OK, I’m the first to admit that I know next to nothing about water heaters. Aren’t they those white cylinders that live in garages, as far as possible from the kitchen and the shower? Well… yes and no. In our Title 24 work, which is architect-designed custom single-family projects, the water heater is usually the last thing on anyone’s mind. However, on many of our analysis projects, the quickest, cheapest way to comply with California’s stringent energy-efficiency requirements has been to upgrade the water heater – and sometimes, to include a solar hot water credit.
This article explains how to assess water heater efficiency numbers, including the use of a handy lookup database at the California Energy Center’s web site.
Why’s water heating so important all of a sudden?
Well… if if a project is behind on heating AND cooling, and it’s already maxed out every other trade-off measure, it can sometimes be salvaged by running it with a more efficient water heater, specifically, a tankless. You’ll really feel the pinch if your project needs to exceed Title 24 by 15% or more. This is a basic requirement for almost all of the “beyond compliance” green standards: GreenPoint Rated, CALGreen optional tiers, LEED for Homes, and incentive programs such as the New Solar Homes Partnership. Local California jurisdictions are adopting various forms of these optional tiers, in some cases requiring very high GreenPoint Rated scores for larger homes.
“The equipment you see with values above about 90% are condensing equipment that have great efficiency values but need different installation practices and cost a bunch more,” warns Ken Nittler, creator of the Micropas Title 24 energy modeling software. “The most common equipment we see are small storage water heaters with energy factors of 0.60 or 0.62. On tankless, we see energy factors of 0.82 or so. On tankless condensing units, we see 0.94 energy factors.”
General Compliance Notes
Radiant Heating. Some basic questions that affect water heaters in Title 24 have to do with the heating system: specifically, whether radiant heat is under consideration. When you run a Title 24 calc with radiant heat as the specified heating type, the water heater drives both the Heating and the Water Heating portions of the score. The Heat score is sensitive to the water heater’s Recovery Efficiency, while Water Heating is associated with the water heater’s Energy Factor.
Solar Hot Water. There’s a very substantial credit for solar hot water in Title 24. You’ll need to know the “insolation fraction” which is the portion of total hot water that you expect to receive from solar thermal. A realistic number may be 25-50%, but that depends on how much sunshine the house will actually receive based on weather, site configuration, surrounding buildings and landforms, house footprint and available space for solar thermal panels, shade trees, and the like. Plan on having a supplemental unit such as an indirect storage tank or an additional water heater, since you’ll need hot water at night as well as during the day.
Radiant + Solar Hot Water Combined. If you’re going for radiant and/or solar hot water, you’ll need a mechanical consultant with strong knowledge of the differences among different manufacturers and unit types. For one thing, radiant heating water temperatures may be different from what you’d need for domestic hot water, so if you’re running them off the same unit or the same tank, specialized installation or components may be needed.
No Electric Water Heaters, Please! Fuel source, such as whether the water heater is gas-fired or electric, has a big impact on Title 24 compliance. Even for all-solar homes. There’s a huge penalty for electric resistance – that’s all you need to know for today’s discussion.
What’s the Highest Efficiency Realistically? The technology inside the water heater, such as whether it’s a two-stage condensing system or a hybrid, can have an indirect impact on Title 24 compliance. Sometimes if something’s not passing I’ll just test some of these numbers to see what happens, but eventually the numbers have to be realistic. This is where I asked around a bit to make sure that a water heater with a .95 efficiency was a real animal. It is – for a tankless. Condensing boilers also can have efficiencies as high as .95. There may also be condensing storage water heaters available, probably at a high premium.
Installation Issues. Do yourself a favor and check with your builder and subs before specifying some of the high-efficiency condensing water heaters. “The equipment you see with values above about 90% are condensing equipment that have great efficiency values but need different installation practices and cost a bunch more,” notes Ken Nittler.
Types of Water Heaters
Water Heater vs. Boiler. What’s the difference between a water heater and a boiler?
- Water heaters cost less and operate at a lower temperature. The “small” version, more typically used for small to medium sized residential dwellings, has an input under 75,000 Btu/hour. Large water heaters have an input that is over 75,000 Btu/hour.
- Boilers make scalding hot water, and more of it, typically around 200 degrees Fahrenheit. That’s why it’s called a “boiler”. A boiler is primarily intended for space heating (radiators), rather than making hot water for daily use. However, boilers are less adaptable to making hot water at lower temperatures at the 120 degrees for showers or 120-135 degrees for radiant flooring.
- They use different efficiency measures, and this last point is what’s crucial for Title 24 compliance.
Indirect Storage Tank. This is a separate, insulated storage tank with a heating element that can be used to collect and store hot water from an intermittent source such as solar thermal. It doesn’t actually heat the water up, it just keeps it hot after it’s already warmed by some other means.
Energy Compliance Calculations – Inputs Needed
Title 24 Defaults. About 90% of the time, we run our Title 24 projects using the software program’s built-in default, which is a small storage water heater with an energy factor of .60. This is a safely conservative assumption, since you can only go upwards from here. Within the Title 24 software, it’s the efficiency that seems to matter, rather than the number of units installed. In reality, however, larger homes are likely to have a boiler, or perhaps several different units at different locations.
Efficiency Measures. Although the array of water heating technologies seem to be almost endless, the only thing that matters for energy compliance are these efficiency ratings, and they’re different depending on whether it’s a water heater or a boiler, whether it’s hooked up to a radiant heating system, and for water heaters the Btu input. There’s some confusion of terms here; I’m sticking to the terms as they are used in the Micropas software inputs.
Energy Factor. Used for “small” water heaters under 75,000 Btu/hour. The energy factor (EF) indicates a water heater’s overall energy efficiency based on the amount of hot water produced per unit of fuel consumed over a typical day. It’s a sort of umbrella measure that includes the recovery efficiency and standby loss measures described below, and is expressed as a decimal fraction between 0 and 1. An EF of .60 is average, while one of .80 will yield noticeable improvement in a project’s Title 24 compliance score.
Recovery Efficiency (sometimes called Thermal Efficiency, or AFUE, although they’re not exactly the same thing). Recovery efficiency is for “large” water heaters and boilers that are 75,000 Btu/hour or above. This is a measure of how effectively the unit turns fuel into heat. It is measured as a fraction between 0 and 1, higher being better. This seems to affect compliance score when radiant or hydronic heat is the main heating system.
Miles Hancock says, “Thermal efficiency is a useless number, measuring the unit’s efficiency at boiling water… what’s key is keeping that water hot, which is the standby loss. Standby loss is what kills the energy compliance.”
Standby Loss. Used only for “large” water heaters with a storage tank, this is the percentage of heat loss per hour from the stored water compared to the heat content of the water. In other words, once the water’s hot, how long does it STAY hot? This is measured as a fraction as well, in this case lower being better. The software built-in for the sample large water heater shows a standby loss of 0.03. Miles
Internal and External Insulation R-Value. Used for indirect storage tanks only. I haven’t tested the effect that this would have on residential compliance, because indirect storage tanks aren’t as common a configuration for the custom home projects that we analyze.
Other Inputs. There are additional inputs for heating element type (gas, electric, heat pump), rated input for gas heaters in Btu/hour, storage tank volume in gallons, distribution type, and for some types of water heaters, pilot light size in Btu/hour. For almost all our projects, the fuel type is always gas, since electric water heaters are discouraged.
The distribution type has to do with the piping configuration, and that’s mainly for larger residential projects. Example values are “standard” (the default), “Point of use”, or various recirculating on-demand varieties. I’ve occasionally run projects with these different distribution types to see what difference it made, and for a typical 4,000 SF single-family residential house, it made almost no difference in the compliance score. Sometimes it can make a big difference in the real world, though, so if you or your client are trying to maximize actual energy efficiency, as opposed to just complying with Title 24, try reading up on it in Ann Edminster’s book “Energy Free”.
Heater/Boiler Systems in the Title 24 Software Model. This is based on Micropas, which is one of two programs you can use for California energy compliance. I haven’t used Energy Pro as much, and it’s organized a bit differently. Here are a few basic types of heater/boiler systems that are set up as default or example entries in Micropas (and why you would pick one or the other):
- Small Storage. 40,000 Btu, 50 gallons, 0.60 EF. Use for legacy systems or if you don’t know what else to specify, and you want to assume the lowest allowable efficiency for the time being.
- Large Storage. 100,000 Btu, 100 gallons, 0.77 RE, Standby loss 0.03. Realistically this would be used for a larger home. However, I have noticed that specifying a large storage as opposed to small one lowers the compliance rating, probably because of the higher input rating.
- Tankless (instant). 195,000 Btu, EF .80, RE .76. This is the equivalent to a small storage water heater, and would be used in comparable applications such as a single family home as a more energy-efficient alternative. Compliance advantage is that it’ll boost the water heating portion of the score by as much as 30%, and it’s often a good choice in the real world as well. Real-world advantages: higher efficiency, theoretically endless hot water supply, smaller footprint. Real-world disadvantages: must be located close to point of delivery in order to realize energy savings.
- Boiler. 250,000 Btu, RE .80.
- Instantaneous boiler. 250,000 Btu, RE .80. Same as a boiler, but without a storage tank.
- Electric Storage. This is an electric resistance water heater. You will never, ever, want to use this type when using the performance method of compliance. Occasionally, if you’re trying to grandfather in some unpermitted basement addition, guest cottage, or in-law unit, and the space qualifies for prescriptive (not too much glazing!) you might be able to get away with stating “existing systems to remain” and bypass the issue. But that’s about it.
- Electric Heat Pump. I’ve never had to model this type of water heater, so I can’t speak to it. I don’t think the penalty is as steep for this type as for the electric storage.
Hot Water for Additions. If we run an addition as “Addition Alone”, water heating is not calculated. However, if the addition won’t pass by itself and we have to model the whole house, then the water heater becomes a potential source of energy trade-off. It’s even more important for additions and partial remodels, because other interventions such as upping all the wall insulation may not be available with projects of limited scope.
Pools and Spas. These are not included in the energy report calculations. There are separate energy standards that apply to them. For now, they are outside the scope of this article.
A Nifty Search Lookup for Current Models
Miles Hancock, one of our Green Compliance Plus Affiliates, walked me through the Appliance Efficiency search database on the California Energy Center’s web site. This database contains listings for all appliances certified to the California Energy Commission as meeting currently applicable efficiency standards – so it won’t be of much use for legacy systems (for remodels or additions). Some of the terms they use for efficiency are different from what’s described in the preceding article, but I’ve tried to list their equivalents above, where possible.
About the author
Rebecca Firestone has been working in the Bay Area since 1998 as a technical writer, business content developer, architectural filing lady, marketing director, and sorcerer’s apprentice.