Cast Iron vs. Steel Boilers

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  #1  
Old 12-02-08, 05:22 PM
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Cast Iron vs. Steel Boilers

It seems like cast iron, sectionalized boilers are more prone to cracking. (Cast iron is less ductile than steel.) Most new residential hot-water boilers seem to be cast iron. (I'm not sure that any steel boilers for residential use are still being manufactured.)

I have an old (nearly 60 years) steel Kewanee boiler that is going strong. It is a two-pass, fire-tube type, natural-gas, with the tubes rolled into the tube sheets.

By flue-gas analysis, I'm getting nearly 80% efficiency. If I were to replace it with, say, a 90% efficiency boiler (which is optimistic), it would take quite a few years to recover the investment.

I'll stick with my steel boiler for the time being.
Doug
 
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Old 12-02-08, 06:02 PM
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Cast iron is so 20th century.

Most of the truly 'new' boilers have cast aluminum or stainless steel heat exchangers. These are the gas-fired modulating/condensing boilers. (Although there is a new cast iron mod/con made by Mestek. It's got a super-thick heat exchanger and is designed to allow sacrificial degradation over a couple decades.) These units run in the 92-98% efficiency range if you run at condensing temperatures. Perhaps more importantly from a savings perspective, they modulate their flame based on the heating load.

A somewhat-popular residential steel boiler is the Energy Kinetics.

Your 80% is basically an instantaneous combustion efficiency. That's not really a number that you can compare to an overall system efficiency, or even an AFUE.

A well-designed and installed gas-fired modcon would probably save you anywhere from 15-50% in fuel per year. Payback on these systems can run well under a decade in some situations. Worth considering.

But of course the simplest way to save fuel, with the shortest payback, is to insulate and air seal the building. Get the load down.
 
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Old 12-02-08, 07:32 PM
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Xphias is right, don't let that 10-12% fool you (80% compared to 90-92%) that 10-12% is HUGE in real life heating situations. Several years ago I took out a boiler that was installed in the early 50's and if my memory serves me correctly I believe it was a 3 zone Arcoliner running at around 76% with a 1375 rpm burner, stack relay, 3 B&G 100 circs, a coil for domestic hot water and a copper hot water storage tank. I replaced it with a 4 section Weil McClain boiler, (Running around 85%) an indirect water heater and 3 programmable thermostats and without exaggeration the homeowners fuel consumption went from 2000 gallons a year to 1100 gallons a year. That is huge!!!
 
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Old 12-03-08, 08:07 AM
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Here are the economics that I calculate for replacing my 80% efficiency hot-water boiler with one rated 90%:

Annual heating fuel cost (nat gas) = $2,000/yr (last winter)

Fuel savings (therms), @ 90% vs. 80% = 12.5%

Annual fuel cost savings = $2,000/yr x 12.5% = $250/yr

Ballpark cost: new 90% boiler, installed = $10,000

Payback period = $10,000 / ($250/yr) = 40 yr

'course, as natural gas prices increase in the future, the payback period would be shorter.
Doug
 
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Old 12-03-08, 10:20 AM
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You're not doing the economics right. That said, it's tough to do and you're definitely not alone. Some things to consider:

1) as mentioned above, you are not comparing apples to apples when comparing one 80% instantaneous combustion efficiency measurement to an AFUE of 90%. In fact, neither is a particularly good measure of overall system efficiency of a well-sized, well-installed, and well-controlled boiler-based heating system. AFUE is a much-derided standard, but it's 'what we got' for now.

A better, but still not ideal comparison would be AFUE(boiler A) vs. AFUE(boiler B). But that still doesn't tell you what the overall system efficiency will be in your house, because you could have a massively oversized, high AFUE boiler that performs poorly in your house for any of a number of reasons, such as oversizing, poor control strategy, etc. etc. etc.

2) You make the implicit assumption that the sizing and control strategy for the current boiler will be the same as the new boiler. Probably not a good assumption. For a gas-fired boiler, you'd want a mod/con, in which case you're looking at a 92-98% AFUE boiler that even when it's not condensing, is still modulating its flame (i.e., gas input rate) based on the heating load as determined by at least an outdoor sensor and more ideally an indoor sensor as well.

The apple to orange comparison here is comparing your "80%" boiler with a fixed gas input rate (let's call it 120,000 BTU/hr)to a boiler that might have an input rate that ranges anywhere from 15,000 BTU/hr to 80,000 BTU/hr. And 80-90% of the heating season, it will probably be in the <60,000 range.

3) You need to explicitly include future fuel costs. There is authoritative (but not perfect) data -- for Illinois -- from the US Energy Information Agency here:

Illinois Natural Gas Prices

Looking quickly at the numbers, it appears that your gas prices rose about 3.1% per year from 1990-2007, and about 4.5% per year from 2000-2007.

So, the basis of your payback goes up by around 4% per year over the life of the boiler. That's 8% in year 2, 12% in year three, 16% in year 4, 20% in year 5, etc. etc. It adds up.


Dealing effectively with issues 1 and 2 is not easy and has vexed most in the industry. What seems to be the case in real-world application is that conversion from older, typically 2x oversized bang-on/bang-off boilers to a mod/con using outdoor reset results in savings of anywhere from 15-50%. In some cases 70%. The crux of the problem is that there are many variables involved and they do not necessarily apply equally from one house to another.

It has also been discussed here on this forum (in a thread from last year called something like "to modulate or not") about how much difference there would be between conversion from an old, oversized, bang-bang system to either a) the mod/con/ODR system above, or b) a properly sized fixed fire cast iron boiler with variable speed injection mixing, ODR, etc. The premise of the argument for the latter is that much of the overall system efficiency that leads to fuel savings is derived from proper sizing, system design, and control strategy. There's a lot of merit to that argument.

But again, if you're looking for short payback, reduce the heating load. Insulate and air seal 'til you can do no more.
 
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Old 12-03-08, 11:40 AM
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Thanks.

Even if I'm off by a factor of two, the payback period would be 20 years instead of 40 years.

I doubt if I've got 20 years left - and I don't have an extra $10,000 either.
Doug
 
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Old 12-03-08, 12:41 PM
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And you probably are (off by a factor of 2).

Assume:

You use 1000 therms per heating season.
Gas is $1.20/therm including fees, etc.
Annual increase is 4%.
New boiler reduces annual fuel cost by 35%.

Cumulative savings in year 10 is $5043.

There are shorter payback options for your situation.

But... if it's really more like 60% (and who's to say without figuring out all the problematic stuff above), then you're paid back during year 12.
 
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