Use Cast Iron Radiators for heat exchanger in storage tank?

If you make sure they don't leak, they'd likely work in some fashion.

They'd likely rust too.

EDIT: I don't think I'd put them in my DHW stream - most are very ugly inside.

I don't think I've heard of anyone even trying it - so you might be on your own, in a lone guinea pig sort of way. Sometimes that's the only way to find out if something will work.

Measure/monitor your temps going in & going out (difference = delta T). Not just once but constantly or periodically over time. Along with your tank temps. A more rigorous efficiency measurement would involve measuring flows too - but just seeing what the temps do over time with constant flow should be a pretty good seat-of-pants measure. If your delta T goes small after short time, that's not good.

One concern is that in a closed system, the oxygen in the water is quickly absorbed by creating a little bit of corrosion, but once that happens that's it. With DHW, you have a continuous supply of fresh oxygenated water, so I'm pretty sure that you'd get continuous corrosion. Now cast iron has much less of an issue than steel, but it would be good to hear from someone who has actual experience of using cast iron in such an application (unlike myself). Doesn't have to be radiators - any cast iron exposed to a steady supply of hot oxygenated water would do.

Nofossil said:

One concern is that in a closed system, the oxygen in the water is quickly absorbed by creating a little bit of corrosion, but once that happens that's it. With DHW, you have a continuous supply of fresh oxygenated water, so I'm pretty sure that you'd get continuous corrosion. Now cast iron has much less of an issue than steel, but it would be good to hear from someone who has actual experience of using cast iron in such an application (unlike myself). Doesn't have to be radiators - any cast iron exposed to a steady supply of hot oxygenated water would do.

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I agree, whats really inside those things i dont think I would drink from it?? This is a first for me,never heard of it,but dont mean it wont work,just few concerns

In my 1916 & 1920 copies of "the Ideal Fitter" there is a small paragraph entitled "for heating liquid in tanks" where it is stated "American Wall Radiators can be used advantageously in hot water storage tanks in place of the ordinary wrought- iron or copper coils,insuring an ample supply of hot water for a very small initial outlay " they give a small chart of using both steam and hot water for heat sources.I could not find this info in the 1929 edition but all 3 editions did give a comparison chart of iron vs brass piping used horizontally in a storage tank to heat water with steam.

So yes it has been done in the past but was obviously discontinued ,but why ?,probably many reasons perhaps the rust issue,the weight of cast iron compared to copper and eventually discontinued as a production item ,and also copper at one time was probably a better bargain price wise.

I have also heard that once cast has a thin layer of rust on it that actually protects the inner layer, the example I have often heard is take a rusted piece of cast and break it and only the outer surface will show signs of rust,of course you could always coat the radiators for extra protection.

In the above example brass vs iron,brass did have the edge requiring less square footage but if the radiators are free,use more of them.

If you are doing an unpressurized tank, you should probably treat the water when using iron in it.
There will be oxygen getting into the tank water regardless and eventually you will see corrosion.

A radiator is not going to work well within the tank in terms of maintaining stratification.
It is thicker walled than copper, which is an issue for heat transfer. Rust is not going to enhance heat transfer.
If you are using a liner, you need to protect it.

One leak in a radiator is going to ruin your day. Taking a rusty radiator out of your tank is not going to be fun.

At least you can pull a copper hx out easily.

As the Car Talk guys say, "it is the stingy man who spends the most".
Copper coils work well and are well documented.
I would save radiators for in room heating. They are excellent for that.

If the cost of hx was just too much to bear, I would treat the water in the tank with corrosion inhibitor and run the boiler and
heating loop unpressurized. It works and eliminates the space/boiler hx.

my 2 cents worth.

tom in maine said:

If you are doing an unpressurized tank, you should probably treat the water when using iron in it.
There will be oxygen getting into the tank water regardless and eventually you will see corrosion.

A radiator is not going to work well within the tank in terms of maintaining stratification.
It is thicker walled than copper, which is an issue for heat transfer. Rust is not going to enhance heat transfer.
If you are using a liner, you need to protect it.

One leak in a radiator is going to ruin your day. Taking a rusty radiator out of your tank is not going to be fun.

At least you can pull a copper hx out easily.

As the Car Talk guys say, "it is the stingy man who spends the most".
Copper coils work well and are well documented.
I would save radiators for in room heating. They are excellent for that.

If the cost of hx was just too much to bear, I would treat the water in the tank with corrosion inhibitor and run the boiler and
heating loop unpressurized. It works and eliminates the space/boiler hx.

my 2 cents worth.

Click to expand...

Amen!

A properly installed ci rad in a closed (i.e., hydronic) system won't have any corrosion or rust inside. Usually, it's just a fine coating of black residue which turns to dust when dried. Steam rads are a whole nother matter. They're full or rust, corrosion, cat hair, etc. As to whether or not the radiators would corrode in a non-pressurized tank, I think that if you seal it off properly (the tank), the ox levels will drop over time and you'll wind up with very little potential for major corrosion issues. Yes, a leak would be inconvenient, but a radiator in good condition that is properly installed and pressure tested is not going to leak. If it does, a little Boiler Stop Leak would probably fix it. Cast iron rads throw out A LOT of heat, thick walls be damned. They're designed to efficiently transfer heat. I don't see what difference there is between air and water in that regard, other than you'll get a lot more transfer (measurable BTU's) with water.

All that being said, I've never done it, so I can't endorse the idea without reservation. However, I would think that if you installed the rad (or rads) in a way that they could be relatively easily inspected and monitored, you should be able to anticipate and compensate for any potential problems if you keep an eye on them (like once or twice a year). I would keep the number of rads to a minimum, however, as the more piping you incorporate into the installation, the more potential for problems. But I would consider stacking them so that you get some stratification in the tank.

Let us know if you decide to move forward with this project. I'm not aware of anyone else who has tried it, and I'm dying to know how it would work.