cast vs stainless combustion chamber

allenh

what are the major differences between cast iron and stainless for combustion chambers?
I have been reading literature from different manufacturers with all sorts of claims.
Which one will last the longer? Is stainless really more efficient when boiler fires for short periods like a fall evening?

Perry_3

It depends what you do with it...

Cast Iron has a limited temperature of operation. It will not tolerate for long cooler temperatures of the system water. This is becasue the combustion condensation gasses will both greatly spead up normal rusting with the moisture and the acid will eat away metal to boot...

This limts the efficiency that a cast iron - or welded steel heat exchanger can achieve.

If you are going to extract the latent head of condensation from the combustion gasses you need a heat exchanger that can stand up to the acidic combustion gasses. SS works admirably for that.

Thus, a condensing SS boiler is inherently more efficient than a cast iron boiler.

Further efficiencies can be gained by modulatin the firing rate so that the boiler does not have to start up and shut down a lot. A common method is to use an outdoor temperature sensor and a "reset" curve so that the boiler only produces the needed temperature of water to heat the house. Many houses can be heated with 90F - 100 F water on their already installed baseboard heating system when the temperatures outdoors is not that cold (whatever "that cold" is for your area). They only need "full" temperature when approaching a design cold day.

Thus, a modulating condensing boiler can achieve the most effieciency for many heating applications. This assumes that the boiler is properely sized - and that the appropriate reset curve is available for the house in question (and reset curves vary greatly depending on house construction and installed heating system). You do not want to oversize a modulating condensing (mod-con) boiler. You want it to run as much as possible and not cycle on and off.

Of course; proper installation and care of the boiler is required. Cleaning the heating loop (or at least the near boiler piping) with a basic cleaner to remove and neutralize any acidic fluxes and gunk. There have been questions raised about chlorine affecting SS heat exhangers - and I several weeks ago mentioned that it would probably not hurt to put an elcheapo carbon filter in prior to the fill valve to remove any trace chlorine. Finally, you should not clean the SS coil with household cleaners - get a cleaner from an industrial supply house that is chlorine free.

Perhaps this is over-reacting; but better to be safe than sorry a few years down the line.

There is also some Aluminum Block mod-con boilers on the market. In my opinion this is a poor choice of material for a boiler HX due to possible galvanic issues with copper and steel (Iron) systems and the fact that the combustion condenstion slowly disolves the HX (and the HX's are designed extra thick in order to allow a certain life while they are being eaten away - not that the Mfr's will tell you what the design life is).

You also have to be very carefull about water chemistry and most companies will not warranty cover replacement if you do not maintain fairly strick control over water chemistry. So - how often do you test your customers system water for PH, and how often do you adjust PH and other additives... Overall - reports from a number of contractors in Europe are that Aluminum block HX boilers are fairly short lived boilers (but cheap) - and that the HX can dissolve from the inside leaving a slimy mess (and I have posted the links to the european sites several times).

I will note that for new radiant floor construction where you do not have existing steel or copper pipes and baseboard - that Aluminum block boilers may be a better fit and would not have the galvanic issues. I assume that you may be able to better control system chemistry in those systems. That does not stop the slow corrosion of the Aluminum block HX from the condensation - but it may last as long as other modern boilers.

I also note that "thick" aluminum is not a good heat conductor - so don't fall for that line. Heat conduction is a factor of both the fixed material heat rate and the thickness of the material. A material that has 1/3 better heat transfor capabilities - but is twice as thick; actually is 1.32 times worse off as a heat conductor than the other material it was being compared against.

There is a ton of information on this site available by searching.


Edited to add: Changing to a condensing boiler will substaintially reduce stack temperatures. You will almost certialy have to reline the stack - or abandon it. Most mon-cons run out the side of the house.

An old style boiler installed as a replacment will probably last decades. No one really knows how long the mod-cons will last. They have not been on the market that long. Probably the cheapest is the Muchkin - Probably the most expensive is the Viessman Vitodens 200. Many models in the middle - and different levels of controls avialable as well (not all have a very adjustible reset curve).

Do you expect the cheapest to last as long as the more expensive models. I don't. And if you want to see a really rugged heavy duty built one look at the Vitodens 200 (which has a premium price).

Note also - I achieved a 45% fuel use reduction by switching from my old beast of a boiler to my Vitodens. Many others report other significant efficiency gains by installing mod-con boilers.


Hope this helps,

Do have a nice day,

Perry

(note I am a homeowner, a plant engineer, and installed a Viessmann Vitodens 200 in my house last fall).

Unknown

Cast iron and cold returns

Reading the postings here for a while, I am aware of the cautions from the manufacturers about cool return temperatures. They all seem to recommend bypasses, etc. to insure that the boiler operates above the condensing range.

Why didnt they worry about this years ago. Reading the old books like the 1940s B&G handbook there is never any mention of bypasses or other means of keeping the boiler above condensing temp. The handbook even promotes the efficiency of cold start systems which modulate the water temp by the run time to satisfy the thermostat. There is a graph which shows boiler temps running in the 90 degree range during warmer weather.

The gas fired cold start system in my parents house operates exactly in this manner, and has been doing so flawlessly since 1955. Return temps even at this time of year usually run 110-120 degrees, well within the condensing range. I check the boiler flues every season and ther is no excessive scale or rust buildup.


This cast iron boiler has operated in this manner for 50 plus years. Whats so different about todays cast iron boilers?

Gordy

Cold start

Mike a cold start system like your parents is probably why thier boiler is not getting thermal shock. Plus the boiler is probably oversized, less efficient so it also probably gets up to temps past condensing range rather quickly. Also the old beasty has alot more cast iron then new generation CI boilers, Another thing is water volume of the older boilers vs new.


I have a WM cgm7 that has a simple bypass that has never seen return temps higher than 98*. But my boiler is also twice as big as it needs to be. I know the owners manual states that if there is a delta T at the boiler of more than 50* constant circulation is recommended to prevent thermal shock. My boiler was installed in 1993 been running like that since then.

I know this goes against proper piping practice. But these are the reasons I have found that is probably letting an overly sized CI boiler survive the harsh enviroment when improperly piped.

Gordy

Perry_3

Cast iron and Water mass

The old boilers were fairly large with high mass cast iron and large water volumns. Running 110 return temps into a boiler that holds 20 - 30 gallons of 140 water is not a problem. Running 110 return temps into a boiler that holds 5 gallons of water is a different situation.

There are several modern boiler designs that do in fact do quite well in putting out 140 F supply and 120 F return temperaturs. They will last a long time.

However, change the system to a mod-con with a properly variable reset curve and it is likely and you will have 90 F supply temperatures and 80 F return temperaturs for many days - and only get to the 140 supply temperatures for 5 - 10 days a year. The vast majority of times you will likely run supply temperaturs about 100 F or so.

Substaintial energy savings in the process; but big difference on what effect it has on a boiler. A cast iron boiler would not last in this scenerio.

So it all comes down to do you want a modern high efficiency boiler and heating system - or just live with an old stlye low efficiency boiler.

I do note that a high efficiency boiler is usually a more expensive install as well. Many of us figure that the cost payback is worth it.

In the end it is your choice. But you cannot get a real high effiency cast iron boiler becasue cast iron will not stand up to the combustion side environment needed to maximimize efficiency.

Perry

G.Kaske_2

Water content

Perry, my cgm 7 only has about a 3.5 gallon water content. Alot has to do with the bypass also at 15 gpm the boiler is maybe only seeing 5 or 6 gpm out of that 15.

While I would love to go MOD/CON one hates to part with a perfectly functional 82% CI boiler. For me its a waiting game also. Hopefully HXs will improve with MOD/CONS the longer I wait. I have a copper ceiling radiant system so all the HX hub bub has me thinking.


Gordy