Offset Electric Boiler energy usage with batteries and solar?

[Roswell Bob]

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.

 

[Hedges]

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.

 

[MichaelK]

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

Large battery banks matched with small solar arrays is the classic mistake that has left a vast wake of ruined solar systems.

 

[mechdriver]

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.

I've been looking for a solar hot water heater with AC assist, there's the Rheem units, but I believe they are targeted for domestic hot water. I could do air to water heat pump as I mentioned but then it mimics the efficiency profile of my existing air to air heat pumps which leaves a gap in very cold weather. I'm never going to know my usage profile of this standby boiler in advance between it being weather and comfort dictated. The weather will also dictate the boiler temp because of my outdoor reset control.

Thanks for the insight, I'm clearly theorizing in the wrong direction and if I do solar it looks like a grid tied system as you suggest for the house itself is my best option. Where I arrived in this crazy idea was using the difference in cost between electric and gas boiler to somehow turn that into a small energy assist setup between solar and battery for the boiler to feed off first in light usage. It would be interesting if I knew it's usage just to maintain the low mass heat in its primary circuit instead of just it's absolute usage at full tilt.

 

[mechdriver]

Large battery banks matched with small solar arrays is the classic mistake that has left a vast wake of ruined solar systems.

Appreciate the benefit of other's experience here, for certain. Trying to avoid the buy now, figure out later chaos.

 

[edcoopered]

Use water cylinders to store heat from excess PV generation and/or off-peak power. Batteries are more expensive, less reliable, degrade over time and will need replacing.

 

[740GLE]

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.

They make pellet hydronic boilers, a ton of pellets will keep forever if you keep the dry, and will provide all the heat you need for multiple days.

My experience comes from needing to heat 2700sqft when it can get -20 out, we have a cord wood boiler that provides all heating and hot water needs and oil boiler as back up.

 

[efficientPV]

The solar boiler only needs to be a pipe with a couple elements installed. Best to avoid panel voltage and grid power in the same box. Grid tied is the way to go as once summer comes then solar power can power home and AC needs. The most reasonable use for solar when wanting to avoid electrical entanglements with the gris is supplemental domestic hot water with 4-6 panels. Hot water is used summer and winter. Being slightly underpowered in panels insures 100% of power creation will be used.

 

[mechdriver]

Thanks to everyone who has read and/or replied as I went overboard explaining at the onset. The enthusiasm behind my curiosity did not match my subject matter knowledge in order to pose an accurate question at the start. Maybe I can do better now.

I was inspired by installations in which people use solar and batteries to charge their EV cars. In my head it was similar to my need: an added load they are offsetting using solar and battery direct to the load itself, rather than refit their entire electric service to solar and battery. The time spent intermittently charging would be lower in my usage case, but the load would be higher and possibly need a threshold when to have the grid supply the difference or just takeover altogether.

So in as few words as I can, is it feasible to connect an electric boiler to a zero-export or AC coupled inverter where below a certain threshold power is provided by battery which is charged via solar? Over a threshold the electric boiler would run from the grid. I could also use a standalone AC charger to top off the battery if solar is insufficient during a given period.

I appreciate the outside the box answers too. But in this case I'm either going with an electric or gas boiler for hydronic heating. Gas refit is at least 4x more expensive and not DIY hence my investigation. I don't think solar thermal would cover what I need unless it had an AC backup which I've found only to be at lower temps than I need. I've thought of refitting the electric boiler with DC heating elements to avoid adding an inverter, but I'd have to start buying equipment to test. I could even split the 4 electric boiler elements into 2 sets- DC which are solar/battery powered then 2 which are AC powered, but that would be farther from anything someone else would expect to find than I'd like to do in my home.

 

[Hedges]

I forget - is grid-tied net metering possible for you?
(and are there terms, such as mandatory different rate schedule, which might make it unattractive?)
This is the cheapest and most efficient way to utilize 100% of available PV.

If no net-metering, then grid-tied zero export is the next best. Only question is, how you can best control discretionary loads to consume 100% of PV and avoid curtailing production, while minimizing draw from grid.

Beyond that it gets more complicated and more expensive.

For electric hydronic heating, perhaps a hybrid heat-pump water heater would be worthwhile (so long as it doesn't draw heat from the room air it heated.)

 

[12VoltInstalls]

I’m late to the show but I read everything and have thoughts.

1st goal appears to be heat in the bathrooms. Electric baseboard is quick and inexpensive to install. It makes nearly instant heat (keep it low unless showering, what have you). Plus, resistance heat is 100% efficient- though not lowest cost.

Cost-wise I’d keep the hydronic system in place. How to power that has multiple options available but consider the cost to upgrade nor change fuel or setups versus the benefit. Generally from least dollar cost per BTUs to highest dollar cost per btu the fuel source expenses are usually as follows (without regional variations considered):
Split cordwood (sometimes coal)
Coal (sometimes wood)
Pellets
Natural gas
Heating oil
Propane (sometimes electric)
Electric (sometimes propane)
Solar, 15-year aged value

Generalizations but sufficient assumptions without trying to be empirically accurate, and general factors for appliance inefficiency (like 85% for oil is assumed).
This came from an in depth cost study i did years ago for 6 buildings from 2000sf to 45000sf. Long Island may vary some.

For whole house heating you merely need a tank to run it. AND it will likely cost less than the ‘splits for heating. Additionally, you can sink solar watts into the hydronic loop to shave whatever solar will provide. However, just doing a grid-tie no export as suggested gives you year-round electric cost shaving without a lot of system gymnastics.

I don’t think you’re going to be able to do anything that will cost less money than that up front, plus the seasonal input will be at the highest you can attain with the AIO Gridtie yielding the lowest annual cost possible in your situation. Another advantage is with a much more limited battery bank you could have some grid-down backup: an oil-fired furnace (your 140-180 modulated furnace is not a boiler) takes way less wattage to heat the house whereas electric heat falls down very quickly.

Those are my thoughts. I applaud your interest and desires but unless cost is no object I don’t see an enjoyable outcome especially when you say you have limited space for your panels. YMMV

 

[mechdriver]

I’m late to the show but I read everything and have thoughts.

Thank you for your reply, you're not late since I haven't started! I'm am really trying to stick with the hydronic system since I put so much effort into re-zoning it and the baseboards are also all new. The heat also feels nicer than the mini splits can manage at lower temps (although my recently added drain pan heaters have made a difference in how well the mini splits heat at low temps). In a future life I could also switch the baseboards to radiant in the bathrooms with a mixing valve.

Why this thread was focused specifically around the boiler was if I could start off with a small solar array that charges a large battery bank that powers the electric boiler intermittently I could go to an electric boiler and shave off any added operating costs. I also felt the outdoor reset control I have would keep requested temps lower. But in reality, it will just reduce the time the electric boiler is on, not necessarily the load.

It has become abundantly clear I just need to commit to a larger solar array, to the same battery backup and pick more loads I want to run in a no export grid tie setup. The biggest blocker is the load of the boiler. Sized to my house's full time needs, the boiler needs to be 16kw (4x4kw elements) and that is not a small battery system. If I was willing to size it as a smaller standby boiler, my idea could work, but I want full redundancy.

I did think of heating the hydronic system with solar thermal (or solar pv to DC heating element) and using an instant gas water heater to make up the difference towards target, but like you said that is a lot of system gymnastics.

Right now I'm looking at the Growatt SPF 5000W ES in parallel with rack batteries and 10 panels. Grid tied but no export and 240v loads only. I would start with the 16kw boiler and re-wire the 2 mini split outdoor units to the panel and add provisioning for electric car charging to the dedicated panel.

This way I could possibly avoid added costs from the electric boiler (unless it was at full load) and lower costs by removing the mini splits and future electric car charging from my grid panel.

Looking forward I could add an autotransformer and have 120V loads moved to the solar/battery panel.

Once I get a little further in my plan, I'm going to start a new thread on my concept.

 

[SolarPrep]

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?

mechdriver: you suffer the same affliction that I tend to exhibit at times. You see a technology you like, and then want to find reasons to justify it. Nothing wrong with that, if it makes you happy, and you end up getting what you want as a result.

I've done most of the things you are doing, in our current house. Mini splits, hot water heat, etc. In your case, I'd FORGET the idea of an electric boiler completely. You live in a cold area, and are going to need reliable heat. Solar is great, but what are you going to do when there are blizzards, overcast, ice, etc? (Pretty much what is going on right now.....)

If you have done a heat load calculation that is reasonably accurate, look for a condensing gas fired boiler that will cover your loads, with a reasonable margin. Look carefully at the pressure drop through the boiler, and if possible, pick a model that has the least pressure drop so you can use smaller pumps. I use TACO controller,s, and love them. Both propane and natural gas work well, and I'd prefer either over oil.

If you want to experiment with PV, or any type of heat exchanger technology, do that after you have a proper boiler installed to cover your loads. You could look at solar water collectors, or heat pumps. Deye markets some tank systems; take a look. It is actually nothing new...they have been around for decades, they are just getting cheaper.

 

[mechdriver]

mechdriver: you suffer the same affliction that I tend to exhibit at times. You see a technology you like, and then want to find reasons to justify it. Nothing wrong with that, if it makes you happy, and you end up getting what you want as a result.

I'll see you at the weekly support group meeting, I'll bring the cookies.

I've done most of the things you are doing, in our current house. Mini splits, hot water heat, etc. In your case, I'd FORGET the idea of an electric boiler completely. You live in a cold area, and are going to need reliable heat. Solar is great, but what are you going to do when there are blizzards, overcast, ice, etc? (Pretty much what is going on right now.....)

Initially I was in a pickle to make a decision quickly for this winter as my heat pumps were not rated below 14 degrees F. However I found LG makes a retrofit kit that allows operation down to 0F. I installed those and I'm running off my mini splits as my primary heat source now- I have 2 outdoor units feeding 8 indoor units so they cover the whole house. The belly pan heaters also seem to help their operation below 40F as before they took a lot longer to produce heat when it got cold.

If you have done a heat load calculation that is reasonably accurate, look for a condensing gas fired boiler that will cover your loads, with a reasonable margin. Look carefully at the pressure drop through the boiler, and if possible, pick a model that has the least pressure drop so you can use smaller pumps. I use TACO controller,s, and love them. Both propane and natural gas work well, and I'd prefer either over oil.

If you want to experiment with PV, or any type of heat exchanger technology, do that after you have a proper boiler installed to cover your loads. You could look at solar water collectors, or heat pumps. Deye markets some tank systems; take a look. It is actually nothing new...they have been around for decades, they are just getting cheaper.

After the last month of running off the upgraded mini splits, I'm confident they can continue to provide primary heat. This makes me want to downsize the hydronic boiler requirements but part of me thinks that's a bad idea. A concern with a gas boiler, especially a high efficiency model, is that by sizing it correctly to serve the whole house, I'm oversizing it to serve as a standby unit. Electric boilers don't care about that apparently. But I'll have no idea how much energy the larger electric boiler will use in standby mode either until I installed it.

I like the idea of the solar assisted heat pumps with grid backup. I have all the tools and build experience on servicing mini splits so I could go that route, but they would match the efficiency model of the existing heat pumps which might now work.

As you've probably surmised I'm leaning towards the electric boiler, sized to my heat calculations as if it was the primary, but relying on my outdoor reset control and intelligently using the thermostats to minimize usage.

Solar will have to wait until I can do a whole house setup, or at least just my 240v loads. It would be cool to run the minisplits, electric boiler and car chargers on their own panel fed by a battery/solar setup.

 

[12VoltInstalls]

This makes me want to downsize the hydronic boiler requirements but part of me thinks that's a bad idea.

Granted as a northern new englander I have a ‘different’ view of things. Having the heat pumps and fossilized energy just feels secure and supportable to me. Add to those a couple cords of split hardwood and an airtight stove and I’d feel secure power or not.

All electric? Another ice storm like a few years back or another Irene and you’ll have nothing.

Solar to power basic lights, hot water circulation, and maybe a short term of a minisplit and that’s about as secure you can be in the city.

Plus in cost per BTU for running an electric hot water furnace (not a boiler unless you have steam heat) I just can’t imagine it! And running a heating pad to make electric heat pumps work to zero is way outside my tolerance.

Not poking mean at you, just showing you how I think (a different way of thinking) and hopefully that’s helpful.

For myself, since being on solar, I’ve not noticed several all-day-long outages and a number of short outages. For 8 warmer months of the year I haven’t ever really needed a backup. Imperceptibly I’ve discovered I’m quite secure whereas I don’t trust the grid nearly as much. I don’t even own a generator. I am using ‘grid backup’ for the winter months but by March I won’t need it.

I’d feel vulnerable being dependent on the grid for heat.

 

[noenegdod]

This is probably not to code where you live but just tie into your NG hot water heater and call it a day. When they are running they are 80ish% efficient.

You tie in with a small pump and throttle the output to match the 40k btu output it probably has so when you are calling for heat the water heater stays lit. You install a thermostat that turns the pump on and off so that if the tank drops below desired temp the pump turns off and preserves DHW for showers etc. It is simple as hell and you already have the tank

You will have to replace the tank more often but so what. They are cheep and if you require an inspection it is simple as hell to plumb it in after the fact.

Lets be clear, This is not a safety issue. Not being code compliant is because they are missing that ~15% efficiency.

 

[SolarPrep]

Mechdriver:

I prefer chocolate/raisin/oatmeal...thank you.

The electric boiler would be a big mistake. Two years ago, I swapped out one that my friend had running just to heat his work shop. His electric bills went down an average of $150/month when running that boiler. Here in the Midwest, our cost per kwh is 13.7 cents.

With the increasing violence of our weather patterns, I think it would be a bad idea to go with that as your primary heart source. Get a wall hung boiler, or even one of those cool HTP combi units with the tank/water heater and boiler built into one unit.

I have high quality mini splits too, and they work down to -12. But they work a lot harder, and use more more energy doing so, while putting out about 40% less heat. They should have periodic cleaning and maintenance too. I have to do that this year, and it is messy.

Apply your energy and knowledge to an efficient boiler, and then tackle other areas where you can shave off loads.

Good luck, what-ever you decide.

 

[Ralphloren]

.

 

[Someguy]

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.

 

[timselectric]

Nice setup