Residential Water Heater Energy Requirements
Julius Ballanco, PE
I recently received a notice from my good friend John O'Reilly (former editor of our sister publication Supply House Times) regarding the introduction of the new Rheem Pronto water heaters. The Rheem Pronto units are instantaneous, tankless water heaters.
The statistics on the Pronto heaters are impressive. The small unit is 22 inches by 13 inches by 5 inches deep. It weighs all of 35 pounds. The small unit can deliver a 4.2-gpm (at 45 degrees F) rise of continuous hot water, while the larger unit can deliver 7.4 gpm. The units are self-modulating to save on the amount of gas used.
This is not a commercial for Rheem, although I am sure the new Prontos are fine instantaneous water heaters. What this shows is a shift by the traditional tank-type water heater manufacturers into the world of small instantaneous water heaters.
“What is going on?” you may be asking.
If you ask the corporate executives, they probably will tell you that they have evaluated the world of instantaneous water heaters and see the need for developing high-quality units that use less energy.
They can list all of the reasons for using an instantaneous water heater. You can probably list the reasons yourself: lower energy demand, no storage loss, no standing pilot light, continuous supply of hot water, smaller unit often wall-hung, etc.
These are all fine reasons, and it is probably why Rheem and other manufacturers are entering the instantaneous water heater market. But I believe their thought process was prompted by some wild ideas a few years ago by our federal government.
It wasn't too long ago that a few Department of Energy bureaucrats came up with the idea of attacking the energy demands of residential water heaters. One of the first thoughts was to ban electric storage-type water heaters. That's right. It was suggested we ban these water heaters since they used too much electric energy and had too high a standby energy loss.
Next on the list were other residential storage-type water heaters, namely 40-gallon, 50-gallon and 75-gallon water heaters. After catching their lunch on an attempt to ban electric storage-type water heaters, the feds modified their thought process regarding higher efficiency of gas-fired water heaters. Some of our energy wiz kids decided that water heaters with an efficiency below 70 percent were inadequate.
Furthermore, these storage tank-type water heaters heat up 40 or 50 gallons of water only to have it sit for hours before it is used. There is energy lost through the tank wall requiring more gas to raise the temperature to an acceptable level. What the wiz kids didn't realize, or forgot about, was that gas-fired, storage-type water heaters only have gas as their energy supply. There is no electrical connection to the water heater. Thus, the standing pilot light is extremely important.
Change In EfficiencyOn Jan. 20 of this year, new federal efficiency requirements kicked in for residential gas-fired water heaters. The DOE mandatory efficiency requirements increased from 0.54 EF to 0.59 EF. The rating is a calculated method of rating a water heater. The higher value equates to a water heater with an efficiency of approximately 70 percent.
The new rating is considered the highest rating for a standing pilot light water heater. Any higher efficiency will do away with standing pilot lights and add electrical requirements to gas-fired water heaters. It is doubtful that such a change will be considered in the future.
The DOE estimates that the new requirements will increase the cost of a 40-gallon water heater $58 on average. They estimate that the energy savings will be $12.74 per year.
I did a quick calculation of my household gas budget. This savings will impact my annual gas bill less than 0.5 percent. Not much of an impact considering any savings will be eaten up by any fuel surcharge or harsh winter cost adjustments. In other words, you'll never notice it.
Water Heater Changes: You will probably not notice any change in the water heater with the higher efficiency requirements. The manufacturer may change the type of insulation, modify the baffles, change the heat trap or reshape the tank. But, on the outside, it will probably look about the same. Hence, you will still be able to get the replacement water heater into the same space and through the existing door.
Most of the units already have been on the market, listed as high-efficiency water heaters. Expect to see new names and new literature on these water heaters.
The concern by some is that the overall impact has not been properly evaluated. When the furnace efficiency was modified, we had numerous vent system failures, some still waiting to happen. The question is, will the higher-efficiency water heaters require another adjustment in the venting requirements? To date, the indication is that no adjustment will be necessary.
There is also the question of whether we are “over-engineering” a simple product. In engineering school, they emphasize the problem of over-engineering, which really means not considering all of the impact of the changes.
Engineers typically hear the story of the World War II German rifle. It was a highly engineered rifle that was accurate up to 2 miles. The Russians had a loosely engineered rifle that was accurate to about 300 yards. When the Germans were fighting in the cold Russian climate, their highly engineered rifles seized up and would not fire. The Russians were still accurate to 300 yards and won the battle.
The German engineers did not take into account the contraction of the metal in the cold climate. Hence, the rifles were no good. Interesting story. I'm not sure how accurate it is, but it drives home the point to engineers to not overkill a simple design. Gas-fired water heaters are a simple design.
Instantaneous Still An OptionGetting back to the number of new instantaneous water heaters. Instantaneous water heaters typically have a higher efficiency than tank-type water heaters. They also have electrical connections. But, when considering their installation, you need to consider the difference between tank-type and instantaneous water heaters.
The flow rate on instantaneous heaters is normally based on a temperature rise of 45 degrees F. That means when the water is down to 50 degrees F in the winter months, the flow rate listed will provide water at a temperature of 95 degrees F. This is lower than a standard shower temperature.
The instantaneous water heater will provide a higher temperature of water; however, the flow rate goes down. Tank-type water heaters list water availability based on a 90-degree F temperature rise.
A lot depends on the amount of hot water you determine is necessary. If you calculate two showers being taken simultaneously plus a load in the washing machine, the hot water demand is between 7 gpm and 9 gpm for an instantaneous water heater. If you calculate a single shower with normal hand washing hot water use, the flow rate is approximately 3 gpm to 4 gpm. These values are based on the instantaneous water heater rates using a 45-degree F temperature rise.
Some whole house instantaneous water heaters can have an input rating as high as 200,000 Btu/hr. Hence, this is a major increase in the gas demand for the house. The gas piping will get larger and sometimes a larger gas meter will be required.
One of the other considerations when using an instantaneous water heater is for the installation of a storage tank to meet the higher demand times for use of hot water. Just don't tell the wiz kids at the DOE because they never entered a storage tank into their calculations of heat loss and energy efficiency.