Sizing
The first thing to realize is water heating is “sized” by first hour delivery at a certain degree of rise. The rise is the change in temperature required between the cold water inlet and the desired hot water outlet temperature. In cold climates the inlet temperature can be significantly lower than in warm climates, and the water heater has to be properly selected to handle the difference.
For example: A water heater being fed 80 ° water and heating it to 120° needs a 40° rise. Depending on make & model, a 50 gallon water heater will produce about 70 to 80 gallons of 120° water in the first hour. If you need a 90° rise from the same unit (e.g., 50° water heated to 140°), you will get about 40 gallons the first hour.
In southern California, I size water heaters to a nominal 60° rise for a year round average, figuring on a required rise of 50 to 60° with an output temperature of about 120° and a constant year round hot water usage.
BTU Rating
The BTU (British Thermal Units) rating refers to the amount of heat the burner puts out. Residential Tank type heaters have, at best, between 32,000 and 40,000 BTU. That BTU rating is also the approximate limit of the exhaust vent that is installed in most homes (3”), and the highest BTU I have seen in a residential unit.
My experience is a regular 40 gallon tank will work for 2 people if they don’t need to fill a large tub in the winter or supply for guests at anytime. 50 gallons is a more appropriate family size.
Rheem made a “50/50” that had 50 gallon tank and 50,000 BTU. It put out about 90 gallons the first hour. However, this unit was not approved for installation in California.
Smaller Tank Size/Higher BTU Water Heaters
Bradford White has really great water heaters in 20, 30, 40, and 50 gallon sizes with high BTU Ratings (about 100,000 BTU) which are approved for residential installations. The higher BTU rating requires a 4” vent (most existing houses have a 3” vent). For new construction the Bradford White is the absolute best for a tank type!
The smaller Bradford-White tank capacities and higher BTU rating produce water hotter than what is needed at the faucet. This smaller tank has a mixing valve factory installed at the outlet and mixes cold water with the heated water from the tank. Delivery is a consistent 120° ( a safe temperature), and because the hot water delivered to the faucet is only partially supplied by the hot water tank itself, the first hour capacity of the tank is greatly increased. (You’ll see that other articles on this page refer to this mixing valve as an ASSE 1017 mixing valve.)
I will refer to this setup as a cross between tankless and tank type water heaters. It’s really the best of both worlds. Some locations can’t provide the 200,000 BTU of gas required for a tankless water heater, so Bradford White unit is perfect because it needs about 100,000 BTU. Further, this Bradford White can be vented with regular type B venting. (More on venting shortly, but this is the most common venting in use.) Tankless water heaters need special direct venting, and that’s expensive.
While I am praising Bradford White, I will say the tank they make is very well glazed inside; the glass lining is second to none. “Hydrojet” is a self cleaning feature they have, and the new code requirement for FVIR (Flammable Vapor Ignition Resistant - having a sealed burner chamber that prevents igniting flammable vapors in the water heater area) is serviceable on Bradford White (but not by all manufacturers). The efficiency ratings on Bradford White tanks is tops.
Indirect Fired Water Heaters
Another option is a water heater with no burners at all, called an indirect fired water heater. This type of water heater has a heat exchanger coil inside that carries hot water from another source – such as the home’s boiler. In this example, the water heater acts as another zone on the home’s heating system and uses the water heating power of the home’s boiler to provide hot water. No venting is required for the water heater (since it has no burner), yet is retains the efficiency of gas, and the high BTU power of the home’s boiler. As an example, a 40 gallon Bradford White indirect fired water heater has a rating of 260 gallons of hot water the first hour, assuming a 157,000 BTU output from the heat source (but not all home boilers will have that much capacity, so the output would be accordingly less).
Venting
The proper venting is critical. Improper venting is life threatening.
There are a few basic types of venting used for residential water heaters. The proper type of venting depends, in part, on the efficiency of the appliance (water heater, furnace, or combination), and the pressure of the appliance’s exhaust gases, which is a characteristic of the appliance’s design.
The higher efficiency appliances produce a cooler exhaust. This sounds great, but the cooler, exhaust does not rise out of the vent as easily as hotter exhaust gasses. The cooler exhaust gasses also form condensation more easily, especially in cooler climates. The condensate is acidic and corrosive, and therefore requires the proper venting material to ensure that leaks don’t form and infiltrate the home. If the exhaust gasses vent into a masonry chimney, installation of high efficiency appliances may require lining the chimney, as the condensate can destroy the mortar between the bricks and lead to the collapse of the chimney.
Some systems have negative pressure exhaust systems, where the hot exhaust gasses are drawn out of the building by their own buoyancy; this is the most common tank type water heater design. Others (especially higher efficiency appliances) have positive pressure exhaust that must be pushed or drawn out with a fan, and must also be properly sealed. Some installations (though not typically water heaters) may also require a condensation drain in the exhaust system. The air-tightness and venting of the house can also affect exhaust venting requirements. This is to ensure that a negative pressure inside the house does not cause outside air to be drawn in through the exhaust vent resulting in the actual exhaust entering the home, a dangerous condition known as “backdrafting.”.
Direct venting is a term that is used for a gas water heater that has a sealed combustion system and gets its air for combustion thru a second passage in the vent system. Direct venting can be used for both tankless and tank-type water heaters.
Type B venting is double wall vent having an insulating effect and twist-lock or snap-lock connections. It is required by code for common gas burning appliances and has been for decades. Type B venting is useable on appliances with lower efficiencies and negative pressure exhaust systems. It is the most common gas water heater vent in use. Type B type venting is not useable for direct vent type or tankless water heaters.
Category 3 venting is for systems with a positive pressure exhaust, but operating at a temperature where condensation is not usually an issue. It is typically made from stainless steel. Noritz tankless water heaters utilize category 3 venting.
Category 4 venting is used for high efficiency–positive exhaust pressure heating systems, where condensation in the flue is most likely to occur.
PVC or CPVC may also be used for some installations with low temperature, positive pressure exhaust.
All venting systems use special connectors, flashings, and installation requirements. Some can be vented horizontally out the side of the building, some cannot. Each appliance has its own requirements and no one vent type works for them all. Check with the manufacturer, who will require the vent system that works properly with each unit, and do not vary from their requirements. It would not just be a code violation; it would also be dangerous, if not fatal. I don’t recommend DIY for most people in water heating, gas work, or venting. I do recommend understanding the difference in the systems so one can decide how to hire someone to get the work done properly.
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Spending money on “in tank” cleaners marketed to make your chores easier and decrease the time you spend scrubbing your toilet is one cost you can cut.
When you buy something cheap and it breaks, don’t expect to get it replaced with the latest and greatest product on the market.
The transfer of the water from the tank to the bowl can be impaired by mineral buildup in the flush ring and siphon jet. The slower the transfer the poorer the flush. The mineral build up and reduces the initial flow of water into the trap, resulting in a poor siphon. The resolution is to quickly poor about a quart white vinegar down the overflow tube (located in the toilet tank). Let the vinegar work for a minimum of about 2 hours (preferably overnight) before flushing. It may help to first remove the small 1/4 inch bowl fill tube that comes from the ballcock and to the overflow tube. The vinegar dissolves the calcium (hard water build up) in the flush ring and the siphon jet, allowing the toilet to once again work as designed. You may need to repeat the process several times. Annual treatment could be necessary. 
The bowl fill tube no longer discharges into the overflow tube. If the bowl fill tube does not empty into the overflow tube, the bowl will not refill after flush. Starting out with a low bowl water level will result in a poor flush. The resolution is to reconnect the bowl fill tube to the overflow with a clip, to keep the bowl fill tube in place. It's also important is to ensure the bowl fill tube does not extend too far down into the overflow tube (1/4 inch) or constant cycle of tank-siphon loss and refill could occur.
The chain from the handle to the flapper valve is not properly adjusted. If the chain is too long, the flapper valve does not lift up enough to move to the fully open position (where it normally floats until the tank water level drops enough to allow the flapper to close). If the chain is too short, the flapper valve may be prevented from fully closing or from opening enough. The resolution is to adjust the chain length so that the slack in the chain is 1 or two links (no more, no less). A loose flush handle can be the cause of inconsistent adjustment of the chain, resulting in the above problems, and is a common cause.
If your toilet is running, and the constant filling is on and off, then it could be one of several things:
The ballcock mechanism (the float in the tank) could be catching or hanging up on something.
You may be surprised to know that I don't even have a plunger among my plumbing tools! While this has been used by many people for many years, using it for a toilet is not a very good idea. 
