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DirtFlinger 06-27-12 02:57 PM

Freezeproof batch preheater from old gas water heater?
We had to get a new water heater awhile back and even though we have natural gas instead of electric, I didn't want to waste the old tank. So, I stripped it down and it wasn't as hard as people make it out to be, not if you have a pipe wrench and saber saw.

So, there I had it all bare and noticed something strange... it has a 4" hole running right through it from top to bottom! Learn something new every day, I say. So, I start with the new, working, water heater and figure out how it works exactly and it's amazingly simple.

The air is heated by the gas jets in the base and it rizes through a spiraling baffel as it rizes up out of the center hole in the take and up through the HVAC tubes running outside through the roof.

But... what about all that wasted hot air? This is why I don't trust architects all that much.

So, with the realization that the initial design is elegantly simple but wastefully flawed, I came up with this:

Introducing the freezeproof solar batch preheater. Here's an angle with the back open and the reflector hidden.

The HVAC tubes aren't connected in this "sketchup" but the spot where the old vent used to sit will now run up and through the preheater, before exiting the new vent on top of the preheater.

Now, I don't expect the hot air to actually heat the water in the second tank, the preheater, but the idea is that it prevents it from freezing during the winter months or on cold nights because the water heater is always running and usually hot.

So, it would function as a simple preheater (the designs for this box with reflectors I got somewhere online, pm if you want a copy) when being heated by the sun, heating water before it enters the main tank, cutting down on the cost, perhaps significantly, of running the main water heater.

Any HVAC guys out there that can tell me what they think of this idea?


strider3700 06-27-12 04:31 PM

I've never had a gas heater. Do they have always on pilot lights or do they relight each time you need extra heat. Even if it's a pilot light I can't imagine the put out enough heat to overcome freezing in cold cold weather.

DirtFlinger 06-27-12 04:43 PM


Originally Posted by strider3700 (Post 22645)
I've never had a gas heater. Do they have always on pilot lights or do they relight each time you need extra heat. Even if it's a pilot light I can't imagine the put out enough heat to overcome freezing in cold cold weather.

That was my first question too. I spent all last winter checking up on it, going out there in the garage and feeling the tubes with my hands and it was always hot to the touch at night and lukewarm in the day. I checked it regularly and at random times and that's what led to me persuing it further.

It seems to me that a significant amount of heat is generated and just wasted. In fact, in the winter you can just go to town and see the houses pumping out huge plumes of heated water vapor, like they're trying to warm the neghborhoods.

I suppose if I don't have time to build it this summer, I'll take actual readings on the thing next winter to get some data behind it.

diyEthic 07-01-12 10:49 PM

I don't always trust mainstream architects and engineers either, but in this scenario the hot gases exiting the top of the water heater rise passively by convection. You don't want exhaust gases hanging around in your house, do you? Convection is the engine which draws the gases out of your house up the flue. This is no more of a "waste" than putting bumpers or a windshield on your car. Think of how much more efficient a car could be if it did not have to bear the weight or wind resistance of any safety feature on it. The engineering of safety features of any product nearly always have to strike a balance with added initial costs of manufacturing and lifetime costs of operation, and inefficiencies incurred.

The more efficient furnaces and water heaters which extract more heat from the exhaust gases require an electric blower just to pump out the exhaust. This makes them more complex and less reliable. One house I rented for 3 years had a furnace that the exhaust blower failed twice during that time period.

While I like your idea of re-using the old water heater AND of re-purposing the exhaust gases to prevent freezing in the pre-heater, please make sure your re-routing of the exhaust doesn't weaken the convection engine which is supposed to draw the potentially poisonous gases safely outside of your home.

In my house, one thing I've done that reduces (somewhat but not dangerously) the exhaust temp of my radiant system boiler and my water heater is to limit the flow of the gas supply by partially closing the valve on the supply pipe approaching the 2 appliances. The reason I originally did this is a bit of a long story, posted on another forum ---> Mysterious gas water heater problem - Community Forums

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