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Offset Electric Boiler energy usage with batteries and solar?

mechdriver

New Member
Joined
Jan 5, 2022
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25
Hello,

I thought about asking this in small bits, but instead I'm dropping my whole story here in the hopes of guidance.

I renovated my Long Island, NY home myself over the last 10 years from its original unimproved 1950s design. Low-E windows, every wall, ceiling, floor and the attic thoroughly insulated. Basement remodeled and double insulated. Home was heated by an oil burner connected to hydronic baseboards which had 2 zones. I replaced every baseboard and re-zoned the hydronic system into 4 proper zones using a Taco zone controller with an outdoor reset control which vastly improved the efficiency of the older oil burner.

For cooling I installed 2 LG Multi Zone Mini Splits with 4 zones each. Initially I used them exclusively for cooling and as I observed their low electricity usage, I used them all year for heating and cooling. I recently upgraded them with base pan heaters so they can operate down to 0 degrees so they are my primary heating/cooling.

This has meant I have not refilled my oil tank in 2 years. I noticed recently the tank developed a leak, so I drained it and now I'm prepared to replace the boiler. I could install a gas burner and be done with it (I already have a gas water heater), but I'm curious about electric boilers that I can offset electricity usage with a battery bank and solar over time

I've looked into air to water mini splits, which are rare in the USA, but they would be expensive and mimic the efficiency profile of my mini splits at lower temperatures which can be a problem for the up to 180 degree water for hydronics. I'm keeping the hydronics running because of how much work I put into it and the minisplits don't warm the bathrooms directly.

So I've settled on either replacing the oil burner with gas and forgetting this foray into solar and batteries or diving into building out an electric system which I can improve over time (to keep upfront costs down)

I completed a Model J calculation showing a 50k BTU heat need. I have 3 choices on electric boilers:

54k BTU 16kw 4 element boiler (4x4kw)
41k BTU 12kw 4 element boiler (4x3kw)
41k BTU 12kw 2 element boiler (2x6kw)

Because the mini splits are run full time, this is really a standby heating system that I'm sizing as a primary for redundancy. It will be running all the time but likely only to maintain the heat relevant to the outside temperature (it will connect to my existing outdoor reset control) in its low mass tank. It's primary role will be to heat 2 small bathroom baseboards and occasionally bump the heat in a room which the mini split does not heat well enough during low temperatures. I could also figure out when the outdoor temp drops below a threshold where electric is more efficient and comfortable than the mini splits (they take longer to heat at lower temps). This is where it would then heat the whole house, potentially.

The electric burner will step up through its elements depending on demand, so I don't see it using the full kw rating all the time. I've done various calculations and I won't really know how much it will average over time until it is installed and running. I think it's wise to use the 54k BTU boiler with the 4kw elements so the staging is lower and it's not undersized.

My home is a split which cannot place a large solar array without making my roof an ugly patchwork of panels. I like the concept of a large battery pack which can power the electric burner and a small solar array that can help charge the batteries. I would likely need to grid charge the batteries on occasion, again depending on usage.

This brings me to the relevant questions:

Do I DIY the $2k electric boiler, connect to grid, observe usage and start building a high kw battery back that I can charge with solar/grid?

Or is this a pie in the sky project that's not worth it and buy the 8k gas boiler and forget it?

I realize a proper 48V battery back and inverter at this size quickly raises costs but I'm not in a hurry and would like to do it in stage such as:
-Boiler connected to grid and operating to understand accurate usage
-Battery pack and inverter powering boiler, grid charging battery pack
-Small solar grid connected to inverter to charge batteries and run boiler entirely
-Larger solar grid connected to inverter to charge batteries and run most of house?

I'm trying to use my flexibility that this is a backup system to develop a really nice setup without needing it to be running 24/7. I could see this growing into a grid tied system that eliminates my power bill.

Thanks for any advice.
 
It's not worth it and you should forget the idea completely.

If you all ready have grid-power, you will never save a single penny. Off-grid solar is simply the highest costing way to produce electricity. I think the conservation improvements you'll already performed are amongst the best things you can do.

Alternatively, a grid-tie system where you reduce your power bill may be a better option for you.

If you goal includes something besides saving money, blackout backup for example, then solar becomes more viable.
 
if waterheating is your goal, why invest in batteries ?

im looking ar this thing and 6 dedicated, older 250 watt panels

 
It's not worth it and you should forget the idea completely.
I keep telling myself that. But my logic that lead me here travels like this:
As a primary heat source, a grid tied 16kw electric boiler would be too expensive to operate vs a gas boiler of the same BTU running full time.
BUT as a secondary or backup heat source its average usage should be much lower than its peak rating. It also makes sense to have an electric boiler off more often than a gas boiler.

Which leads me to ask questions like:
If I inserted solar and battery before the electric boiler, could a small solar array trickle charge the batteries which could then run the electric boiler for the short periods I actually use it?
 
if waterheating is your goal, why invest in batteries ?

im looking ar this thing and 6 dedicated, older 250 watt panels


I have a gas water heater that I'm happy with and hydronics need higher temps to heat properly than many of these setups can provide.

This guy did something similar with DC heating elements in his water heater from PV:
The electric boiler I'm looking at is AC and I can find DC elements like he did, but that's a larger rabbit hole and also won't heat to the higher temp ranges.
 
Which leads me to ask questions like:
If I inserted solar and battery before the electric boiler, could a small solar array trickle charge the batteries which could then run the electric boiler for the short periods I actually use it?
What part of "Off-grid solar is simply the highest costing way to produce electricity" did you not understand? Anything you can do with solar can be done more cheaply with grid-power.

If this is about learning how to work with solar, then yes, sure. If this is about saving money, then absolutely not.
 
What part of "Off-grid solar is simply the highest costing way to produce electricity" did you not understand? Anything you can do with solar can be done more cheaply with grid-power.

I understand the uptick in costs, I did mention the electric boiler being 6k less than the gas boiler and I can DIY the electric. So I could just leave it like that, saving the difference for any increase in my electric bill over time or spend the difference now to take steps to keep the electric bill where it is (or lower) while also learning a new skill.

I realize when sized to maximum output a 16kw battery, inverter and small solar array incinerate that difference, but as I thought the loads would be smaller and intermittent, I have been trying to understand if I could start small and build out. Or use battery/solar for the lower intermittent usage and switch to grid for peak. Nothing bad has ever happened when someone has uttered "How hard can it be?" right?

The electric boiler has the ability to manually shut down heating elements and heat with as little as 4kw from one element which I could size my battery pack to that for start. I'd imagine the inverter would need to be 16kw capable, then add small battery and solar at first. Or do I size the battery to maximum kw and start smaller with inverters that I can combine? This is where my mind drifts to.

As you can figure I struggle with scope creep and over focus on details in my projects, sometimes that works out magnificently (like learning HVAC and installing my own mini splits or learning plumbing and rezoning my heating system) and other times I get bogged down.

Reducing overhead costs in electric bills would be a side benefit of having completed a customized mixed source 12 zone heating/cooling system that can be efficient to the ambient temperature and be very comfortable for my family.
 
Leave batteries out of it. You can store energy in water, but why. Just use the energy when it is generated. I heat water with PV efficiently and it is super cheap for me, being just the cost of the panels. But, winter is just a bad time for solar. Just feed it into some electric baseboard heaters. If you want to try it just for fun and science, PM me.
 
I heat water with PV efficiently and it is super cheap for me, being just the cost of the panels.

I read through the Direct to water heating thread, and your projects are cool. I guess I was trying to marry an off the shelf grid tied electric boiler with solar/battery/inverter to reduce peak usage by drawing from a larger sized battery which the solar had slowly charged (as I won't have room for a large array). Expecting with low usage the solar battery could feed all it entirely. I'll have to eat the usage at peak points or in the event of a mini split failure. But basically this 4 zone hydronic system should only be used sporadically for a few weeks out of the year (and even then maybe just at night) and to bump the heat in 2 small bathroom baseboards twice a day during the winter.
I may just be overly concerned that the boiler being rated at 16wh but that's again if both my mini splits are down and the temp drops below 0 for an extended period of time (unlikely in Long Island, NY). I felt if I did this I had to size it as a standby unit that could do the whole house and not just a tandem unit to bump the performance of the mini splits at low temp. And I can manually shutdown elements if my outdoor reset control doesn't manage the boiler temperature enough to reduce usage.
I don't know why there isn't more direct PV to DC water heater with AC backup units that do more than domestic hot water. It would seem an opportunity to combine PV, DC heating elements, Optional Battery Backup, and grid tied AC failover into a combo boiler unit with outdoor reset control.
 
This all sounds like a lot of money doing it with hot water. The solar calculator is your friend. Multiply the daily solar hours it gives by the total watts of the array. This just isn't that big a number even with a multi KW array. If panels are cheap it can be somewhat practical. This is a little oil filled heater I used. These are rated at 1.2KW, but only in extreme cold. It has two resistance elements and in parallel they work well at 60V. This is not touch safe with only 400W going into it. This runs with a little NANO.
heater1.JPG
 
How old is the furnace? Seems a shame to get rid of it and spend more money, due to a leaky tank especially if it’s used only as a back up.

Modular plastic oil tanks are pretty darn cheap, also sounds like you won’t need a full 250 gallon either, .
 
How old is the furnace? Seems a shame to get rid of it and spend more money, due to a leaky tank especially if it’s used only as a back up.

Modular plastic oil tanks are pretty darn cheap, also sounds like you won’t need a full 250 gallon either, .

The oil burner is a thermodynamics unit from roughly the 1980s which on a good day is 80% efficient. I have it modulated through the taco zone controller’s outdoor reset control so it at least is not just set to 180 degrees in the winter and 140 in the summer like they were doing to the old lady who lived here before. The burner itself is actually an ancient Texaco unit which itself is a knockoff of a beckett model. I’ve disassembled it, replaced most of the burner parts and re-insulated the steel boiler. It took me 2 years to empty my old 250 gallon tank with the mini splits covering most of my usage. This is a partly why I’d expect the electric burner to also be low usage.
I could replace the tank, but I’d want to move it outside of my basement but then I’m committing myself to oil and replacing this oil burner with another one when it finally gives up the ghost.
 
Electric resistance heating is the least efficient way to heat and paired with the most expensive electricity is just crazy. I would replace the boiler with a tankless water heater 90+% efficiency 100k + BTUs lot less cost. Use the leftover money to start grid tide solar. Or enough off grid to run that tankless and pumps so you have heat when grid goes down.
 
Electric resistance heating is the least efficient way to heat and paired with the most expensive electricity is just crazy. I would replace the boiler with a tankless water heater 90+% efficiency 100k + BTUs lot less cost. Use the leftover money to start grid tide solar. Or enough off grid to run that tankless and pumps so you have heat when grid goes down.

Thanks for presenting a solution. I know my electricity is expensive, hence my investigation into offsetting it. I was only thinking of full on gas boilers and not gas tankless heaters to feed the hydronic system. I’ll have to research that.
 
Thanks for presenting a solution. I know my electricity is expensive, hence my investigation into offsetting it. I was only thinking of full on gas boilers and not gas tankless heaters to feed the hydronic system. I’ll have to research that.
Not all of them can deal with hot return water so do the research.
 
If you have recently insulated your home then you don't need a lot of BTUs. And you certainly don'd need super heated steam for hydronics. Buy a small point of load water heater if you have emergency. they are dirt cheep and many use them.

I can heat my slab with pv panels. Slab is on 2nd floor.
 
If you have recently insulated your home then you don't need a lot of BTUs. And you certainly don'd need super heated steam for hydronics. Buy a small point of load water heater if you have emergency. they are dirt cheep and many use them.

I can heat my slab with pv panels. Slab is on 2nd floor.

The original oil burner was 110,000 BTUs. My recent Model J I did myself estimated 50k BTU considering the various improvements.

The Taco outdoor reset control has very rarely gone over a boiler call for 160 degrees, but we also have not had particularly cold weather in the last 2 years where I had been watching it closely.
 
PV panels cost $0.025/kWh
Batteries cost $0.50/kWh
Any questions?

That’s what I was here to ask.

Can a battery pack, topped off by a smaller solar array, offset the intermittent, seasonal usage of a large kWh load?
 
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