Thermostat + Timer

I guess I'm missing something here? Doesn't the water heater already have a thermostat? All you need to do is apply power. When the water heats up, the thermostat opens. And if it doesn't, then the eventually the safety switch (or valve) will.

For setting the on/off window (within which the existing thermostat will operate), one off-the-shelf idea might be to use one of those fancy water sprinkler timers and a 240-volt high-current relay. Most give you 4 to 6 watering cycles per day, 7-day programming, etc.. Highly flexible.

That said, I do understand the need to "experiment". A PIC with the appropriate precautions (as noted above) should work nicely. Good luck.

No, but I've been meaning to. Ours is a manual (wall switch) operation now, and I've been meaning to put (at least) a real timer on it. Empirically, if we forget to hit the switch enough times during the month, the power bill is indeed higher.

The weather here is always pretty hot anyway (spring, summer and fall), and just very warm otherwise (winter). So, the hot water heater could almost be considered an optional accessory item. :)

It only takes about 10 minutes or so to get it up to temp enough for a shower. And the dishwasher has it's own internal heater (though it could probably benefit from even hotter water?)

The calculations must involve the frequency with which significant volumes of water will be taken from the tank. You'd also need to know the dt/dv over time. (Temperature vs. volume of water removed over time) to really make any sense of it. Cold water replenishing the system will be heated up when mixed with the tank contents. And if that resulting temperature over time is acceptable (i.e., you can still father children afterwards!), then no need to turn the tank on.

What would be nice is a Hot Water controller that knows the system capacity, and can monitor both the tank temperature and "remember" at what times, and in what volumes hot water will be required, and adjust it's ON/OFF time accordingly. In short, a hot water heater timer with some artificial intelligence.

A 10K@25 NTC and a pull up resistor on the 0-5v ADC of a pic will give you around 0.2c resolution. A good stable Vref would help too, Like a LM4040 instead of the supply voltage.

Cheers

It always costs more to maintain bulk water at the higher temperature - entropy sees to that. Losses scale with the temperature difference x surface area. The most cost effective solution if you are stuck with electric water heating are the flash boilers used in showers etc. They heat a small amount of water to temperature and deliver it as needed.

More insulation will help. The mirror finish bubble wrap is remarkably effective for a very thin layer ~ 1cm. The LCD thermometer strips and paint sold for checking thermal solutions are ideal for checking this.

Why not use a solar water heater then? At US latitudes they should be commonplace and available relatively cheaply.

It is madness to use electricity for resistive bulk water heating. Some places are using the waste heat from fuel cells for their water heating

- the Japanese Prime Ministers residence has such an installation. the hot water is a by-product of his electricity generation.

Good hot water tank designs try to stratify the water so you can use almost the entire tank volume of hot water before cold gets sucked up. This also improves the thermodynamics for the heater.

Regards, Martin Brown

Martin Riddle Inscribed thus:

Thats a useful bit of info. I have a number of 10k thermistors I could use. Thanks:

Yes, the math has been done many times by many people. Back when I was in high school, it was used as a math question. It is fairly easy to do the math if you assume that the losses from the water heater are linear with the amount its temperature is above room temperature.

That will work. If your schedule is fairly constant, you can use a programmable timer to do the job.

It's only cheaper if you don't factor in the cost of a new roof, or a whole new house! You can loosely translate that as "Hurricanes".

Martin is a clueless European. He has no idea what Florida is really like. Hail storms will destroy a solar water heater as well. Large areas have no natural gas, so that leaves propane, fuel oil or electricity as the only way to heat water.

My well was 100 feet from the house at my last home. The pipe was about 18 inches between the exposed 30 gallon tank and the house. Two people could take a hot shower in the late afternoon before you ran out of hot water. You had to let the 'cold' water run for a couple minutes to let it cool down enough to get it to a comfortable temperature.

The only time the water heater was turned on was during the few cold winter days.

The well here is in the shade most of the day, and the underground pipes are in real dirt instead of sugar sand. The water heater is on a manual switch that is turned on about 15 minutes before a shower, and turned off when you finish. That leaves enough hot water to wash the dishes in the morning, except on the coldest days. Of course there are more of those each year, thanks to 'Global Warming'. I use the AC less each year, and the heat more days.

The Japanese seem to manage OK with them although they call their "Hurricanes" as Typhoons the effect is pretty much the same.

Are you saying that US homes are so shoddily built that the additional load of a solar water heater on top would cause them to collapse in the wind of a hurricane ?

Can't do much about tornadoes most things fly apart in those.

Regards, Martin Brown

It's the wind force loading, not the weight that causes the problems. To meet local code, it's VERY expensive. And regardless, my hot water bills are no so high as to justify the risk of losing the whole house.

I wonder if Japan mandates solar use.

I don't remember seeing any Japanese houses fall down and we lived there through several powerful direct hit typhoons.

That is a problem. Even though the capital cost is relatively low the total cost of hot water is tiny compared to space heating/cooling.

I doubt it. This was long before AGW was even on the agenda. They like gadgets though and Japanese baths require copious hot water.

Regards, Martin Brown

Dude, respectfully, you are not listening. Capital cost is NOT low. No matter what you compare it to.

As for the "doubting Thomas" attitude... Shouldn't be too difficult for you to Google images of the thousands of homes destroyed by Hurricane Andrew.

Here's one to get you started.

I submit to you that the roofs in the photos are not substantially strengthed by the addition of a solar hot water heater.....

Think: Wing.

And not a prayer.

Sounds more like a 22V10 task to me. But i always prefer hardware to firm ware.

=46er crying out loud, we are talking about a water heater for domestic use. 2 degrees C is plenty good control.