Noise from Attic
The first time I heard loud noise from attic direction was on an independence day, I thought that it was the sound of firecrackers! Continuing banging or popping sound from attic after showers made me to investigate the source of the noise. It was the (gas fired) water heaters in our attic!
A literature search quickly revealed the cause of the noise:
1) No maintenance of the water heater for long time leads to the buildup of hard water sediment, or mineral scales (primarily composed of calcium and magnesium carbonate), forming a hard skin on the bottom of the heater, interfering with heat transfer.
2) After extensive use of hot water, e.g. after the whole family had showers in a short time span, cold water filled the tank, the water heater starts to heat the water tank. For a gas fired water heater, the gas fire heat the water tank from below. Due to the poor heat transfer of the scale, the water between the scale and the bottom of the tank heats up much faster than water above the scale, leading to cavitation - explosion of water bubble - thus the banging or popping sound.
This loud noise is not only a nuance, it is also an indication of upcoming more serious problems - higher gas bill due to poor heat transfer, tank leak due to the mini explosions (which cause the popping sound) that weakens the bottom of water heater. On rare occasion, the water tank can become a "rocket" during its failure process.
One way to eliminate the scales, and thus the noise from the water heater, is to flush the water tanks regularly. We flushed our water tanks several times with limited success at reducing the nosies - though we actually saw lot sediments flushed out.
Note: tank water heater manufactures typically recommend be drained, flushed and cleaned regularly (e.g every 6 months) to remove sediment which may build-up during operation.
Tankless Water Heater
Due to limited success from flushing water tanks, we decided to replace the water tanks instead of hiring professional plumbers to clean them. So the questions became: tank or tankless water heater?
We went out to seeks bids from several plumbing services for both tankless and tank water heaters in summer of 2009. Here were two quotes we got: (tank (2x50 Gallon), tankless (for 4 showers)
Both plumbers provided quotes on the exactly the same brand tanks (American) and tankless (Rinnai) water heaters. Tankless water heater is much more expensive than tank water heater - 50% more (compared to two water tanks - our case), but the no worry about leak was a big plus for us. Plus, with the huge TAX credit for energy efficient appliances at the time (2009-2010), the final expense for the tankless water heater would be about the same as the tanks.
Plumber 2 had clear price advantage over plumber 1, but we were hesitant to buy Rinnai from him. The reasons were: plumber 1 claimed a need for gas upgrade for our installations as well as the need to acquire a permit from city; plumber 2 said that there were no need for either. We wondered if plumber 2 was eager to sell and thus omit necessary work. I did additional research on gas pipe sizing, and went back to ask plumber 1 specifically on the two issues. I was assured that there was no need for gas upgrade, and acquiring permit or not was up to us.
We had not made up our mind yet on the installation of Rinnai from plumber 2 at the end of the year. Then we got an email from the plumber offering a huge promotional discount for Rinnai tankless water heater, the price dropped from $3100 to $2500 for a December installation! Adding his reassurance of no need of gas upgrade for proper operation and purchasing installation permit for us, we had the tankless water heater installed on December 28, 2009.
Note: it turned out that city inspection was superficial. A city inspector came to our house after installation. He made a quick visual inspection of the water heater and vent in less than 2 minutes. The quality of the installation is essentially dependent of the plumber's craftsmanship and homeowner's diligence in checking on the installation.
Tankless Needs Maintenance as well
The Rinnai Tankless water heater worked as advertised - quick supply of hot water, no more noise from attic, no more worry of tank leak. After a year's use, I compared 3 month gas bills between summer 2010 and 2009, the gas usage in the three months (6,7,8) in 2010 was about half or less of that in 2009 - validated the manufactures claim of 40% more energy efficiency. We had more gains in efficiency because of heavy scale buildup in our old water tanks.
We were told at the installation, that the water heater needed an annual flush to maintain its efficiency. So we contacted the plumber for the annual maintenance in the last week of 2010. His employee came to flush the tankless unit on January 3, 2011. It took 45 minutes to flush the unit with descaler solution (vinegar), and then 15 - 20 minutes to flush the unit with water. The price for the flush was $199! Expensive!
With foresight, I observed the whole maintenance process, asked the service technician many questions related to the flush. I also wrote down the name/brand of the kit he used "Whitlam Flow-Aide System Descaler Kit". The detailed descaling procedure is available at the manufacturer's website. The kit costs about $150, the solution $20.
I will buy a descaler kit, flush the tankless annually and try to keep it running efficiently for a long time.