Economics of condensing versus convential boiler


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Old 08-10-11, 08:40 PM
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Economics of condensing versus convential boiler

I can't decide which to install: gas condensing or conventional boiler. If my objective were to be impressive by using the least fuel, regardless of cost, then the choice would be easy - condensing. But since I want to save money, both in the short and long run, the negatives of condensing boilers seem to me to outweigh the positives. Positives- slightly higher efficiencies, 84 versus up to 96 ( though with my radiator system, the highest efficiency would never be possible). - no cost for chimney ( though direct vent conventional boilers don't need chimneys either). Negatives -higher equipment and installation costs. -shorter warranty periods ( 10, 12, 15 versus 25 years or limited lifetime ) - anecdotal reports of less reliable service and higher maintenance costs. - anecdotal reports of manufactures' denial of expensive warranty claims for minor or unrelated reasons. Am I wrong?
 
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Old 08-11-11, 08:43 AM
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As far as it goes, anecdotes are just stories. For every horror story I've seen, I've found others that say it's not true. The horror stories are oft-repeated, the WOW stories are fewer and even hard to come across. It mostly goes back to the installer - what he did or didn't do, and who he was. If you have someone dependable, trustworthy, conscientious, and knowledgeable, you'll get a good system that will give you many years of reliable service. If you think you're going to get something less, that's where the problems crop up. I've called for service for my boiler just to scope out the service department of the potential installer. So far, they've all failed to meet my standards. I want someone good, better than I am. I do my homework and more, they just don't seem to.
A three-hour labor charge for draining the system, before replacing the circulator, doesn't cut it. Then there was the charge to refill it. They didn't get the work.
As for your situation, is this a new construction, a retrofit, or a changeover from an existing system?
In doing the project what do you expect for an outcome? Yes, its low or lower TCO (projected), but in what context? for family size - more hot water, for house - more space?
 
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Old 08-11-11, 03:06 PM
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I made the same choice a couple years ago. I went for the conventional over the mod/con. The main reason is that they need to be cleaned every year to keep things in tip top shape. A conventional cast iron doesn't need such high level of attention. The cost of that service every year seemed like it would offset any savings in fuel I would gain from it. Your best bet to save money is to insulate and air seal your home, then get the new boiler. I wish I had done that myself, but I didn't.

So tell us about your radiator system and why you think you won't get the highest efficiency possible?
 
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Old 08-12-11, 06:30 AM
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Radiators are a fantastic match for mod-con boilers. Second only to radiant floors.

Mod-con also makes sense in older houses where you will be improving the building envelope. Good insulation and air sealing can easily cut an older home's heat loss by half or more. Been there, done that. Typically you address the heat loss first, then size the boiler. Especially if you are going with a conventional boiler. Kind of stinks to size a boiler, then cut the heat loss by 30-50% and have it be twice the size it needs to be.... Doh!

As you reduce heat loss, two things happen: 1) you need a smaller boiler, and 2) your heat emitters (radiators, whatever) become relatively oversized, which allows you to use lower water temperatures to meet the heat loss and thus keep a mod-con in the condensing range nearly all the time.

Case in point, my house. House has fin-tube baseboard that was originally sized for the old heat loss decades ago. Insulated and air sealed. Reduced heat loss by 50%. Even on the coldest nights, the supply water temp doesn't go above about 130-135F (return temp at that supply temp is around 120F), and it spends the rest of the heating season using supply temps between 85-120F. Around here, that's about 5800 hours and 5700-6000 degree days.

IMHO, a good install of a good mod-con in a building that can benefit from it, like an older home upgraded with insulation and air-sealing, especially one with big old cast iron radiators, is a smart investment for the short- and long-term. And also incredibly comfortable.
 
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Old 08-12-11, 12:21 PM
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I installed a cast iron boiler 34 years ago in the time that I used it, 28 years, I cleaned out the heat exchanger once and replaced the thermocouple. As far as I know it is still working.
It may have been only 80% efficient but it worked my base board heating a treat.....I will buy another.
 
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Old 08-14-11, 10:48 AM
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The cost of operation from one to the other is not that much different dependent of the operation water temperature. If the mod/con will operate in a condensing mode most of the time it will save you about 9% than when not condensing. If it does not operate in the condensing mode most of the time the cast iron over the mod/con will only be about 4 - 5 % more to operate. Assuming you spend $1000 after installing a new boiler, 5% of $1000 = $50. Can you clean it annually for $50. This coupled with annual maintenance will make the cost about the same. If you look at life cycle cost being installation, operation, maintenance and repairs the cast iron unit will be cheaper. This is all assuming the boilers are properly sized and have OD reset.
 
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Old 08-14-11, 01:45 PM
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What about modulation and cycle length?
 
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Old 08-15-11, 05:00 PM
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Modulation is not a huge money saver as the flue passes are not completely filled with flue gasses. Testing has proven that thermal transfer goes down with modulation. With that said it is about a wash between modulation and thermal transfer. When flue passes are filled is the best thermal transfer within the boiler. Short cycling is another issue. With properly sized cast iron boilers that may not be micro-zoned and thermal mass/water volume will not short cycle as much as anticipated. A 7 - 10 minute run time on a cast iron boiler is a good run time. This is all considering it is not a chimney vented boiler. If you chimney vent the boiler it changes the argument some. Sidewall vented boilers have less standby losses than chimney vented boilers.
The largest factors in fuel savings is proper boiler sizing and system efficiency. The boiler efficiency has minimal affect on fuel bills with todays new boilers unless the mod/con is operating less than 130f most of the time so you can take advantage of the extra 9% you gain from latent heat.
 
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Old 08-15-11, 05:46 PM
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Boiler efficieny needs to be put in context. I pay about $1,000 per year for natural gas for space heating (conventional hot-water boiler). That needs to be compared to my other recurring costs:

Snow removal
Water and sewer
Lawn care
Electricity
Cable TV
Internet service
Telephone (cell and landline)
Tree trimming
Driveway maintenance
Etc.
 
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Old 08-20-11, 08:42 AM
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May I say that the advantage so often mentioned with a condensing boiler is that heat is recovered from the exhaust gas to pre heat the incoming colder water, making them slightly more efficient.
This only works from a cold or almost cold start up, once the boiler has reached operating temperature this benefit disappears.
There is no doubt that if you already have a working boiler, you will save money by keeping it running, there are still plenty of spares out there for most.
 
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Old 08-20-11, 08:30 PM
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Perry, your concept of how a condensing boiler works is not quite correct.

They don't 'preheat the incoming cooler water' .

The design is such that the flue gases are condensed in the heat exchanger, extracting the normally lost latent heat from the gases and transferring that heat in a useful manner to the water.

If the system is properly designed, with the correct amount of heat emitters (which can meet the heat loss of the building with cooler water), they can run in the condensing mode most of the heating season.
 
 

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