Interesting water heater application

Friends (in the US) have a mountain cabin with an electric water heater. When they leave the cabin, they drain the water heater to avoid potential freezing problems. When they return, they must allow the water heater to fill to a point where the water covers the heater coils before turning on the power to the water heater - otherwise they could burn out the heater element. They have had problems with guests who do not turn on the water first, or forget to close the drain, even if they do turn on the water.

The obvious solution would seem to be a water detector near the top of the tank. But, getting a wired detector into the tank, without leaks might be hard. The best approach seems to be a pressure sensor tapped into the output pipe. As water enters the tank, it will compress, and eventually displace the air, raising the pressure in the outlet pipe. The pressure sensor (a simple pressure operated switch would suffice) would connect to an I/O-Linc or EZIO. The water heater would be connected to a 230 volt insteon switch. The I/O-Linc could then directly control the heater or do so via Indigo. A side benefit would be automatic shutdown when the tank was drained (or ruptured).

So... Does this idea sound reasonable? If not, I'd love to hear other thoughts. If so, any ideas where to find such a pressure sensor at a reasonable (I.e. less that the cost of a Mac-Mini) price?

EDIT: Duh, I was thinking electronic and totally forgot about standard pump equipment like this.

Still, what other ideas do you folks have?

Nuclear reactors have a similar basic problem -- if you lose water pressure, the water inside the core will boil, which means that there will be hot fuel rods with no coolant contacting them, which means you are in trouble. The mechanism you suggested (pressure sensor) is precisely the mechanism used to trigger a scram and preserve the reactor in the event of a primary coolant leak.

After some Googling, I wasn't able to find a part that I thought would serve the purpose better than the one you linked to.

So I guess this isn't a terribly useful post.

You could also add an electric valve on the inlet, and one on the drain. That way you could control the entire process coming and going.

Departing:

1) Turn off power
2) close inlet
3) Open drain
4) disable pressure trigger

5) bonus points
turn on anti-freeze wrap around piping. I use this in my pump house at my cabin.

Arriving:

1) close drain
2) enable pressure trigger
3) open inlet

Pressure trigger:
1) turn on power

Seems like we already went here with the aquarium filler question... I would need to see the tank to understand exactly what would work but I would suggest a side-mounted level switch. One of these:

http://www.omega.com/toc_asp/subsection ... book=Green

Make sure it's rated for the temperature. Some of these can be installed by drilling and tapping even when you only have access from the outside of the tank.

I don't see any reason to bring an IOLINC into the picture. That is just adding a lot of complexity and extra points of failure. There is probably a low-current thermostat loop already in the heater that the switch could be hooked into. If not, all that is needed is a relay.

Would be nice to see pics of the tank, inside and out if possible.

A problem with a pressure switch (besides that it's more complex than a level switch) is that it can only tell you that the incoming water valve is open. It will not tell you whether the substance in the tank is water or air.

The air will not be flushed out until someone opens a hot water tap.

A problem with a pressure switch (besides that it's more complex than a level switch) is that it can only tell you that the incoming water valve is open. It will not tell you whether the substance in the tank is water or air....

That is a good point. Though, I am not sure if these cheap switches will even respond to anything but liquid pressure. In typical pump/pressure tank applications, the pressure switch is located on the input to the pressure tank. That way the switch is always "under water." So, it might be a good idea to move the pressure switch to the cold water input.

You could also add an electric valve on the inlet, and one on the drain. That way you could control the entire process coming and going.

Nice thought, a ToggleLinc by the door: On = "hello honey I'm home", etc.

Nuclear reactors have a similar basic problem... ...So I guess this isn't a terribly useful post.

Confirmation of concept is always good to hear.

I'm no expert on nuclear power but isn't the water just sitting there under the pressure of its own weight? That's why you can put a pressure sensor at the bottom of the tank to measure the depth.

A home water system is different. The pipes are closed and pressurized from one end. You'll have 50 psi (or whatever the regulator is set to) at all times, except perhaps lower when a faucet is open. Whether there is water or air in the pipes it will read the same.

As a side note: for a hobby project I developed a telemetry system for my community's mutual water company. I use pressure sensors in the bottom of the tanks to monitor the water level in each of four large (100K+ gal) storage tanks. Here are a couple that I have used.

http://www.globalw.com/products/levelsensor.html
http://www.stevenswater.com/catalog/ste ... 2793720%27

They give an analog output which is quite accurate, although we still use the simple microswitches for pump control. An interesting feature of these sensors is that they have vented cables, which expose the back side of the sensor to atmospheric pressure. This cancels out the air pressure that is pushing on the water from the top of the tank.

These would not work in this hot water application because the pressure does not vary with level, I just thought you might like to see them.

I'm no expert on nuclear power but isn't the water just sitting there under the pressure of its own weight? That's why you can put a pressure sensor at the bottom of the tank to measure the depth.

I believe it builds pressure from the heat of the reaction. If it were open to atmospheric pressure it would instantly turn to steam. Come to think of it, thats how they drive the turbines. You may be thinking of the ponds used to dump excess heat.

I believe it builds pressure from the heat of the reaction. If it were open to atmospheric pressure it would instantly turn to steam. Come to think of it, thats how they drive the turbines. You may be thinking of the ponds used to dump excess heat.

Gotcha - yes I was picturing the ponds.

...I use pressure sensors in the bottom of the tanks to monitor the water level in each of four large (100K+ gal) storage tanks...

So, effectively you have a scale that is measuring the weight of a column of water, from which you can calculate the height of the column.

...I use pressure sensors in the bottom of the tanks to monitor the water level in each of four large (100K+ gal) storage tanks...

So, effectively you have a scale that is measuring the weight of a column of water, from which you can calculate the height of the column.

Exactly. Then multiply by the cross section of the tank and I have gallons. Then differentiate over time and I get gallons/minute of flow!

Here you can see what the monitoring system looks like: http://www.seanadams.com/temp/shmwc.pdf
There are a lot more graphs for each site showing the raw input/output signals and other things.

The grey shading on the graph shows the times of day when electricity is more expensive so we do not run the pumps. The pumps used to be hardwired, using 24VAC control circuits leased from the phone company. Those liens had been installed 50 years ago and you can't even order them any more. Now everything is under software control which gives us much greater control and also lets us get alerts when any of the hardware fails.

I vote for bobeast's idea.

Could even make it a manual valve (dual for drain and fill) then use an ezio or just basic relay logic to turn on the electric. They have delay on relays, just dial in twice the normal time it take to fill and you should be ok)

(of course the manual valve means the guest needs to remember to turn it off when they leave. Insteon means you can check on it from anywhere)

I'm no expert on nuclear power but isn't the water just sitting there under the pressure of its own weight? That's why you can put a pressure sensor at the bottom of the tank to measure the depth.

Can't say the way things work currently, but I can tell you stuff that's about fifty years out of date and relevant only to the Navy. The primary reactor system is closed and pressurized to about 2000psi. This pressure is maintained by a big elevated water tower-looking thing called a pressurizer, at a slightly lower temperature than the rest of the primary coolant system, which forms a steam bubble at the top. Pretty good self-regulatory system.

bobeast's comment was slightly inaccurate... the primary coolant is used to heat the water in the secondary system, and that turns to steam -- the primary coolant does so only in the pressurizer. Or at least in my experience, which again is significantly out of date.

But this is off-topic -- I just meant to tell the O.P. that from what I could tell, his basic idea was sound. Not trying to be an Internet Expert... just saying I couldn't think of a better way of doing it

A home water system is different. The pipes are closed and pressurized from one end. You'll have 50 psi (or whatever the regulator is set to) at all times, except perhaps lower when a faucet is open. Whether there is water or air in the pipes it will read the same.

That's very true. A low-pressure condition should trigger a cutoff... a suitable-pressure condition should not trigger a cut on I apparently didn't have my thinking cap on.

How do people normally tell if their emptied hot water heater no longer has air in it? Isn't the outflow pipe at the bottom?

Friends (in the US) have a mountain cabin with an electric water heater. When they leave the cabin, they drain the water heater to avoid potential freezing problems. When they return, they must allow the water heater to fill to a point where the water covers the heater coils before turning on the power to the water heater - otherwise they could burn out the heater element. They have had problems with guests who do not turn on the water first, or forget to close the drain, even if they do turn on the water.

The obvious solution would seem to be a water detector near the top of the tank. But, getting a wired detector into the tank, without leaks might be hard. The best approach seems to be a pressure sensor tapped into the output pipe. As water enters the tank, it will compress, and eventually displace the air, raising the pressure in the outlet pipe. The pressure sensor (a simple pressure operated switch would suffice) would connect to an I/O-Linc or EZIO. The water heater would be connected to a 230 volt insteon switch. The I/O-Linc could then directly control the heater or do so via Indigo. A side benefit would be automatic shutdown when the tank was drained (or ruptured).

So... Does this idea sound reasonable? If not, I'd love to hear other thoughts. If so, any ideas where to find such a pressure sensor at a reasonable (I.e. less that the cost of a Mac-Mini) price?

EDIT: Duh, I was thinking electronic and totally forgot about standard pump equipment like this.

Still, what other ideas do you folks have?

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