Offset Electric Boiler energy usage with batteries and solar?

[efficientPV]

New member here, but figured I'd share. I live off-grid in a very cloudy location (Big Island Hawaii). One of my goals when setting up our ~10kW setup was to not ever have to mess around with a generator (we have none). As such, we are dramatically oversized for our fairly modest electrical needs on ~85% of the days and since there's not grid any power that could be made after the batteries are full is just lost. So... that energy is basically free to put to use in some fashion and we chose to dump it into a 120 gallon water tank that has a 120V x 2kW heating element on the bottom and a 240V 2kW heating element on the top. My outback charge controller turns on a relay to put ~55V battery float voltage to the bottom element, which makes ~420W at that voltage. This relay is only on while the batteries are at the target voltage so it automatically comes on in the morning around 9am and goes off around 4-5PM (I did say I was oversized!). Up top, I have a mechanical timer set for the hours of 10-3 that puts 120V from my inverter in to the 240V AC element (which runs around 500W at that voltage). That top element regulates temperature and is only on about an hour a day from what I can tell. I found that even without any hot water usage, the bottom element won't push the water temp much past 120F so the system needs both the do the job. My wife and I have enjoyed domestic hot water from this system for 4 years now. However, there are a couple weeks a year where the PV is right at the limit (winters can rain for weeks on end here) and for that we do unplug the top element (off the inverter) and just live with luke-warm showers. Batteries are just (8) 8D AGMs at 48V, but it works fine for us.

I think the issue is the tank is too large. Stratification is your friend. Having a tall smaller tank like 40 gallons would give you faster recovery. An extra large tank like 120 gallons is good for long term storage and tempering tank. That is far too much water to heat with a small lower element. Lower section doesn't have a penalty because there is very little heat loss due to temperature differential. Unfortunately, tanks with more than two elements are rare. I have a 40 gallon tank just for laundry operating at only 500W and 1 to 2KWH with stratification definitely works well. House has used a small 6 gallon point of use being fed by another the other tank works well.

In the off season when AC is not used for the upper element, I'd be connecting to the lower element. Many have the upper element on a plug so they can switch back and forth. That extra 100W doesn't seem like much, but that will really bring up the temperature. 900WH will raise 6 gallons up 60F and you already have pre heated water.

 

[Ralphloren]

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[mechdriver]

Any progress with your plan?

The boiler replacement is in progress this week actually. I went with the electric boiler and redesigned my plumbing around the boiler into primary/secondary hydronic loops. I also upgraded my circulators to delta T models (no zone valves, each zone has a circulator as an independent pump). Combine those with the outdoor reset control and the multi-stage nature of the electric boiler I hope it will use a lot less electricity than one would expect. Once I finish the install and wire it up I'll start testing the power consumption to determine if solar/battery is feasible.
My house has 200A service with two 100A sub panels. I'm tossing around how I could wire my electric boiler, mini splits and my electric car charger into a zero export off grid solar/battery array off a sub panel using a transfer switch.

 

[mechdriver]

Any progress with your plan? It seems to me that you are exaggerating a little and complicating everything. This can be solved much easier and faster without such costs. In any case, I don't understand why to throw away a good boiler just because of a leaking tank. Is a new boiler worth less than just a tank repair? It doesn't make any sense to me. I'm just wondering how long does it take to service a gas boiler? I don't think it's very long. I see only such a normal output. Just call a specialist to repair your tank. Plus, you can ask him questions that interest you. It will be much faster and safer.

Okay, I guess I need to reply twice since your edit. As I've explained ad infinitum, I have used my multi-zone mini splits to heat and cool my home for the last 2 years with great success. I have a indoor unit in every room. The oil burner was part of the previous primary heating system. It was the only old part as I had re-zoned the hydronic system to a circulator pump per zone design and wired in an outdoor reset control through a zone controller. The old oil furnace was 100k BTU but with all the insulation upgrades my house's Model J calc is now at 50k. The burner and furnace were old, I had thoroughly serviced the oil burner with new parts (ignition, sensors, electrodes, jets, etc) and courtesy of a local oil tech even ran exhaust gas combustion efficiency calculations against different jet sizes and the draft of my chimney. The burner was a Texaco (knock off Beckett clone) and the furnace was an old Thermodynamics steel unit, 86% efficient with the best of our efforts. The oil tank was single wall unit inside my basement. When I got rid of it, I installed an egress window against that wall to brighten and add value to my basement. Now that I'm disassembling my oil furnace, I'm glad I got rid of it. The electric burner is 1/5th the size. I would have had to buy an expensive outdoor dual wall outdoor oil tank to re-use it. Also, in this configuration of low, intermittent usage I have determined modern high efficiency condensing gas or oil burners would have been very unhappy. I would have had to buy what is basically a more insulated version of my old furnace. This way I can have the electric burner off until I want to use it. And before you panic about frozen pipes, every pipe was moved into a wall and insulated for the entire home run for all zones from the basement and over the last two years when the oil burner was basically off, I've had no issue.
Why I'm here in this forum was my curiosity around the burgeoning server rack battery market and how it could store solar energy to be used to power an electric boiler that would be used very infrequently. I'm honestly surprised by how little interest there has been in offering solar/battery solutions. I understand the extra cost, but I'm in a position that my DIY efforts offset a lot of that upfront cost. I also understand the peak usage of this electric boiler (16kw) is very high for most solar/battery setups. So, I'm going to finish my install and determine usage to make a call if a zero export solar/battery setup can eliminate any added electricity cost from this boiler, while maybe also incorporating the load from my mini splits and electric car charger (which I do know usage for both) to make the cost worthwhile.

 

[Hedges]

Why I'm here in this forum was my curiosity around the burgeoning server rack battery market and how it could store solar energy to be used to power an electric boiler that would be used very infrequently. I'm honestly surprised by how little interest there has been in offering solar/battery solutions. I understand the extra cost, but I'm in a position that my DIY efforts offset a lot of that upfront cost. I also understand the peak usage of this electric boiler (16kw) is very high for most solar/battery setups. So, I'm going to finish my install and determine usage to make a call if a zero export solar/battery setup can eliminate any added electricity cost from this boiler, while maybe also incorporating the load from my mini splits and electric car charger (which I do know usage for both) to make the cost worthwhile.

Solar production much lower in the winter. For some locations, 1 to 2 hours effective sun vs. 6 or 7 in the summer. Tilt for winter might improve that (except straight up is apparently best for overcast conditions.)

"used very infrequently" - If you cycle the battery once per day and it is supposed to last 6000 cycles, that's 16 years.
Does "infrequently" mean only on an occasional night? You end up buying battery cycles you never use, boosting cost.
Sized for heating in winter, will you also get much use out of it in summer? If not, that could double or quadruple effective cost per kWh.
(The battery will probably also die of old age at some point.)
The server rack batteries seem to have reached $0.05/kWh, assuming you use them up, so even getting only a fraction of their cycle life may be cost effective.

Note that they are apparently designed to power a rack of computer CPU and discs, have problems with inrush and surge loads. For off-grid solar/inverter use, need to size battery bank accordingly.

16kW isn't excessive. In the summer daytime I could run that 2/3 on, 1/3 off. I power AC and house no problem. Winter not as much PV production but could run a lower percentage of the time.

"Boiler" - is that resistive heating element, or heat-pump? If resistive you should be able to rewire for lower wattage, longer run time. Less battery and inverter current.

 

[mechdriver]

Solar production much lower in the winter. For some locations, 1 to 2 hours effective sun vs. 6 or 7 in the summer. Tilt for winter might improve that (except straight up is apparently best for overcast conditions.)

"used very infrequently" - If you cycle the battery once per day and it is supposed to last 6000 cycles, that's 16 years.
Does "infrequently" mean only on an occasional night? You end up buying battery cycles you never use, boosting cost.
Sized for heating in winter, will you also get much use out of it in summer? If not, that could double or quadruple effective cost per kWh.
(The battery will probably also die of old age at some point.)
The server rack batteries seem to have reached $0.05/kWh, assuming you use them up, so even getting only a fraction of their cycle life may be cost effective.

Note that they are apparently designed to power a rack of computer CPU and discs, have problems with inrush and surge loads. For off-grid solar/inverter use, need to size battery bank accordingly.

16kW isn't excessive. In the summer daytime I could run that 2/3 on, 1/3 off. I power AC and house no problem. Winter not as much PV production but could run a lower percentage of the time.

"Boiler" - is that resistive heating element, or heat-pump? If resistive you should be able to rewire for lower wattage, longer run time. Less battery and inverter current.

The electric boiler is a resistive heating element with 4 x 4kw elements which are electronically controlled to stage usage but can also be individually turned off. That gives me options to cap peak usage. I’m also wiring this to the outdoor reset control which should keep boiler temps low and the delta T circulators should help manage efficient heat usage of the zones.
It’ll take time to understand how much energy I use in real life. Toasty bathrooms on a cold day I don’t mind paying extra for!
I was drawn to the concept of a zero export inverter which can use grid power when needed if solar is low and the batteries have worn down. But perhaps besides when that happens it can be powered by solar/battery.
The hydronics have two loops- a small primary loop with a boiler circulator that would be running when the boiler is on. That’s the first load to understand usage for. Then the zones are each their own parallel secondary loop and their usage will depend on how long they call for heat, etc. It’s going to be interesting to figure this all out!

 

[Azeem]

The electric boiler is a resistive heating element with 4 x 4kw elements which are electronically controlled to stage usage but can also be individually turned off. That gives me options to cap peak usage. I’m also wiring this to the outdoor reset control which should keep boiler temps low and the delta T circulators should help manage efficient heat usage of the zones.
It’ll take time to understand how much energy I use in real life. Toasty bathrooms on a cold day I don’t mind paying extra for!
I was drawn to the concept of a zero export inverter which can use grid power when needed if solar is low and the batteries have worn down. But perhaps besides when that happens it can be powered by solar/battery.
The hydronics have two loops- a small primary loop with a boiler circulator that would be running when the boiler is on. That’s the first load to understand usage for. Then the zones are each their own parallel secondary loop and their usage will depend on how long they call for heat, etc. It’s going to be interesting to figure this all out!

Did you install the electric boiler? If yes, would you mind sharing the model and consumption data. I am trying to do something similar since I already have PV system installed e.g. 1000kwh per month in winters generation.

 

[Ampster]

I am trying to do something similar since I already have PV system installed e.g. 1000kwh per month in winters generation.

If you haven't considered a large Heat Pump Water Heater it is four time more efficient that a resistive element. I do not know your use case but I just wanted to mention it in case it is a possibility.

 

[Azeem]

If you haven't considered a large Heat Pump Water Heater it is four time more efficient that a resistive element. I do not know your use case but I just wanted to mention it in case it is a possibility.

Thanks Ampster, the upfront cost of a heatpump, existing installed aircons (with heat) and gas fired boiler with radiators just puts me off from heat pump (what am I going to do with all of the installed equipment, can I utilize my existing indoor units of AC with a heatpump?). I installed a 11kw PV system earlier this year and I am happy with its performance that I haven't paid a penny to utility company this summer.
So I was thinking of utilizing solar in winters for my heating needs, my PV system will generate 1000 kw/month.
On top of it there aren't many professional installers of heat pump in my country who could assess my requirement for heat and cooling system and I haven't seen the system working where I live. I already have an oversized heating system boiler that is highly inefficient (keeps cycling in winters!) and my usage is 15mmbtu per month in peak winters for 8 hours use daily. Rising gas prices ($15/mmbtu incl taxes) is a huge expense So I was thinking now that I have solar why not get rid of gas boiler and put a resistive element to circulate water in existing radiators. I haven't got a clue how bigger heatpump do I require for my needs. I have 4 x 18000 btu aircons installed for cooling in my home so based on HP efficiency factor alone (4 time more efficient than aircon) I could do with 2.5 ton heatpump for my needs? Does this sound about right

 

[Shimmy]

@Azeem how cold does it get in the winter in your location? Does the boiler always short-cycle throughout the heating season or does it run all-out for part of the winter? Do you know what your return water temperature to the boiler is?

Adding a hot water coil to the air conditioner likely will not give you satisfactory results.

If you want to convert to electric it might just be easiest to go with a new tankless water heater, but you would need to check that the maximum inlet temperature works for your system.

 

[Azeem]

Thanks for your thoughts. Our winters are not that cold barring a spell of 2 to 3 weeks but it doesn't go below subzero. Most of the time temperature stays in single digits (deg C). I have a non condensing heating only boiler with radiators, I did try to balance it last year (without thermometer though) using flow and return philosophy (10deg delta between flow and return).
My problem is that I don't know how to work out how many kw boiler I require if I want to go electric?

 

[timselectric]

1.5kw for each radiator being fed.

 

[Azeem]

timselectric: Thanks someone gave me a figure of 2kw for large radiator. I have 4 radiators covering 4 rooms so I need 6kw unit to cool and heat the house? That's a lot of outlay in $$.

 

[Shimmy]

Our winters are not that cold barring a spell of 2 to 3 weeks but it doesn't go below subzero. Most of the time temperature stays in single digits (deg C). I have a non condensing heating only boiler with radiators, I did try to balance it last year (without thermometer though) using flow and return philosophy (10deg delta between flow and return).
My problem is that I don't know how to work out how many kw boiler I require if I want to go electric?

If you have moderate winters and that low of a delta-T then a heat pump is a very good solution. The radiators are more of a hassle than they are worth; they make a lot of sense when it is -10C outside, but +5C outside means you don't need high temperature air to feel warm inside. (Heat pumps will generally produce cooler warm air than say a gas furnace. They aren't a great solution when you have big windows that radiate the cold into the home.)

But, an actual solution comes down to your budget. A tankless water heater in the US is $200-600 plus installation, not sure if you would need a new circulator as well.

 

[32 volt boater]

Thanks for any advice.

Move off LongIsland. Long time girlfriend was from that place nothing but taxes, state - county - town - village hassles and massive corruption from the parasite class on every level.

A dump load controller, also known as a diversion load controller, diverts electricity from a battery bank once the charging source (wind turbine, PV, hydro turbine, etc.) has fully charged the battery bank. www.google.com/search?q=dump+load+controller

Choose from a wide variety of submersible DC water heating elements for direct connection to your batteries or solar panels.

dc hot water heater elements - Google Shopping

www.google.com

You can covert any hot water heater to lower voltage DC.

Put the hot water heater in the attic for the summer where its already hot for obvious reasons.




I am thinking maybe an AC/DC hot water heater. Probably putting the solar DC into the bottom element would be best.

But anyway its insane inverting solar produced DC to AC, alternating current to make heat.

Tesla was cool but in the solar off grid world alternating current is just a major hassle.

 

[32 volt boater]

$200-600 plus installation

This is a DIY forum. We skip that part.

 

[Hedges]

But anyway its insane inverting solar produced DC to AC, alternating current to make heat.

Not so crazy if your hardware has the capacity.
It lets you located dump load anywhere, without separate power wiring. Readily available safe technology. (I've seen AC thermostats packaged and sold with a DC label on outside.)
How about second thermostat on water heater, boost temperature when you have surplus? (and then tempering valve.)
If you want more heat/efficiency, HPWH could be used.

 

[32 volt boater]

Anyway maybe the best solution,

Hybrid solar panel 250 Wp / 24 volts and 1.58 absorption surface m2 for electricity and hot water simultaneously.​

Thanks to the assimilation of heat, the photovoltaic cells are constantly cooling, increase their productivity and operate with optimum efficiency.

You can now have electricity and hot water in one panel increased efficiency and saving on both bills.

Hybrid solar panel 250W hot water and electricity

Hybrid solar panel 250 Wp / 24 volts and surface absorption of 1.58 m2 for electricity and hot water at a time

www.todoensolar.com

 

[Hedges]

649 Euro for 250W

Better to pay $50 for 250W (used), and just do electric heat.

Yingli Solar 240W 60 Cell Poly Solar Panel

240W Yingli Solar, only $50 ($0.21 per watt)! Some cosmetic blems. Perfect for a DIY solar panel kit. Best wholesale solar prices available!

store.santansolar.com

Many is the good idea that died due to economics.

 

[32 volt boater]

Not so crazy if your hardware has the capacity.
It lets you located dump load anywhere, without separate power wiring. Readily available safe technology. (I've seen AC thermostats packaged and sold with a DC label on outside.)
How about second thermostat on water heater, boost temperature when you have surplus? (and then tempering valve.)
If you want more heat/efficiency, HPWH could be used.

If anything a hot inverter with the fan blowing is an electric heater.