that can be mitigated with smaller AC elements and/or time clocks to prevent the water heater during times when it's likely to be running many others loads.

Also, solar panels connected directly to a water heater will never help with other energy needs in your house. Just some food for thought.

Cheers!

 

[murmeister]

Also, solar panels connected directly to a water heater will never help with other energy needs in your house. Just some food for thought.

I agree with this. Having just powered up my house with solar, I start eyeing a way to get power to my shop when the batteries are full and I start exporting to the grid. You may wonder what to do with the extra solar power once your water is hot and the sun is shining brightly in the sky!

 

[TM48]

You need DC rated thermostats or you will fuse the AC ones and potentially have a run a way condition. DC arcs badly because you have no zero crossing condition occurring which extinguishes the arc. There are dc rated devices available but as it's already been said just use a regular water heater and feed it AC.

 

[mciholas]

You need DC rated thermostats or you will fuse the AC ones and potentially have a run a way condition. DC arcs badly because you have no zero crossing condition occurring which extinguishes the arc. There are dc rated devices available but as it's already been said just use a regular water heater and feed it AC.

One bizarre way to do it:

Have thermostat on water heater turn on RSD transmitter (which will be low volts and current, so contacts will be fine) and use one or more RSD in the panel system to shut it off when the thermostat is done.

The RSD is not a relay, so no arcing.

This also provides a shutdown signal for safety, just depower the RSD transmitter and you PV wires (outside of the array) aren't powered any more.

Mike C.

 

[Giantkiller]

Thanks much for quick responses.
Im going to use this for my experiment. I have done everything for solar hot water heating my driveway. I live in Wisconsin. And I hate shoveling snow. I installed 8000ft of pex on top of 2"foam insulationm myself. I made all 3 of my manifolds myself. And poured 5" of concrete over that. 11 truck loads. I also built 50' of lean to on the south side of my house myself. I had planned on using solar hydronic panels. But the cheapest are $900 all the way up to $2600. I have a permit to do this as long as I don't tap into electric. Will be self contained inside of the insulated lean-to. Pumps water heater tanks. Going to have return water going into the first water heater cranked up to the max. The second tank. Will feed the outgoing Set at 55°to 65° so it doesn't shock the concrete when it's cold. Plus you only need 40° to 55° in the concrete to melt the snow well. I have built the lean-to to the best angle for winter solstice.

 

[justinm001]

Thanks much for quick responses.
Im going to use this for my experiment. I have done everything for solar hot water heating my driveway. I live in Wisconsin. And I hate shoveling snow. I installed 8000ft of pex on top of 2"foam insulationm myself. I made all 3 of my manifolds myself. And poured 5" of concrete over that. 11 truck loads. I also built 50' of lean to on the south side of my house myself. I had planned on using solar hydronic panels. But the cheapest are $900 all the way up to $2600. I have a permit to do this as long as I don't tap into electric. Will be self contained inside of the insulated lean-to. Pumps water heater tanks. Going to have return water going into the first water heater cranked up to the max. The second tank. Will feed the outgoing Set at 55°to 65° so it doesn't shock the concrete when it's cold. Plus you only need 40° to 55° in the concrete to melt the snow well. I have built the lean-to to the best angle for winter solstice.

Wow that's sweet. Wouldn't geothermal be better tho? Then you're just pumping 50ish temp from the ground which id think is perfect for snow melting.

 

[Giantkiller]

Wow that's sweet. Wouldn't geothermal be better tho? Then you're just pumping 50ish temp from the ground which id think is perfect for snow melting.

I installed my geothermal in my house. I only have a 1 acre lot used one end of it for the loops I paid $5,000 to have the loops installed. I don't have an excavater. Used up most of the open underground land for that. No powerlines storm sewer ect. 6-7ft down 56ish degrees. But if you don't replenish some of that heat during the summer running AC. Eventually can permanently cool it down. And you still run a deficit. Because during the summer you put less back than you pull out during the winter. Unless you way over size the loops. Alot of people have problems with poor performing geothermal because of undersized loops.
That being said I would need a severely large amount of space to oversize the loops for the 8000ft of pex that I have.

Thanks it took me 5 years to build myself almost all by myself. The first 4 years I never missed a day working on it for atleast a couple of hours after my 12hr shifts. And all day on my days off

 

[mciholas]

I had planned on using solar hydronic panels. But the cheapest are $900 all the way up to $2600.

And they don't work that well, especially in the cold.

PV panels heating water electrically will work better, IMO.

Pumps water heater tanks. Going to have return water going into the first water heater cranked up to the max. The second tank. Will feed the outgoing Set at 55°to 65° so it doesn't shock the concrete when it's cold. Plus you only need 40° to 55° in the concrete to melt the snow well. I have built the lean-to to the best angle for winter solstice.

Is the strategy to store heat during a sunny day to melt snow on a cloudy one? Or do you only intend to melt snow when sunny?

In my mind, what you want is a large insulated tank. When you have sun, pump water past an electric heating element driven by the PV panels and heat the tank up as hot as you dare. The larger the tank and the hotter the water, the more energy you store.

When you want to melt, a second pump flows water through the driveway piping. Prior to the pump, you install a mixer that mixes the tank hot water with the return water bypassing the tank. This way, you can set an output temperature that is sane (say 60 F) and you will use only that hot water you need to raise the return water to the target. This maximizes the effectiveness of the heat transfer. This can occur even when there is no sun as long as you have pump power. If there is sun, then the first pump can be also running topping up the tank.

I ran a few numbers (which might be wrong). For a 2000 SF driveway covered in 5 inches snow (0.5 inch water equivalent), that is 623 gallons of frozen water which will require 210 KWH of energy to melt it and that's just the phase change. You won't have perfect heat transfer, and you do have some temp rise to deal with, so maybe 400 KWH to do the job. That is a lot! I compute you need about 1200 gallons of 80 C water to have stored enough energy to do this.

You might be better off using the solar electricity to run a geothermal heat pump which would give you a COP of about 4. Every watt you put into it results in 4 watts of heat.

I would carefully run the numbers to see how practical this will be. The guidelines for a snow melt system are about 40 watts per SF, so 2000 SF driveway is 80 KW. It doesn't really work to put in small amounts of energy as the heat dissipates before it can melt anything, you really have to up the power to get the job done.

Man, I sure hope my numbers are wrong and I've made a mistake somewhere.

Mike C.

 

[justinm001]

I installed my geothermal in my house. I only have a 1 acre lot used one end of it for the loops I paid $5,000 to have the loops installed. I don't have an excavater. Used up most of the open underground land for that. No no powerlines storm sewer ect. 6-7ft down 56ish degrees. But if you don't replenish some of that heat during the summer running AC. Eventually can permanently cool it down. And you still run a deficit. Because during the summer less back than you pull out during the winter. Unless you way over size the loops. Alot of people have problems with poor performing geothermal because of undersized loops.
That being said I would need a severely large amount of space to oversize the loops for the 8000ft of pex that I have.

Thanks it took me 5 years to build myself almost all by myself. The first 4 years I never missed a day working on it for atleast a couple of hours after my 12hr shifts. And all day on my days off

That's crazy man. I have a waterfurnace on my house on 3 acres of wooded. Previous owner put in and have zero clue where the loops are. Seems like an amazing system. Interesting enough I have radiant flooring in basement concrete but it's a separate all electric hot water tank. I never understood why they didn't use that for it. They did hook it up to my normal hot water tank which I guess is good.

 

[Giantkiller]

And they don't work that well, especially in the cold.

PV panels heating water electrically will work better, IMO.


Is the strategy to store heat during a sunny day to melt snow on a cloudy one? Or do you only intend to melt snow when sunny?

In my mind, what you want is a large insulated tank. When you have sun, pump water past an electric heating element driven by the PV panels and heat the tank up as hot as you dare. The larger the tank and the hotter the water, the more energy you store.

When you want to melt, a second pump flows water through the driveway piping. Prior to the pump, you install a mixer that mixes the tank hot water with the return water bypassing the tank. This way, you can set an output temperature that is sane (say 60 F) and you will use only that hot water you need to raise the return water to the target. This maximizes the effectiveness of the heat transfer. This can occur even when there is no sun as long as you have pump power. If there is sun, then the first pump can be also running topping up the tank.

I ran a few numbers (which might be wrong). For a 2000 SF driveway covered in 5 inches snow (0.5 inch water equivalent), that is 623 gallons of frozen water which will require 210 KWH of energy to melt it and that's just the phase change. You won't have perfect heat transfer, and you do have some temp rise to deal with, so maybe 400 KWH to do the job. That is a lot! I compute you need about 1200 gallons of 80 C water to have stored enough energy to do this.

You might be better off using the solar electricity to run a geothermal heat pump which would give you a COP of about 4. Every watt you put into it results in 4 watts of heat.

I would carefully run the numbers to see how practical this will be. The guidelines for a snow melt system are about 40 watts per SF, so 2000 SF driveway is 80 KW. It doesn't really work to put in small amounts of energy as the heat dissipates before it can melt anything, you really have to up the power to get the job done.

Man, I sure hope my numbers are wrong and I've made a mistake somewhere.

Mike C.

Only when the sun shines. I don't care how long it takes. I'll just drive over it until it does. I figured it would run all the time when the sun shines. And would store some heat in the driveway. I pretty much just drive on it now until it melts although I did have my friend plow it once this winter. After a blizzard. It's all just an experiment. To see if I can get it to work. I have 3 zones in the driveway. So could turn off the less important parts.

 

[mciholas]

Only when the sun shines. I don't care how long it takes.

You have to put in some minimum amount of power per SF to have any useful effect. You have to overcome the heat loss to the environment to get the snow to melt. If the heat input is too slow, your temp rise is too small to matter.

I have 3 zones in the driveway. So could turn off the less important parts.

I think it will be necessary to heat only one zone at a time so you can concentrate enough heat to make it work.

Snow melt system take an incredible amount of power. I considered it for my company building but the power requirements and cost were simply too great to make it feasible.

Found this snow melt design guide online:

https://www.uponor.com/getmedia/ad591b16-0881-4f43-9b71-30a995f0ff73/snow-and-ice-melting-design-and-installation-?sitename=Canada

Has lots of guidelines and design data.

Your system will have to use antifreeze as well. The fluid will get below freezing when the pump isn't running (and maybe even when it is).

Mike C.

 

[Giantkiller]

That's crazy man. I have a waterfurnace on my house on 3 acres of wooded. Previous owner put in and have zero clue where the loops are. Seems like an amazing system. Interesting enough I have radiant flooring in basement concrete but it's a separate all electric hot water tank. I never understood why they didn't use that for it. They did hook it up to my normal hot water tank which I guess is good.

My geothermal runs my infloor heat 6zones. 3 inslab downstairs in 2400sqft of shop and storage. And 3 zones upstairs 1000sqft of living space. 3 chilled water fan coils for cooling. Installed it all myself infloor, inslab, fan coils, bosch heat pump,(it has desuperheater that heats domestic water) boiler buddy, pumps, controls everything. 3400sqft heated and cooled at all times. Pics of insulation and pex I installed by myself I put insulation inside and outside of the foundation walls. And pumps on the wall for the 6 zones.

 

[Giantkiller]

You have to put in some minimum amount of power per SF to have any useful effect. You have to overcome the heat loss to the environment to get the snow to melt. If the heat input is too slow, your temp rise is too small to matter.


I think it will be necessary to heat only one zone at a time so you can concentrate enough heat to make it work.

Snow melt system take an incredible amount of power. I considered it for my company building but the power requirements and cost were simply too great to make it feasible.

Found this snow melt design guide online:

https://www.uponor.com/getmedia/ad591b16-0881-4f43-9b71-30a995f0ff73/snow-and-ice-melting-design-and-installation-?sitename=Canada

Has lots of guidelines and design data.

Your system will have to use antifreeze as well. The fluid will get below freezing when the pump isn't running (and maybe even when it is).

Mike C.

Yes planned on using 42% e glycol.
Heated driveways used to be illegal in Wisconsin. Because they used to much power. I plan on using 4 2000 Watt heater inserts in the 2 water heaters. I can put up to 24 390watt panels. I just purchased 30 @ $118 apiece. Delivered free on sale at the end of the year.

 

[justinm001]

You have to put in some minimum amount of power per SF to have any useful effect. You have to overcome the heat loss to the environment to get the snow to melt. If the heat input is too slow, your temp rise is too small to matter.


I think it will be necessary to heat only one zone at a time so you can concentrate enough heat to make it work.

Snow melt system take an incredible amount of power. I considered it for my company building but the power requirements and cost were simply too great to make it feasible.

Found this snow melt design guide online:

https://www.uponor.com/getmedia/ad591b16-0881-4f43-9b71-30a995f0ff73/snow-and-ice-melting-design-and-installation-?sitename=Canada

Has lots of guidelines and design data.

Your system will have to use antifreeze as well. The fluid will get below freezing when the pump isn't running (and maybe even when it is).

Mike C.

Concrete holds quite a bit of heat and snows a great insulator so I'd assume it work pretty well as every btu will be used since it's literally melting it's insulation.

 

[Giantkiller]

Thanks for the uponor read. I know for most snow melt systems. They expect to put in massive amounts of heat. Because they have to melt in a short period of time for Businesses for safety/liability reasons. And on Private property Because only a rich Person. Could afford to heat their driveway. And they want it to "melt it right away. What did I pay you all that money for!"
Hopefully it'll melt it off enough. So when I get to where I can't shovel. In a couple of years. And if I can ever get to where I can think about retiring. I Won't have to pay anything.

 

[Zwy]

I think DC water heating will become a dinosaur. I recently installed a heat pump water heater and run it on heat pump mode although I could run it with the electric 4500W element if desired. I originally installed it as a dump load but found the heat pump water heater efficiency is pretty amazing. A COP of 3 to 4 will greatly increase the usable btu's per kwh.

Normal day we use 3 to 3.5 Kwh of power to heat the 50 gallon tank plus I have propane water in series after the heat pump water heater with the thermostat on the propane water heater turned down much lower than the heat pump water heater. All 100 gallons is heated by the heat pump water heater. I left the propane in place for increased water heater storage capacity and for times in winter with very minimal sun.

Granted there are some conversion losses by changing DC to AC to run the heat pump. The COP efficiency by far exceeds those losses.

 

[Giantkiller]

Thanks for the idea. But. Heat pump water heater are great especially in southern climates. Since like all heat pumps they remove the heat from the air and put it into the tank. They in turn cool the air around them. Not really a great idea when up north. Especially when the only time I would be using it is in the middle of winter. And It would be running constantly. During any day that the sun is shining. AC in an insulated area you want to keep as hot as possible. Heat pumps are awesome there efficiency is great. But it declines when trying to pull heat out of -20° air that's why they aren't used too much up nort der hey. They do make northern climate heat pumps that can somehow pull heat out of really cold air. But again at major loss of efficiency and durability. Due to having to run constantly, during the winter.
Thats why I installed my geothermal. Heat pump that only has to take heat out of 56° water. And for that matter it gets to put heat into 56° water. During the summer. Way less work. That's why geothermal heat pumps can last 30- 40 years. They aren't trying to pull heat out of -20° air and trying to put heat into 90° to 100° air.

A geothermal heat pump would be the best to heat my driveway. But again lack of space in my yard for loops. And cost. I got 3 estimates for geothermal for my house/shop. All 3 had different ideas for equipment. And all 3 came back estimate of just over $45,000. Like they all got together and said lets tell him $45,000. I installed it myself for a little over $22,000. With Warmboard infloor heat upstairs. And that includes paying $5,000 to have the loops installed. It took me a month to figure it all out and still working 65+ hrs driving a forklift to pay for it. If I knew what I was doing. I probably could have installed it in a day maybe 2. So the geothermal guys where making $23,000 + for a couple of days work.

Sorry I got off on a rant.

Anyway thanks everyone for the information. It's greatly appreciated.

 

[mciholas]

Since like all heat pumps they remove the heat from the air and put it into the tank. They in turn cool the air around them. Not really a great idea when up north. Especially when the only time I would be using it is in the middle of winter.

Geothermal (aka ground source) heat pumps are not like that. They work well in the winter. You can get water to water GSHP.

But again lack of space in my yard for loops.

You want vertical wells if you can find the right driller. This is particularly true in northern climates where earth temperatures are cold even quite deep. My office building uses 32 wells, each 300 ft deep. My entering water temperature (EWT) is about 63 F year round.

This boils down to a simple equation, what gets you the most hot water for the system cost, a smaller PV array with a water to water GSHP, or a larger PV array with resistive heat? You need about a 4 to 1 ratio of PV array size for the equivalent performance.

I could easily see the larger PV array with resistive heat winning that race. For one thing, it will tolerate intermittent sunlight better than a GSHP.

I like the idea of DC direct couple to the heating elements. No inverter to buy or fail. You just need a good way to have the thermostat cut off the power so you don't have a melt down. Hence my idea of using an RSD device to avoid any mechanical contacts being involved. There are also SSR (solid state relays) that work on DC voltages if that would be better.

Mike C.