preventing corrosion in a gas fired cast iron boiler

jcd457

I have a Burnham Series 2, M/N 205NI-A2 to heat my home. It's start temperature is set to 140F with an high limit of 180F and differental of 15F (factory settings). What constitutes a "sustained" low boiler water temperature? From a cold start the boiler temperature reading recovers to above 140 F in less than 10 minutes and reaches 180 F in 15 to 20 minutes. Does this pose a threat of corrosion for the boiler from flue gas condensation?

Tim McElwain

The theoretical dew point temperature for flue gas is 140 degrees any temp returning to the boiler below that temp on a continuing basis could cause some condensation. We typically strive for something around a 20 degree temperature difference between supply pipe sending heated water out into the system and a return temperature of at least 140 returning to the boiler. Ideally if the boiler was set to operate at around 180 degrees the a return temp of 160 would be pretty close to normal.

hot_rod

jcd457 said:

I have a Burnham Series 2, M/N 205NI-A2 to heat my home. It's start temperature is set to 140F with an high limit of 180F and differental of 15F (factory settings). What constitutes a "sustained" low boiler water temperature? From a cold start the boiler temperature reading recovers to above 140 F in less than 10 minutes and reaches 180 F in 15 to 20 minutes. Does this pose a threat of corrosion for the boiler from flue gas condensation?

The goal is to get that return temperature above 140F within 10 minutes of firing. Also look for short cycling, oversized boilers with multiple small zones can cause excessive cycling.

j a_2

Post a picture...you could be in need of a boiler bypass or primary secondary piping

jcd457

Thanks to all for responding to my question. I've ordered a pipe thermometer to monitor the return temperature. If I understand your advice correctly, the return temp should be at or above 140 F within 10 minutes of the boiler firing to prevent boiler corrosion from flue gas condensation. Thanks again!

mikeg2015

Minimum return water temp of 140F, is usually a setpoint of around 150F, since at that temp, the delta T is usually 10F assuming you use primary/secondary piping or a single zone, primary loop.

hot_rod

All boilers run in condensing mode from a cold start, there is no "official" time period to get them out of condensing mode, most would say 10 minutes or so. The important thing is the boiler and flue gets above dew point during the run cycle to keep corrosion at bay.

If you want to watch and record that get an Azel differential temperature sensor. It displays both temperatures and records delta T, fairly inexpensive, find them on Amazon and other places.

Formerly

boiler protection valve goes a long way, if installing one is in the cards (especially on a new boiler). I have a caleffi, works great (thanks rod)