Finned vs Cast Iron Boiler -- home heating

Ryan Flaherty

I need to replace a boiler in a home owner installed hot water retrofit. Has anyone got an opinion on finned copper tube boilers vs cast iron? I've only seen these type boilers on swimming pools.

The system is a mess. At about 8300 ft elevation. in a dirt cellar. He used PEX flex plastic that leaks frequently, bringing in much feedwater (and its O2). Control is by single T-stat on gas valve w/ constant circulator. Basically a single loop.

Thought a finned boiler might react faster, stand up to introduced O2 better. Will freeze better too (though may be able to replace finned wet element). Cost seems to be about same.

Ryan

John@Reliable

Boiler Type

You can't beat a cast-iron boiler,still work on units that are 75yrs old.

Boilerpro

Cast iron boilers and systems have changed in 75 years.

The biggest problem I see today with cast iron boilers is themal shock. With a modern baseboard system, the water temperature drops very quickly whenever the pump turns off. However, a cast iron boiler stays hot after the cycle, especially if it has a standing pilot and stack damper. When the pump turns on for the next cycle, it dumps all tthat cold system water into the hot boiler. With an old cast iron boiler, it had losts of water volume to help moderate that cold wate coming back. However, new boilers hold a very small amount of water (usually less than 5 gallons for home sized units) and the system piping and heating units hold this same amount. As soon as the pump turns on the boiler temperature usually drop 80F to 100F in less than a minute causing losts of stress on the castings.
Copper tube boilers, on the other hand cool quickly and the heat exchanger tend to deal with rapid temperature changes alot better, but they are much pickier whe it comes to installation.
I believe a properly installed cast iron boiler will probably last longer than a copper tube,(even though I've seen some 30 to 40 year old coper tube units running just fine), however, return tmmperature protection will need to be installed in the form of injection pumps, thermostatic mixing valves with P/S piping, or at minimum a system bypass.

A copper tube can usually be installed without this extra piping, but you must make sure adequete flow is provided, the boiler is properly sized to the load, a flowswitch installed, and usually only in single zone systems unless it is a high mass system. IMO

Boilerpro

Mike Kraft

I'm a little confused

Ryan
You say the system has"PEX flex plastic that leaks frequently, bringing in much feedwater ".Can the leaks be repaired?You also said that,"Control is by single T-stat on gas valve w/ constant circulator. Basically a single loop."

It seems to me that repair and design need adressing first.Otherwise you'll be putting good into bad.I am a fan of high mass boilers.CU HX boilers have a definite place in the field.If you go ahead with a changeout of boilers and dont address the leaking system you have a recipie for an early failure.Better check the water condition too........for examle,lets say you install a CU boiler (with system leaks)and you have a low PH ...........uhoh!

cheese

Unknown

Cast Iron Boilers and Cold Return Water

Modern copper baseboard systems do cool down rather quickly upon shutdown. Particularly those with pipes buried in outside walls (yes, folks really do that). However, the water content ratio of a cast iron boiler to these types of system is sometimes up to 3 to 1. Most cast iron boiler manufacturers products can handle this small amount of cool return water very well with little or no negative effects. Once the system water content begins to approach the same if not greater water content than that of the boiler is when the adverse effects begin to take place. You will generally see this with a cast iron radiator system or radiant floor system. Add to this the fact that these systems operate with much lower water temperatures and the concern grows. This is when boiler bypass piping and methods such as primary-secondary or injection piping are used to help overcome the thermal imbalance issue. These same effects can also occur with a copper tube type boiler and special care is needed as well to overcome them. Hope this helps.

Glenn Stanton

Burnham Corp.

Steamhead

I agree

if the system is messed up it will strain the boiler and kill it prematurely. Fix those leaks first!

This would also be a good time to go with PUMPING AWAY, multiple zones, outdoor reset etc. Many times if the system is being overhauled it doesn't cost that much more to make the changes, and you'll end up with a much better system as a result.

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Ryan Flaherty

Leaks / Controls

Thanks Guys,

I have repaired the leaks -- several times, thus I fear this will continue to be a problem. I don't know if this is because of age, poor installation, or inferior/defective flex. It may have been subjected to too high water temp. Repair by copper relacement is the only cure I can see, but not all connections are accessible.

The house is old and drafty, so continuous circulation is a good precaution against freezing (I've already had to thaw it once due to pump failure!). Also should help with thermal shock.

Mike Kraft

Ryan............

What type of pex?Manufacturer?Age of pex?Have any ratings on it?Color of tube?

cheese

Ryan Flaherty

pex

As I recall, it is blue-grey, about the color of plastic elec conduit. Trade name and specs were in gold letters.
If I get back by the job, I'll get the proper info. I'm guessing it to be 15 - 20 yrs old, but don't know. Most leaks occur at copper / flex transition. The copper is stubbed out Vertically, with the tubing making a U bend to next base board or a longer run Horz. to next room.

The tubing either leaks at connection, or cracks right at the clamp. Seems to be imbrittled right there. (they didn't support any of the flex. longer runs lay on ground most of way, short runs just loop down.)

Haven't collected the courage to crawl through the tight spaces to address that yet, nor convinced the owner it is necessary. No TLC for rental properties.

Ryan

Mike Kraft

Grey/gold letters........

sounds like poly butelyne.MAN name maybe Quest or Brass Craft?This tube has no 02 protection.I would give the HO an estimate including replacement of all of that tubing.Not sure what the temp. rating is on poly b.

cheese

Bill Clinton

polybutylene on baseboard

Been there, done that. I'm sure you're operating at fairly high temperatures, probably well within the ratings for that tube (I'm sure its pb. What happens at the points where the tube is crimped on the adapters to copper is called "plastic creep": The crimp ring pressure on the tube causes it to slowly flow (creep) away from the pressure point. My experience is that there is no cure. Gotta replace it. Sorry.

Bill