My comments to others coments.
40 gallons enough? Items that affect hot water usage and volume- Is there a dip tube.
- Is the hot and cold pipes connected properly
- Baths instead of showers
- This depends on the life style of the occupants a. Length of showers b. Is hot water running while brushing teeth
- Number of occupants
- Hot or warm laundry
- Recovery rate of the water heater
- Are the shower heads 1.6 gpm or 2.5 gpm or much higher
- The distance of piping between the water heater and the usage points
- Are the pipes fully insulated
- The temperature of the water heater
- Amount of sediment in the tank
Any time the occupants presently run out of hot water and a new system in installed and there is plenty of hot water the amount of gallons will then go up. As for the increase of cost, this depends on the amount of increase of hot water in gallons and the efficiency increase of the new unit.
Tankless water heaters: Generally the only information on efficiency is verbal from the salesperson. The efficiency of water heaters is in AFUE and most tankless units are not rated in this manner. I agree there is no standing loss due to the lack of a tank. The efficiency on large usages can be ok the efficiency on the small usages can be very poor. This would only make it a fair choice.
There are only a few that are above 90% or what we call condensing units. The units that are designed to keep a constant output temperature are expensive and complicated. And hope no repairs are needed. If the water in your area is full of sediments they need regular cleaning. They should have a sediment filter to keep particles from clogging their filter. If one wants a on-demand circulating pump they will not work with tankless units. They have a minimal flow rate to turn on.
Contrary to belief they do not get water to the faucet faster, actually a small amount longer. On has the added wait of the burner to ignite and warm up the heat exchanger.
They are expensive for the good ones. They normally need a larger gas line and in some homes this is real expensive to install. The stainless vent pipe is expensive.
And to think I used to recommend these units.
I would recommend looking at a unit such as a Phoenix ( there are other companies). This type of unit is 90%+ AFUE. They have a high recovery rate. They do require a large gas line and are expensive. The up side is they can reduce the cost of heating water about
30-40%. In most parts of the world where the heating load is not to big they can also heat the home.The older style of tankless I used to have would shut off on the low pressure of our water pump, ever take a shower from Michigan well water in the winter? I would hope the newer units are better.
Q: What happens when half a tank is used up? Does ice cold water rush in and cool everything down? Is it thus better to gang two together somehow to have the 2nd take over when the first is refilling? A: Ice water will not rush in. The way the two tanks in series works is. The cold water is plumbed into the cold inlet of the first water heater. The hot outlet from the first water heater is plumbed into the cold of the second water heater. The hot outlet of the second goes to the hot water to the house. When the faucet is turned on the cold water immediately starts flowing into the first tank and hot water from the first tank goes to the second tank. As the first tank cools down the burner (assuming gas) turns on and heats the water in the first tank. If the rate of water used is higher than the first tank can heat then cooler water enters the second tank. The burner of the second tank will turn on once nit is cooled past its set point. If enough water is run at a rate high enough then the outlet temperature will decrease from the second tank
Q: Cold water sinks so it pushes the hot water up, although you will get some "warm" water before cold as the hot water runs out.
A: Cold water will sink but... In a water heater there is a dip tube and water is directed to the bottom of the tank when entering.
Q: They say tankless is 80% more efficient than an electric tank. That one is more expensive, but it recoupes in cost in two to three years.
A; are we comparing electric to gas? If not and your are going to put a tankless electric water heater in that will supply enough flow rate... wait till you get a price on a electric service and wiring to handle this type of unit (OUCH).
Q: Spare us the tankless marketing bullship. Standard water heaters DO NOT run constantly. They are well insulated and have a large thermal mass of water inside.
A: I would question well insulated. The temperature difference is greater from the inside of a water heater to the space it is installed in. We put R-30, 38, 50 in our attics and maybe R-15-20 in water heaters. Has anyone ever looked at the vent pipe temperature when the unit is not running (nice and warm). Or the temperature of the hot and cold pipe when no water is being used. And guess what there is almost nothing that meets code we can insulate the most important two pieces of pipes in the DHW system. Code requires 3 inches minimal for combustible for standard vent and fiberglass and foam will not meet this requirement.
The amount of thermal mass has nothing to do with the loss from the tank. Heat loss by conduction is Q=U * A * Delta T * H (Q is the amount of BTU's, U = 1/R-value, A = Area, Delta T= temperature difference, H = hours). Heat loss by radiation is determined by the surface temperature, texture and emissivity. Convection, is based on the surcace temperature, the temperature of the air around the tank and the amount of the insulating value of the air film against the tank.
Q: Oil fired tankless is the way to go. All the hot water you want, never run out and it stops running just as soon as you cut off the faucet. I know because I had one.
A: I do not know the cost of gas vs. oil in your area. Assuming oil at $3.00 per gallon and gas at $2.00 per therm (100,000btu) and both units at 90% AFUE. The cost per million BTU's for oil is $31.97 and for gas $22.22. I would think gas is less than $2.00 per therm, I've never seen it that high, Oil (anyone know how much per gallon). And Oil is unlikely 90% efficient.
What kind of oil unit do you have that shuts off when the faucet is shut off, sounds like a tankless unit, I was not aware of such type of unit.
Q: and the tankless boys can stick them. I like HOT water. Not lukewarm water. A: A properly sized tankless water heater can give you scalding water.
I try to deal in facts AndyEnergy Residential energy consultant