Gas Condensation in Oil Boilers

Christopher Haley

Viessmann says that the design of it's Vitola 200 prevents the creation and accumulation of harmful gas condensate on the heat exchanger, making it the ideal boiler for radiant applications. I am wondering how serious a problem this can be for boilers that lack this mechanism. I've heard claims that it reduces the life of heat exchanger but by how much? (5 years, 10 years?).

Christopher Haley

Another quick question. Do new boilers featuring corrosion resistant cast iron solve this problem?

Kevin__Flynn

Corrosion

Perhaps more important than the condensation corroding the boiler over time is the fact the this condensation when dried leaves behind scaling. This scaling slows heat transfer which produces higher stack temperatures and decreases combustion efficiency.


Kevin Flynn

Constantin

A matter of degrees?

The Vitola is unique in that it can tolerate running at temperatures at which other non-condensing boilers would start to condense. It can also tolerate very low return temperatures without the flue gases condensing.

Ms. Vitola here has managed to run at 125-160°F all year, the higher temps mainly for DWH creation. Our stack temps are usually in the 240-275°F range, or about as low as you can safely go without a DV system. Even so, I would not attempt to run flue temps that low without a right-sized, insulated, and stainless-lined chimney. Viessmann allows you to raise the stack temp in 25°F increments by boring holes into the Vitola combustion target.

As for the life expectancy of other boilers under the same conditions, I think it depends on how you stress them and their designs. Some boilers like the Burnham MPO or the Buderus G215 are also designed to run at low return temps (though not on gas, and the G215 is a pretty "big" boiler). Other designs would presumably rot out in much less time, just look at the fine print in the manuals.

Considering how easy it is to protect older boiler designs from flue gas condensation, thermal shock, and other radiant-induced possibilities (P/S piping, bypass valves, etc.), I would not shy away from other boilers per se, I would simply account for the extra work that is required to make an older high-temp boiler function well in a low-temperature application.

The extra labor and materials may or may not result in a boiler plant that approaches the cost of a Vitola system. I doubt that older, high-temp boiler designs can combust as efficiently or have standby losses approaching those of a Vitola simply on the basis of flue gas temprature and boiler water temperature.

If your load is low enough, you could consider one of the two condensing oil boilers on the US market, i.e. the Monitor FCX or the Peerless Pinnacle. Those boilers are rated at 75kBTU/hr and 80 kBTU/hr DoE output, respectively and would offer an additional 5%-7% efficiency over the Vitola on the basis of latent heat. Right-sizing any of these boiler plants is likely to be even more important, efficiency-wise.

For gas-fired radiant-heat applications, I would consider a modulating-condensing boiler like the Vitodens. You reap not only the 12-14% latent heat efficiency gains, but also benefit from a HX design which is truely condensate-proof and a modulating burner. All these factors taken together ensure that the boiler runs at peak efficiency all year.

I chose the Vitola for my home solely because I wanted to burn oil, not gas, to heat my home.