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Copper and Black Pipe

doitall's picture

OK, so the problem with dissimilar metals - copper joined to galvanized pipe, is understood.  But what about copper joined directly to black pipe.  For example, a residential situation with old upright cast iron radiators, exisiting black pipe supply and return lines, a new boiler, and copper tying the new boiler into the existing black pipe lines.  Is corrosion still an issue to be concerned about.


 


Thanks for your input. 

(post #58935, reply #1 of 14)

Yes.  Black pipe is steel with black paint on it.  Galvanized pipe is steel with zinc on it.


 


-- J.S.


 

 

 

-- J.S.

 

(post #58935, reply #2 of 14)

The Black vs. Galvanized is not really an important factor here.  The Zinc coating on galvanized will sacrifice itself first to save the iron, but this is also not really important in this case.


Corrosion is not generally a problem for closed hot water heating systems.  Mixing copper and iron for near boiler piping is more common than not.  The reason is that the potential electrolytic reaction between the two is starved for oxygen.  The same water is typically used for many years, and the dissolved oxygen is quickly expelled and depleted.  I have observed systems that have been in service for several decades with no problems.  In my own home, I have black iron pipes from around 1910 that were mixed with some copper in 1962, and no problems today.


Potable water systems have an endless supply of fresh dissolved oxygen, so they have big problems.  Steam heating systems can have problems as well.


Galvanized pipe is not recommended for hydronic heating applications, due to potential problems with bits of zinc collecting in undesirable places.  Forget the exact details on this...

(post #58935, reply #3 of 14)

I'd agree with csnow's analysis here.  My old place has a 70-yr old closed-loop hydronic heating system with cast iron rads and boiler, black steel pipe, and copper tubing/sweat soldered fittings where it was too tight for them to fit black pipe.  The carbon steel and cast iron are intact, even at the joints where brass or copper meets iron or steel, and the water always has that mild sulphurous smell which indicates that it's gone anaerobic (i.e. run out of oxygen).  Even though there's a big galvanic potential to drive the generation of rust, there's no oxygen to complete the other "half-cell" and let the rusting proceed, so the steel stays happy and intact.


I've wondered how those systems which give both heat and hot water using the same boiler are configured- do they have separate exchangers for heat and hot water, so that the water in the heating system remains closed-loop?  Or is there a possibility for heating water to end up in the hot water supply?  That's not very appetizing...

(post #58935, reply #4 of 14)

"I've wondered how those systems which give both heat and hot water using the same boiler are configured- do they have separate exchangers for heat and hot water, so that the water in the heating system remains closed-loop?  Or is there a possibility for heating water to end up in the hot water supply?  That's not very appetizing..."


That's a whole other matter.


Some folks do advocate so called 'open' heating systems, where the heating water and DHW are one and the same.  Even advocates recognize that one cannot have any iron in this config.  Too much free O2. Circulators must be bronze or stainless, Pex or copper tubing throughout, water heater or stainless steel boiler, and so forth...


As you mention, stagnant water is a potential health issue, and there are several other consideration.  Open systems are illegal on many jurisdictions in the US.


The conventional method with a boiler is to use a 'tankless coil' or an 'indirect tank' to exchange heat to the DHW.  For heating systems that use water heaters, plate heat exchangers seem to be the mainstream solution for segregating the DHW.


If you end up getting yourself a condensing boiler, adding DHW is a great way to 'leverage' your investment in having efficient heating equipment.  Typical gas water heaters are notoriously inefficient.

(post #58935, reply #5 of 14)

Please 'splain me what you mean by a "tankless coil" for producing the direct hot water- sounds good, but I don't know what you mean.  The nice modulating condensing boiler you recommended to me has only one heat exchanger- so- are you suggesting that I short-cycle my heating boiler year-round to produce hot water on demand via an exchanger, and valve all my hydronic heating loops so that they're only open to circulation when I want to heat the house?  Will that really be more efficient than my crappy existing tank-type hot water heater?

(post #58935, reply #7 of 14)

A 'tankless coil' is sort of like a heat exchanging coil of pipe within a pipe.  A 'very small tank', would be a better description.  The idea is 'hot water on demand' and 'no standing losses'.  Personally, I do not care for them, they would not work well with a low-mass boiler, and HTP does not recommend them.


A much better design, IMHO, is an indirect tank.  A low-mass boiler works well for an indirect because there is very little 'stranded heat' in the boiler.  Also, the boiler ramps up very quickly.  Standard boilers are rather inefficient until they reach operating temperature.  Standing losses through the tank are not a significant factor with a good indirect.  High mass boilers can be ok for this if they are loaded with insulation, like some of the German models.


Generally, the indirect is piped like any other heating zone, but is wired for 'priority' at the zone controller, meaning all other zones shut off when the indirect zone calls for heat.  Otherwise, the boiler would need to be sized to handle both loads at once, which would make it way oversized most of the time.  This is the typical indirect setup regardless of boiler.


With the latest [optional] HTP controls, the indirect zone can be setup to receive a different water temperature than the heating zones, which is great for low temp radiant applications.  The controls even allow the heating zones to run on 'full outdoor reset', meaning heating zones get the [relative] 'reset' water temp, while the DHW gets a specific [higher] set temp.  Am I making sense?  This is of particular advantage for a condensing boiler, since it runs more efficiently at low temps.


This setup is way more efficient than an ordinary gas water heater.  Using it for DHW leverages your investment in an efficient heating appliance.


Best of luck.

(post #58935, reply #10 of 14)

Thanks, csnow.  I understand exactly what you mean.  You definitely need the flexibility to run the controls in the way you've described so you'd be able to get your hot water on demand up to the right temperature with a modestly-sized circulator and indirect exchanger. 


These Grundfos circulators that people talk about:  are they single mechanical sealed units?  The catalog picture I've got for the UP series closed-system circulators look suspiciously like they're single mech sealed.  I've been looking at high-temp bronze magdrive pumps from Little Giant, March etc. as an alternative to sealed pumps because I have a well-deserved hatred of mechanical seals (and I can get the magdrive units at OEM prices through work, so they're cheap too).  How long do the Grundfos units last before you have to tear the little buggers apart and service them?

(post #58935, reply #11 of 14)

The circ on a DHW zone with a low mass boiler is larger than one might expect.  You need a good amount of flow through a low mass Hx.


I should mention that an indirect tank can keep up with a substantial DHW demand even after all of the stored hot water has been depleted.  You are throwing the full force of the boiler at the task, which is generally far beyond the heat transfer capabilities of most gas water heaters.  Obviously depends upon the boiler in question, and the design of the indirect.  I can take endless extra-high-volume showers with my 140kbtu unit.  Could probably serve 3 low-flow showers just as well.


I do not know much about those other circ brands you mention.  Taco and Grundfos own most of the residential market.  Both have excellent reputations for longevity.


Edited 11/26/2003 10:54:01 AM ET by csnow

(post #58935, reply #12 of 14)

What is the basic difference between a low mass boiler and an on demand hot water heater?

From what I have read here there function is very similar.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #58935, reply #13 of 14)

"What is the basic difference between a low mass boiler and an on demand hot water heater?"

They are very similar in many ways.


Some folks do use instant HWHs for space heating.


Boilers are generally not rated for potable water, and may not be up for handling the free O2.


There are even 'hybrids' that do both right out of the box, like the MZ 25s.


http://www.mzboiler.com/products/mz/mz25s.html

(post #58935, reply #14 of 14)

Thanks

I have a downdraft furnace for the first floor, updraft for the 2nd floor, and HW heater all in a small untility closet.

Once of these days the furances are going to need to be replaced. The downdraft unit is GROSSLY oversized. However, I don't get wide temp swings the heating ducts are in the concrete slab and it gives some tempering.

But the idea of combining all 3 units into one heat source with hydro/air sounds interesting.

I think that a more continous low heat would work better for the first floor so I think that the oposite type of system with a hot water resivor would be better for my application.

But this is just in the back of my mind "think about" stage. At this point I don't have any numbers.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #58935, reply #6 of 14)

"Some folks do advocate so called 'open' heating systems, where the heating water and DHW are one and the same.  Even advocates recognize that one cannot have any iron in this config.  Too much free O2. Circulators must be bronze or stainless, Pex or copper tubing throughout, water heater or stainless steel boiler, and so forth..."


I agree.  I was rushing to finish our RFH cabin and threw in a cast-ron circ pump because I had it lying around.  A few months later, I'd gotten around to replacing it with a brass-bodied one and there was noticeable rust inside and outside the junction of copper/iron (open system).  Otherwise it was all PEX, copper, and brass and no problems.  Still none.


But, to take this question further:  What about the HWH?  Doesn't it see oxygenated water in the very common application: heating incoming city/well water for domestic hot water?  Would you expect / does anyone see a reduced service life when an open hydronic heating uses the HWH?  Certainly has a higher duty cycle and burns a lot more gas each year.  But that effect life?  Or accelerate the anode replacement schedule?


David Thomas   Overlooking Cook Inlet in Kenai, Alaska
David Thomas   Overlooking Cook Inlet in Kenai, Alaska

(post #58935, reply #8 of 14)

"But, to take this question further:  What about the HWH?  Doesn't it see oxygenated water in the very common application: heating incoming city/well water for domestic hot water?  Would you expect / does anyone see a reduced service life when an open hydronic heating uses the HWH?  Certainly has a higher duty cycle and burns a lot more gas each year.  But that effect life?  Or accelerate the anode replacement schedule?"


I would predict a shorter service life from more burn time.


I would think an open heating system would be exposed to more moving free O2 more of the time, but I really cannot say if this is significant, since reactions with the tank may never be starved for O2 anyways.


If it were setup to exchange DHW for space heating in a closed system, I'm still doubting the reaction with the tank would ever be starved for oxygen.


Regular gas water heaters tend to fail when the bottom rots out.  It would be logical to presume that this would happen faster if the burner is on more of the time. (more heat + more acid =more reaction)


Not sure how greater usage impacts the super-efficient water heaters that are popular for heating applications. These generally have stainless tanks.

(post #58935, reply #9 of 14)

An alternative theory I have heard is that the bottom fails when the repeated expansion and contraction cycles produce fine cracks in the glass lining of the tank, allowing water to get to the metal and produce corrosion.  If this is the case, then longer life would be had with long burn times if they gave fewer total cycles.  However, I don't really know what the exact mechanism is, just throwing out something I have heard.