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 s 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.
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.
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.
The original oil burne
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.
r 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.
I have 80,000 btu lp heating slab in basement floor. Uses about 2 gallons/day when 0C outside. The slab isn't well insulated. We have slab on the second floor too. I am thinking a 5kW electric would do nice. If we get some cloudy day I can use backup LP generator to charge batterys and heat floor. That will load it up nice. On sunny days I have a 18kWh surplus. We have lots of glass facing directly south so we do well to begin with.
I am familiar with the controller you are using. I'm sure you can get all the data you need from it.
What does it cost you per BTU
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.
What is Lilco charging you for a kWh? I thought LI had the highest rates at one time. Are you still paying for Shoram?
I mis-read and thought you were radiant floor. I run hydronics at <100 in floor. I know you are hotter with baseboard. I suppose you know how much PV array you need to heat your place. How much do you need? I am thinking that my 18kWh/61,000btu/hr surplus should heat 2nd floor. This is our last winter on the ground floor.
As has already been said it makes no sense to store in battery or invert to ac then heat. I don't suppose you can get some geothermal going on over there. Based on your electric rates you can do a thumbnail calculation to see how much money you lose with solar. If you can store the heat in stone or paraffin then you heat it when the sun is out and use it overnight.
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?
Why a battery?
You could have a solar array heat water in the boiler, reducing need for utility power.
If you put PV generated power in a battery then later draw power from the battery to heat water, you loose some percentage due to inefficiency.
But more importantly, a battery (top name brand), by the time it wears out, will have cost you $0.50/kWh. More than just buying the electricity.
(DIY and off-brand made of recycled might cost only $0.05/kWh)
PV is most cost effective either consumed immediately or used for "net metering" where you spin the meter backwards, use the utility bill as a "battery".
H2O is cheaper than Pb or Li.
Store the energy you collect as heat, not as charge on a battery.
Resistance heating is cheap and simple, but less efficient.
Heat pumps will produce more heat for less power (except in case of extreme cold.)
It sounds like your split units are serving as heat pumps.
If your backup is resistance heat of boiler, that could be powered by PV.
Easiest way to use PV for heat pumps is with a grid-tie inverter. Configured for "zero export" if necessary.
With zero export, your PV can offset grid for heating, cooling, all household electric loads.
Small battery system is appropriate for backup of communications, a couple lights, etc., but expensive for more than that.