hot water flow sensor

[jessey]

Hello Everyone,

I'm looking to build a tank-less on demand electric water heater for myself. I just replaced my 40 gallon hot water tank about 5 months ago and it just started leaking yesterday. The hot water tank is still under warrantee and will be replaced at no cost (but not the installation) by the manufacturer but I thought this would be a good time to build my own on demand one and selling the replacement one would more than pay for this project.

I've already built a programmable thermostat circuit to control an electric space heater using a DS1820 chip so I should be able to utilize one for this project. I'm thinking that I'll have to use a water flow sensor of some sort to sense when the hot water tap is turned on so the microprocessor will know when to turn on/off the heating elements and also a DS1820 or similar chip to monitor the temperature and if it gets too high to shut off the elements. I've never had any experience in using a flow sensor and there are so many different ones on the market. Would anyone know of a good water flow sensor that would suit my application and would be easy to interface to a pic chip?

Also if anyone has done this sort of project I would be really interested in any do's or don'ts or comments about this type of project. I was a welder/heavy duty mechanic for years before I got into appliance repair (my current vocation) and have a big shop in my back yard with all the equipment that I'll need to accomplish this project on my own so I should be able to build one at a very low cost.

Can anyone help me to get started with this project?

Thanks
jessey

[Normnet]

PID
Proportional Integral Derivative Control
page 145

http://www.parallax.com/dl/docs/books/edu/ic.pdf

Norm

[jessey]

Hi Normnet,

Thanks for the information but I don't think I'll have any problems controlling the temperature of the water and was thinking of just using the temperature sensor as a sort of high temperature limit switch to shut off the heater elements in the event that the temperature exceeds a preset high temp level then turn them back on when the temp reaches a preset low temp level. The elements will be either on or off using a relay or a triac. I feel confident and shouldn't have any problems writing code for that part. Actually I think I'll be ok with the flow sensor as well once I find one that would be suitable for this application. I was also thinking of having the micro control a Proportioning Valve to control the amount of water passing through the heaters so that if I use a solar pre-heater for the in coming water supply then the water temperature would control the flow rate. The warmer the incoming water then the more flow rate I would have and when the incoming water was cold then the micro could limit the flow rate some to maintain a preset temperature for the water. I'll just use a manual valve to experiment with to see how it works and when I get it to work good then I'll use a proportioning valve. Do you think this would be a good approach?

Thanks
jessey

[Dave]

jessey, Use an inexpensive diferential pressure transducer across a fixed orifice. There are many different company's out there that supply small diferential pressure transducers. Motorola MPX2100 first comes to mind but there are many more... LOL

Dave Purola,
N8NTA,

[Darrel Taylor]

Jessey,

If you can accurately control the temperature of the water, then you don't need a flow sensor. Just maintain the water at a slightly elevated temperature, under no flow conditions. Once the tap is turned on, the water temperature will suddenly decrease. Detect the change to turn on the heaters. When the tap is closed the heaters will turn off, due to normal temp control. If they are off for a certain amount of time, revert to standby mode, which allows it to slowly cool off again.

That seems to be how our's work. It has a relay that clicks on for about 10-15 seconds every 20-30 minutes, just to keep it warmed up.

.

[jessey]

Hi Darrel,

That sounds really good as it would certainly make it more affordable and simpler to build. I'm not sure if I'd really need it but what do you think of having the micro control a Proportioning Valve on the inlet side of the heating unit which could control the amount of water passing through the heaters so that if I use a solar pre-heater for the in coming water supply then the incoming water temperature could control the flow rate. The warmer the incoming water then the more flow rate I could have and when the incoming water temperature was cold then the micro could limit the flow rate some if needed to maintain a minimum preset output temperature for the water? Something to think about.

I managed to fix my hot water tank yesterday with an epoxy glue for now so I won't be rushed to get this done and can now take my time. It sure is nice to have hot water again!

I was thinking of just welding a rectangular box together just big enough to house the heating elements with an inlet and outlet for the water and of course a pressure relief valve and possibly a sacrificial anode for corrosion protection. I'll do a search for some kind of anti rust protective coating for the box but that can be later. Thanks to all for the comments about doing this.

Thanks
jessey