Switch from gas to electric forced air heating after solar?

[Sunking]

Assuming a house in isolation:
If you use natural gas, you're emitting co2.
If you use solar, you're not emitting co2.

Am I missing something?

Yeah a lot. Sun does not shine at night when you need heat in SD, you are buying power. Nor does a small 6Kw system generate enough energy to run a heat pump during most of the day. Generating electricity to use as heat is extremely inefficient on the order of 20% or less from plant to heat pump. Using NG at Point Of Use is nearly 100% efficient. Using electricity takes 5 units of energy to make 1 unit of energy from NG. Your way pisses away a lot of money and resources with much higher emissions.

So why do you advocate pissing away money, using precious resources, and a heavy polluter. You ought to be ashamed of yourself.

[Sol Jolla]

Curious, who is determining that a ~20 year old system needs to be replaced? We're also in coastal San Diego and our current "Payne" is ~30 years old and works great. I have a 1980 vintage rental in hot Riverside with the original unit that still does the job. Granted the 1980 unit is overdue and likely expensive (probably a SEER 6 or 7) to run, but the tenant hasn't complained. Our home unit is probably a SEER 8 or 9 and does fine, especially since we only use it a few days a year. I barely maintain it, service maybe once every 4-5 years.

Thanks very much, everyone. A few more details: The house is ~110 years old. It would be difficult to seal up windows and doors without compromising historic integrity/charm, though the attic (where the furnace is located) could be better insulated. When we purchased the home a few months ago, during a pre-sale inspection, we were told that we couldn't expect much more life from the furnace based on it being 20 years-old.

[silversaver]

Installing a solar system to save money, but using electric heating over gas to waste money. Where is your saving after all?

[DanKegel]

The house is ~110 years old. It would be difficult to seal up windows and doors without compromising historic integrity/charm, though the attic (where the furnace is located) could be better insulated.

Also pretty similar to my situation - my house is 104 years old and a designated cultural landmark. Look up bronze spring weatherstripping - that's period-appropriate and can help with a lot of leaks.
Do you have / want air conditioning?

[Sol Jolla]

Also pretty similar to my situation - my house is 104 years old and a designated cultural landmark. Look up bronze spring weatherstripping - that's period-appropriate and can help with a lot of leaks.
Do you have / want air conditioning?

Yes, home came with central AC. To clarify, my goal with switching to electric would be to cut costs by maximizing solar use, since the system should make in excess of what we are currently drawing from the grid and in the ideal case would eliminate natural gas consumption and still have net zero or negative consumption from the grid.

[silversaver]

Yes, home came with central AC. To clarify, my goal with switching to electric would be to cut costs by maximizing solar use, since the system should make in excess of what we are currently drawing from the grid and in the ideal case would eliminate natural gas consumption and still have net zero or negative consumption from the grid.

If the area isn't large, try searching for "mini splits"

[DanKegel]

If the area isn't large, try searching for "mini splits"

Good point. Minisplit heat pumps can be more efficient than central air -- they don't have to work to push air through all those ducts.

To SK's question: yeah, I'm leaning on net metering to make it make sense, I should have said that. I'm also assuming that I can reduce energy usage by reducing leaks and adding insulation. So it's as much an efficiency story as a fuel switch story.

Whether one can save money by switching from natural gas to solar + efficiency is an open question. If your furnace is old and inefficent, and there are incentives for improving efficiency and switching to solar, it might be doable. But my goal isn't saving money; it's demonstrating that one can get even a big old historic house close to carbon neutral.

[ncs55]

"Just installed a 4.5 kW system (15 LG300N1K-Gw panels, P300 power optimizers, Solar Edge 3.8 kW inverter) in a small home in coastal San Diego and awaiting for permission to turn on. It's a somewhat small system but more than adequate for our home and lifestyle. We have a ~20 year old forced air gas central heating system nearing the end of its life. We're thinking about replacing it with an electric forced air central heating system so that we can take advantage of unused solar electric capacity. Anybody done anything similar? Thoughts on pro's vs. cons? Thanks- "


If you install a new electric unit, expect an electric bill sooner than you had originally calculated. Check and see what your demand charges are and your NEM rate. It is never a good idea to install large electric appliances after you install solar unless you had planned for it and oversized accordingly for one, your system was never calculated for that offset. For two, oversizing the PV system is usually done to hedge you against any future utility increase and or demand charges and to compensate for the system degradation. We are seeing degradation for systems varying widely, depending on how the installation was done and what equipment was used. The system design is also a very important factor for annual harvest and system longevity. You should consider all of these factors before jumping to electric for your heating needs. If you are going to replace your unit, stay with NG it is always less expensive than electricity.
We have had great success retrofitting in a solar thermal system and specific type of heat exchanger with existing heating systems and or heat pump systems to either use as a preheat when it gets cold or as the only heat when you need to knock the chill off of your home. This technique can be added to an older less efficient heating system and the end result would be the same as replacing to a more efficient heating system.

[DanKegel]

The OP already has PV, so unless he didn't size properly, he's already sunk that cost and doesn't want to consider a solar thermal system.

[ncs55]

The OP already has PV, so unless he didn't size properly, he's already sunk that cost and doesn't want to consider a solar thermal system.

It seems that the system was sized correctly and the question is whether to go Electric or Gas for a new heater.
Well most people would not consider what I just suggested because not many are looking at energy management of a household or can do it and show the savings. We can and do. Adding a thermal system to augment a heating system is less expensive than replacing an older unit that is functioning perfectly. And is still in place and functioning when a new unit is required. There are many advantages to this type of add on with heating systems. Also Thermal heating is around 90% efficient. PV is not even close to that. The other advantage is that you can use your existing hot water heater as a heat dump for this system during the summer months. That is two savings / paybacks now. It pays off faster than PV and maybe it should have been considered in the initial energy management design. Adding PV is NOT just slapping modules on the roof. It is an energy management retrofit and you have to consider and evaluate the home usage, customer peak usage, usage times, existing equipment, budget etc.. as a whole and how the E.M. retrofit will work with the customer and their usage. We couple other types of tech with PV to make it a more viable solution for our customers in the long run. We also couple this with educating our customers on how to get even more savings by changing their lifestyle a little.

[J.P.M.]

Assuming a house in isolation:
If you use natural gas, you're emitting co2.
If you use solar, you're not emitting co2.

Am I missing something?

Well, for starters, the last time I checked, most houses with grid tie PV are not in isolation.

[J.P.M.]

Thanks very much, everyone. A few more details: The house is ~110 years old. It would be difficult to seal up windows and doors without compromising historic integrity/charm, though the attic (where the furnace is located) could be better insulated. When we purchased the home a few months ago, during a pre-sale inspection, we were told that we couldn't expect much more life from the furnace based on it being 20 years-old.

Start by insulating the sill plate and around windows and doors. I believe caulking was around 100 years ago. Big payback on materials. The rub with sealing is the required attention to details takes time. This is the type of stuff best done before adding expensive solar. Any subsequent PV will be smaller and less expensive as a result. Cart before the horse stuff for those still contemplating solar.

[FFE]

I foresee this question being asked in the future quite often. There will be people that sized their system for an electric car or a pool and no longer need all the kWh their PV system produces. Or, a previous owner installed the system and they now can save money by using the excess for heating. Mini splits would be awesome but might not look right in a historic house. A central heat pump might work but might use too many kWh. If that is the case, switching from a gas water heater and/or gas clothes dryer to electric might save money when they need to be replaced. If you need to "waste" excess kWh before you need to replace something, inefficient resistance heaters would reduce your NG use while you figure out what you need.

[J.P.M.]

Gas dryer too.

And gas water heater.

[J.P.M.]

Whether one can save money by switching from natural gas to solar + efficiency is an open question. If your furnace is old and inefficent, and there are incentives for improving efficiency and switching to solar, it might be doable. But my goal isn't saving money; it's demonstrating that one can get even a big old historic house close to carbon neutral.

Been done lots of times. Seems we keep reinventing the wheel. Check out a tome titled "Home Remedies". I got pretty close back in the '80's in Buffalo, but not all the way there, reducing energy use by about 75% or so on an annual basis from historic use records with a real craftsman house (from Sears) built in the '20's. The trick there was attention to the sealing details, a lot of insulation, a few lifestyle changes and some architecturally appropriate but cost ineffective passive solar.