i would probably go with a POU at the bathroom and a whole house to feed everythign else. but put the POU inline with the existing hot water line. this way it only has to heat until the hot water from the whole house unit makes it there.

that is my plan when we replace our electric tank heater. i may even use tankless to supplement a solar WH, i still have a bunch more research to do on that one though.

Hi,

This is probably not what you want to hear, but I don't think that electric tankless heaters offer much saving over a modern electric tank type water heater.
A good quality and up to date tank type electric heater will have an energy factor of around 0.95 -- this means that only 5% of the supply energy is wasted, and that the maximum saving you could get with a tankless would be 5%.

On gas water heaters, the saving is much larger because tank style gas heater have high losses from the flue that goes up the middle of them --typically the energy factor for a tank style gas heater is about 0.65 and for a tankless gas heater the energy factor is about 0.85 -- so there is a pretty good potential saving on the gas side.

Have you thought about a gas tankless heater?
This would save you money (gas is a cheaper fuel), and it would also reduce greenhouse gas emissions.
CO2 emissions for electric water heating is high because so much of our electricity comes from coal fired electric power plants.
Gary

Thanks for your responses. A gas water heater is unfortunately not an option, mainly because we have no gas line to our house or even on our street. Also, in our area, natural gas is more expensive than electric, and continues to go up (not that I am complaining since we are all-electric).

I think for our purposes, the point-of-use unit will be fine. We are very interested in conserving water, especially since we are currently in an exceptional drought, and I don't like running cold water down the drain waiting on the hot to arrive. So, it sounds like a point-of-use heater for those problem places in the house is what we want, since a central tankless system will not eliminate the problem of cold water in the pipes.

Actually, I have a follow-up question: I have heard several places that electric tankless water heaters don't offer any energy savings over electric tank heaters, and I don't really understand this. Assuming that both units had identical energy efficiency ratings, that rating provides information about how efficient the device is when performing a certain amount of work, such as heating a gallon of water for example. Whereas a tank water heater operates fairly continuously to maintain a certain water temperature, the tankless only heats the water once, when it is used. The tank heater may reheat the same water several times if the unit is standing idle (such as when you are at work all day). So even though both may use the same amount of electricity to heat a gallon of water, the tankless unit will only heat it once, whereas the tank unit may reheat it several times (though hopefully from a warmer starting point each time!).

So why heat the water all day if you aren't even there? Wouldn't a tankless heater be somewhat equivalent to setting your thermostat higher (in summer) while you are gone from the house?

Hi,
I think all that you say is true about the standby losses, but I believe what has happened is that good electric tanks are insulated well enough that the heat losses are low.
My understanding is that the energy factor (basically an efficiency) that is shown on the tag on new water heaters includes realistic use with appropriate standby losses, and on good electrics it up around 0.95 -- so they are not losing much heat.

I do think that your point about point of use heaters giving instant hot water and saving water is a good one. This also saves energy as you don't lose the heat in the pipe line after each use.

Gary

Solarguy is correct, the energy factor takes into account the actual use of the heater and the standby heat loss. An electric tankless has an energy factor of .98 or .99. An electric tank model has an energy factor of .95 or so. It amounts to about a 3% savings.

There is another issue with the whole house electric tankless that has to do with low flow rates and large electrical requirements that make whole house tankless not very practical.

Instead, I think you should consider a well insulated electric tank unit and a recirculating pump system on the line furthest from the heater. All of the hot water pipes should be insulated.

You would install the small pump, an insulated return line and a motion detector to activate the pump. The pump should also have a thermostat on the pipe.

When you go into the master bath, for example, the recirculating pump turns on until the water is hot in the pipe at which point the thermostat turns it off.

If you don't actually use the hot water, there would be a little stand-by loss.

A recirculating system is a common way to deal with this problem. The motion detector reduces heat loss when there's no one to use the water.

Our Plumbing Apprenticeship class has been working with the local gas company on a project for two years to study what the actual standby loss is from a gas water heater per month.

Twelve different 60gallon water heaters were all set up side by side with a common header for cold water supply and one hose bibb on the output side of the tanks. Each water heater has a separate gas meter. All twelve water heaters were filled, the the thermostats set to the code maximum of 125degF and left standing with absolutely no load coming off the heaters.

At the end of each month the gas meter is read and the cost of operation is computed at the local residential gas service rate.

In mid summer when the ambient air is warmer the average is $4.44/mo and the average in mid winter when the ambient temps drop to about 45deg at the test site the average is $5.31/mo.

The overall average per anum worked out to $5.02/mo.

Now considering that the electric water heater has an even better insulation and no flue losses it would stand that the standby loss from a modern electric water heater would be almost negligible.

The final conclusion was the the standby loss from the water heater is equal to about the price of one cup of coffee from Starbucks, hardly worth the cost of any high tech bells and whistles to try to reduce it.

Hi,
Thats and interesting experiment.

Can you tell us what the gas cost per therm or 100CCF is so that the actual energy use can be calculated?

If there is is a website or report that describes the test setup and results, that would also be of great interest.

I guess that $5 a month or $60 a year is not huge, but it seems like a worthwhile saving to me. You could invest $500 to save $60 a year and be doing better than the the stock market does while saving about a 1000 lbs of CO2 emissions a year.

Gary