Electric or Gas On Demand Hot Water?

Inspired by Brendon's comments at this blog entry, I took a deeper look at our decision to go with electric on demand backup hot water heating. Brendon maintained that electric hot water heating is  much more inefficient than gas because most electricity (around 45% this  year) in California is generated from gas, and electricity generation is only about 33% efficient, as opposed to heating the water with gas directly, which uses most of the heat for actually heating the water. My response to Brendon was that since we were offsetting all of our electric use with PV anyway, hot water was another electric use that we could offset, just like the lights we use at night when we don't generate any solar power. We are not storing the solar electrons in batteries, then using them later, as would be the case with a non-grid tie system. And also the electric grid is projected to get greener over time but the gas grid won't.

After thinking about it, though, I decided that we could equally well offset the carbon from gas by generating solar electricity. It would not directly offset the cost, of course, but starting next year, PG&E is going to reimburse people on their net metering schedule for unused solar electricity credit. Currently, any net metering credit simply expires at the end of the year. So we could conceivably be reimbursed for the gas cost indirectly by the amount we receive for any solar electricity that we don't use. The reimbursement rate PG&E is proposing isn't equivalent to the amount you would receive through net metering, but it is certainly better than just losing the accumulated credit. And I wanted to check my assumptions about the greening of the electric grid, to see which type of backup water heater would actually have a lower carbon footprint over the long term.

So I did some calculations. Our current gas fired tank water heater is a 40 gallon Bradford-White, circa 2001 vintage. The Bradford-White web page on energy efficiency has an efficiency of around 0.6 for their "natural" gas fired tank water heaters. I don't know our exact model (it is currently packed behind several layers of insulation and plastic curtains for the remodel) so I took that number as sufficient. We used an average of 6.1 therms per month for hot water over the past few years, of which only 3.66 therms actually ended up heating the water due to the low efficiency. The rest either went up the flue or was radiated out by the tank while the water was in storage. Since we will be using backup hot water for 7 months  of the year, this amounts to 25.6 therms or 750.67 kwh per year for backup hot water heating.

The on demand electric hot water heater we are thinking of installing is the Steibel-Eltron  DCH-E. Electric hot water  heaters have an efficiency of 1, so all  of the energy goes to hot water heating. I could not find any data on backup energy use by the Steibel-Eltron, so I assumed therefore that it is negligible. Electric hot water heating would require 750.67 kwh. The carbon footprint for that amount in 2010 in California is 222.26 kg/yr.

Our architect recommended a Takagi gas hot water heater, the T-H2 is their greenest model. It has an efficiency of 0.93, which would result in 27.55 therms per year for water heating over the 7 month season during which we need backup. But it also requires electricity for standby and for ignition. The standby energy use is 196 Wh/day. Assuming that the heater comes on 4x per day for about a minute, it would use 10.1 Wh/day for ignition. So the total daily electricity use during the season when we need backup is 206.93 Wh/day, or 44.90 kwh for the 7 month season when backup is needed. The carbon footprint of the T-H2 for both gas and electricity in 2010 is 159.66 kg.

Now, I've assumed in the above calculations that during the 7 months the backup heater is in use, all of the hot water will be provided by the backup water heater. This is obviously not the case, since the solar thermal hot water heater will provide some heat, though in December, January, and February, it will be minimal. In addition, even in the depths of winter the solar thermal will raise the temperature of the incoming water from 60 degrees as it comes out of the main to around 80-100 degrees, thereby reducing the amount of heat input from the backup water heater. The calculations above scale by the amount of energy needed from the backup heater. If the backup heater only comes on 10% of the time, then, with the exception of the standby electricity use for the T-H2, the carbon footprint for both types will be reduced by 90%. So the calculations should still be good for comparison  purposes.

Today, in 2010, the gas on-demand heater clearly has the lower carbon footprint, but not by much. The difference is only 62.6 kg, and that will scale down by how much heat the solar thermal provides. The carbon footprint of the gas water heater is therefore about 28% smaller than the electric. However, the situation is likely to be different as the electric grid (hopefully) gets greener. The plan in California is for the grid to have 33% renewable content by 2020. Today, the renewable content is around 15%. Gas-fired power generation is practically the only fossil carbon generating source of electricity in California of any consequence today. Coal has been completely phased out by PG&E, and oil has not been of any consequence since the energy crisis in the 1970's. Assuming that the planned increase in renewable content goes to offsetting gas-fired power generation and not to reducing the contribution from other non-carbon based sources such as large hydro or nuclear, the result would be a reduction in carbon generated by the grid from 0.302 kg/kwh in 2010 to 0.137 kg/kwh in 2020. Our carbon footprint for hot water if we chose an electric heater would then be reduced to 103.17 kg. in 2020, making the electric backup's carbon footprint 32% smaller than the gas. Note that this calculation accounts for the reduction in carbon footprint in the Takagi electricity use. 

The crossover point - where the gas and electric hot water heater generate the same amount of carbon - is in 2013, when the grid should have 22% renewable content, if of course the renewable content is introduced in an incremental, linear fashion between 2010 and 2020. This is surprisingly soon. The crossover carbon  footprint is 155.94 kg. The graph below summarizes the calculations.  The vertical axis is kg/yr carbon for backup water heating, assuming 100% of the water heating in  the 7 month season is supplied by the backup,  the horizontal axis  is the year:

That said, the assumption of a nice, linear increase in renewable content may be too optimistic. PG&E's record in introducing renewables has not been linear, and  the political climate at the moment isn't very good for continued policy pressure on them to do so.  Valero, the big oil company, is sponsoring a proposition on the November ballot to suspend AB32 until the unemployment rate goes down to 5.5%, and Meg Whitman, the Republican candidate for governor,  has said she will deemphasize renewable energy should she win the election. But PG&E seems to be forging ahead signing power purchase agreements with different renewable providers. The graph below shows the historical and  projected (until 2011, when PG&E's current policy on renewable purchases should complete) trend for percent renewable content in the northern California grid (data from PG&E's web site):

Gas on-demand hot water heaters have some other undesirable characteristics:

• Ignition is quite noisy and combustion is also not entirely quiet. The Takagi installation manual recommends not installing it next to a room used for sleeping or meditation. In contrast, the electric on demand heater makes a clicking noise when the relay goes on to start it, but is otherwise quite quiet. The most convenient place to install the backup heater is where our current gas-fired tank heater is located, which is across the hall from our bedroom. If we were to get a gas  on demand heater, we would probably have to pay to have another space modified for it.
• Complex flue and venting is required to support combustion. The T-H2, for example, uses PVC venting because the condensing unit required for high efficiency generates combustion byproduct air that is considerably cooler than is the case  for less efficient gas appliances such as our hydronic boiler, and the byproduct air would condense out on the sides of the metal flue that vents our hydronic boiler if it were vented through that, causing it to rust. In addition, air  must be vented into the area where the water heater is located for combustion, and it must come from an unihabited space or outside. An electric heater requires no such complex venting.
• Gas on demand  heaters require 3/4" to 1" gas piping due to the huge draw. Our house has 1/2" from where the main enters the house to the combustion closet. This would require another $4-5K to replace the gas piping. The electric heater requires a 220 line, which will likely cost a fraction of what the gas piping would cost to install.

Based on considerations of cost and convenience of installation, the electric heater wins hands down.

Considering that we will only be using the backup  heater for a small fraction of our water heating in winter, except for December, January, and February, and that the greening of the grid (if it happens on schedule) should result in a break-even carbon footprint  sometime roughly around 2013, we've decided to go with electric.

What's Behind the Walls?

Ever wonder what was behind your walls? If you've ever had any remodeling work done, you know that sometimes you can be in for a surprise. Our walls have been off for a couple weeks now,  and  I did a photo expedition into the curtained off part of the house. There were some interesting findings.

The first one was this:

and it certainly came as a surprise. What you see is one of the biggest No-Nos in remodeling: a large hole cut through a load bearing header. The previous owner installed a hall half-bath, and he cut the hole to run the toilet vent through. In California, with our out-sized earthquake activity, this can lead to a major structural failure during a severe earthquake.

The result is some extra expense and time needed to fix the problem. It should be fixable by running a metal strap across the header and another strap at an angle from the header to the top of the wall. But I've not had a full structural report yet, so I don't know precisely what the structural engineer will recommend.

Here's another one, that I sort of knew would be there:

The blackish stain you see around the ridge is mold. The house originally had a shake roof, but we had it replaced with a composite roof when we moved in . According to buildingsciences.com, a composite roof with fiberglass batt insulation requires a ventilation space under the roof decking so condensation doesn't form in the winter. Our contractor told us nothing about this, and I didn't really know enough about construction  at that time to ask (I know more now, and I don't trust contractors to know anymore either). So there are areas on the roof, like this one, that have some mold on them. Not a lot of mold, but still some. The ridge is particularly bad since it is the highest point in the house, so all the hot air collected there and the water condensed out as the heat radiated out through the ridge (increasing our heating carbon footprint). We of course need to get the mold treated, so more time and expense.

Unlike fiberglass batt, closed cell foam does not let air through so it can be installed directly in contact with the roof decking. Closed cell is not only more energy efficient than fiberglass batt, but it also is much less complicated than having to install vents at intervals along the roof, like some of our neighbors have.

Portugal and China

The New York Times today had an article about Portugal's conversion to  green  energy. About 45% of the country's electricity comes from renewable sources, including large hydro. Several years ago, a new government came into power with a large majority and simply decided to convert, since the country was importing the fossil fuel for electricity generation. Contrast that with the totally abysmal performance in this country since the 2008 election, and you can see the advantages of parliamentary government. The new system required a new kind of grid in which dispatchers act kind of like air traffic controllers, directing power from areas where the wind is blowing strongly to areas where the power is needed. The number of grid  dispatchers - a well paying job - approximately doubled.

The Chinese, too, seem to be pushing strongly on policy to improve the carbon efficiency of their economy. Will it take the US another 20 years to finally wake up?

Thermal Imaging and Insulation

Around two years ago, we had a thermal imaging study done of our exterior walls and ceiling of our house. In a thermal imaging study, the walls are photographed using an infrared camera. Areas that are colder show up dark blue to purple, areas that are warmer show up light red to white. The study also involves a blower door test, where the house is otherwise sealed and a blower is installed on one door to exhaust air out of the house. Infrared photos are then made of areas where there might be air leaks, for example around light fixtures, plugs, etc. The study showed that our house had average insulation compared to other houses, with several areas where the insulation had failed or was not properly installed resulting in thermal holes. There were also numerous places where the blower door test showed extensive air leaks. "Average" sounds fine, except the baseline is very low. Most houses built before 1970 in California have little to no insulation. After all, the climate here is mild compared to the Midwest and East Coast. Of course, we get subfreezing temperatures in winter at night and 100 degree temperatures in summer periodically, but back in the 50's and 60's, energy was cheap and people who moved from the East Coast were happy with running the furnace.

I subsequently built a spreadsheet model of our house and calculated that we could get about a 30% improvement by doubling the insulating power (R-value) of the insulation  using closed cell foam. That prompted us to move forward with the current plans to seal up the thermal envelope of the house in order to reduce natural gas usage for heating. With the exception of a couple areas, the drywall and insulation are now off the inside of the house. The temperature inside is about twice as warm during the day as beforehand (and maybe twice as low at night). Just as for the solar hot water tank I measured last fall (reported on here), insulation does seem to make a real difference in temperature control. This is good news, since our primary effort on this job is to substantially increase the insulation.

Here's some pictures of the inside of our house without drywall. Here you can see the pellet stove and the fireplace without insulation around it:

The fireplace area was one of the worst areas in the thermal imaging study. There was essentially no insulation around the fireplace. This thermal image shows the area above the fireplace:

The colored bar on the side shows colors corresponding to temperatures. Further toward purple is colder,  further toward red is warmer. The horizontal  light green to yellow lines are the studs you see in the picture above with the drywall off. They are warmer than the cavities, which are the turquoise areas between the lines. Typically, in a well insulated wall, the cavities are warmer than the studs because the insulation material filling the cavities transfers heat much less efficiently than the studs.

Here is what the west wall of the family room looks like with the drywall off:

This shows the wall between the sunroom (protected from construction dirt by the plastic curtain) and the side door.

In the thermal image below, you can see that the insulation in the stud bay next to the door was either omitted or was so poorly installed that it collapsed:

In the thermal image above, you can see that the insulation along the header at the top and the stud in the middle has sagged away from the stud.

The thermal images point out a major problem with the most common form of insulation used in the US: fiberglass batting. Batting is very difficult to install correctly and even when installed, has a tendency to sag with age. Any areas where the batting detaches from the studs results in thermal holes or air leaks. In California, the likelihood that batting will sag is relatively high because of the high amount of ground movement (i.e. earthquakes). Even small movements can result in tears to the paper surrounding the fiberglass, eventually causing the paper to fail and the insulation to sag. Tightly packing the stud bays with batting, blown fiberglass or, even better, cellulose is a superior way to get the same amount of insulating power (R-3 per inch). Closed cell foam is yet better because it seals air leaks tightly and has twice the insulation power (R-6 per inch) but is more expensive. Open cell foam provides the same tight air seal but has the same insulating power as blown fiberglass or cellulose.

Air leaks cause even more heat loss than simple conductive transfer through the walls. In the picture below, you can see some insulation peeking out from under the drywall that was not removed from the area in front of the upstairs bathroom:

 In the thermal image below done with the blower on, you can see the air being pulled in around the light on the ceiling right outside the bathroom where the insulation above is at:

Notice how the pink fiberglass is discolored by black dirt? That is from 30 years of air flow entering and leaving the  house in winter when the heat is on (and taking the heat energy with it). The fiberglass acts as a filter, removing all the dirt from the air. While it is nice to have fresh, filtered air inside the house, pulling it randomly through the ceiling and walls doesn't seem the right way to do it.

Our plans for the system remodel are to seal the thermal envelope and instead ventilate the house through the heat recovery ventilation system. This will pull fresh air into the house and exhaust stale air  out in a controlled manner, through a heat exchanger which transfers heat from the inside air to the outside, so the heat doesn't get lost. It also filters the air so that the black dirt you see above doesn't end up on random inside parts of the wall, but rather on a removable filter that can be washed periodically.